tethers in space handbook

TethersInSpaceHandbook Edited by M.L.Cosmo andE.C.Lorenzini Smithsonian Astrophysical Observatory for NASA Marshall SpaceFlightCenter GrantNAG8-1160 monitored byC.C.Rupp M.L.Cosmo andE.C.Lorenzini, Principal Investigators ThirdEdition December 1997 TheSmithsonian Astrophysical Observatory isamember ofthe Harvard-Smithsonian Center forAstrophysics FrontCover: (left)PhotoofTSS-1takenfromtheShuttle cargobay,1992; (right)PhotoofSEDS-2 inorbittakenfromtheground, 1994. ' FOREWORD Anewedition oftheTethers inSpaceHandbook wasneeded afterthelastedition published in1989.Tether-related activities havebeenquitebusyinthe90's.Wehavehad theflightsofTSSIandTSS1-R, SEDS-1 and-2,PMG,TIPSandOEDIPUS. Inlessthanthree yearstherehavebeenoneinternational Conference onTethers inSpace,heldinWashington DC,andthreeworkshops, heldatESA/Estec intheNetherlands, atISASinJapanandatthe University ofMichigan, AnnHarbor. Thecommunity hasgrownandwefinally havereal flightdatatocompare ourmodels with.Thelifeofspaceborne tethers hasnotbeenalways easyandwegotourdoseofsetbacks, butwefeelprettyoptimistic forthefuture. Wearejust stepping outofthepioneering stagetostarttousetethers forspacescience and technological applications. Aswearewriting thishandbook TiPs,aNRLtetherproject is flyingaboveourheads. Thereisnoemphasis inaffirming thatasoftodayspaceborne tethers areareality and theirpotential isfarfrombeingfullyappreciated. Consequently, alargeamount ofnew information hadtobeincorporated intothisnewedition. Thegeneral structure ofthehandbook hasbeenleftmostly unchanged. Thepasteditors havesetastylewhichwehavenotfeltneeded change. Thesection ontheflightshasbeen enriched withinformation onthescientific results. Thecategories oftheapplications have notbeenmodified, andinsomecaseswehavementioned theexistence ofrelated flightdata. Wefeltthatthesection contributed byJoeCarroll, calledTetherData,should be maintained asitwas,beinga"classic" andstillveryaccurate andnotatallobsolete. Wehaveintroduced anewchapter entitled SpaceScience andTethers sinceflight experience hasshownthattethers cancomplement otherspace-based investigations. Thebibliography hasbeenupdated. Duetothegreatproduction inthelastfewyearswe hadtorestrict oursearchtoworkspublished inrefereed journal. Theproduction, however, is muchmoreextensive. Inaddition, wehaveincluded thesummary ofthepaperspresented at thelastInternational Conference whichwasaforumforfirst-hand information onallthe flights. Wewouldliketothanktheprevious editors, W.Baracat andC.Butner, P.Penzo andP. Amman, forhaving donesuchagoodjobinthepasteditions thathasmadeoursmucheasier. Thecompletion ofthishandbook wouldnothavebeenpossible without thecontributions fromthefollowing people: A.Allasio A.Jablonski J.Puig-Suari F.Angrilli L.Johnson W.Purdy S.Bergamaschi K.Kirby C.C.Rupp M.Candidi J.Longuski D.Sabath J.Carroll M.Martinez-Sanchez J.Sanmartin K.Chance P.Merlina A.Santangelo S.Coffey L.Minna S.Sasaki D.Crouch J.McCoy N.Stone R.Estes A.Misra B.Strim L.Gentile V.Modi T.Stuart F.Giani P.Musi G.Tacconi M.Grossi M.Novara G.Tyc D.Hardy K.I.Oyama F.Vigneron R.Hoyt P.Penzo M.Zedd ii Also,wewouldliketothankthestaffoftheScience MediaGroupatSAOfortheirhelp. NASA support forthisworkthrough GrantNASS-1160 fromNASA Marshall SpaceFlight Centerisgratefully acknowledged. MarioL.Cosmo Enrico C.Lorenzini Smithsonian Astrophysical Observatory Cambridge, Massachusetts December 1997 iii Tethers inSpace Handbook -Third Edition Table ofContents Pa_e Foreword ..................................................................................................................... ±i SECTION 1.0TETHER FLIGHTS 1.I 1.2 1.3 1.4 1.5TheTethered Satellite System Program: TSS-1 andTSS-1R Missions ............................................................................. 2 TheSmallExpendable Deployer System (SEDS): SEDS-1 andSEDS-2 Missions .............................................................. 15 ThePlasma MotorGenerator (PMG) .................................................. 21 TheTether Physics andSurvivability Spacecraft (TIPS) ...................... 25 TheOEDIPUS Tethered Sounding Rocket Missions .............................. 28 SECTION 2.0PROPOSED TETHER FLIGHTS 2.1 2.2 2.3 2.4 2.5 2.6 2.7Electrodynamic Tethers ForReboost oftheInternational SpaceStation ...................................................................................... 37 AnUpperAtmospheric Tether Mission (ATM) ................................... 41 TheNavalResearch Laboratory's Advanced Tether Experiment .......................................................................................... 46 TheAIRSEDS-S Mission .................................................................... 49 TheRAPUNZEL Mission ................................................................... 51 Tether Mechanism Materials andManufacture Project......................... 52 TheSpaceTether Experiment (STEX) ............................................... 53 SECTION 3.0TETHER APPLICATIONS 3.1 General ............................................................................................... 55 3.2 Tether Applications Listing ................................................................ 56 3.3 Tether Applications ............................................................................ 58 SECTION 4.0TETHER FUNDAMENTALS 4.1 4.2 4.3GRAVITY GRADIENT ...................................................................... 119 4.1.1 General ....................................................................... 119 4.1.2 Controlled Gravity ....................................................... 124 4.1.3 Constellations ............................................................. 128 ROTATION OFTETHER SYSTEMS ................................................. 132 4.2.1 General ....................................................................... 132 4.2.2 Controlled Gravity ....................... ............................... 132 MOMENTUM EXCHANGE ............................................................... 134 4.3.1 General-Conservation ofAngular Momentum ............ 134 4.3.2 Tether Payload Deployment ....................................... 135 4.3.3 OrbitVariations ............................... 136 iv Page 4.4 4.5ELECTRODYNAMICS .................................................................... 137 4.4.1 General ..................................................................... 137 4.4.2 Electric PowerGenerators ........................................ 137 4.4.3 Thrusters .................................................................. 146 4.4.4 ULF/ELF/VLF Antennas ........................................... 148 4.4.5 Constellations .................................... ....................... 150 REFERENCES ................................................................................ 151 SECTION 5.0TETHER DATA 5.1 5.2 5.3 5.4 5.5 5.6General ................................................................................................. 153 Generic Issues ....................................................................................... 154 OrbitEquations andData...................................................................... 155 5.3.1 OrbitsandOrbital Perturbations ................................... 155 5.3.2 Orbital Perturbations .................................................... 157 5.3.3 Aerodynamic Drag....................................................... 159 5.3.4 Thermal Balance .......................................................... 161 5.3.5 Micrometeoroids andDebris ......................................... 163 Tether Dynamics andControl ............................................................. 165 5.4.1 Gravity Gradient Effects ............................................... 165 5.4.2 Dumbbell Libration inCircular Orbit............................. 167 5.4.3 Tether Control Strategies .............................................. 169 5.4.4 Momentum Transfer Without Release .......................... 171 5.4.5 OrbitTransfer byRelease orCapture ............................ 173 5.4.6 Energy andAngular Momentum Balance ...................... 175 Tether Material Consideration ............................................................. 177 5.5.1 Tether Strength andMass............................................ 177 5.5.2 Tether Impact Hazards ................................................ 179 Electrodynamic Tethers .................................................................... 181 5.6.1 Interactions withEarth's Magnetic FieldandPlasma ...181 5.6.2 Electrodynamic OrbitChanges .................................... 183 5.6.3 Tether ShapeandLibration Control ......................... 185 SECTION 6.0SPACE SCIENCE ANDTETHERS 6.1 6.2Overview ........................................................................................... 188 Synergy ............................................................................................ 191 SECTION 7.0REFERENCES 7.1 7.2 7.3General .............................................................................................. 195 TableofContents oftheFourth International Conference on Tethers inSpace ................................................................................. 196 Bibliography ....................................................................................... 210 SECTION 8.0CONTACTS ................................................................................. 218 Acknowledgments .............................................................................. 23/4 V SECTION 1.0TETHER FLIGHTS 1.1TheTethered SatelliteSystemProgram: TSS-1andTSS-1RMissions S-BandCo._unk:atkx_ ToSatellite KU-Band Tracking OfSamlUte Figure 1.1TSS-1Satellite andTetherAttached to12MeterExtendible Boom TheTethered Satellite System (TSS)wasproposed toNASA andtheItalian Space Agency (ASI)intheearly1970's byMario Grossi, oftheSmithsonian Astrophysical Observatory, andGiuseppe Colombo, ofPadua University. Ascience committee, the Facilities Requirements Definition Team(FRDT), metin1979toconsider thepossible scientific applications forlongtethers inspaceandwhether thedevelopment ofatethered system wasjustified. TheFRDTreport,published in1980,strongly endorsed aShuttle-based tethersystem. ANASA-ASI memorandum ofunderstanding wassignedin1984,inwhich NASA agreedtodevelop adeployer system andtetherandASIagreedtodevelop aspecial satellite fordeployment. Ascience advisory teamprovided guidance onscience accommodation requirements priortotheformal jointNASA-ASI Announcement of Opportunity forscience investigations beingissuedinApril,1984. Thepurpose oftheTSSwastoprovide thecapability ofdeploying asatellite onalong, gravity-gradient stabilized tetherfromtheSpaceShuttle whereitwouldprovide aresearch facility forinvestigations inspacephysics andplasma-electrodynamics. Nineinvestigations wereselected fordefinition forthefirstmission (TSS-1) inJuly,1985. Inaddition, ASI agreedtoprovide COREequipment (common tomostinvestigations) thatconsisted oftwo electron guns,current andvoltage monitors andapressure gaugemounted ontheOrbiter, and alinearaccelerometer andanammeter onthesatellite. NASA agreed toaddahand-held low lightlevelTVcamera, fornight-time observation ofthedeployed satellite. TheU.S.Air ForcePhillips Laboratory agreedtoprovide asetofelectrostatic charged particle analyzers, mounted intheShuttle's payload bay,todetermine Orbiter potential. 2 Figure1.2TSS-1Configuration onOrbiter During TSS-1, whichwaslaunched July31,1992onSTS-46, theItalian satellite was deployed 268mdirectly abovetheOrbiter whereitremained formostofthemission. This provided over20hoursofstabledeployment inthenearvicinity oftheOrbiter--the region ofdeployed operations thatwasofgreatest concern priortothemission. TheTSS-Iresults conclusively showthatthebasicconcept oflonggravity-gradient stabilized tethers issound andsettled several shortdeployment dynamics issues,reduced safetyconcerns, andclearly demonstrated thefeasibility ofdeploying thesatellite tolongdistances-which allowed the TSS-1R mission tobefocused onscience objectives. t,"_'-",-'-"""""---''""""""" """"-'" Figure1.3TSS1andTSS1R Timelines TheTSS-1R mission waslaunched February 22,1996onSTS-75. During thismission, thesatellite wastohavebeendeployed 20.7kmabovetheSpaceShuttle onaconducting tetherwhereitwastoremain formorethan20hoursofscience experiments, followed bya second stopforanadditional seventoninehoursofexperiments atadeployed distance of 2.5km. ThegoalsoftheTSS-1R mission weretodemonstrate someoftheuniqueapplications of theTSSasatoolforresearch byconducting exploratory experiments inspaceplasma physics. Itwasanticipated thatthemotion ofalongconducting tetherthrough theEarth's magnetic fieldwouldcreatealargemotional emfthatwouldbiasthesatellite tohighvoltages anddriveacurrent through thetethersystem. Thecircuitforthetethercurrent wouldbe closedbyalargeexternal loopintheconducting ionospheric plasma whereanarrayof physical phenomena andprocesses wouldbegenerated forcontrolled studies. Although theTSS-1R mission wasnotcompleted asplanned, theItalian satellite was deployed toadistance of19.7kin--making TSS-1R thelargest man-made electrodynamic structure everplaced inorbit. Thisdeployment wassufficient togenerate highvoltages across thetether andextract largecurrents fromtheionosphere. These voltages and currents, inturn,excited several spaceplasma phenomena andprocesses ofinterest. Active tether science operations hadbegunatsatellite fly-away andcontinued throughout the deployment phase,whichlastedmorethan5hours. Asaresult,ahigh-quality datasetwas gathered andsignificant science activities hadalready beenaccomplished priortothetime thetetherbroke. Theseactivities included themeasurement ofthemotional emf,satellite potential, Orbiter potential, current inthetether, charged particle distributions, andelectric andmagnetic fields. Significant findings include: (1)Currents, collected bythesatellite atdifferent voltages during deployment, that exceeded thelevelspredicted bythebestavailable numerical models byfactors ofup tothree(seefigure1.4). (2)Energetic electrons, thatarenotofnatural ionospheric origin andwhose energy ranged ashighas10keV,wereobserved coincident withcurrent flowinthetether. These datasuggested possible energization ofelectrons bywave-particle interactions(see figure1.5). (3)Alargeincrease ofthetethercurrent, aprecipitous dropofthesatellite biasvoltage, veryintense andenergetic ionfluxesmoving outward fromthesatellite's high-voltage plasma sheath, andastrongenhancement oftheacelectric fieldinthe200Hzto2 kHzrange-all observed tobeconcurrent withasatellite ACSyawthruster firing. Theseobservations implyaplasma density enhancement byionization oftheneutral gasemitted bythesatellite thrusters. SL.II),,S.**I,,_&.,.I... 0200400_8001000'1201 smakv_o_is)3/03:_6:05 tO_1/08.'_ 107'.:........ •..... )u_'L.,o,t 1 .t 10"e:-_---.=_.--...... ._r 1 10 100 tO00 Figure 1.4Measured TSS-1R and theoretically predicted I-VcharacteristicsFigure 1.5Energetic electron Population measured atthesatellite's surface. Itisalready apparent, therefore thatthedatagathered duringTSS-IR havethepotential tosignificantly refinethepresent understanding ofthephysics of(1)thecollection of current andproduction ofelectrical powerorelectrodynamic thrustbyhigh-voltage tethered systems inspace,(2)theinteraction ofspacecraft, andevencertain typesofcelestial bodies, 4 withtheirlocalspaceplasmas, and(3)neutral gasreleases inspaceplasmas andtheireffect onbothoftheaboveprocesses. (Rm3B mmmm& _mh) gtmUm Figure 1.6TSSFunctional Schematic Thesensor package ontheboomwaselectrically isolated fromthesatellite, andits potential wascontrolled bytheROPEfloating powersupply. Forsatellite potentials upto 500V,thesensor package wasmaintained nearthelocalplasma potential toallow unambiguous measurements tobeobtained. Thepotential ofthesensorpackage couldalsobe swepttoallowthepackage itselftoserveasadiagnostic probe. TSS-1R Science Investigators TSSDeployer CoreEquipment and Satellite CoreEquipment (DCORE/SCORE) CarloBonifazi, Principal Investigator Agenzia Spaziale Italiana TheTethered Satellite System Core Equipment willdemonstrate thecapability ofatethered system toproduce electrical energy andwillallowstudies oftheelectrodynamic interaction ofthetethered system withtheionosphere. TheTSS CoreEquipment controls thecurrent flowing through thetether between the satellite andtheorbiter andmakes a number ofbasicelectrical andphysical measurements oftheTethered Satellite System. Deployer CoreEquipment consists of several instruments andsensors onthe starboard sideoftheMPESS inthe payload bay. Amaster switch connects thetetherconductor toscience equipment intheorbiter payload bay;apower distribution andelectronic control unit provides basicpower, command, anddata interfaces forallDeployer Core Equipment exceptthemaster switch; anda voltmeter measures thetether potential withrespect totheorbiter structure. The Core Electron Accelerator hastwo electron-beam emitters thatcanejectup to750milliamperes ofcurrent fromthe system. Two other instruments complement theelectron accelerator's operations: avacuum gaugetomeasure ambient gaspressure andtoprevent operation ifpressure conditions could causearcingandadevicetoconnect either generator headtothetetherelectrically. Satellite CoreEquipment consists ofa linear three-axis accelerometer andan ammeter. Theaccelerometer (alongwith thesatellite's gyroscope) willmeasure satellite dynamics, whiletheammeter will provide aslowsampling monitor ofthe current collected ontheskinoftheTSS- 1Rsatellite. Research onOrbital Plasma Electrodynamics (ROPE) NobieStone,Principal Investigator NASA Marshall SpaceFlightCenter Thisinvestigation isdesigned tostudy thebehavior oftheambient ionospheric charged particle populations andofionized neutral particles around theTSS-1R satellite underavariety ofconditions. Sincethecollection offreeelectrons from thesurrounding plasma produces current in thetether, knowledge ofthebehavior of charged particles isessential to understanding thephysics oftethercurrent production. From itslocation onthesatellite's fixedboom, theDifferential IonFlux Probemeasures theenergy, temperature, density, anddirection ofambient ionsthat flowaround thesatellite, aswellasneutral particles thathavebeenionized inthe sateUite's plasma sheath andaccelerated outward radially. Inthisinstrument,anelectrostatic deflection system, which determines thecharged particle direction ofmotion overarangeof100degrees, routesparticles toaretarding potential analyzer, whichdetermines theenergy of theionstream, measuring particle energies from0to100electron volts(eV).The directional discrimination ofthe Differential IonFluxProbe willallow scientists todifferentiate between the ionospheric ionsflowing around the satellite andtheionsthatarecreated in thesatellite's plasma sheath and accelerated outward bythesheath's electric field. The Soft Particle Energy Spectrometer insmanent isacollection of fiveelectrostatic analyzers thatmeasure electron andionenergies from1to 10,000 eV.Three analyzer modules provide measurements atdifferent locations onthesurface ofthesatellite's hemispherical Payload Module. These sensors determine thepotential ofthe satellite andthedistribution ofcharged particles flowing toitssurface. Twoother SoftParticle Energy Spectrometer sensors, mounted withtheDifferential IonFlux Probeontheendoftheboom, measure ionsandelectrons flowing bothinwardand outward from thesatellite. These measurementscanbeusedtocalculate the localpotential oftheplasma sheath. Thesensor package ontheboom is electricallyisolatedfromthesatellite,and itspotentialiscontrolled bythefloating power supply.Forsatellite potentials up to500V,thesensor package willbe maintained nearthelocalplasma potential toallowunambiguous measurements tobe obtained. Thepotential ofthesensor package alsocanbeswept, allowing the package itselftoserveasadiagnostic probe. 6 ResearchonElectrodynamic Tether Effects(RETE) Marino Dobrowolny, Principal Investigator Agenzia Spaziale Italiana Thebehavior ofelectrostatic waves andplasma intheregionaround atethered satellite affects theabilityofthatsatellite tocollect ionsorelectrons and, consequently, theability ofthetetherto conduct anelectric current. This investigation provides aprofile ofthe electrical potential intheplasma sheath andidentifies waves excited bythis potential intheregionaround thesatellite. probes, placeddirectly intotheplasma in thevicinity ofthesatellite, map alternating current (ac)anddirectcurrent (dc)electric andacmagnetic fields produced asthecurrent inthetether is changed byinstabilities intheplasma sheath orastheFast-Pulse Electron Accelerator orCoreElectron Accelerator orCoreElectron Accelerator isfiredin thepayload bay. Theinstruments aremounted intwo canisters attheendofapairof2.4m extendible booms. Asthesatellite spins, thebooms areextended, andsensors measure electric andmagnetic fields, particle density, andtemperature at various angles anddistances inthe equatorial plane ofthesatellite. To produce aprofile oftheplasma sheath, measurements ofdcpotential andelectron characteristics aremadebothwhilethe boomisfullyextended andasitisbeing extended orretracted. Thesame measurements, takenatonlyonedistance fromthespinning satellite, produce amap oftheangular structure oftheearth. Oneboom carries awavesensor canister, whichcontains athree-axis ac electric fieldmeteranda coilacmagnetometer to fieldsandelectrostatic characterize theintensitytwo-axis search identify electric waves andto ofsurrounding magnetic fields. Highly sensitive radio receivers andelectric fieldpreamplifiers within thecanister complement the operations oftheprobes.Ontheopposite boom, aplasma package determines electron density, plasma potential, andlow-frequency fluctuations inelectric fieldsaround the satellite. ALangmuir probe withtwo metallic sensors samples theplasma current; fromthismeasurement, plasma density, electron temperature, andplasma potential maybedetermined. This potential isthencompared tothatofthe satellite. Twootherprobes measure low- frequency electric fields. Magnetic FieldExperiment forTSS Missions (TEMAG) Franco Mariani, Principal Investigator Second University ofRome Theprimary goaloftheTEMAG investigation istomapthemagnetic fields around thesatellite. Ifthemagnetic disturbances produced bysatellite interference, attitude changes, andthe tether current canberemoved from measurements oftheambient magnetic fields,theTethered Satellite System will proveanappropriate toolformagnetic fieldstudies. Twotriaxial fluxgate magnetometers, veryaccurate devices designed tomeasure magnetic fieldfluctuations, arelocated on thefixedboom. Onesensoratthetipof theboom andanother atmid-boom characterize ionospheric conditions attwo distances fromthesatellite, determining themagnetic signature thatisproduced as thesatellite moves rapidly through the ionosphere. Combining measurements fromthetwomagnetometers allowsreal- timeestimates tobemadeofthemagnetic fieldsproduced bythepresence ofsatellite batteries, power systems, gyros, motors, relays, andpermanent magnets. The environment atthetipoftheboomshould belessaffected bythespacecraft subsystems thanthatatmid-boom. After themission, thevariable effects of switching satellite subsystems onandoff, ofthruster firings, andofotheroperations thatintroduce magnetic disturbances will bemodeled byinvestigators inanattempt 7 toremove thesespurious signals fromthe data. Thetwomagnetometers willmake magnetic fieldvector readings 16times persecond toobtain thegeographic and temporal resolution needed tolocate short-lived orthinmagnetic structures. Thereadings willbemadetwotimesper second toallowdiscrimination between satellite-induced magnetic noise, the magnetic signals produced bythetether current, andtheambient environment. Themagnetometers willalternate these rates: whiletheoneonthetipofthe boomoperates 16timespersecond, the midpoint magnetometer willoperate twice persecondandviceversa. Datagathering beginsassoonaspossible afterthesatellite isswitched oninthepayload bayand continues aslongaspossible during satellite retrieval. Shuttle Electrodynamic Tether System (SETS) BrianGilchrist, Principal Investigator, University ofMichigan Thisinvestigation isdesigned tostudy thecurrent-voltage characteristics ofthe orbiter-tether-satellite system andthe fundamental controlling parameters inthe Earth's ionosphere. Thisisaccomplished through control ofthetether system electrical loadimpedance andtheemission ofelectrons attheorbiter endofthe system. Theexperiment alsoexplores the useofspacetethers asscience tools. Orbiter charging processes aremeasured usingelectron emissions plusthetethered satellite asaremote electrical reference. Plasma wavesgenerated byelectron beams aremeasured byreceives atthesatellite. Ionospheric spatial structure is investigated bysimultaneous in-situ measurements atboththeorbiter and satellite. Also,electrodynamic tetherlow- frequency radiowavereception, emission, andtransient response areinvestigated. Thehardware islocated ontheMPESS nearthecenter ofthepayload bayand adjacent tothedeployer pallet. ATether Current-Voltage Monitor measures tethercurrent andvoltage, while controlling tethercircuit loadresistance. TheFast- Pulse Electron Accelerator emits an electron beam.of100or200milliamperes atanenergy of1000electron volts. The beamcanbepulsed withon/off times ranging from400nanoseconds to107 seconds. Thebeambalances thetether current andisusedtocontrol thelevelof charging oftheSpaceShuttle orbiter. In addition, thebeamisusedasanactive stimulus oftheplasma neartheorbiter in support ofseveral scientific objectives. TheSpherical Retarding Potential Analyzer, mounted onastematone corner ofthesupport structure, records plasma iondensity andenergy distribution inthepayload bay.Similarly, aLangmuir Probe measures electron plasma temperature anddensity andismounted on thetoweralso. Atthecenter ofthe support structure, theCharge andCurrent Probemeasures thereturn current tothe orbiter, recording largeandrapidchanges inorbiterpotential, suchasthosethatare produced whenelectrons areconducted fromthetether totheorbiter frame or whenanelectron beamisemitted. A three-axis fluxgate magnetometer measures themagnetic field,allowing the magnetic fieldlinesinthepayload bayto bemapped, whichiscrucial sinceelectron beams andtheflowofplasma spiralin response tothese fields. Using this information, scientists canaimthe electron beamatvarious targets, including orbiter surfaces, tostudythefluorescing thatoccurs. Shuttle Potential andReturn Electron Experiment (SPREE) DavidHardy, Principal Investigator Department oftheAirForce,Phillips Laboratory SPREE willmeasure thecharged particle populations around theorbiter for ambient spaceconditions andduringactive TSS-1R operations. SPREE supports the TSS-1R electrodynamic mission by determining theleveloforbiter charging withrespect totheambient spaceplasma, 8 bycharacterizing theparticles returning to theorbiter asaresultofTSS-1R electron beamoperation, andbyinvestigating local waveparticle interactions produced by TSS-1R operations. SPREE ismounted ontheportsideof theMPESS. Thesensors forSPREE are twopairsofelectrostatic analyzers, each pairmounted onarotary tablemotor drive.Thesensors measure thefluxofall electrons andionsintheenergy range from10eVto10keVthatimpact the orbiter attheSPREE location. The energy rangeissampled eitheronceor eighttimespersecond. Thesensors measure theelectrons and ions simultaneously overanangular fieldof viewof100x10degrees. Thisfieldof view,combined withthemotion ofthe rotarytables,allowsSPREE measurements overallangles outofthepayload bay. TheDataProcessing Unit(DPU) performs allSPREE command andcontrol functions andhandles alldataandpower interfaces totheorbiter. Inaddition, the DPUprocesses SPREE dataforusebythe crewandtheground support team. A portion oftheSPREE dataisdownlinked inrealtime,andthefulldatasetisstored ontwoSPREE Flight DataRecorders (FDRs). EachFDRholdsupto2gigabytes ofdataforpostflight analysis. Tether Optical Phenomena Experiment (TOP) Stephen Mende, Associate Investigator Lockheed Usingahand-held camera system with imageintensifiers andspecial filters, the TOPinvestigation willprovide visualdata thatmayallowscientists toanswer a variety ofquestions concerning tether dynamics andoptical effects generated by TSS-1R. Inparticular, thisexperiment willexamine thehigh-voltage plasma sheathsurrounding thesatellite. Inpaceoftheimage-intensified conventional photographic experiment package thathasflownonnineprevious Shuttle missions, acharge-coupled device electronic system willbeusedinstead offilm. Thisnewsystem combines the imageintensifier andthecharge-coupled device inthesame package. The advantage ofcharge-coupled devices over filmisthatthey allow real-time observation oftheimage, unlike film, which hastobeprocessed afterthe mission. Thesystem alsoprovides higher resolution inlow-light situations thando conventional videocameras. Theimaging system willoperate in four configurations: filtered, interferometric, spectrographic, and filtered withtelephoto lens. Thebasic system consists ofa55mmF/1.2or135 mmF/2.0lensattached tothecharge- coupled device equipment. Various slide- mounted filters, anair-spaced FabryPerot interferometer, and spectrographic equipment willbeattached tothe equipment sothatthecrewcanperform various observations. Inonemodeofoperation, thecurrent developed intheTethered Satellite System isclosedbyusingelectron accelerators to returnelectrons totheplasma surrounding theorbiter. Theinteraction between these electron beamsandtheplasma isnotwell understood. Scientists expect togaina better understanding ofthisprocess and howitaffects boththespacecraft andthe plasma byusingthecharge-coupled device tomake visual, spectrographic, and interferometer measurements. Thruster gasses alsomayplayacritical rolein Tethered Satellite System operations. By observing optical emissions during the buildup of the system-induced electromotive force(emf)andduringgas discharges, scientists canunderstand better theinteraction between acharged spacecraft andtheplasma environment andwillincrease theirknowledge ofhow thecurrent system closesatthepolesof thevoltage source. Investigation ofElectromagnetic Emissions bytheEleetrodynamic Tether (EMET) Robert Estes,Principal Investigator Smithsonian Astrophysical Observatory (SAO) 9 Observations attheEarth's Surface of Electromagnetic Emissions byTSS (OESEE) Giorgio Tacconi, Principal Investigator University ofGenoa Onegoaloftheseinvestigations isto determine theextenttowhichwavesthat aregenerated bythetether interact with trapped particles andprecipitate them. Wave-particle interactions arethought to occurintheVanAllenradiation belts where waves, transmitted fromEarth, "jar"regions ofenergetic plasma andcause particles to"rain" intothelower atmosphere. Although poorly understood, wave-induced precipitation isimportant because itmayaffect activity inthe atmosphere closertoEarth. Various wave phenomena thatneedtobeevaluated are discrete emissions, lightning-generated whistlers, andsustained waves, suchas plasma "hiss." Wave receivers onthe satellite detect andmeasure the characteristics ofthewaves, andparticle detectors sensewave-particle interactions, including those thatresemble natural interactions inradiation belts. Ground stations maybeabletodetect faint emissions produced aswaves disturb particles andenhance ionization. Furthermore, thecurrent iscarried away fromthetethered system through the ionosphere byelectromagnetic waves. Also,investigators wanttoknowwhat typeofwavepredominates inthisprocess andwhether thetether-ionosphere current closure occursnearthesystem orhundreds ofkilometers away. Ground=based measurements maybeabletoshedlighton thisquestion. Another goalistodetermine howwell theTethered Satellite System can broadcast from space. Ground-based transmissions, especially thosebelow 15 kHz,sufferfrominefficiency. Sincelarge portions ofground-based antennas are buried, mostofthepowersupplied tothe antenna isabsorbed bytheground. Because ofthelargeantenna sizeand consequent highcost,veryfewground- basedtransmitters operate atfrequenciesbelow 10kHz. SincetheTethered Satellite System operates inthe ionosphere, itshould radiate wavesmore efficiently. Forfrequencies lowerthan15 kHz,theradiated signals froma1kW spacetransmitter mayequalthatfroma 100kWground transmitter. Waves generated bythetether will moveinacomplex pattern within the ionosphere andintothemagnetosphere. EMET andOESE science teams will operate ground stations equipped with magnetometers atremote sitesalongthe TSSground track. TheEMET siteson Mona Island (Puerto Rico) andBribie Island(Australia) arecapable ofmeasuring frequencies fromneardcto40kHz.The OESEE sitesintheCanary islands and Kenya utilize Superconducting Quantum Interference Devices (SQUIDs) andcoil magnetometers tomonitor frequencies below100Hz.Researchers atthesesites willtrytomeasure theemissions produced bytheTSSandwilltrackthedirection of wavesthataregenerated whenelectron accelerators intheorbiter payload pulse thetether current astheorbiter passes overhead. Theincoherent scattering radar andantenna attheArecibo Radio Telescope facility willattempt toobserve theionospheric perturbations produced by theTSSsystem. Investigation andMeasurement of Dynamic NoiseintheTSS(IMDN) Gordon Gullabom, Principal Investigator Smithsonian Astrophysical Observatory Theoretical andExperimental Investigation ofTSSDynamics (TEll)) SilvioBergamaschi, Principal Investigator Institute ofApplied Mechanics TSS-1R willbethelongest structure everflown inspace, anditsdynamic behavior willinvolve oscillations overa widerangeoffrequencies. Although the majordynamic characteristics arereadily predicted, future applications oflong tethers demand verification ofthe theoretical models. Moreover, higher 10 frequency oscillations, which are essentially random, aremoredifficult to predict. Thisbehavior, called"dynamic noise," isanalogous toradiostatic. An understanding ofitsnature isneeded if tethered platforms aretobeusedfor microgravity facilities andforstudying fluctuations inthesmall-scale structure of Earth's gravitational andmagnetic fields. Thesegravitational fluctuations arecaused byvariations inthecomposition and structure ofEarth's crustandmaybe related tomineral sources. These twoinvestigations willanalyze datafromavariety ofinstruments to studyTethered Satellite System dynamics. Theprimary instruments willbethe accelerometers andgyrosonboardthe satellite; however, tether tension and length measurements andmagnetic field measurements alsowillbeused. The dynamics willbeobserved inrealtimeat theMarshall SpaceFlightCenter (MSFC) Payload Operations Control Center (POCC) andwillbesubjected todetailed postflight analysis. Basicmodels and simulations willbeverified (andextended orcorrected asneeded); then,thesecanbe usedconfidently inthedesign offuture tethered missions, bothoftheTethered Satellite System andofotherdesigns. The dynamic noiseinherent tothesystem will beanalyzed todetermine iftethered systems aresuitable forsensitive observations ofthegeomagnetic and gravitational fieldsand,ifrequired, to develop possible damping methods. Theory andModelling Tethered Satellite O MST)inSupport of Applications AdamDrobot, Principal Investigator Science Applications International Corporation (SAIC) This investigation willdevelop numerical models ofthetether system's overall current andvoltage characteristics, oftheplasma sheaths thatsurround the satellite andtheorbiter, andofthe system's response totheoperation oftheelectron accelerators. Alsoofinterest are theplasma wavesgenerated asthetether current ismodulated. Alldatacollected on themission willbecombined torefine thesemodels. Two- and three-dimensional mathematical models of the electrodynamics ofthetether system will bedeveloped toprovide anunderstanding ofthebehavior oftheelectric and magnetic fieldsandthecharged particles surrounding thesatellite. Thesestudies are expected tomodel theplasma sheath (through whichthesatellite travels) under avariety ofconditions. Thisincludes thoseinwhichthemotion ofthetether andneutral gasemissions from the thrusters arenotconsidered, thosethat incorporate theeffects oftether motion, andthosethatfactorinthegasemissions. Thesheathsurrounding theorbiter has several uniquefeatures thatarerelated to theabilityoftheelectron accelerators to control theorbiter's potential. Models of theorbiter's sheath, whensmallcurrents areflowing inthetether, willconsider the potential oftheorbiter tobenegative; for largecurrents, models willbedeveloped assuming apositive orbiter potential. In thisway,thesheath structures and impedance characteristics ofthe orbiter/plasma interface canbestudied. Theresponse ofplasma tothe electromotive force produced bythe motion ofthetether system through the geomagnetic fieldisanother focusofthe TMST investigation. Using datafrom otherstudies, kinetic plasma processes will beanalyzed ornumerically simulated by computer tomodel thereaction ofthe ionosphere tothepassage ofTSS-1R. Thisinvestigation alsomodels the relationship between theefficiency of wavegeneration andtheamount of current flowing through thetether to examine howthetether antenna couples totheionosphere andhowultra-low- frequency (ULF) andvery-low frequency (VLF) wave propagate through the ionosphere. These models will complement theinformation gathered by TSS-1R instruments atground stations. 11 TheSubsatellite element (TSS-S) oftheTethered Satellite System (TSS) Figure1.7TSS-SduringTSS-1R Mission TheTSS-S isaShuttle-tethered instrumented platform supporting dynamic and electrodynamic investigations; itthusavailstheuniqueopportunity offered bythetether complex. TheTSS-Shasflowntwice,f'LrStinAugust, 1992,theninFebruary, 1996andits performance hasexceeded expectations bothtimes. Asshowninfigure1.7,theSatellite hasaroughly spherical shapewithanouterdiameter of 1.6m;itfeatures twof'Lxedandtwodeployable/retractible booms. Oneofthef'Lxedbooms (withstruts)isonemeterlonganditismeantforscientific instrument accommodation atits tipandmidpoint (2.5kgoverall), whiletheotherfixedboomsupports thesubsatellite's RF communications antenna. Thetwodeployable/retractible booms (DRBs) aredesigned totake science instrument packages weighing upto1.5kgperboomupto2.35mawayfromthe satellite shellin14mmsteps. Asshowninfig.1.8,thesatellite isfunctionally divided intothree"modules", namely the Service Module (SM),theAuxiliary Propulsion Module (APM) andthePayload Module (PM). TheSMisthehemisphere located onthetether' sideanditaccommodates all subsystems butforthepower andcommand-data handling unitsinterfacing withthe experiments; these,together withthescience experiment equipment, arehoused insidethe PM,i.e.thehemisphere opposite tothetether. TheSMandPMareseparated by,andjoin at,theAPM,whichismadeupbytheequatorial plane,thepropellant tankandallthevalves, pipingandpropellant management equipment. TheTSSSatellite hasanoverall massofabout 521kg,outofwhichupto66kgmadeupbyscience instruments and61kgbythegaseous nitrogen propellant (GN2) forsatellite attitude andratecontrol andfortethertension augmentation tosupport earlyTSS-S deployment andtokeepthetether tautduring proximity operations, whenthegravity-gradient-originated tension istooweaktoguarantee thatthetetherdoesnotbecome slack.Yawthrusters areprovided attheSatellite's equatorial planeforyawattitude andyawratecontrol; eachyawthruster hastwonozzles andprovides 0.5Nmpuretorqueaboutthe"vertical" axisusingabout1.7g/sofon-board propellant. Yaw attitude control accuracy is 12 about1degofthedesired anglewhileyawratecontrol isaccurate to÷/-0.1RPMfor ratesintherange-0.7to+0.7RPM.Thereference yawangleandrateaswellasthe associated control deadbands canbeselected bytelecommand. Figure1.8TSS-SExploded view Thrusters arealsopresent closetotheequatorial planetocontrol pitchandroll oscillation rates;in-plane andout-of-plane thrusters control thepitchandrollrates, respectively; theyprovide a0.8Nmtorque abouttherelevant axis.Thein-plane (pitch control) thrusters alsogenerate pureforcesalongthex(roll)andz(yaw)axes,about0.7and 1Ninmagnitude, respectively. Likewise, theout-of-plane (pitchcontrol) thrusters giverise topureforcecomponents alongthey(pitch) andzaxes,about1.9and1Ninmagnitude, respectively. In-plane andout-of-plane thrusters useabout4.4and3.8g/sofGN2each, respectively, andcanbeactuated oneatatimeonly.Theycanoperate underexternal command orundercontrol fromtheTSS-Sonboardsoftware intheso-called AutoRate Damping (ARD) mode.TheTSS-S's tether-aligned thrusters, in-line 1and2,eachproviding 2 Npurethrustalongthezaxis,useabout3.2g/swhenactiveandcanbeactuated upon external command eitherindividually ortogether. Theyaremeant fortether tension augmentation andsupport TSS-Sseparation fromtheOrbiter duringearlydeployment and close-in approach totheOrbiter duringfinalretrieval. TheSatellite isprovided withacomplete setofattitude detrmination sensors, i.e.4rate- integrating gyroscopes, twobolometer-based optical Earthsensors (ES)andfourDigital Sun Sensors (FDSS). Satellite attitude determination iscarried outonboardhesatellite witha+/-1 degaccuracy whenever thesatellite isinattitude holdmode; theon-board attitude determination algorythm isbasedongyroscope outputandmakesuseofESoutput forgyro driftcompensation. Ground-based algorythms havebeendeveloped byAlenia Spazio tomore accurately reconstruct theSatellite attitude history, evenwhileinspinandpassive mode,to support post-flight science dataanalyses; undernormal operating conditions anddata availability, theycanprovide TSS-S attitude history reconstruction tobetterthan1'. TheTSSSatellite element alsocarries onboardasetoffourAg-Zn batteries tosupport thedeployed mission; theycanprovide upto10.6kWh,depending ontheirdischarge profile andthermal conditions, asascertained bybothground testing andflightexperience. Outof thetotal,science experiments areallocated about2.5kWhoverall, witha100Wmaximum overall powerlevel.Twelve individually switched andfusedpowerlinesareprovided foruse 13 bytheTSS-Sscienceexperiments, 4with5Aratingand8with1.5Arating, at30+/-6V inputvoltage. TheTSS-Sprovides a16kbpscontinuous telemetry stream, outofwhichabout4kbps subsystem housekeeping, 10.25kbpsscience instrument telemetry (housekeeping andscience data)andabout1.75kbpsservice (sunchronisation) words.Discrete, analog and16-bitserial monitors canbeacquired fromthescience instruments andinserted intothetelemetry stream; analog monitors areA/Dconverted to8-bitwords. TheTSS-Ssupports a2kbps maximum telecommand bitrate; thecorresponding telecommand ratedepends onwhether the telecommands require processing bytheSatellite on-board software, andcanreachthe maximum valueofabout20commands/s incasenoprocessing isrequired.Science experiments canbeprovided relay-driving, discrete commands aswellas16-bitserialdigital commands; noOBDH processing isprovided onscience experiments commands butrouting totheenduser.TheTSS-Shasa40-slot TimeTagged Command Buffer (TTB), where commands canbestoredforexecution atalatertime.Outofthese,upto30canbeallocated tothescience experiments; TTBtimetagresolution isabout32sec,i.e.timetagscandiffer by32secasaminimum, butcommands withthesametimetagareexecuted within 128msec ofeachotherinaFIFOorder. Besides engineering resources andcapabilities, theexperiments onboardtheTSS-Sare provided withamagnetic cleanliness program whichensures thatDCmagnetic fieldgenerated bythesatellite doesnotexceed about20nTatthefixedboomtip,withaveryhighstability (afewnT).Additionally, theTSS-Soutershelliscoated witha100-120 micron-thick conductive paintlayerapplied directly ontheshellbaremetal(AI);thepainthelpskeeping theresistance opposed bytheskintotheelectric current flowtoafewtensofOhms, the exactvaluedepending onpaintthickness andapplied voltage. Ground testing andflightdata haveproved boththemagnetic cleanliness levelandtheTSS-Soverall conductivity tomatch orexceed thescience requirements. TheTSS-Sisequipped withtwo"standard" science support equipment items,namely the Satellite Ammeter (SA)andtheSatellite LinearAccelerometer (SLA). TheSAisafourscale (+_5,_+0.5,+-0.1,_+0.02A),auto-ranging instrument capable ofproviding measurements ofthe electric current flowing inthetetherwitha7=bitaccuracy overeachrange; SAdataare provided 16timesasecond inthesatellite telemetry stream. Theinstrument, however, also hasa1kHzbandwidth analogoutput, allowing othersatellite experiments todirectly sample current impulse waveforms. TheSLAisathree-axis accelerometer withinductive-mechanical (coil-spring) control loopandcapacitive pick-off; theinstrument provides threemutually orthogonal acceleration measurements intherange-60-+20miHi=g (zaxis)and-20-+20 milli=g (x,yaxes),eachavailable 16timesasecond insidethesatellite telemetry, with accuracies ranging from100micro-g (zaxis)to10micro-g (x,yaxes).Theinstrument measurement bandwidth is4.5Hz. Theexperience acquired withthetwoperformed flights hasallowed veryaccurate characterisation ofallTSS-Sperformance characteristics andhasprovided Alenia Spazio with expertise andSfWtoolswhichallowtheCompany toprovide in-depth andextensive support tobothdynamics andelectrodynamics analyses aswellastomission analysis, preparation andsupport. Contacts fortheTSSProject: •M.Calabrese, R.Carovillano, T.Stuart-NASAHqts. •C.Bonifazi, M.Dobrowoiny -ASI •F.Giani, B.Strim, -Alenia •N.Stone, tLMcBrayer -NASA/MSFC •TSSInvestigator Working Group 14 1.2TheSmallExpendable Deployer System (SEDS): SEDS-1 andSEDS-2 Missions TheSEDSproject started asaSmallBusiness Innovative Research contract awarded to JoeCarroll byNASA MSFC. SEDShardware proved tobeabletosuccesfully deploy a20km tetherinspace.BothflightsofSEDS-I (March 29,1993)andSEDS-2 (March 9,1994)flew assecondary payloads onDeltaIIlaunches ofGFSsatellites. Afterthethirdstageseparation theend=mass wasdeployed fromthesecond stage.SEDS-I demonstrated thecapability of deorbiting a25kgpayload fromLEO.SEDS-2, ontheotherend,demonstrated theuseofa closedloopcontrol lawtodeploy atethered payload alongthelocalvertical. SEDS' hardware, asshown infigure 1.9,consists ofadeployer, brake/cutter and electronics box.Allthecomponents thatareincontact withthetether,except forthebrake post,arecoated withteflon. Thedeployer consists ofbaseplate, core,tether andcanister. Thetether iswound around thecore.Inaddition therearethreeLightEmitting Diodes (LED). TwooftheLED'sareusedtocounttheturnsofdeployed tether, whilethethirdis usedtocheckwhenthetether isalmost completely unwound. Thecanister provides a protective coverforthetetherandrestrains itduringdeployment. Thetether material is SPECTRA- 1000. Figure1.9SEDSandEndmass ontheDeltaSecond Stage Thebrake/cutter components are:brakepost,stepper motor, tensiometer, temperature sensor, pyrocutter, exitguide. Thetetherpostiscoated withhardanodize. Thestepper motorisusedtowraporunwrap thetethertovarythedeployment tension andtheresulting deployment velocity. Thebrakemechanism isafriction multiplier andthemultiplier function isproportional tothefriction surface areabetween thetetherandbrakepost.SEDS functional diagram isshowninfigure1.10. 1S Figure 1.10SEDSFunctional Diagram Themaindifferences between SEDS1andSEDS-2 areshownintable1.SEDS-2 closed loopwasimplemented bydeploying thetether according toapre-mission profile. The deployment control logicactedonthebrakemechanism byincreasing ordecreasing the deployment velocity tofollowtheprofile andbringthepayload attheendofthetether deployment toasmooth stopalongthelocalvertical. Table1.Maindifferences between SEDS-1 andSEDS-2 Tether Cutter Pyrotechnics Control Law Tether SolderLumps Tether Fabrication Mission Initiation BrakeUsage Tether StabilizationSEDS-1 Active OpenLoop StudyTension Pulses Tether Application PriortoDepletion Burn Minor NoneSEDS-2 Inactive Closed Loop None Cortland/Hughes AfterDepletion Bum Significant after1Km Yes Theend-mass payload (EMP) wasdeveloped byNASA LaRCinordertomonitor the dynamics ofatethered susbsatellite. EMPconsisted ofthreeprimary science sensors: athree- axisaccelerometer, athreeaxistensiometer andathreeaxismagnetometer. TheEMP measured 40.6X30.SX20.3 cmandweighted about26kg.Theend-mass wascompletely autonomous andcarried itsownbattery, electronics, computer andS-band telemetry system. Asschematic ofEMPisshowninfig.1.11.Thethreeaxistensiometer wasalsodeveloped atNASALaRC. SEDS-1 mission objectives weretodemonstrate thatSEDShardware couldbeusedto deploy apaylod attheendofa20km-long tetherandstudyitsreentry afterthetetherwas cut.Theorbitchosen hadaninclination of34degrees andaperigee altitude of190kmand 16 Figure1.11Schematic ofSEDSEMP anapogee altitude of720km.TheEMPtransmitted over7900seconds ofdata beforeburning intotheatmosphere (1Hzsampling rateforthemagnetometer and8Hzfor thetensiometers andaccelerometers). Aspredicted, SEDS-1 reentry wasoffthecoastof Mexico (seefig.1.12a). NASAstationed personnel atCaboSanLucas, Puerto Vallarta and Manzanillo tomakephotographic andvideoobservations. ThePuerto Vallarta sitewasable toobtainobservational dataasshowninfigure1.12b Figure 1.12aSEDS-1 EMPreentry trajectoryFigure1.12bObservational DataofSEDS- 1reentry SEDS-2 mission objectives weretodemonstrate thefeasibility ofdeploying apayload withaclosed-loop control law(i.e.apredetermined trajectory) andbringittoasmallfinal angle(<10degrees) alongthelocalvertical. Asecondary objective wastostudythelongterm evolution ofatethered system. Theorbitthistimewaschosen tobecircular withanaltitude ofabout350kin.TheSEDS-2 tetherwasallegedly cutbyamicrometeroid ordebrisafterfive days.TheEMPtransmitted over39,000 seconds ofdatabefore thebattery died(1Hz sampling rateforallthethreeprimary science sensors). 17 SEDS-1andSEDS-2FlightData SEDSdatabaseisavailablethroughanonymous [email protected] (128.183.76.209) SEDS1 dataareinthesubdirectory/pub/project_tether/SEDSMissionl and SEDS-2 dataareinthedirectory /pub/projects/tether/SEDSMission2. Eachdirectory is organized indifferent subdirectories withdeployer data,EMPdata,radar,etc..Eachcontent ofadirectory isdescribed inaread.me file. SEDS-1 Theturncounter dataareshowninFigure 1.13a,thetension atthedeployer isshown in figure1.13bandthetetherratein1.13c.Inordertocompute thetetherlengthanditsrate, theturnshadtobemapped andconverted intodeployed length.Notethatthevelocity atthe endofthedeployment wasabout7ngsexplaining thehugejumpintension andthe consequent rebounds. i "......IIHi.....i_iiiiiiiiiiiiiiiiiiii!.............,/l -1_ | lO_ /41 N I N mo _mmui_._NI Figure 1.13a.SEDSDeployer Turns Counts8U6-11q.IQH'r DATA i.i.......i....i'¸¸i [ -tON 0 tl moo arstso _ Imm• ill• 7909 TNIHPflOliliN0Id4dlSMKUIASL NtCOlitO$ Figure1.13b.SEDSDeployer Tension 8EDit FLIGHT DATA _2 0 i i i 0_ 1000 _ID00 _000 2S00 IION N00 4000 4600 TIMEFROMEN0MAGISRELF.A&E° _rr,CONOS1000 Figure 1.13c.SEDSDeployer 10-secAverage Length Rate 18 Themagnetometer andtension moduli attheEMPareshown infigures 1.13dand 1.13e,respectively. Notethatthemagnetometer wasaffected byabiasestimated tobe3065 nT,-3355nTand-4188nTonthex,yandzaxes,respectively. Procedures onthedata calibration andvalidation aregivenattheftpsiteaswellasaredescribed inseveral papers presented attheWashington Conference. ........... :................... _............ _........... _......._..¢........... .:....... 4........... ,Z........... i............ .:-.......... "......._............ ;-- .........._...........i.......i..........._...........i...........T..........i:........ tliiiiiiio Figure 1.13d.EMPMagnetometer ModulusiiiililLtL!i "'J/JLJIJJJ..... Figure 1.13e.EMPTension Modulus SEDS-2 Thetetherdeployment rateandthetension atthedeployer areshown infigure1.14a, and1.14b,respectively. Thedeployment lawwassoeffective thatthefinaltetherratewas about2cm/s.Ascomputed bythemodulus oftheEMPtension, showninfig1.14c, thefinal libration wasabout4degrees,-and itwasconfirmed alsobytheradartracking. EveninSEDS-2 themagnetometer signalwasaffected byabiasanomaly thatwasestimated tobe-1128nT, 1312nT,and2644nTonthex,yandzaxes,respectively. $ 4.0 __:!!!iiI 4 3.0 0.0_ 0 0 2800 50000Time(s_ Figure 1.14a.TetherRate 19 IIfl ! m Figure 1.14c.EMPTension Modulus Contacts fortheSEDSProject: •J.Harrison, H.Frayne Smith,K.Mowery, C.C.Rupp-NASA/MSFC •J.Carroll -TetherApplications •J.Glaese -Control Dynamics •M.L.Cosmo, E.C.Lorenzini, G.E.Gullahom -SAO •T.Finley ,R.Rhew, J.Stadler -NASA/LaRC •W.Webster- NASA/GSFC 20 1.3ThePlasma Motor Generator (PMG) ThePMGexperiment wasdesigned totesttheability ofahollow cathode assembly (HCA) toprovide alowimpedance bipolar electrical current between aspacecraft andthe ionosphere. The500m-long tetherwaschosen toassurecomplete separation between the grounded ends,forcing current closure through theionosphere ratherthanwithlocaloverlap ofthetwoplasma clouds. Inordertofunction properly, anelectrodynamic tetherneedstobe effectively "grounded"on bothends.Theexperiment aimedatdemonstrating thatsuch configuration couldfunction eitherasaorbit-boosting motororasagenerator converting orbitalenergy intoelectricity, asshowninfigure1.15. Figure 1.15PMGinvestigation ofanelectrocl3_amic tether Themission objectives were: HCAOperation End-mass separation greater than200m Induced voltage of30Vorhigher PMGPlasma cloudscompletely separated Achieve currents inthe0.1-1Amp Reverese current intetherusingbiasvoltage Observe tetherstability forgravity gradient vs.IXBforces i1Collect I-VCharacteristics forfullorbit Asshowninfigure1.16,PMGconsisted offourmajorsubsystems: TheFar-End Package (FEP), theNear-End Package (NEP), anelectronics box(SEDS) andthePlasma Diagnostic Package (PDP). Thesystem waslaunched asasecondary payload onaDeltaIIonJune26, 1993.Afterthethirdstageseparation, PMGwasleftinanelliptical orbit(193X869) at25.7 21 deginclination. TheFEPwasejected upward withaninitialvelocity ofabout2-3m/s.PMG wasprogrammed tooperate inthreedifferent datamodes, byusingamicroprocessor to control selectable loadresistors, tochange 1)biasvoltage levels, 2)polarity reversal 3) bypass relays. Figure 1.16PMGMajorhardware components TheSEDSdeployer fixedspoolconcept wasadapted foruse,without thebrake mechanism toprovide minimum friction deploymnet oftherelatively massive tether. The PDPexperiment, developed byNASA/LeRC/U. ofNewHampshire, wasaddedinorderto measure thedeployer potential. TheNEPincluded apower"ON"relay,amicroprocessor basedcontrol/data module and elecgometer, andatetherbiasvoltage powersupply. EachHCAwasequipped witha1litergasbottle,on/offsolenoid, gasmetering blockand powersupplies toproduce aweakly ionized xenoncloud.Bothendplatforms carried a28volt silvercellbattery foranominal 3-6hourslifetime. Thetether wasa#18AWGteflon insulated copper wire. Afterdeployment, duringthefirst150minutes, setsofIvs.Vperformance datawere obtained byapplying biasvoltages of+65Vto-130V inserieswiththeIXBinduced emf, whilevarying loadresistance instepsfrom200to700ohmstotaltether current path internal resistance (seefigure1.17).Totaltether voltage wasmeasure byplacing a2.2 MOhms resistance inserieswiththetether. ThePMGcurrent showed tobefullyreversible, operating eitherasagenerator system withelectron current flowdownthetetherorasamotorwithelectron current drivenupthetether. 22 .:.: i ? ;b b iI -7:_ I II 4t_.i.. I II "'_'" I II "#_"*_'"IJ" o.li_ •¢._a. IT/J"__L"*'" ""'* '*" ':*_q .,,a¢. •o•_'IW._ ** "*_'_ Figure1.i7PMGI-VCurves Thedataconfirmed thattheHCAswereabletocomplete PMG's current loopand100- 300mAcurrents wereobserved inthedaytime portion (probably duetotheenhanced plasma denity) oftheorbitand10-50mAonthenightime side,asshowninfigure1.18. l............. _l "..J :! 1 .L' •_'--- II I 11 • 4 • • l • • • Figure 1.18PMGElectrometer Reading of2.2MOl_s Loadandallbiasvoltages Theinduced emfwas measured withtotalvoltage biasedfrom+150to-90volts,andalso withthebiasturnedoff,byusingonlytheinduced emf.Variability oftheinduced emfhas beenmatched against different models ofelectrodynamic interactions. Acontingent studywasthedetection withground-based radarsandsquidmagnetometers (seeOESEE experiment onTSS1,G.Tacconi PI)oftheplasma disturbances, ELFwaves radiated bythesystem, theHCAplasma clouds andtheirassociated plasma/ionosphere currents. Theexperiment duration, intermsofplasma contactor operation andconsequential activeenvironment interaction, lastedaboutsevenhours,untilthebatteries expired. 23 PMGdatabaseisavailable through anonymous [email protected] (128.183.76.209) underthedirectory/pub/projects/tether/PlasmaMotorGenExp. Contacts forthePMGProiect: *J.McCoy -NASA/JSC •J.Carroll -TetherApplications •M_D.Grossi, R_Estes -SAO •R.J.Jost -System Planning Corporation •R.C.Olsen-Naval Postgraduate School •l.Katz -S-Cubed •G.Tacconi, L.Minna-U.ofGenoa •D.C.Ferguson-NASA/LcgC •R.Tolbert -U.ofNewHampshire •W.Webstcr- NASA/GSFC 24 1.4TheTether Physics andSurvivability Spacecraft (TIPS) TiPSProl_'arn Overview TheTether Physics andSurvivability (TIPS) Experiment wasconceived asaquick response, simple experiment tostudythelongtermdynamics andsurvivability oftethered spacesystems. Theknowledge gainedfromthisexperiment willhelpDODandthenation gainexperience withtethered systems foreventual useinoperational spacecraft. TheNaval Research Laboratory's, theNavalCenter forSpaceTechnology (NCST) designed, builtand nowoperates theexperiment fortheNational Reconnaissance Office (NRO). The experiment isafreeflyingsatellite consisting oftwoendbodiesconnected bya4kilometer non-conducting tether. Inthisrespect itisdifferent fromothertether experiments, like thoseflownontheShuttle, whereoneendmass wasconnected toamassive hostvehicle. Figure1.19ArtistRendition ofTIPSinOrbit.Ralphisonthebottom. TIPShasbeeninthis orientation sincedeployment. TiPSwasjettisoned fromahostspacecraft onJune20,1996,withthedeployment ofthe tether occurring shortly afterjettison. TheTIPStether isintact through thiswriting (12/9/96), whilenootherspacetetherhaslastedlongerthanfivedays.TiPSisthesixth known orbital tethered system flowntodate. Tomeetanearlylaunchopportunity, TIPShadtobedesigned andbuiltinapproximately oneyear.Duetoaverytightbudget, theexperiment objectives werelimited toonlythose thathadthehighest payoff, thesewere:1)Longtermorbitandattitude dynamics and2) tethersurvivability. TIPSHardware Theprogram constraints oflimited timeandmoney dictated thattheexperiment design beassimple aspossible andconsist largely ofexisting component hardware and/ordesigns. 25 Theexperiment goalsonlyrequired a simple electrical powersystem thatuseda battery forallitselectrical needs. The battery supported theinitiation of deployment, recording ofdataonthe deployment characteristics and transmitting thisdatatoground stations. TiPSconsists oftwoendbodies, dubbed Ralph andNorton, connected bya4 kilometer tether. Ralphcontains aSmall Expendable Deployer System (SEDS) tetherdeployer, abattery consisting of10 Lithium Thionyl Chloride Dceils,atimer toinitiate deployment, aSEDS data acquisition electronics boxanda transmitter andantennas todownlink the deployment data. TheSEDSdeployer, SEDSelectronics andthetransmitter were existing flightsparehardware fromthe SEDS 2tether experiment thatwas successfully flown inspace byNASA. NASA provided thishardware toNRLfor theTiPSprogram. Thiswasinkeeping withtheapproach ofusingofftheshelf components sothattightbudgets and shortschedules couldbemet.Norton isan inertbodycontaining thetenspring cartridges usedtorapidly pushthetwo bodiesawayfromeachother,pulling the tether outofitsdeployer mounted on Satellite Tracking andDynamics Themotion oftheendbodies is observed byaground basedSatellite Laser Ranging (SLR) network andbyground basedvisualobservations. Fig.1.21isan imageofTIPStakenasthetwoendmasses wereseparating. Thetracking dataconsists largely ofrangedataprovided bytiming thetwowayroundtripdelayforalaser bounced offtheretroreflectors onthe spacecraft. Thedataistransmitted across theInternet toNASA Goddard in Greenbelt, Maryland. Thetracking dataisanalyzed, atNRL, todetermine thedynamic motion ofthe tethered system. Since,thereisnoobject atthecenter ofmassofthetethered system, boththeattitude motion andtheRalph. RalphandNorton eachhave18 optical retroreflectors mounted onthem. Fig.1.20isapicture ofthecompleted satellite without itsthermal blankets. In thisfigure, several ofthesmallround retroreflectors canbeseen. ® Fig1.20TIPSSatellite Without Thermal Blankets. Thesmallroundobjects arethe laserretroreflectors. "Figure 1.21Telescope image ofTiPS during deployment (Image taken by Starfire Optical Range attheAirForce's Phillips Laboratory). 26 orbitalmotion areinferred fromobservations oftheendmasses only.Thishasproved tobea difficult taskrequiring frequent observational dataandnewestimation algorithms thatwere incorporated inthetraditional orbitdetermination system usedbyNASA. Thetracking data isalsoprocessed toprovide updates tothestatevector usedtopredict themotion ofthe endmasses. Thesepredictions areusedbytheSLRsitesforsubsequent observations. Therequirement ofdetermining thetethersurvivability willbemetthrough ground based radartracking ofTiPSwhichwilldetermine whenorifthetetheriscut. TiPSFindings toDate Thef'mdings oftheTiPSprogram arebestsummarized bysaying thattheyprovide "confidence" thattethertechnology willbeviableforfutureoperational missions. Thereis stillalotoftechnology development required beforeoperational systems wouldbereadyto incorporate thistechnology, butTiPShasprovided asignificant stepinthatdirection. The initialresults showthattethers canbemadetobesurvivable. Withregardtothelibrational motion, ourestimates nowindicate thatthetetherislibrating withasmaller amplitude than atdeployment. Visualobservations madeshortly aftertheinitialseparation oftheend- bodiessuggested thatthetetherwaslibrating withanamplitude of47degrees withrespect to vertical alignment. Overthecourse ofthenextthreemonths, wehavedetermined withhigh confidence thattheamplitude ofthatmotion hasdecreased toapproximately 12degrees. At thisloweramplitude, thetetherbehaves muchmorepredictably. During themonth of October, 1996,wewereabletovalidate ourabilitytopredict tethermotion 6to12hours intothefuture. Whilethiswasonlypossible duringaperiodwhenanabundance ofdatais available, thisprovides agreatdealofconfidence inourabilitytomodeltetherdynamics. NRLhassetupaWebsitewhereinformation anddatacanbeobtained. TheURLis http://hyperspace.nrl.navy.mil/tips. Contacts fortheTipsProiect: •Shannon L.Coffey, William E.Purdy,NRL 27 1.5TheOEDIPUS Tethered Sounding Rocket Missions OEDIPUS A OEDIPUS standsforObservations ofElectric-field Distribution intheIonospheric Plasma -a Unique Strategy. Canadian activities inspacetethers beganwithOEDIPUS Awhichwas designed asalargedoubleprobeforsensitive measurements ofweakelectric fieldsinthe plasma oftheaurora. Itwaslaunched usingaBlackBrantX,3-stage sounding rocket. The OEDIPUS program wasajointprogram between National Research Council ofCanada and NASA withparticipation oftheCommunication Research Center inOttawa, Canada (Principal Investigator), various Canadian universities, andtheUSAirForce Phillips Laboratory, thepayload primecontractor wasBristol Aerospace Ltd.Themajorobjectives oftheOEDIPUS-A mission were: •tomakepassive observations ofauroral ionosphere, inparticular, thenatural magnetic- field-aligned dceleclric fieldEll,utilizing alargedoubleprobe; •tomeasure response ofthelargeprobeintheinospheric plasma; •toseeknewinsights intoplane-andsheath-wave rfpropagation inplasma. Therocket payload OEDIPUS Awasflown onJanuary 30,1989fromAndoya inNorway. Thetethered payload consisted oftwospinning subpayloads withamassof84and131kg, withtheirownexperiment complement andtelemetry systems, thatwereconnected bya thin0.85mmdiameter conductive andalsospinning tether. Themission achieved its scientific objectives todetectthenatural magnetic-field-aligned dcelectric fieldF1utilizing a largedouble probe, andtocarryoutnovelbistatic propagation experiments. Theflight established arecordforthelengthofanelectrodynamic tetherinspaceatthattime:958m. Although themission wassuccessful, flightdataindicated thattheaftsubpayload experienced arapidincrease initsconing angletonearly 35degrees (halfangle). Apost-flight investigations concluded thatthedynamic behavior wascausedbyinteraction ofthetether withthesubpayloads. Thisobservation wasunexpected duethefactthatthetethermasswas negligible relative tomasses ofbothsubpayloads, andthetether dynamic interaction was expected tobenegligible intherelatively shorttime(11minutes) ofasuborbital flight. TheOEDIPUS-A payload configuration isshowninFig.1.22.Thetwosubpayloads were initially connected andejected fromaBlackBrantXwithaspinrateaboutthelongitudinal axis.Theradialbooms ontheforward andaf_payloads wereusedasdipoles forscience experiments. TheACSmodule, located attheaftendoftheaftsubpayioad, wasusedtoalign thespinaxistowithin1degreetotheEarth's magnetic field.Thetetherwasatefloncoated stranded tin-copper wireanditwasdeployed fromaspool-type reellocated ontheforward subpayload. Toseparate thesubpaylaods anddeploy thetether, aspringejection system was provided andfollowed bythecoldgasthruster system intheforward subpayload. Amagnetic hysteresis brakewasprovided tocontrol thetetherspoolbyapplying asmallconstant torque, tosmoothly decelerate therelative motion ofsubpayloads. 28 attitude(_#__j/ . orgivefairing Figure 1.22Somesubsystems intheOEDIPUS-A payload configuration Shortly aftermotorbum-out, thefairing wasjettisoned alongwithanumber ofexperiment doors,andthetwosetsofradialboomsweredeployed. AtT+121 seconds, theACSmaneuver wasinitiated whichaligned thepayload within1degreeofthelocalgeomagnetic fieldline.At thispoint,theseparation usingcoldgassystem wasinitiated. AtT+448s,apogee wasreached (about512km)andthepayload separation wascompleted withtetherlengthof958m.This configuration wasmaintained fortheremainder oftheflight.Duetogravity-gradient torque onthetwo-body system, theentireconfiguration experienced aslightrotation through-out theflight.Atapproximately T+800 sthepayload re-entered theatmosphere andwasnot recovered. Flightdynamics dataarepresented inFig.1.23.Theseareprocessed magnetometer dataused tocompute theangular deviation ofeachpayload's spinaxisfromthemagnetic fieldvector. Dataisshownforbothforward andaftsubpayloads. Theforward subpayload experienced a tip-offatseparation thatcausedaconing angleofapproximately 7degrees whichvariedonly slightly duringtheflight.Theaftsubpayload, whichalsoexperienced asmalltip-off, hadan increase intheconing angleanditapproached almost35degrees attheendoftheflight.The post-flight investigations concluded thattheincrease coning wastheinteraction between tetherandtheaftsubpayload. OEDIPUS C Thesecond flightofOEDIPUS configuration, namely, OEDIPUS CtookplaceonNovember 6,1995fromthePokerFlatResearch Range, located nearFairbanks, Alaska. Thescientific objectives ofthemission weresimilar totheprevious onebuttherewere 29 Forward Payload 20......... •............. .----........ T........................... "_.......... -'_......... ! 15_"t"--_"-m--_ .......'_............... ':....................... _DUETOC_ING PEDZOn_.-I,J :|__ . i0¢z_-l_S_C0NeS i '"7;t............-..."-'l-_-'cl?s.s s) .....-.............-......... _............... i 14I............ _...._--'-i.... _....... ;..... ";....... J"!.............. ! i ! js 2 0 ttmefromleuech (s) AftPayload 40 3S 2S 2O 15 10 S_ _! I_GUI.._OSCZLLAT]CI".I i .....mSA.--.._.--.:.----- ......................................... '_uETOOONZmer_zIO_ - ¥I_I,SUD-lmmTLOmO GE_TZO,, : ; _1l|_ ............. .................. i"v""i i i i i : i , , i Figure 1.23OEDIPUS-A flightdynamics datashowing thetimehistory oftheanglebetween spinaxisandthedirection oftheearth's magnetic field,forbothsubpayloads important differences andextensions. TheOEDIPUS-C payload waslaunched, usingthe BlackBrantXIISounding Rocket, toahigher trajectory withapogee of843kmandthe length ofdeployed tether was1174m.Thus,thetrajectory hadagreater rangeinplasma density thanOEDIPUS Aandprovided anextended perspective onplaneandsheath waves andtheirinteraction withspaceplasma. Tounderstand theimportance oftheelectrically conducting tetherforthepropagation offfwaves between thesubpayloacls, thetetherwascut frombothendsonthedownleg partoftheflight.Theexperiments werealsodesigned tohelp understand howcharged particles associated withtheaurora affect satellite transmissions. Therewere13experiments (threeinstruments fromtheCanadian Space Agency, sevenfrom theNational Research Council ofCanada, andthree others fi'om theUniversity of Saskatchewan andtheUSAirPhillips Laboratory intheUSA). TheOEDIPUS-C payload was sponsored bytheCanadian SpaceAgency andthepayload contractor wasBristol Aerospace 3O Ltd.Oneofthemaininvestigations onOEDIPUS Cwastheproject funded bytheCSA's Space Science Program which involved controlled radio-wave experiments. Theradio instruments (HEXandREX)werebuiltbyCALCorporation andRoutes Inc.,bothfrom Ottawa, Ontario, andthePrincipal Investigator wasfromtheCommunications Research Center(CRC) inOttawa, Canada. TheOEDIPUS-C configuration ispresented onFig.1.24.Thetethered payload consisted of twospinning subpayloads withamassof115and93kg,respectively. Theywereconnected bythesametypeofthetetherasusedintheOEDIPUS-A mission (0.85mmdiameter). The subpayloads eachhadfourlongbooms (Be-Cu BI-STEM elements), forming aV-dipole antenna (13m,tip-to-tip, ontheaftsubpayload; and19montheforward subpayload). The tetherwasa24gaugewireperMIL-22759/32 whichhasa19strand, tin-coated copper conductor withwhitetefloninsulation (radiation cross-linked, modified ETFE) ratedat600 V.Bothsubpayloads hadvideocameras forthedetermining theandtheattitude solution and therelative position between theforward andaftsubpayloads. Thepayload performance wascaptured inspacebyanaftpayload videocamera. Thesubpayloads telemetered datatoa ground station forabout15minutes beforetheylandedintheArcticOcean(non-retrievable). tN._TETHER __ DEPLOYER ITETHER AFTPA_OAD--_ pT_ TF$ Figure 1.24OEDIPUS--C configuration withlocation ofthe"ITSAuniqueTether Dynamics Experiment (TDE) wasoneoftheexperiments flownduring that mission. Itwassponsored bytheSpace Technology Branch oftheCSA in collaboration with Bristol Aerospace, University ofManitoba, University ofBritish Columbia, McGi11 University, Carleton University andNASA Langley Research Center. Adescription oftheTDEispresented inthefollowing section. OEDIPUS-C TETHER DYNAMICS EXPERIMENT fiDE) The planning forthis technological experiment wasinitiated in1992, and culminated withthesub-orbital flight on November 6,1995.Themainobjectives were asfollows: derivetheory anddevelop simulation and animation software foranalyses ofmulti- bodydynamics andcontrol ofthespinning tethered two-body configuration; provide dynamics andcontrol expertise, forthesuborbital tethered vehicle andforthe science investigations, develop anattitude stabilization scheme forthepayloads and support OEDIPUS Cpayload development; acquire dynamics dataduring flight, andcompare withpre-flight simulations to demonstrate thatthedesigntechnology isvalid. 31 Figure 1.25TEther LABoratory Demonstration System -TE-LAB, atDFL, CSA Figure L26Tether ForceSensor (TFS) flexureTheTDEadvanced spacetethertechnology significantly. Thefollowing arenoteworthy. •Several types ofmathematical model wereinvestigated, including bothlinear andnon-linear approaches. •Alaboratory 'hanging spintest'facility wasestablished attheUniversity of British Columbia, which wasableto demonstrate theessential dynamic stability characteristics ofspinning tethered systems. •ATEther LABoratory Demonstration System (TE-LAB), developed in conjunction withgraduate engineering program ofCarleton University, supported precise ground simulation of theOEDIPUS dynamics. TheTE-LAB facility stimulated advances ingimbals suspension andinnon-contact attitude measurement techniques, tomeet stringent requirements ofzero-g simulation inthe one-g earth environment (Fig.1.25). •Aunique precision 3-axis tether Force Sensor (TFS) wasdesigned byBristol Aerospace Ltd.inconjunction with NASA Langley Research Center. The design derived from theNASA's experience withmulticomponent wind tunnel balances fortheaerospace industry. TheTFShadtwosetsofstrain gauges: foilgauges andpiezo-resistive gauges. TheTFSwasmanufactured by Bristol Aerospace Ltd.,andModem Machine &ToolCo.,Newport News, Virginia, andwascalibrated byNASA, CSAandModem Machine &ToolCo. (Fig.1.26). During theflightthesubpayloads andallon- board instnnnents metandexceeded expectations. Thedeployment ofthebooms andtether, including severance ofthetether fromthepayloads, wascaptured inspaceby theaftpayload camera, andprovided an overall eonfh-mation ofstability ofthe spinning subpayloads andtether dynamics. Anexample oftheprocessed flightdynamics data-nutation anglesofbothsubpayloads are presented inFig.1.27. 32 4.5 4 3.5 3 !, i1.s 1 0.5 0 100 2.5 1.5 m o1 Z0.5IiI,IiItI_IiIi! 200300400500600700800900 AftPoyloodt 1000 ,I,!tIII,IItIIItIi I00200300400 500600700800 900I000 Tuneafterbunch(sec) Figure 1.27Nutation (coning) angles asfunction oftimeforOEDIPUS-C payload Thetimehistory ofthetotaltetherforcecalculated basedonthefoilgaugeoutputs is presented inFig.1.28.Thedeployment profilebasedontheflightdataisshowed inFig.1.29. Themajor achievement wastheimplementation anddemonstration ofthemajor axis spinner stabilization forthetethered OEDIPUS-C subpayloads. Theground testsalsoserved verywelltounderstand thecomplicated dynamics ofthespinning tethered two-body configuration andtheinteraction between therigidandflexible bodymodes. Theanalysis of thedamped gyroscopic modesofspinning tethered spacevehicles withflexible booms turned tobeaveryeffective tooltounderstand thedynamics ofthesystem. 33 4 0 iTi i 150 _ 35O 4_ "Pmdlq_'Imxlb(,w,) Figure 1,28Tether tension vs.timemeasured byTFSduring OEDIPUS Cflight 1250 --.-1000E -=750 C .?500 Im 0"250_am WTheactualspinrateduringflightwas0.084 Hzwellwithin thestablerange. Full3-D computer animation ofthetethered system's dynamic behaviour andofthedamped gyroscopic modes alsoserved verywellin understanding thedynamics ofthis configuration. TheOEDIPUS-C tether deployment system ispresented inFigure 1.30.Itwaslocated intheaftendofthe forward subpaylaod anditiscomprised ofa rotating spool, supporting structure, a magnetic hysteresis braketocontrol tether tension, aslipring,highandlowresolution shaftencoders, awireguard/snare retainer, andforward tethercutterassembly. I I I I I_*,_'_'-- I ----d------L-----L------I....../11_-- ----I L---- I I I I_1 I I I I I J_ I I I I C--T--j_J---T--_---i--- I I L_ II I I I__/___/ ...... I___/__/ _L__ I I_1 I I I I I I_" I I I I I ----'1------.--'------.------.------I------.---- I /I I I I I I •..•s.... |.••.i•I.•s.. •,i.,,•i,.•i|••., 100150200250300350400450500 timeafterlaunch(sec) Figure1.29OEDIPUS-C tetherdeployment profile fromthespoolencoder data Figure 1.30 TheOedipus-C TetherDeployer 34 Contacts fortheOEDIPUS Project: •H.Gordon James-Communications Research Centre •Alexander Jablonski -Canadian SpaceAgency •George Tyc-Bristol Aerospace •FrankVigneron -Consultant, Canadian SpaceAgency Contacts fortheOEDIPUS-C Tether D_amics Experiment (TDE): •AlexJablonski, FrankVigneron -Canadian SpaceAgency Tether Dynamics Experiment, Payload Stabilizazion andTE-LAB •George Tyc-BristolAerospace Tether Dynamics Experiment, Payload Stabilizazion •ArunK.Misra-McGill University Tether Dynamics •VinodJ.Modi-University ofBritishColumbia Tether Dynamics •Douglas A.Staley-Carleton University TE-LAB •RayRhew-NASA LaRC TFS 35 SECTION 2.0PROPOSED TETHER FLIGHTS 36 2.1Electrodynamic Tethers ForReboost oftheInternational SpaceStation Propellantless Reboost fortheISS:AnElectrodynamic TetherThruster Theneedforanalternative tochemical thruster reboost oftheISShasbecome increasingly apparent asthestation nears completion. Anewtypeofelectrodynamic tetherattached totheStation (Figure 1)could bedeveloped togenerate anaverage thrustof 0.5-0.SNewtons for5-10kWofelectrical power. Bycomparison, aerodynamic dragon ISSisexpected toaverage from0.3to1.1N (depending upontheyear). Theproposed system usesatether witha kilometers-long uninsulated (bare) segment capable ofcollecting currents greater than10A fromtheionosphere. Thenewdesign exhibits a remarkable insensitivity toelectron density variations, allowing ittooperate efficiently evenatnight. Arelatively shortandlight tether(10kmorless,200kg)isrequired, thus minimizing theimpact ontheISS(center of massshiftlessthan5m).Orbital velocity Plasma -'. Con_r Pow,_Supply Deploy_r Station 4----- I]"'-current flowElectrons ejected,,÷Stationmaintained 4.--- atlowbias. 4.---- Insulated segment 4.---...- /oftether 4----- Tetherdeployed Geoma_etic 4--- /vertically downwardfieldexerts thrusting force4----- (deflected somewhat proportional _ byreboost force). tocurrent ¢'--" g,_ allalong 4-----_-_tether. '%Electrons collected _fromionosphere ___¢__alongpositively •-_._biasedbaresegmentoftether. Figure 1.Anelectrodynamic tetherreboost system fortheInternational SpaceStation. HighTetherCurrents forISSReboost ISSreboost (thrust forcesoforder1N)withatethernolongerthan10kmrequires tether currents oforder10A.Thecritical issueishowtodrawionospheric electrons atthatrate.The standard tethercarries insulation alongitsentirelength, exchanging current withtheionosphere only attheends:TSS-1R carded apassive metallic sphere asanode; PMGcarried anactive (plasma- ejecting) contactor. Current collected toapassive, biasedsphere inamagnetized plasma calculated bythestandard Parker-Murphy (PM)model(taking intoaccount magnetic effects, whicharedominant) growsasthe square-root ofthebiasvoltage, animportant factforfixed-area collectors. Apreliminary analysis ofthemeasured TSS-1R currents indicates thattheyweretypically greater thanthePMmodelpredictions (usingvaluesoftheelectron density andtemperature estimated from ionospheric models andasatellite voltage calculated withsomeuncertainty). TheTSS-1R datado not,however, appear topointtoadependence ofcurrent onvoltage greatly different fromthatof PMforhighervoltages. Eventhough, forexample, aTSS-1R current of0.5Aat350Vbiasmay surpass PMmodelestimates, itcouldstillimplyavoltage ofroughly 35kVtoreach5Aforthesame plasma parameters (which wouldrequire over175kWforathrustof0.7Nwitha10-km-long tether!). Activeanodes (plasma contactors) havebeendeveloped inanattempt tosolvebothspace-charge shielding andmagnetic guiding effects bycreating aself-regulating plasma cloudtoprovide quasineutrality andbyemitting ionstocounterstream attracted electrons andproduce fluctuations thatscatter thoseelectrons offmagnetic fieldlines.TheonlytetherexPeriment touseanactive anodesofarwasthePMG,whichreached 0.3Ainflightundera130Vbiasandthebestionospheric conditions. Unfortunately, thereisnowaytoscaletheresultstohighcurrents. Thediscouraging fact wasthatcollected eurrent decreased sharply withtheambient electron density atnight. Fortunately, thereisanother tetherdesignoption--the baretether-asproposed bySanmartin. 37 TheBare-Tether Breakthroush. Thebare-tether designrepresents abreakthrough thatmakes short- tetherelectrodynamic reboost withmoderate powerrequirements fortheISSapossibility. Towork ontheISS,areboost systemmustnotonlybecapable ofdelivering adequate thrust(preferably night andday);itmustdosowithsmallimpact ontheISSenvironment whilerequiring minimal accom- modation bythebaseline ISSsystems. Itshouldalsobesimpletooperate andmaintain, anditmustbe competitive intermsofitsuseofresources forthebenefits itprovides. Ourproposed designusesthetetheritself,leftuninsulated overthelowerportion, tofunction as itsownveryefficient anode.Thetetherisbiasedpositively withrespect totheplasma alongsomeor allofitslength. Thepositively biased, uninsulated partofthetetherthencollects electrons fromthe plasma. Thefollowing features argueinfavorofthebare-tether concept. 1.Thesmallcross-sectional dimension ofthetethermakes itamuchmoreeffective collector of electrons (perunitarea)fromthespaceplasma thanisalargesphere (suchastheTSS-1R satellite) atequalbias.Thisisbecause thesmallcrossdimension ofthetetherallowsitscurrent collection totakeplaceintheorbital-motion-limited regime, whichgivesthehighest possible current density. 2.Thelargecurrent-collection areaisdistributed alongthetetheritself,eliminating theneedfora large,massive and/orhigh-drag sphereoraresource-using plasma contactor attheupperendof thetether.Thissubstantially reduces thecenterofgravity shiftinbothcasesandreduces thecost andcomplexity inthecaseoftheactivecontactor. 3.Thesystem isself-adjusting tochanges inelectron density. Thisisaccomplished byanatural expansion oftheportion ofthetether thatisbiasedpositively relative totheionosphere whenever thedensity drops(Figure 2). Features (1)and(2)combine toprovide an abilitytocollect largecurrents withmodest inpmpowerlevels.Wepresent belowa candidate system thatcanproduce average thrusts of0.5-0.8 N,forinputpowerof 5-10kW. Developing anISSReboost System. Ourpre- liminary design foranelectrodynamic tether thruster capable ofdelivering 0.5-0.8N of thrusttotheISSatacostof5-10kW of electrical power consists ofan10-km-long aluminum tetherintheformofathickribbon (0.6mmby10ram). Despite itslength, the tetherwouldweighonlyaround 200kg.Since thebareportion ofthetetheristoactasour electron collector, adownward deployment of thetetherisdictated bythephysics oftheeast- ward-moving platform. Theupperpartofthetetherwillbeinsu- lated.There aretworeasons forthis.First, thereisthenecessity forpreventing electrical contact fromdeveloping across theplasma between theupperportion ofthetetherandthe Space Station, which (when thesystem is operating) areseparated byanelectrical potential difference ofaround akilovolt. Beyond that,theinsulation provides forgreater thrustatagiveninputpower. Thiscomesfrom thefactthatthelargest tether-to-plasma bias occursattheupperend,anddecreases downthetether.Acompletely baretetherwoulddrawthe maximum Orbitalvelocity Power Supply\LD_I ELECTRON Plasma DENSITY Contactor ° + _Direction of Geomagnefc .o....Iasulatedfieldexertssegment tlmasfiag force_ oftether Innpordonal tOcul_nt 4--- allalong tetheto t-_collection __W_ collection Figure2.Abare-tether thruster designed to adjusttolowerelectron density (asatnight). A shiftinthezeropointofbiasfurther downthe tether increases thecollecting surface and maintains anearly steady thrustforconstant inputpowerandinduced e.m.f. 38 current through thepowersupply, butthecurrent wouldbestrongly peaked attheupperendofthe tether. Keeping theinputpowerconstant, wecansubstantially increase theaverage current inthe tether, andhencethethrust, byinsulating thetetherovermuchofitsupperportion, collecting current withthelowerportion, andhaving aconstant current intheupperpart. Determining theoptimal fraction toinsulate ispartofthedesigneffortfora"bare" tether reboost system. Ourpreliminary designhastheupper50%ofthetetherinsulated. Evengreater thrustduringdaytime operation couldbeobtained withahigherfraction, butthenight-time adjust- abilitywouldsuffer. Thesystem provides flexibility, inthesensethatthethrustobtained depends almost linearly on theinputpower, asseeninFigure4. Thebare-tether designhasessentially solvedtheproblem ofday/night thrustfluctuations. But fluctuations inthrustduetofluctuations intheinduced e.m.f,asthesystem encounters avarying geomagnetic fieldaround theorbitareafactoflifeforanytether-based system. Figure5showthe thrustvariations around theISSorbitwithdifferent inputpowerlevels. 1.2 C.6 0.6 _"o.4 0.2I Electrondensity _'O.8 2)(1012 _, _0.6 z 0.4 e- 0.2 4000 6000 8000 10000 12000 14000 InputPower(Warns) Figure 4.Variation ofthrust withinput powerfornominal 10-kinsystem. Motional e.m.f.: 1.2kV.f" ,_ /'x__/' / 5kW Avg.0.46N 050 i00 150200 Time(minutes) Figure5.Comparison ofthrustgenerated for inputpowers of5kWand10kW Giventhelevelofthecurrent thesystem maydraw,thesystem willalmost certainly require its owncathodic plasma contactor attheStation end.Theeontactors currently underdevelopment at NASA LewisResearch Center shouldbewellsuitedforthisfunction. Ifthrustsover0.5Naredesired, itislikelythatthesystem willalsohavetorelyontheISS'splasma contactor aswell,oronasecond dedicated contactor, sincecurrents overthe10Aratingofthecontactors couldberequired. Before anoperational electrodynamic tetherreboost system fortheISScanbedesigned, aseries ofground andspace-borne experiments andcomputer simulations mustbeperformed. Inaddition, thorough systems analyses mustbeperformed todetermine thephysical integration andoperational issuesassociated withitsimplementation ontheISS. Among theissuestobeaddressed intheanalyses ofthereboost system aretheattachment location forthetether, needforretrieval capability, microgravity impact, power interfacing, and safety. Theseareinaddition todesign issuesspecific tothetether itself,suchastethermaterial, length, andgeometry. Assessment ofSpaceApplication andBenefits totheISS 1)Mission Benefit. Thevalueinanelectrodynamic tetherreboost system liesinitsability tocouple powergeneration withthrust. Heretofore theelectrical andpropulsion systems havebeeneffectively totally separate entities. Outfitting ISSwithaneleetrodynamic reboost tether seversthemost critical andconstraining dependency onEarth-propellant resupply. TheStation cansupply itsown powerbutnotitsownpropellant. Without anelectrodynamic tether,thespecter ofSkyLab andthe 39 words"reentry" and"atmospheric burnup" willforever hauntthemindsofanyone whohasan interest intheprogram. Addatetherandsomeadditional storage capacity forsupplies, andsuddenly aoneyearinterval between visitstotheStation becomes conceivable. Evenifthecurrent frequency ofresupply flights totheStation ismaintained, withan electrodynamic tethertheStation Program hastheoption totradekilowatts forincreased payload capacity. Resupply vehicles candeliver usefulcargolikepayloads, replacement pans,andcrew supplies ratherthanpropellant. Within therangeof5to10kW,acrudeapproximation of1,000kg ofuserpayload gainedperkWexpended peryearappears reasonable; further analysis willrefinethis estimate. Asabonus, propellantless reboost isexhaustless reboost: external contamination around the Station isconsiderably reduced. TheStation reboost propellant ishydrazine. Anyconsumption of propellant mayresultinresidual chemical deposits andcontamination ontheStation's exterior surface. AnelecU'odynamic tether provides ameans toreboost theStation without the complications ofchemical combustion. Thepurityoftheexternal environment forscience payloads isenhanced, andbeneficial operational impacts ofreduced propellant exhaust onexternal systems andopticswillberealized. Electrodynamic thrustuulyrepresents solarpoweratitsfinest. Yetanother dimension topropellantless reboost mustbeconsidered. Station usershavebeen allocated aminimum of180daysofmicrogravity peryear. Current planning essentially halts science activity during reboost maneuvers. Lowthrust electrodynamic tetherreboost couldbe performed overlongduration, asopposed toshortduration, highthrustpropulsive maneuvers. The 0.5to0.8Nthrustprovided bya10kmtethermorethancounteracts theStation's atmospheric drag onadailybasis.Thusthequestion arises,cananelectrodynamic tethercompensate forthedrag whileitisoccurring, without disrupting themicrogravity environment? Fluctuations intheinduced voltages fromtheEarth's magnetic fieldandinelectron densities willcreate"turbulence" through which theelectrodynamic tether driven Station mustfly;canload-leveling control systems compensate forthesepockets andmaintain microgravity levels? Inthiscaseanewrealmof possibilities opensupforlong-duration microgravity experiments. Theallureofthisself-propelled spacefacility iscertainly remarkable, andofferspotential advantages. 2)RiskReduction. Asidefromreplacement offailedcomponents, anelectrodynamic reboost tether ontheStation makesthevehicle itselfessentially independent ofpropellant resupply fromEarth. Theprimary resupply consideration becomes theinhabitants oftheStation andnottheStation itself. Thisisanewviewfordevelopment ofspaceoperations. Thereceasestobeconcern overthe "lS0-day countdown toreentry at150nautical miles" whichcurrently permeates everyaspect of Station mission planning. Withthemulti-billion dollarinvestment inthevehicle virtually secured andfreefromconcern overlongresupply vehicle launchdelays, particularly Russian Progress orFGB tankerdelays, theProgram willbeabletofocusmuchmorestrongly ontheISSmission ratherthan onISSitself. 3)CostPayBack. Thecostoftheproposed system comesintheformofthedevelopment, launch, andinstallation ofanoperational tetherreboost system ontheStation. Thepayback comesinthe formofreduced propellant uprnass requirement. For2003to2012,nearly90,000 kgofpropellant mustbelaunched. Usingafigureof$20,000 perkg,thisrepresents asumof$1.8billion. An electrodynamic tethersupplying 90percent ofthisrequirement wouldreduce theoperational costby $1.6billion, paying foritselfmanytimesover.Moremodest estimates stillresultinareturnon investment tensoftimesthecostofdevelopment andoperation ofanelectrodynamic reboost tether. Contacts: LesJohnson, NASA/MSFC JoeCarroll, TetherApplications Company JuanSanmartin, Polytechnic University ofMadrid Robert D.Estes&EnricoLorenzini, Smithsonian Astrophysical Observatory BrianGilchrist, TheUniversity ofMichigan AnnArbor Manuel Martinez-Sanchez, Massachusetts Institute ofTechnology 40 2.2AnUpper Atmospheric Tether Mission (ATM) Introduction TheAtmospheric Tether Mission (ATM) isaShuttle basedscientific experiment thatwill deploy asetofeleveninstruments tocollect valuable atmospheric dataneverbeforeobtained. This setofinstruments willbehoused inanendmass/spacecraft thatisdeployed downward fromthe Shuttle bya90kmtether. Theinstrument package willcutthrough theatmosphere, collecting data, atthreedifferent altitudes overasixdaymission. Ateamwasformed attheMarshall SpaceFlight Center (MSFC) toconduct apreliminary concept studydefining asystem thatwouldaccomplish the objectives oftheATM.Adetailed reportwillbepublished bytheteamattheconclusion ofthestudy. Science Instrument Requirements AScience Definition Team(SDT)wasformed byNASAHeadquarters todefinethescientific objectives oftheAtmospheric Tether Mission (ATM). TheSDTproposed asetofeleven science instruments thattogether wouldmeetalloftheATMmission objectives. Theinstruments, their requirements andlocations areshowninTable1andinFig.6,respectively. Instrument ._t_or J_ecfro_c$ ._trtsor E-Box I_trument TG_mcfry Dexcr_g_n Directions Dmwnsions Mass Mass Power Rate IoaD_I_ 12dia 21112xl6 0.9 2.3 3 2000 Me_r 7deep Retm_ng 12dia 21x|2x160.92.3 410O0 Potential 7deep A_dyz_ IonM&_.s 18x12xl i 18xl2xl6 1.8 2.0 6 500 l_mgmmr I_a 15x15x10 0.35 3,0 4 5600 Probe 15loa8 boommount Neulral Wiad 16dia 18x12x16 2.1 2.2 8 10(30 Met_ 19¢kep Neutral Mass 18x12xl I [Sx12x16 2.0 2.5 10 10(_ Sp_-_vmeu_ Eaes'ge_c 19xlSx18laclud_d2.2N/A 28000 Particle inScasor Spccuumeta E-Field 20c_mdia 12x12x8 18.0 3.0 t0 50K Double 6deep (3x6) Probes IR lOxl0x21 18xlSx13 7.0 2.0 13 128K Spe_'_ometef UV 10x10x25 iDc,in 2.8 iac. 5 320 Pho(omelef $easof sens 3-Axis 8x8x21 18xlSx13 1.0 2.5 2 1600 MaSae_me_ TotalPayload 39.1 21,8 67 199KN*ltttralMini T_ S_lctromtot _ _ MaQnl_tomo_t iIt"...... "_oNo Figure 6.Preliminary drawing ofthe endmassATM Table1.Science Instrument Requirements Mission Scenario Thebaseline mission scenario isthattheOrbiter willentera220kmcircular orbitata57 degreeinclination. Thetetherlengthforthisscenario is90kmandwilloperate inadeploy only mode.Onthefirstdaythetethered endmass willbedeployed downward 50kmto170kmaltitude andremain therefortwodays.Ondaythree,anadditional 20kmwillbedeployed, lowering the endmass toanaltitude of150kmfortwodays.Ondayfive,thefinal20kmoftetherwillbe deployed, lowering theendmass toitsfinal130kmaltitude andwillremain atthisaltitude fortwo 41 days.TheOrbiteraltitudewillbemaintained byuseofthePrimaryReactionControlSystem (PRCS)thrustersontheOrbiter.Ondayseven,thetetheriscutandtheendmassbeginsareentry course.Thecurrentestimateoffuelrequiredforthisscenariois1996kg(4400lb.). Fiveofthescienceinstruments arerequiredtofacetheRAMdirection withtwointhewake. A seriesofE-field doubleprobesandLangrnuir probesareplacedatspecific locations around the1.6m diameter satellite shell.Thisconcept showsanaerodynamic tailusedtoincrease yawstability. Aerodynamic Analysis ofEndmass Thedragforaspherical shaped endmass of1.6mindiameter rangesfrom0.92N at130km altitude to0.11Nat170kmaltitude. Abulletshaped endmass wasconsidered toeasepackaging constraints oftheendmass subsystems. Thedraganalysis showed thatthedragforasphere is20 percent lowerthantheequivalent bulletshapeendmass. Thediameter ofthespherical endmass was increased from1mto1.6mindiameter toalleviate packaging constraints. Endmass Attitude Control System Thereareseveral constraints impacting theendmass attitude control system design. Twomajor constraints onthesystem are;avoidance oflargetorques thatwilldisturb theendmass forceand acceleration measurements, andtheinability ofusing magnetic torquers because theycause disturbances inthemagnetic fieldfluxmeasurements. Thescience instrument requirements statethat theendmass shouldbepointed withinplusorminus3degrees ofRAMwithaplusorminus 0.1 degreepost-flight knowledge requirement. Anattitude control system combining theuseofreaction wheels andstrategically placedcoldgasthrusters isthecurrent proposed baseline. Thelocation of thrusters willbedetermined usingDirect Simulation Monte Carlo(DSMC) analysis toavoid instrument andendmass contamination. Thecontrol system isestimated toweigh15kg. Electrical PowerSystem Themission lifetime ofsixdaysrequires sevenLi/SOCL2 typebatteries weighing 105 kg.Theadditional cables, harnesses and distribution weights bringtheelectrical power system toanestimated 155kg.Thetotal desired power loadsareestimated at176.6 wattsincluding a25percent contingency. Thistotalincludes thescience instruments and electronics, andtheendmass majorsubsystem equipment. Asummary oftheelectrical powersystemmassversusmission durations is seeninFigure7.250,Projected ATMEPSMASS (DataBasedonTSS-1R Config) 200 150 100 Mass(_ 50 0Ban:Li/SOCL2 Mission Duration: 6-Days Pwr@S/CLoads: 180w 155 106 48 96 144 192 (2-Days) (4-Days) (6-Days) Mission Duration (Hours) Fig.7.ATMElectrical PowerSystem MassEPs 42 Thermal Control System Aflowfield temperature analysis was performed atanaltitude of130km.The temperature variations occurinshock-layers ranging fromg00Kto12000 K.The maximum aero-heating ontheendmass surface isshowninFigure8. Acombination ofthermal blankets and heaters comprise thecurrent endmass thermal control system. Theestimated weight ofthe system is7kgrequiring 4Wofpower.! 10I ,'w "w c 16"Endmass Thermal Control •Mmdmum AMo-HalffilB on SUrface (Stlgnltlon Point) All Mcx-l-k_t Flux 130km 135kw/mZ-$ 150km 0.4ekw/mZ-s 170km 0.19kw/ma-s •ForThomsll Control Surfw:m HiltFluxDll;trlbutlon Is Avenlble fromDSMCSTC 10-: I,i. i L,..I,,,I O0 100 120 140 160 180 200 Air,km 19 DSMC Stagnation PointHe_RuxCalculations forSphere Fig.8.ATMendmass thermal control Endmass Structure Therecommended material fortheendmass structure isAluminum 2219.Theendmass structure iscomposed ofanequatorial ringwithamounting panelwithtwohemispheres offourflanged quadrants each. Localstiffening willberequired forthemounting ofdeployables andsome instruments, andattachment oftheaerodynamic tail.Asmooth surface isdesired foraerodynamics requiring theuseofcloseouts. Theestimated weight oftheendmass structure is81.9kg. Baseline Tether Concept Thecurrent tetherconcept isa1.65mm diameter Kevlar strength member surrounded byaNomex jacket withatotaldiameter of 2.16mm.Amagnification ofthebaseline tetherisshowninFigure9. Thetetherhasabreakstrength of2892 Nandweighs 4.03kgperkm.Thetetheris non-conducting andiscurrently 90kmin length. Theprobability ofsurvival ofthe baseline tether overasixdaymission, assuming acritical particle sizeof0.3ofthe tetherdiameter, isapproximately 0.93. The probability ofsurvival ishighly sensitive to critical particle size. Agraph showing a particle sizeof0.2,0.3, and0.5ofthetether diameter isseeninFigure 10.Nomex Jacket (OD2.1Smm) 1.6SmmDiameter Keviar Strength Member Drawing nottoscale Fig.9.Amagnification ofthebaseline tether. 43 Several alternate tetherdesigns arebeing considered liketheHoytape (seeFailsafe Multiline Tethers forLongTether Lifetimes in theApplication section). Thesurvival probability usingaparticle sizeof0.3ofthe tetherdiameter jumpsfrom91percent fora singlelinetether to99.99percent forthe Hoytape. WithaHoytape typeoftether, thereisincreased surface areaincreasing the overall dragonthetethered system. Other Hoytape designs using smaller diameter members willimprove thedragconcern while maintaining anear100percent survival. Fig.10.Comparison ofcritical particle sizes andtheprobability ofsurvival versustime. Atmospheric DragandTether Dynamics Theatmospheric dragonthetetherandendmass willinducelibration oscillations ofthetether. This isduetothefactthattheatmospheric density isnotconstant thusaffecting thein-plane libration of thetether. Basedonthecurrent analysis, alibration and/or satellite pitchattitude control scenario maybe required. Deployment Dynamics Therearetwotypesofdeployers considered fortheATMmission. Amodified Tethered Satellite System (TSS)deployer andaSmallExpendable Deployer System (SEDS). TheTSSdeployer existsandhasflowntwicebutmustbemodified fortheATMmission. TheSEDSdeployer issmaller butisnotOrbiter qualified andwouldrequire extensive modification. Thecurrent baseline deployer oftheATMsystem isamodified TSStypedeployer. Deployment dynamics arestableandhavebeen demonstrated inearliermissions. TheTSSdeployment control strategy isproven andsuitable for theexpected endmass altitudes required intheATMmission. Theproposed ATMsystem willbe mounted onaSpacelab Palletinadesignated location intheOrbiter payload bay. Weight Statement The total estimated weight (without contingency) oftheendmass is325.3kg.The deployer reel,electronics, support structure andSpacelab palletaddanadditional 2940kg andthetetheradds500kg.Witha30percent contingency thetotalweight oftheATM system is4895kg.Table2detailstheATM weight statement. Table2.ATMweight statement.•Endmass -ScienceInstruments &Electronics Boxes 60.9kg -Structures 81.9kg -ElectricalPowerSystem 155.0kg -C&DHSystem 5,5kg -ThermalControl 7.0kg -AttitudeControlSystem 15.0kg •Deployer -Reel,Electronics, SupportStructure,SLPallet2940.0kg -Tether(120Ion) 500.0kg •Contingency (30%) 1129.6kg •Total 4894.9kg 44 ATMDevelopment Schedule FromAuthority ToProceed (ATP), thedevelopment oftheATMisplanned totake fouryears. Asixmonth PhaseAstudyforengineering designwouldbeginimmediately followed byaninemonth PhaseBdefinition. Parallel tothebeginning ofthePhaseA,an Announcement ofOpportunity (AO)wouldbereleased forthescience instruments. The selection oftheinstruments wouldoccuratthebeginning ofthePhaseBandthescience instrument design, development, fabrication andtesting wouldbegin. Thedevelopment of theendmass andtetherwouldbeginparallel totheinstrument development withthedeployer development starting withinthenextqua_er. Allhardware wouldbedelivered andintegrated intotheOrbiter inthebeginning ofthefourth yearwithaprojected launch inthethird quarter oftheyear. Contacts: •LosJohnson, NASA-MSFC •B.Carovillano, T.Smart,NASA-Headquarters •R.Heelis, U.Texas 45 2.3TheNaval Research Laboratory's Advanced Tether Experiment TheNavalResearch Laboratory (NRL) planstoflyitssecond tetherexperiment, called ATEx, in1998. ATExstandsforAdvanced Tether Experiment. Thetether system isa simplegravity-gradient dynamics andsurvivability research experiment. Majorprogram objectives include adding tothetether community's understanding of deployment dynamics andcontrol viaaconstant-speed motor, in-andout-of-orbit plane libration control viathrusters toexcite anddamplibrations, andinvestigating the survivability oflong-life tether materials. Isometric ViewsofATEx UpperEnd-Body Lower __ij_,_ nd'B°dy ATExDeckWhich Remains Attached To TheHostSatellite Upper End-Body:7.6x62.2x52.0cm Lower End-Body:60.9x48.2x38.1cm Mechanical Overview The83kgtethersystem willflyasapayload onahostsatellite inacircular altitude of 425NM.Apassive upperend-body's massofabout12kghasnoinstrumentation otherthan green-filtered retroreflectors. A6km(12kg)tetheriscomposed of0.004inchthickby1 46 inchwidelowdensity polyethylene with3singlestrands of215denier Spectra ®1000 uniformly spacedacrossthewidth. Thelowerend-body, of30kgmass,remains attached toa 29kgelectronics deckforthe90-day attached phaseofthemission. Attheendofthe90- daytether experiment, thelowerend-body isseparated fromtheelectronics deck,which remains withthesatellite. Thelowerend-body andportions ofthesatellite arecovered with IR-filtered retroreflectors. Toaccomplish someofthemission's science objectives, thelowerend-body is instrumented witha3-axistensiometer atthetetherattachpoint,a3-axisaccelerometer, a reelturn-counter, andasensortodetectsomediscrete anglesoftetherdeparture withrespect tothelowerend-body. Deployment Scenario ARerachieving anear-circular orbit,the3-axisstabilized momentum-bias satellite will orientwithATEx radially awayfromEarth. ATEx's upperend-body willseparate awav fromthelowerend-body viaaconstant speedmotorat2cm/s.Thestepper-motor willdrive apairofpinchrollers pulling thetetheroffalevel-wound reel;but,themotor andreel cannot reverse direction. Theentiredeployment sequence hasbeenspecifed andincludes satellite pitchmotions tomaintain atetherdeparture anglenearlyperpendicular tothelower end-body. Analysis showed thein-plane system libration anglewillinitially befiftydegrees and throughout thedeployment oscillate atsignificantly loweranglestoresultinafmallibration anglenearzerodegrees. Libration Control Demonstrations Fortheremaining 87daysofpost-deployment activities, tetherdynamics willfocuson exciting anddamping in-andout-of-plane librations. Thesatellite hasthrusters located on allfoursidesofthevehicle toforcethesatellite andlowerend-body (nowactingasonelarge end-body) forward-and-back intheorbitplaneandleft-and-right outtheorbitplane. Details oftheseactivities havenotbeendefined; however, athruster wouldbefiredandobservations madeoftension, acceleration, satellite attitude perturbations, andend-body positions. The resultswouldbeinterpreted inaquick-look scheme viathedynamics simulations. Satellite LaserRanging (SLR)Tracking Eachend-body has43retroreflector opticsor"comer cubes". Aretroreflector returns lightbacktothesourceindependent ofretroreflector orientation thuspermitting theend- bodiestobeobserved bytheglobalSLRnetwork. Thedifferent coating oneachend-body is sensitive toadifferent laserfrequency toassistinidentifying theend-body. Earlyinthe mission, telescope observations willguidethelaserbeamtotheend-body. Laterinthemission, perhaps thetether motions willrepeat regularly andorbit determination willbestraightforward suchthatalasercantargettheend-bodies eveninlocal daylight. TheSLRground stations require pointing information givenbyinter-range vectors (IRVs). NRL willenhance thetether system's orbit determination from USSPACECOMMAND byincluding end-body motions. Initially, thetetherdynamics models ofthein-plane andout-of-plane librations willbeusedtoaugment theIRV. Later,asSLRdatabecomes routinely available, estimates oftheorbitandrefined tetherdynamics models fromtheSLRdatashould substantially improve end-body position andrateestimates. TheIRVcanbefittotheobserved tetherdynamics toenhance the acquisition andtracking, perhaps theSIRsitescanacquire (indaytime) without telescope assist. Thiswillincrease around theglobeviewing opportunities. 47 TheGoddard Space Flight Center coordinates SIRobservations within their international network anddistributes theIRVstoeachsite.Weexpect tocollect tether data forapproximately oneyear. Afterthat,weplantooccasionally request aseriesofSIR trackstoconfirm long-term tethermotion andthatthetetherisstillintact. Tether Survivability Afterthe87daysoflibration control research, thelowerend-body isseparated fromthe satellite. Atthistime,ATExiscompletely unpowered, passive, andcanonlybeobserved by theground methods: SLR,radar,optical telescops. Atthispoint,theATExmission issimilar totheTiPSproject described inchapter 1ofthishandbook. Analyses indicate thatATEx willreenter intotheatmosphere in3-4years. Themodelincluded theatmospheric heating effects ofthesolarcycle. Contacts: •D.Spencer, M.P.Zedd-NRL 48 2.4TheAIRSEDS-S Mission Several organizations haveexpressed aneedforlowcosttethersolutions forthespace shuttle, International SpaceStation andunmanned launch vehicles. Furthermore, NASAand ASIhaveexpressed interest inflyingatethered satellite system inadownward deployed mission calledTSS-2.tThescientific andengineering information tobegainedfromsucha mission wouldallowadvances inourunderstanding andmodeling capabilities ofatmospheric andionospheric phenomena including satellite drag,theenergy deposition from magnetospheric currents andparticle precipitation, andthespatialandtemporal gradients in ionospheric properties. Moreover, thenextgeneration oftethered satellites andhypersonic vehicles arebeingplanned toflythrough thisatmospheric region. Before undertaking a mission ofthesizeandcomplexity ofTSS-2itmaybeprudent toexplore thepossibility that alesscomplex mission mightbeperformed, whichutilizes manypresent tethertechnologies andoptions forcommercial sponsorship, toachieve alimited setofscience andengineering goals. Inthefallof1994TheMichigan Technic Corporation (TMTC) wasawarded by NASA Headquarters andMarshall SpaceFlight Center Phase Afunding toconduct a preliminary design oftheAIRSEDS-S probe andmission plan. AIRSEDS-S, Atmospheric/Ionospheric Research SmallExpendable Deployed Satellite, willtestand demonstrate tether system dynamical interactions, flightqualify deployer systems and reusable components forapplication totheSpaceShuttle andtheInternational SpaceStation (ISS),verifymodels oftetherandsatellite aerothermodynamic behavior, anddetermine lower thermosphere chemistry andcomposition. Figure 1shows thesystem conceptually integrated withtheHitchhiker-C Crossbay Structure. AIRSEDS=S, basedonNASA's successful andproven SEDSprogram, isa90kmtethermission designed tocollect atmospheric information inthealtitude rangeof230-130 kmviaatethered satellite lowered fromthe SpaceShuttle Orbiter toaltitudes whichcannot currently beexplored usingballoons or aircraft. TheAIRSEDS-S mission willprovide thef'u_-thorizontal in-situ sampling atlow altitudes intheEarth's upperatmosphere. Inaddition, thesuccessful flightdemonstration of theAIRSEDS-S probeanddeployer system couldresultinthefuturedevelopment ofalow 49 costdeployertoconductfurther exploration oftheEarth's upper atmosphere and ionosphere, andconduct payload returnoperations, ISStowing operations andmicrogravity experiments fromthespaceshuttle andtheInternational SpaceStation. Thespecific objectives oftheAIRSEDS-S Mission areto: (a)Flightqualify tethered satellite hardware ontheSpaceShuttle Hitchhiker-C andfor use,byinference, ontheInternational SpaceStation. (b)Conduct aninvestigation ofthehorizontal distribution ofneutral atmosphere composition anddynamics inthelowerthermosphere. (c)Understand thelocalatmospheric environment ofthetethered probe,andcompare withcurrent predictions. (d)Testanddemonstrate tethersystem dynamical interactions. Thisincludes studying thebehavior ofatethered satellite system andanalyzing theflightcharacteristics ofthe probeintheEarth's upperatmosphere andcomparing withcurrent models. (e)Toprovide educational opportunities tostudents inbothpre-college andcollege level. ThelongtermgoalofTMTC andtheparticipants oftheA/RSEDS-S mission including theUniversity ofTexasatDallas, theUniversity ofIowa,theUniversity ofNewHampshire, TheAIRSEDS Institute, Tether Applications, Tethers Unlimited, TheSmithsonian Astrophysical Observatory, andNASA Marshall andGoddard SpaceFlight Centers, isto provide alow-cost reusable modular tetherfacility fortheInternational SpaceStation (ISS) andtheSpaceShuttle Hitchhiker-C programs. Suchafacility maybefurther developed to support experiments conducting remote sensing, electrodynamic operations, microgravity studies andpayload return. Mostofthecomponents fortheAIRSEDS-S mission willhave directapplication onfutureISSapplications andshuttlebasedmissions including thedeployer system, thetether, avionics, payload ejection andpayload support systems including data systems, endmassattitude control, communication anddatacollection. Forfurther information pleaserefertotheAIRSEDS lnternet Central websiteat http://www.airseds.com/. Contacts: •A.Santangelo -TheMichigan Technic Corporation 50 2.5TheRAPUNZEL Mission Thesmalltether project RAPUNZEL wasstarted in1991bytheInstitute of Astronautics, Munich Technische Universitat (TU)andtheKayser-Threde Company to designalowcosttether experiment. Incollaboration withtheSamara StateAerospace University (SSAU), Russia, theinitialmission intended toflytheGerman re-entry capsule MIRKA onaRussian Photon capsule. Lateron,incollaboration withSSAUandtheformer NPOEnergia, theproject splitintothreedifferent missions onResurs, Photon, andProgress spacecraft, respectively. TheTUteamdesigned andbuiltadeployer basedontextile technology, whichwould ensurebothhighreliability andlowcost(Fig.1).SSAUisbuilding asmallre-entry capsule to flyonResurs. Lately, themainefforthasbeenthe development andtestofthedeployer. In November 1995,acampaign ofparabolic flights tested thedeployer under microgravity conditions. Thef_sttests haveshown goodresults andproven the concept feasibility. Thelaboratory tests werefollowed bynumerical simulations of thepayload deployment anditsre-entry in theatmosphere. Figure1.Breadboard modeloftether deployerFig.2shows aschematic ofthe mission sequence. Whentheendmass is ejected bysprings (1)thedeployment starts(2).Afterreaching thefulltether lengthof52km,thetetheriscut(3)and thecapsule reenters theearthatmosphere (4)andlands onparachute (5). Preliminary simulations haveshown that eventhough theatmospheric draginduces smalloscillations inthesystem, the endmass landssafely intheKasakstan region. 2152km 3 O ,4 260km o Figure2.Schematic viewofthe deployment sequence Contacts: •Manfred Krischke, Kayser-Threde GmbH, Munich, Germany •DieterSabath -Technische Universitat Munich, Germany 51 2.6Tether Mechanism Materials andManufacture Project TheESAfunded Tether Mechanism Materials andManufacture (TMM&M) project has beenperformed byAlenia Spazio (Italy), asprimecontractor, andSABCA (Belgium) and SENER (Spain) assubcontractors. Oneimportant classoflow-cost tethermechanisms andrelated spacemissions was identified inthedevelopment ofexpendable tethersystems thatdidnotrequire complex mechanism operations andtheassociated technology development. FortheTMM&M ESA technology development activity, aEURECA-based tetherinitiated material orsample re- entrymodelmission, witha150-kg masscapsule anda20-kin tether, wasadopted forthe expendable tethermechanism designanditsbreadboard modelselected tobemanufactured andtested. Aparticular challenge inthe expendable tether mechanism design, associated withanear-horizontal tether deployment operation, wasrepresented by thedeployment control, tension andrate rangesandaccuracy requirements. Various simple tethermechanism designsolutions were traded-off andaspool-reel configuration solution, inwhich no (passive) control isapplied intheearly tether spooldeployment operation and active reel-brake (rate-feedback) actions areimplemented tocontrol theremaining partofdeployment accurately, was adopted andbread-boarded. TheTMM&M Project expendable tether mechanism bread-board model(fig.1)wasfunctionally tested onasuitably designed and manufactured testfacility capable of performing tetherdeployment testing for avastrangeofpreselected length, rateand tension reference profiles.Figure1.Expendable tethermechanism breadboard model Contacts: •R.Licata, P.Medina -Alenia •J.M.Gavira-ESA/Estee 52 2.7TheSpaceTether Experiment (STEX) Description TheSpaceTether Experiment (STEX) hasbeenproposed byISAStoflyonboard theSpaceFlight Unit(SFU) follow-on mission asoneofthescience and technology experiments. SFUorbitis500krnandcircular and thespacecraft attitude issun-oriented. SFUcancarry1000kgofpayload. The majorobjective ofSTEXistoassessthe tether technology forfuture scientific missions. Mission Scenario Inordertoevaluate theperformance ofdifferent control logics, a40-kg subsatellite willbedeployed upto10km andretrieved several times. During stationkeeping thesusbatellite willbe stabiLzed alongthevertical withimpulsive thrusts.Figure1.STEXonboardSFU Instrumentation Thesubsatellite willbeequipped withavacuum gauge, plasma probes andwave receivers tostudySFUelectromagnetic environment. Atetherdeployment andretraction system hasbeendeveloped forlaboratory tests,aschematic isshown infigure2.The deployment/retrieval speed,tethertension andreponse ofthefeedback system havebeen analyzed usingthissystem. _,an,ta_m,aIi .v4em_•rmwda¢ o_t'ele uef._c4mml_x : .... o.....;]l(m_m| Figure2.Schematic ofSTEXDeployment/Retrieval System Contacts: •K.I.Oyama, S.Sasaki, ISAS 53 SECTION 3.0TETHER APPLICATIONS 54 3.1General Thissection provides asummary ofvarious tetherapplications proposed thusfar,concentrating onnear- term,mid-term, andinnovative applications. Insomecases,theseapplications aregeneral ideas,andin others, theyarewell-defmed systems, basedondetailed studyandcomputational analysis. Theseapplications havebeendivided intoeightgeneral categories. Incaseswhereanapplication canbelogically placed inmore thanone,ithasbeenplacedintheoneconsidered mostappropriate. Toavoidredundancy, variations ofa particular system concept arenotdescribed separately. Instead, Section 3.2contains alisting ofthe applications bycategory, pagenumber, andpossible crossreference toothercategories. Descriptions of proposed applications followthislisting. Forthesedescriptions, astandardized format isusedtoallowquick andeasycomparisons ofdifferent applications. Thisformat isdesigned toeffectively serveaswidea readership aspossible, andtoconveniently convey thepertinent detailsofeachapplication. Readers with different interests andneedscanfindtheinformation andlevelofdetailtheydesireataglance. TheCategory andtitleofeachapplication ispresented atthetopofthepage.The"Application" subsection provides abriefstatement oftheapplication, andthe"Description" subsection provides abrief description ofthesystem designandoperation. Apicture islocated intheupperrightofthepageto supplement thedescription, byproviding adiagrammatic representation ofthesystem anditsoperation. The "Characteristics" subsection exhibits themajorsystem designandoperation parameters inbulletform.The lastcharacteristic isalways abulletentitled "Potential forTechnology Demonstration". Thisentryattempts toclassify boththeconceptual maturity ofanapplication, andtheamount oftechnological development required todemonstrate theparticular application. Whenapplicable wehavementioned theavailability of flightdatathatsomehow maysupport thefeasibility oftheapplication. Threedescriptors havebeenusedto indicate thedemonstration time-frame: •Near-Term: 5yearsorless, •Mid-Term: 5-10years,and •Far-Term: 10yearsorgreater. Thedateofthisprinting maybeassumed tobethebeginning oftheNear-Term period. Together, these subsections present abriefandcomplete summary ofthesystem's application, design, andoperation. The"Critical Issues" subsection, fiststhedevelopmental andoperational questions andissuesofcritical importance totheapplication. The"Status" subsection indicates thestatusofstudies, designs, development, anddemonstrations related totheapplication. The"Discussion" subsection presents moredetailed information aboutallaspects oftheapplication. Following this,the"Contacts" subsection liststhenames of investigators whoareinvolved withworkrelated totheapplication, andwhomaybecontacted forfurther information. (See"Contacts" section, foraddresses andtelephone numbers.) Finally, the"References" subsection liststhereference usedinthepreparation oftheapplication description. Manyoftheapplications thatfollowaresubject tosimilar critical issueswhicharemoreorless"generic" totethers. These areissuessuchasdamage frommicrometeoroids orotherspacedebris, dynamic noise induced onplatforms, highpowercontrol electronics technology, rendezvous guidance andcontrol, tether material technology development, andsystem integration. Manyofthefigures presented inthe"Tether Data"section address thesecritical issues. 55 3.2Tether Applications Listing Following isalistofabbreviations usedtoidentify crossreferences toother categories. Theapplication listinghasbeenarranged inalphabetical orderby category andapplication withineachcategory. AE AERODYNAMICS PL CN CONCEPTS SC 03 CONTROLLED GRAVITY EL ELECTRODYNAMICS TRPLANETARY SCIENCE SPACE STATION TRANSPORTATION Catego_/Title Page CrossReference AERODYNAMICS Station Tethered Express Payload System Multiprobe forAtmospheric Studies Shuttle Continuous OpenWindTunnel59 60 61SC$3 SCS_ SCTR CONCEPTS Gravity WaveDetection UsingTethers Tethered Lifting Probe External TankSpaceStructures Alfven Engine forInterplanetary Exploration Earth-Moon Tether Transport System MarsMoons Tether Transport System62 64 65 66 68 69SC AE O3 EL PL PLTR SS PL TR TRTR CONTROLLED GRAVITY Rotating Controlled-Gravity Laboratory Tethered SpaceElevator71 73SCPL SSSC ELECTRODYNAMICS Electrodynamic PowerGeneration Electrodynamic Thrust Generation ULF/ELF/VLF Communication Antenna75 77 79SSPL _BPL SCSSTR TR PLANETARY Aerocapmre withTethers forPlanetary Exploration Comet/Asteroid Sample Return Jupiter InnerMagnetosphere Maneuvering Vehicle MarsTethered Observer Tethered LunarSatellite forRemote Sensing81 83 85 87 89AE SC EL AE SCTR TR SC 56 Category/Title Page CrossReference SCIENCE Science Applications Tethered Platform Shuttle Science Applications Platform Tethered Satellite forCosmic DustCollection90 92 9303EL 03EL PLSS SPACE STATION Microgravity Laboratory Shuttle Deorbit fromSpaceStation Tethered STVLaunch Variable/Low Gravity Laboratory Attitude Stabilization andControl94 96 98 100 10203 TR TR 03 03SC SC TRANSPORTATION Generalized Momentum Scavenging fromSpentStages Internal Forces forOrbital Modification Satellite BoostfromOrbiter Shuttle Docking byTether Tether Reboosting ofDecaying Satellites Tether Rendezvous System UpperStageBoostfromOrbiter Tether Assisted Transportation System (TATS) Failsafe Multiline Tethers forLongTetherLifetimes03 05 07 09 10 11 12 14 16SS PL SC $8 SS PL PL $8 PLSg $8 57 3.3Tether Applications 58 -AERODYNAMICS - Station Tethered Express Payload System (STEPS) APPLICATION: Provides awaytoreturnsmallpayloads fromtheInternational Space Station toearthbetween shuttle flights, without thesafetyhazards ofhandling rocket motors orpropellants. DESCRIPTION: Payloads aretieddowninsidea mini-Apollo capsule smallenough tofitthrough therobotic airlock intheJapanese Experiment Module. Thecapsule isejected downward and deploys usingaSEDS-I (deploy-swing) strategy. Thetetheriscutfreeatthestationend,andit orients thecapsule forreentry beforeburning off. (This"kitetail"effectwasvalidated bySEDS-1.) CHARACTERISTICS: •Tetherlength: 30-33km •Payload: 30kg,100liters •Timescale: Near-Term CRITICAL ISSUES: •Tether deployment control forproper swing •Implications ofmicrometeoroid cut(--0.7% risk) •Accelerations of--4microgee onstation duringswing STATUS: •TetherApplications hascontract todeliver protoflight capsule &deployer Feb1998. •Capsule canbetestedasDeltaorProgress secondary payload; bothareunderstudy. DISCUSSION: Thetetherdeployer isasmaller easilyreloadable version ofSEDS. It mounts inareusable capsule balancer/ejector/deployer assembly thatremains withthe station. Fortestflights, thedeployer andflightcomputer mountinsidethecapsule. Thissimplifies integration onthehostvehicle andmaximizes hardware recovery forinspection and potential re-use. Baseline recovery scenario involves softmid-air capture ofgliding parachute byhelicopter. CONTACTS: •JoeCarroll •ChrisRupp •PaulKolodziej REFERENCES: AStation Tethered Express Payload System (STEPS), available fromTether •Applications 59 -AERODYNAMICS - Multiprobe forAtmospheric Studies APPLICATION: Measurement ofspatialgeophysical gradients. DESCRIPTION: Aone-dimensional constella-tion ofprobes islowered bytheShuttle orSpaceStation intotheatmosphere inorderto provide simultaneous datacollection atdifferent locations. CHARACTERISTICS: • Physical Characteristics: Mission related • Potential For Technology Demonstration: Near-Term CRITICAL ISSUES: • Crawling systems mightbenecessary • Operational sequence fordeployment andretrievalOBE STATUS: Configuration studyperformed bySmithsonian Astrophysical Observatory Analysis ofscientific applications performed atUniversity ofTexas, Dallas DISCUSSION: Thisconstellation configuration couldproveveryvaluable inlowaltitude measurements requiring simultaneous datacollection at thevarious probepositions. Goodtimecorrelation ofthe measurements isonebenefit of thissystem. CONTACTS: • Enrico Lorenzini • RodHeelis REFERENCES: Proc.ofFourth International Conference onTethers inSpat%Washington DC, 10-14April1995 60 -AERODYNAMICS - Shuttle Continuous OpenWind Tunnel APPLICATION: Obtain steady- stateaerothermo-dynamic research data underrealgasconditions without experiencing limitating effects inherent inground-based wind tunnels. DESCRIPTION: Atethered aerodynamically shaped research vehicle isdeployed downward formtheSpace Shuttle toobtaindatainthefree molecule, transition, andupper continuum flowregimes. Characterization oftheflee-stream, measurement ofgas-surface interactions, flowfieldprofiling, anddetermination ofstatevectors aretobeaccomplished. CHARACTERISTICS: • Length: 100-120 km • Mass: Variable, dependent onmission requirements • PowerRequired: TBD,forinstruments anddatahandling only • Potential For Technology Demonstration: Near-Term CRITICAL ISSUES: • Quantitative definition ofdatarequirements • Define method forflow-field profiling • Quantitative analysis oforificeeffects vs.altitude STATUS: Prototype experiment andinstrument package proposed forATMmission DISCUSSION: Unique measurements arepossible duetolowReynold;s number andhigh Machnumber regime. Measurements inreal-gas willprovide moredependable data regarding fluidflow,turbulence, andgas-surface interactions. CONTACTS: • Giovanni Carlomagno • Franck Hurlbut • George Wood REFERENCES: 61 Proc.ofFourth International Conference onTethers inSpace_Washington DC, 10-14April1995 --CONCEPTS - Gravity WaveDetection UsingTethers APPLICATION: Todetectgravity wavesfromsources suchasbinarystars, pulsars, andsupernovae. DESCRIPTION: Thesystem wouldconsist oftwo masses oneachendofalongtetherwithaspringatits center. Asthistethersystem orbitstheEarth, gravitational waveswouldcausethemasses tooscillate. Thismotion wouldbetransmitted tothespring, which wouldbemonitored byasensing device. Analysis ofthe springdisplacement andfrequency couldthenleadtothe detection ofgravity waves. CHARACTERISTICS: ° Mass: 20kg(EachEnd Mass) • Tether Length:25 km • Tether diameter: • Spring Constant: • Orbital Altitude:s 0.6mm Ks=2.3x103dyne/cm _>1000km •Potential ForTechnology Long-TermDemonstration: CRITICAL ISSUES: • Existence ofgravity waves • Gravity wavenoiselevelfromotherbodies • Excitation ofoscillations fromothersources STATUS: Preliminary calculations havebeenperformed atSAO,Caltech, and Moscow StateUniversity DISCUSSION: Thisgravitational wavedetector wouldoperate inthe10-100MI-Iz frequency bandthatisinaccessible toEarth-based detectors because ofseismic noise. If gravitational wavesdoexistinthisregion, asimplesystem suchasatether-spring detector wouldproveofgreatvalue. CONTACTS: • K.Thorne • Marino Dobrowolny REFERENCES: V.B.Braginski andK.S.Thorne, "Skyhook Gravitational WaveDetector," Moscow StateUniversity, Moscow, USSR, andCaltech, 1985. 62 B.Bertotti, R.Catenacci, M.Dobrowolny, "Resonant Detection ofGravitational Waves byMeans ofLongTethers inSpace," Technical Note(Progress Report), Smithsonian Astrophysical Observatory, Cambridge, Massachusetts, March 1977. 63 -CONCEPTS - Tethered Lifting Probe APPLICATION: Theliftingbodycontrols the altitude oftheprobe inatmospheric tether missions. DESCRIPTION: Ahypersonic lifting bodyis usedfortheprobeinanatmospheric mission. Changes inliftforcesontheprobecanbeused tocontrol theprobealtitude without changing thelength ofthetether. Required changes in probeattitude canbeaccomplished usinga movable tether attachment point or aerodynamic control surfaces. CHARACTERISTICS: •Tether Length: 10-200 km •ProbeArea: 10-50m2 •Potential For Technology Demonstration: Mid-Term CRITICAL ISSUES: •Development ofcontrol lawstomaintain probeattitude. STATUS: •Preliminary results indicate thefeasibility ofusingliftasacontrol mechanism for probealtitude. •Current studies favortheuseofamovable tetherattachment pointasasimpleand highly effective attitude control mechanism. DISCUSSION: Theliftingprobeprovides anidealcontrol mechanism forthealtitude ofan atmospheric tethersystem. Thealternative istoslowly change thetetherlengthbyusinga reelmechanism. Thismaynotbeeffective insituations where probealtitude mustbe maintained inthepresence ofatmospheric uncertainties. Inaddition, theuseofalifting bodycanincrease theatmospheric penetration oftheprobewithout increasing itsmass. Thisconcept canbeapplied toawiderangeoftetheratmospheric missions fromupper atmosphere research toaerocapture. CONTACTS: •JordiPuig-Suari •BrianBiswell REFERENCES: Biswell, B.,andPuig-Suari, J."Lifting BodyEffects ontheEquilibrium Orientation ofTethers intheAtmosphere,"AIAA-96-3597, AIAA/AAS Astrodynamics Conference, SanDiego, CA,1996. Keshmiri, M.,andMisra,A.K."Effects ofAerodynamic LiftontheStability of Tethered Subsatellite System," AAS-93-184, AAS/AIAA Spaceflight Mechanics Meeting, Pasadena, CA,1993. 64 -CONCEPTS - External TankSpaceStructures APPLICATION: Utilize Shuttle external tanksinaraftformat to formastructure inspace. DESCRIPTION: Tethers are usedtoseparate raftscomposed of external tanks. Thesecaneitherbe usedasa"Space Station" oras structural elements inanevolving SpaceStation. CHARACTERISTICS: • Tether Length: 10-20 km • Potential For Technology Demonstration: Long-TermTe_d _ • | I |Shutl_EmmaTmks | _--_--_--__-t_-'__T°g'WI Pizt_rm CRITICAL ISSUES: • Spaceoperations required toadapttankstoproposed applications • External tankinduced contamination environment • Stability/controllability ofproposed configuration • Assembly/buildup operations • Dragmakeup requirements STATUS: Preliminary analysis performed Further analyses effortdeferred DISCUSSION: Mostlikelyuseofthisconcept wouldbeasa"spaceanchor" fortether deployment concepts. CONTACTS: • JoeCarroll REFERENCES: Carroll, J.A.,"Tethers andExternal Tanks,Chapter 3ofUtilization ofthe External TanksoftheSpaceTransportation System," California SpaceInstitute, LaJolla,California, Sept.1982. Carroll, J.A.,"Tethers andExternal Tanks: Enhancing thecapabilities ofthe SpaceTransportation System," Dec.1982 65 -CONCEPTS - Heliocentric Alfven Engine forInterplanetary Transportation APPLICATION: Generation of propulsion forinterplanetary travel byusingtheelectromagnetic interaction ofaconducting tetherand theinterplanetary magnetic field. DESCRIPTION: Aninsulated conducting tether, connected toa spacecraft andterminated atboth endsbyplasma contactors, provides interplanetary propulsion intwo ways.Thecurrent induced inthe tetherbythesolarwindmagnetic fieldisusedtopowerionthrusters. Theinteraction between thetether current andthemagnetic fieldcan alsobeusedtoproduce thrustordrag.400gM/S CHARACTERISTICS: • Tether Length:1000 km • Cooling: Helium (2°K) • Current: 1000A • Power: 2MW • Materials: Superconducting Niobium-Tin • Potential ForTechnology Demonstration: Far-Term CRITICAL ISSUES: • Howdoesthissystem compare withothers, suchasnuclear orsolarsail • Feasibility andcontrollability havenotbeenestablished STATUS: TSS-1R flighttodemonstrate electrodynamic interaction withsurrounding plasma Moredetailed studyandevaluation ofthisapplication arerequired DISCUSSION: Thesolarwindisamagnetized plasma thatspirals outward fromthesun witharadialvelocity ofabout400km/sec. Themagnetic fieldofthesolarwindis5x10-5 Gauss,producing anelectric fieldof2V/kin,asseenbyaninterplanetary spacecraft. Ifa conducting tether, connected tothespacecraft andterminated atbothendsbyplasma contactors, werealigned withtheelectric field,theemfinduced initcouldyieldanelectric current. Thiscurrent couldbeusedtopowerionthrusters forpropulsion. Thecurrent couldbemaximized byusingsuperconducting materials forthetether. (Thissystem was proposed byI-Iannes Alfven in1972). Ithasbeencalculated thata1000km superconducting wireofNiobium-tin couldgenerate 1000A(2MW). Toachieve superconduction temperatures, thiswirecouldbehoused inanaluminum tubewithflowing 66 supercooled (2°K)helium. Thetubewouldbeinsulated andcapped ateachendwitha refrigeration system. Inaddition totheionthrusters, theinteraction ofthetethercurrent andsolarwind magnetic fieldwouldproduce thrustordrag.Ascurrent flowedinthetether, themagnetic fieldwouldexertanILxBforceonthetether. Ifthespacecraft weremoving awayfrom thesun(withthesolarwind),apropulsive forcewouldbeexerted onthetether asits electrical powerwasdissipated. Adragwouldbeexerted onthetetherifcurrent froman on-board powersupplywerefedintoitagainst theinduced ¢mf.Whenmoving toward the sun(against thesolarwind),theopposite conditions wouldapply. Thissystem couldbeusedtospiralawayfromortoward thesun,ortomoveoutof theecliptic. Theoretically, suchaspacecraft couldattainthesolarwindvelocity of400 km/sec. Useoftheelectromagnetic interaction between aconducting tethersystem and thesolarwindmayallowmuchshorter transfer timesandlargerpayloads forplanetary missions. CONTACTS: • MarieGrossi • JimMcCoy • NobieStone REFERENCES: Applications ofTethers inSpace_Vol.1,2Workshop Proceedings, NASA CP- 2365,March 1985 H.Alfven, "Spacecraft Propulsion: NewMethods," _ Vol.176,pp.167- 168,April14,1972. 67 --CONCEPTS - Earth-Moon Tether Transport System APPLICATION: Transportation ofmaterial from lunartoEarthorbit. DESCRIPTION: Material (probably Moonrocks)inlunar orbitiscollected bytheLOTS (Lunar Orbiting Tether Station), halfistransferred toanAFV (Aerobraking FerryVehicle) whichtransports ittoLEO, whereitistransferred totheTAMPS (Tether AndMaterials Processing Station). TheAFV thenreturns totheMoonformorelunarmaterial.AFV "TAMPS.I/ ee_ LOTS CHARACTERISTICS: • Physical Characteristics: • Potential ForTechnology CRITICAL ISSUES: • UndeterminedUndetermined Demonstration: Far-Term STATUS: Nodetailed studyonthisapplication hasbeenperformed DISCUSSION: Material (probably Moonrocks)inlunarorbitcouldbetransported toEarth orbitwithout theuseofpropellants withthistethertransport system. (Thematerial in lunarorbitcouldhavebeenplacedtherebytheLunarEquator Surface Sling;Application "Lunar Equator Surface Sling"). Itcouldbecollected inorbitbyaLunarOrbiting Tether Station (LOTS). TheLOTSwouldproceed asfollows: (1)catchtherocks,spin-up, catch anAerobraking FerryVehicle (AFV); (2)LoadtheAFVwithhalfoftherocks;(3)spin-up, throwtheAFVintotrans-Earth injection; (4)de-spin, loadtheotherrocksonatether;and (5)spin-up anddeboost therocksformomentum recovery. TheAFVwouldproceed toEarth,whereitwouldaerobrake intoLEOforcapture bythe TetherAndMaterials Processing Station (TAMPS). TheTAMPS wouldproceed as follows: (1)catch,retrieve, andunloadtheaerobraked AFV;(2)process moonrocks into LO2,etc;(3)refuelandreboost theAFVtoward theMoon;(4)recover momentum withan electromagnetic tether;and(5)alsocapture, refuel,andreboost AFV'sgoingtoGEOand deepspacewhenrequired. TheAFVreturning totheMoonwouldbearocketboosted into trans-hmar injection andfinallunarorbitforrecapture bytheLOTS. CONTACTS: • JoeCarroll REFERENCES: Applications ofTethers inSpace,NASA CP-2422, March 1986. 68 -CONCEPTS - MarsMoons Tether Transport System APPLICATION: Transportation ofmanned vehicles and spacecraft fromlowMarsorbit outtoescape, orfromescapeto lowMarsorbit,usingtethers attached totheMoons ofMars. DESCRIPTION: Long tethers (Kevlar strength or better) areattached aboveand belowbothPhobos andDeimos toferryvehicles andotherpayloads between lowMarsorbitandMarsescapewithout the useofpropulsion. Forexample, avehicle istethered upward fromalowMarsorbitstation, released, andthencaught byadownward hanging tetheronPhobos. Thepayload isthen transferred totheupward deployed tetherandreleased. Theprocess isrepeated atDeimos, andresults inescapefromMars.Theprocess isreversible. CHARACTERISTICS: • Length: • Tether Mass: • Tether Diameter: • Power: TBD • Materials:940km(up),1160km(down) atPhobos 6100km(up),2960km(down) atDeimos 5000kgto90,000 kg 2mm(orgreater) Kevlar, orhigher strength material Payload Mass:20,000 kg Potential For Technology Demonstration: Far-Term CRITICAL ISSUES: • Tether dynamics analysis • Comparison withotheradvanced propulsion methods • Rendezvous feasibility • Operations andcost • Tethersevering bymicrometeoroids ordebris STATUS: Aconceptual studydefines thetetherlengthandstrength requirements, but doesnotaddress construction, placement, andoperation ofthetether station. DISCUSSION: Thetwomoons ofMars,Phobos andDeimos arenearequatorial, andcan function asmomentum banksinthetransfer ofmassfromMarsloworbittoMarsescape (orthereverse). Therequirement istoplacelongtethers, upward anddownward, oneach ofthetwomoons ofMars. Example usesmightbetotransfer Deimos orcometmaterial 69 totheMarssurface ortotransfer astronauts fromMarssurface toawaiting interplanetary lowthrustvehicle atDeimos, ortosupport materials processing inMarsorbit. Tether stations onPhobos andDeimos mayhavetobemanned forconstruction, operation, andmaintenance. Therefore, otherhuman functions atthesesatellites wouldbe necessary tomakethisconcept viable. Itisbestsuitedtoahighactivity scenario with departures andarrivals atMarsdailyorweekly. Astation onPhobos alonewouldbe sufficient fornearMarsoperations, andcouldevenbeusedforescape withasufficiently longupward tether. Themassofthetwobodiesissogreat,(>1015kg)thattheirorbits wouldnotbeaffected fordecades orlonger. CONTACTS: • JoeCarroll • PaulPenzo REFERENCES: Penzo, P.A.,"Tethers forMarsSpaceOperations," TheCaseforMarsII, McKay, Vol.62,Science andTechnology Series,p.445-465, July1984.Ed.C.P. 70 -CONTROLLED GRAVITY - Rotating Controlled-Gravity Laboratory (Tethered Platform) APPLICATION: Provideareadily accessible variable/controlled gravity laboratory, capable ofgeneratingartificial gravity levels ofuptoIgandover,inEarth orbit. DESCRIPTION: Atethered platform composed oftwoendstructures, connected bya deployable/retractable 10kmtether. Oneend structure includes thesolar arrays, related subsystems, andtetherreelmechanism. Theother includes twomanned modules andapropellant motor. Artificial gravity iscreated inthemanned modules byextending thetether andfiringthe motor, rotating theentiresystem aboutitscenter ofmass(thesolarpanels arede-spun). Tether lengthisusedtocontrol thegravity level. CHARACTERISTICS: • Length: Upto10km • g-Level: Upto1.25 • Rotation Rate:Upto0.75rpm • Potential forTechnology Demonstration: Far-Term CRITICAL ISSUES:""TETHER PLATFORM CONCEPT TE1HER SOLARARRAnt8 Am. (De-Sm_) Susceptibility tomicrometeoroid/debris damage STATUS: Adetailed dynamic analysis hasbeenperformed atSAO ASystem studyhasbeenperformed atStanford University DISCUSSION: Access toanorbiting variable/controlled-gravity laboratory, capable of providing artificial gravity levelsofupto1gandover,wouldallowvitalexperimentation inthisimportant gravity range,andprovide anappropriate facility, shouldartificial gravity bedetermined tobeaphysiological requirement forextended manned orbital missions. Artificial gravity (intheform"ofcentrifugal acceleration) wouldbecreated byrotating the laboratory. Themagnitude oftheresulting centrifugal acceleration isequaltothesquareof theangular velocity timestheradiusofrotation. Three basicrotating labconfigurations arepossible -atorusorcylinder (centrifuge), arigidstation, andatethered platform. Thecentrifuge istheleastattractive because ofitsrelatively smallvolume, largeCoriolis force,andlargedynamic disturbance levels. Oftheremaining two,thetethered system hasseveral advantages overtherigid one.Itwouldprovide alargerradiusofrotation, reducing therotational raterequired to produce adesired g-level. This,inturn,wouldreduce unwanted sideeffects, suchasthe Coriolis force. Thevariable tetherlengthwouldalsoallowalargevariety ofartificial gravity environments. Tospinthesystem, thetetherwouldbeextended toitsfull10km 71 length, andthemotor fired.(Theminimum necessary Delta-V hasbeencalculated tobe 125m/s.) Thetether length wouldthenbeadJusted toprovide thedesired g-level. Assuming theendmasses areequalandrotating aboutacommon center, 0.08gwouldresult fromatetherlength of10kmataspinrateof0.12rpm,0.16g(lunargravity) froma lengthof8kmat0.20rpm,0.38g(Marsgravity) fromalengthof6kmat0.33rpm,Ig fromalengthof4.3kmat0.65rpm,and1.25gfromalengthof4kmat0.75rpm.The solararrayswouldbede=spun andsun-oriented. However, adisadvantage isthehighDelta- Vrequired tostartandstopthisspin.Another isthefactthattherotation wouldprobably havetobestopped toallowdocking withaspacecraft. Thislabwouldallowexperimentation atgravity levelsranging fromlowgravity, through Moon, Mars,andEarthgravities, tomorethan1g.Theeffects ofgravity on plantandanimal growth, andonhuman performance andmedical processes (suchasthose related tothecardiovascular, skeletal, andvestibular systems) couldbestudied forprolonged periods oftime.Gravity conditions ontheMoonandMarscouldbesimulated, andthelab couldbeusedtoprepare forthepossible useofartificial gravity onmanned interplanetary missions. Itcouldalsoprovide Earth-like habitability atpartialg.Suchphysical processes ascrystal growth, fluidscience, andchemical reactions couldbestudied atvarious gravity levels. CONTACTS: • Enrico Lorenzini • PaulPenzo • ChrisRupp REFERENCES: Applications ofTethers inSpacerNASA CP-2422, March 1986 B.M.Quadrelli, E.C.Lorenzini, "Dynamics andStability ofaTethered Centrifuge in LowEarthOrbit", TheJournal oftheAstronautical Sciences, Vol.40,No.1,1992, pp.3-25 Powell, J.David,Systems StudyofaVariable Gravity Research Facility, Final Report toNASA (GrantNo.NCA2-208), April1988. 72 -CONTROLLED GRAVITY- Tethered SpaceElevator APPLICATION: TheSpaceElevator may beusedasaSpaceStation facility totapdifferent levelsofresidual gravity, andatransportation facility toeasilyaccesstethered platforms. DESCRIPTION: TheSpaceElevator isan element abletomovealongthetetherina controlled waybymeansofasuitable drive mechanism. Theprimary objectives ofthe microgravity elevator mission arethe achievement ofanewcontrollable microgravity environment andthefullutilization oftheSpace Station support whileavoiding themicrogravity disturbances onboardtheSpaceStation. A shorterandslackcablecouldbeusedasbotha poweranddatalink. Aballastmassrepresents theterminal end ofthetethersystem. Itcouldbeanymass(e.g.,a Shuttle ET)oratethered platform. The objective ofthetransportation elevator application istoaccesslargetethered platforms formaintenance, supply ofconsumables, or module andexperiment exchanges. CHARACTERISTICS: • Length: 10km • Elevator Mass: 5,000kg • Ballast Mass: Upto50,000 kg • g-Level: 10-7to10-3 • PowerRequired: Upto10kWbyTether PowerLineLink • LinkDataRate: Upto40Mb/sbyTether Optical FiberLinkPotential For Technology Demonstration: Mid-Term CRITICAL ISSUES: • SpaceStation impacts • Dynamic noiseinduced onthetetherdrivemechanism • Gravity-measuring instrmnentation • Powerlinktechnology • Optical fiberslinktechnology STATUS: ASI/Aeritalia Elevator Definition Studyininitialdesignassessment phase, FinalReport issued inMarch 1988 73 Analysis ofdynamics duringdeployment, station-keeping, andtransfer maneuvers carried outbytheSmithsonian Astrophysical Observatory under contract toNASA/MSFC DISCUSSION: Themostpromising feature offered bytheSpaceElevator istheunique capability tocontrol withtimethegravity acceleration level.Infact,sincetheradial acceleration changes withposition alongthetether,theElevator wouldbeabletoattaina continuous rangeandadesired profilevs.timeofresidual gravity levelbythecontrol of theElevator motion. Moreover, theElevator isabletofullyutilizetheSpaceStation support (power, communications, logistics) andtoavoidtheSpaceStation contaminated environment, fromamicrogravity pointofview,bytethermediation. Another waytoexploit theSpaceElevator capabilities isitsutilization asatransportation facility. Theideaofusinglargetethered platforms connected totheSpaceStation by powerlineandcommunication link(viatethertechnology) makesunrealistic frequent operations ofdeployment andretrieval. Ontheotherhand,theplatform mayrequire easy accessformaintenance, supplyofconsumables, module andexperiment exchange. The SpaceElevator, asatransportation facility abletomovealongthetethertoandfromthe platform, maybethekeytotethered platform evolution. CONTACTS: • Franeo Bevilacqua • Enrico Lorenzini • PietroMerlina REFERENCES: Applications ofTethers inSpacerNASA CP-2422, March 1986 F.Bevilacqua andP.Merlina, "TheTethered SpaceElevator System," Second International Conference onTethers InSpace,Venice, Italy,1987. SATPDefinition Study,Mid-Term Report, Aeritalia, TA-RP-AI-002, March 21, 1986. Tethered SpaceElevator Definition andPreliminary Design, FinalReport, Aeritalia, TA-RP-AI-009, 1988. L.G.Napolitano andF.Bevilacqua, "Tethered Constellations, TheirUtilization as Microgravity Platforms andRelevant Features," IAF-84-439. S.Bergamaschi, P.Medina, "TheTethered Platform: AToolforSpaceScience andApplication," AIAA-86-0400, AIAA24thAerospace Sciences Meeting, Reno, Nevada, January 6-9,1986. Lorenzini, E.C.,M.D.Grossi, D.A.Arnold, andG.E.Gullahorn, "Analytical Investigation oftheDynamics ofTethered Constellations inEarthOrbit(Phase II),"Smithsonian Astrophysical Observatory Reports forNASA/MSFC, Contract NAS8-36606. Quarterly Reports Lorenzini, E.C.,"AThree-Mass Tethered System forMicro-g/Variable-g Applications," Journal ofGuidanee_ Controlr andDynamics, Vol.10,No.3,May- June1987.(pp.242-249) Applications "Microgravity Laboratory" and"Variable/Low Gravity Laboratory" 74 --ELECTRODYNAMICS - Electrodynamic Power Generation (Electrodynamic Brake) APPLICATION: Generation of DCelectrical powertosupplyprimary powertoon-board loads. DESCRIPTION: Aninsulated conducting tetherconnected toa spacecraft andpossibly terminated with asubsatellite. Plasma contactors are usedatbothtetherendsorwiththe baretether(seesect.2).Motion through thegeomagnetic fieldinduces a voltage acrosstheorbiting tether. DC electrical powerisgenerated atthe expense ofspacecrafFtether orbital energy.jRASII_celffaflm CHARACTERISTICS: • PowerProduced: 1kW-1MW • Length: 10-20km • Mass: 900-19,000 kg • Efficiency: -90% • Materials: AluminumPotential For Technology Demonstration: Near-Term CRITICAL ISSUES: • Flightexperiment validation ofthecurrent-voltage characteristics of plasma contactor devices andoperating atcurrents ofupto50Ainthe ionosphere areurgently needed tovalidate resultsfromchamber testsand theoretical models inspace • Flightexperiment validation ofthecurrent-voltage characteristics of thebaretetherconcept • Flightexperiment determination oftheroleplayed byignited mode operation intheionosphere • Ground andflightexperiment validation ofthetheoreti;cally predicted role ofplasma contactor cloudinstabilities • Characterization ofthemagnetosphere current closure pathanditslosses • Characterization oftheeffects oflargeelectromagnetic tethersystems on theLEOenvironment andotherspacevehicles • Assurance oflong-term insulator life • Characterization ofmassive tetherdynamics • Development ofspacecompatible insulation methods andpowerprocessing electronics formultikilovolt operation • Susceptibility tomicrometeoroid/debris damage • Understanding ofcurrent collection effectsatresulting insulator defects and theirimpacts onsystem performance (asinTSS1R) 75 STATUS: TSS-1and-1R,PMGflights Awidevariety ofworkisactively underway intheareasofelectrodynamic demonstrations, hollow cathodes, tethermaterials, andhardware technologies including ademoflight(seesection 2and"baretether" concept) DISCUSSION: Anorbiting insulated tether, terminated attheendseither byplasma contactors orbyabaresection oftether, canbeusedreversibly asanelectrical power or thrustgenerator. Motion through thegeomagnetic fieldinduces avoltage inthetether, proportional toitslengthandderived fromthevxBelectric fieldanditsforceoncharges inthetether. Thisvoltage canbeusedtoderiveaDCelectrical current inthetether. Electrical powerisgenerated atarateequaltothelossinspacecraft orbitalenergy duetoa dragforceofmagnitude (ilB)whereiisthetethercurrent and1isthelength. Ithasbeen shownthatthisdragforcefunctions asanelectrodynamic brakeandcanbeusedtoperform orbitmaneuvering inLEOorintheionosphere ofplanets suchasJupiterorSaturn. Threebasicplasma contactor configurations havebeenconsidered inthestudies performed todate:(1)apassive large-area conductor atbothtetherends;(2)apassive large-area conductor attheupper(positive) endandanelectron gunatthelower(negative) endand(3)aplasma-generating hollow cathode configuration. Hollow cathodes asflownon PMGareconsidered tobesaferforspacecraft systems, sincetheyestablish aknown vehicle ground reference potential withrespect tothelocalplasma. Theyalsoallowsimple reversibility ofthetethercurrent forswitching between powerandthrustgeneration. CONTACTS: • LesJohnson • Joseph Kolecki • JimMcCoy • JuanSanmartin • NobieStone REFERENCES: Proc.ofFourth International Conference onTethers inSpacerWashington DC, 10-14April1995 76 --ELECTRODYNAMICS - Electrodynamic ThrustGeneration APPLICATION: Generation ofelectro- magnetic propulsive thrusttoboosttheorbit ofaspacecraft. DESCRIPTION: Aninsulated conducting tetherconnected toaspacecraft andpossibly terminated withasubsateUite. Plasma contactors areusedatbothtether ends.Current fromanon-board power supply isfedintothetetheragainst theemf induced bythegeomagnetic field,producing a propulsive forceonthespacecra£t/tether system. Thepropulsive forceisgenerated at theexpense ofprimary on-board electric power. CHARACTERISTICS: • Thrust Produced: Upto200N • PowerRequired: Upto1.6MW • Length: 10-20km • Mass: 100-20,000 kg& powersupply • Efficiency: -90%Materials: Aluminum Potential For Technology Demonstration: Near-Term CRITICAL ISSUES: • ThesameaslistedinElectrodynamic PowerGeneration application STATUS: ThesameaslistedinElectrodynamic PowerGeneration application DISCUSSION: Aninsulated conducting tether,terminated attheendsbyplasma contactors, canbeusedreversibly asanelectromagnetic thruster orelectrical power generator. Apropulsive forceofILxBisgenerated onthespacecraft/tether system when current fromanon-board powersupply isfedintothetetheragainst theemfinduced init bythegeomagnetic field. Recommendations havebeenmadethrough theyearstouseelectrodynamic tethers to provide dragcompensation andorbitalmaneuvering capability fortheInternational Space Station, othersolararraypowered satellites, andtousehigherpowertethers (uptoabout1 MW)fororbitalmaneuvering oftheSpaceStation andotherlargespacesystems. Design tradeoffs werealsorecommended, including: 77 Useofcounterbalancing tethers deployed inopposite directions toprovide center-of-mass-location control Useofshorter tethersoperating atlowvoltage andhighcurrent versus longertethers operating athighvoltage andlowcurrent Definition ofelectrical/electronic interface between thetetherandtheuser bus. CONTACTS: • Marino Dobrowolny • LesJohnson • Joseph Kolecki • JimMcCoy • JuanSanmartin • NobieStone REFERENCES: Proc.ofFourth International Conference onTethers inSpace_Washington DC, 10-14April1995 78 -ELECTRODYNAMICS - ULF/ELF/VLF Communications Antenna APPLICATION: Generation of ULF/ELF/VLFwavesbyanorbiting electrodynamic tetherforworldwide communications. DESCRIPTION: Aninsulated conducting tetherconnected toa spacecraft, andterminated atbothends withplasma contactors. Variations in tethercurrent canbeproduced to generate ULF/ELF/VLF wavesfor communications. Thistetherantenna canbeself-powered (usingthecurrent induced initbythegeomagnetic fieldfor primary power) orexternally powered (fedbyanon-board Iransmitter).° la CHARACTERISTICS: • Length: 20-100 km • TetherCurrent: 10A • Potential ForTechnology Demonstration: Near-Term CRITICAL ISSUES: • Characterization ofthetransmitter • Characterization ofthepropagation media(including theionosphere at LEOaltitudes, theloweratmosphere, andoceanwater) • Analysis ofthesources ofbackground noiseandthestatistical structure of thatnoiseatthereceiver • Characterization oftheinstabilities andwaveduetolargecurrent densities intheAlfven wings • Moreadvanced mathematical models arerequired foranadequate understanding oftetherantenna systems, including theneedtosupersede thepresent cold-plasma basedmodels withmoreaccurate warm-plasma basedmodels • Determination ofoptimum ground stationlocations, including the possibility ofmobile receivers • Correlation ofsignals received atdifferent ground station locations to subtract outnoise STATUS: TSS-1andTSS-1Rflights DISCUSSION: Whenacurrent flowsthrough thetether,electromagnetic wavesare emitted, whether thecurrent isconstant ortime-modulated. Thetethercurrent canbethat induced bytethermotion through thegeomagnetic field,oronegenerated byanon-board transmitter. Modulation oftheinduced current canbeobtained byvarying aseries 79 impedance, orbyturning anelectron gunonthelowerendonandoff,atthedesired frequency. Waves areemitted byaloopantenna composed ofthetether,magnetic field lines,andtheionosphere. ULF/ELF/VLF wavesproduced intheionosphere willbeinjected intothe magnetosphere moreefficiently thanthosefrompresent ground-based man-made sources. Thesewavesmayprovide instantworldwide communications byspreading overmostofthe Earthviatheprocess ofducting. Witha20-100 kmtetherandawirecurrent oftheorder of10A,itappears possible toinjectintotheEarth-ionosphere transmission linepower levelsoftheorderof1Wbynightand0.1Wbyday. CONTACTS: • Robert Estes • MarioGrossi • Giorgio Tacconi REFERENCES: Grossi, M.D.,"AULFDipole Antenna onaSpacebome Platform ofthePPEPL Class," Report forNASA contract NAS8-28203, May,1973. P.R.Barmister etal."Orbiting Transmitter andAntenna forSpacebome Communications atELF/VLF toSubmerged Submarines", AgardConference Proceedings 529,May1993,pp.33-1-33-14 Proc.ofFourth International Conference onTethers inSpacerWashington DC, 10-14April1995 80 -PLANETARY- Aerocapture withTethersforPlanetary Exploration APPLICATION: Mayprovide significant masssavings whenusedin theexploration oftheatmosphere- bearing planets andsatellites inthe solarsystem.Orbiter DESCRIPTION: Thebasicconcept involves anorbiter andaprobe connected byalong,thintether. The probeisdeployed intotheatmosphere ofaplanet whereaerodynamic drag decelerates itfrom hyperbolic approach speedtocapture speed.The tension onthetether provides the braking effectontheorbiter, thuseliminating theneedforaretro-propulsion maneuver. During themaneuver theorbiter travels outside theatmosphere anddoesnotrequire heat shielding. CHARACTERISTICS: •Tether Length: 10-100 km •Tether Diameter: 0.5-1.5 mm •Orbiter Mass:1000kg •ProbeMass:1000kg •ProbeArea:500-3000 m2 •Potential forTechnology Demonstration: Mid-term CRITICAL ISSUES: •Reducing theprobeareawithout causing significant bending inthetether. •Assessing theeffectofparameter uncertainties (suchasatmospheric density, targetaltitude, ballistic coefficient andspinrate)ontetherandmaneuver design. •Developing guidance andcontrol lawsandmechanisms tohandlethese uncertainties. STATUS: Preliminary analyses demonstrate thefeasibility oftheconcept. Reentry ofSEDS-1 provides insight intothedynamics ofatetherinan atmosphere. 81 DISCUSSION: Analytical andnumerical studies haveconsidered thepossibility of usingtheaerobraking tetherfortheexploration ofVenus, Mars,Jupiter, Saturn, Uranus, Neptune andTitanaswellasforreturning toEarthfromMars.Onestudycompares the propellant massofatypical rocket propulsion system tothetethermassrequired forthe aerobraking system. Ineveryinstance inthisstudy,thetethermassturnsouttobeless thanthepropellant mass. Thefeasibility ofthedesignissupported bystudies thatinclude flexibility, out-of-plane effects andparameter uncertainties. Asapassive system, theaerobraking tether isless sensitive toparameter uncertainties thanthetypical aerobraking configuration. Forprecise guidance, thesystem seemswellsuitedtofeedback control byadjusting the tetherlength. CONTACTS: •JamesM.Longuski •JordiPuig-Suari •StevenG.Tragesser REFERENCES: Puig-Suari, J.,"Aerobraking Tethers fortheExploration oftheSolarSystem," Ph.D. Thesis, School ofAeronautics andAstronautics, Purdue University, WestLafayette, IN,August 1993. Proc.ofFourth International Conference onTethers inSpace_Washington DC,10-14 April1995 82 -PLANETARY - Comet/Asteroid Sample Return APPLICATION: Collection and returntoEarthofcometorasteroid samples. DESCRIPTION: Tethered penetrators arelaunched froma spacecraft during itsrendezvous witha cometorasteroid. Theypenetrate the body'ssurface, collecting samples of surface material. Theyarethenreeled aboard thespacecraft forreturnto Earth. Usingseveral penetrators, samples couldbecollected from different spotsononebody,orfrom morethanonebody. CHARACTERISTICS: •Tether Length: 50-100 m •TetherSystem: SingleReel •Penetrator System: Multiple Chambered Turret •Penetrators:Core Drilling and Surface •Deployment: SpringandSolidRocketw\ u,m \ IqlMTIIA'mml •Potential forTechnology Demonstration: Far-Term CRITICAL ISSUES: • Long-range, remote-controlled maneuvering andrendezvous • Design anddevelopment ofthepenetrators, tether-reel subsystem, and penetrator turretsubsystem STATUS: Preliminary definition ofthemission andhardware hasbeenperformedat JPL Detailed Analysis anddesignperformed byAleniaforESA'sROSETTA DISCUSSION: Theconventional approach tocollecting samples fromcomets and asteroids wouldbeforaspacecraft torendezvous withthemandrelease alander. The landerwouldattachitselftothebodyinsomeway,drillforacoresample, andreturntothe spacecraft. Thesample wouldthenbereturned toEarth.Atypical scenario wouldrequire thefollowing capabilities: (1)closerangeverification ofasuitable landing anddrilling site; (2)automated andhighlyaccurate softlanding; (3)landerattachment tothebody(since somewouldhaveverylowgravity); (4)adrillunitwithsufficient powertocoreasample; 83 (5)landerseparation fromthebody;(6)automated rendezvous withtheorbiter; (7)sample transfer; (8)launch stageejection; and(9)Earthreturn. Atetherapproach wouldconsist ofthefollowing sequence ofevents: (1)the spacecraft rendezvous withthecometorasteroid; (2)atethered penetrator isshotatthe targetfroma50-100 maltitude; (3)onimpact, sample material entersholesinthe penetrator shellandfillsthesample cupinside;(4)anexplosive sealsthecupandejectsit fromthepenetrator shell;(5)thecupvelocity creates atension inthetetherasitrotates it;(6)spacecraft thrusters control thecupretrieval asitisreeledaboard; (7)othertethered penetrators retrieve samples fromotherareasorbodies; and(8)thespacecraft returns the samples toEarth. Inaddition tothepenetrator designdescribed above, anothertype,inwhichthe penetrator contains acoredrill,couldalsobeused.Forthisversion, flanges wouldbe extended uponimpact, tosecurethepenetrator shelltothesurface whilethecoresample is beingdrilled. Thesurfaces hardness woulddetermine whichtypetouse.Bothtypescould belaunched fromthespacecraft byaspringandthenpropelled byattached solidrockets to theimpact point. (Thisshouldimpartsufficient momentum topermitagoodsurface penetration.) Toallowasingletetherreelsubsystem tohandlemanypenetrators, a rotatable turretwithmultiple, chambered ponetrators couldbeused. Thistethersystem hastheadvantage ofbeingsimpler thanalandersystem (not requiring manyofthecapabilities listedforalandersystem), andofallowing thecollection ofsamples frommorethanonespotorbody.Thecostofsuchatethermission hasbeen estimated tobeabout$750M,asopposed toabout$1-2Bforalandermission. However, thetwomethods arecomplementary inthatthelanderprovides asingleverydeepsample andthepenetrator provides smaller samples fromdifferent areasorbodies. CONTACTS: • PietroMedina • PaulPenzo REFERENCES: "Tether Assisted Penetrators forComet/Asteroid Sample Return," byPaulA. Penzo(JPL); paperpresented at1986AIAA/AAS Astrodynamics Conference. "Feasibility Assessment ofaTethered Harpoon fortheROSETTA backup Sampling", Alenia Spazio, SD-RP-AI-040, January 1990 "CSNR, Mission andSystem Definition Document", ESASP-1125, June1991 84 -PLANETARY - Jupiter Inner Magnetosphere Maneuvering Vehicle APPLICATION: Generationofelectro- magnetic thrust ordragformaneuvering within theinnerJovian magnetosphere. DESCRIPTION: Aninsulated conducting tetherconnected toaspacecraft andpossibly terminated withasubsatellite. Plasma contactors areusedatbothtetherends.Whenused selectively withanon-board powersupply (probably nuclear) oraload,itinteracts withthe Jovianmagnetic fieldtoproduce thrust,dragand electrical powerasrequired tochange orbital altitude orinclination.TO StmRR CHARACTERISTICS: • Physical Characteristics: Undetermined • Potential ForTechnology Demonstration: Far-Term CRITICAL ISSUES: • Successful operation ofhollow cathodes orrelated activecollectors asplasma contactors • Assurance oflong-term insulator life • Susceptibility tomicrometeoroid/debris damage • Successful operation ofapowersupply(probably nuclear) with sufficient outputpower density • Characterization oftheperformance ofanelectromagnetic tether intheJovianMagnetosphere STATUS: TSS-1, demonstrating electrodynamic applications, isscheduled for a1991launch Nodetailed system designstudyforthisapplication hasbeen performed DISCUSSION: SinceJupiter's magnetic fieldisabouttwenty timesthatofEarth,an electromagnetic tethershould workwellthere.Because ofJupiter's rapidrotation (period --10hrs),atdistances greater than2_.Jovianradiifromitscenter, the Jovianmagnetic fieldrotates fasterthanwouldasatellite inacircular Jovianorbit. Atthesedistances, themagnetic fieldwouldinduceanemfacrossaconducting tether,andthedissipation ofpowerfromthetetherwouldproduce athrust(not drag)onthespacecraft/tether system. Atlesserdistances, thesatellite wouldrotate fasterthanthemagnetic field,anddissipation oftetherpowerwouldproduce drag (notthrust). Examples ofinduced tethervoltages are: -10kV/km (fordrag)inLJO;and+108,50,21,and7v/kin(forthrust)atIo, Europa, Ganymede, andCallisto, respectively. 85 InsidetheJovianmagnetosphere, atdistance >2.2Jovianradii,the spacecraft coulddecrease altitude (decelerate) byfeeding powerfromanon-board powersupplyintothetetheragainst theinduced emf.Below2.2radii,powerfrom thetethercouldbedissipated. Toreturntohigheraltitudes, theprocess couldbe reversed. Sincethegravitational attraction ofJupiter issostrong,theenergy required todescend to(orclimbfrom)averylowJupiter orbitisprohibitive forany conventional propulsion system. Todescend tothesurface ofJupiter froma distance of,say,100Jovianradii,anenergy density ofalittleover200kW-hr/kg wouldberequired forpropulsion. Usingthisasaconservative estimate ofthe required performance ofatethersystem, itshouldbewellwithinthecapability ofa nuclear powersupply. Recommendations weremadeattheTetherWorkshop inVenice (October 1985)foraJupiter innermagnetosphere surveyplatform tooperate intherange fromonetosixJovianradii.Theelectromagnetic tetherinthisapplication would beusedprimarily fororbitalmaneuvering. ItcouldalsoassistaGalileo-type satellite tour(allequatorial), sampling oftheJovianatmosphere, andrendezvous withaGalilean satellite. CONTACTS: • PaulPenzo • JamesMcCoy REFERENCES: Applications ofTethers inSpace,NASA CP-2422, March 1986. Gabriel, S.B.,Jones,R.M.,andGarrett, H.B.,"Alfven Propulsion at Jupiter," Tether Int.Conf.1987. Penzo, P.A.,"ASurvey ofTetherApplications toPlanetary Exploration," AAS86-206, AASInt.Conf.1986. 86 -PLANETARY- MarsTethered Observer APPLICATION: Provide instrument accesstolow orbitalaltitudes forperiodic in-situanalysis oftheupper Martian atmosphere. DESCRIPTION: Aninstrument package attached byadeployable tether(upto300kminlength) toan orbiting MarsObserver spacecraft. CHARACTERISTICS: • Length: Upto300km (Tether isLLITE(350kin) INSTRUMENTS __\ \ GROUND TRACK""_ '/ / notvertical) • Satellite Altitude: 350km • Potential For • Instrument Technology Altitude: Downto90km Demonstration: Mid-Term CRITICAL ISSUES: • Tether material (graphite isapotential candidate) andOrbiter fuel consumption STATUS: System performance analysis forvarious altitudes anddifferent mission scenarios oftheprobeperformed bytheSmithsonian Astrophysical Observatory DISCUSSION: Thepurpose ofthemission itselfistoanalyze thecomposition and chemistry oftheMartian atmosphere foroneMartian year.Thetetherwouldallow instruments tobelowered periodically forin-situmeasurements atloweraltitudes and collection martian dustduringstormsthussavingonlanders's costs.Atether(Upto300 kmlong)couldbeusedwiththeobserver asitorbitsMarsatanaltitude of350kin.The instrument package wouldbedeployed forafewhoursatatime,perhaps everytwo months, orso.Additional propulsion capability wouldberequired fortheobserver for altitude maintenance. Although addition ofthetethersystem wouldincrease themission cost,itshould greatly enhance itsscientific value. CONTACTS: • Enrico Lorenzini • PaulPenzo • Monica Pasta 87 REFERENCES: Proc.ofFourth International Conference onTethers inSpace_Washington DC, 10-14April1995 Lorenzini, E.C.,MD,Grossi, andM.Cosmo, "LowAltitude Tethered MarsProbe," ActaAstronautica, Vol21,No.I,1990,pp.1-12. PastaM.andE.C.Lorenzini, "Optimization ofaLowAlitude Tethered Probefor Martian Atmospheric Collection", TheJournal oftheAstronautical Sciences, Vol. 44,No.2,1996,pp.191-205 88 -PLANETARY - Tethered LunarSatelliteforRemoteSensing APPLICATION: Provide instrument accesstolow, unstable, lunarorbitalaltitudes. DESCRIPTION: Aninstrument package atlow altitude, suspended byatetherfromasatellite ina higher, stable,polarorbitaround themoon. CHARACTERISTICS: • Tether Length: 90-250kin • Instrument Altitude: upto50km • Potential ForTechnology Demonstration:_.__TELLITE (350kin) INSTRUMENTS" v_\ \ aaoua. I Far-Term CRITICAL ISSUES: • Assurance ofacceptable strength andflexibility forthetethermaterial • Susceptibility tomicrometeoroid/debris damage STATUS: PROTEUS (PRObe Tethered forExploration ofUncovered Satellites) study performed byALENIA Spazio. Analysis ofmission scenarios andscientific objectives DISCUSSION: DuetoSunandEarthperturbations, closelunarsatellites wouldbeunstable andshortlived(perhaps afewmonths). However, asproposed byGiuseppe Colombo, accesstolowlunarorbitscouldbeachieved bytethering aninstrument package toa satellite inastablelunarorbit.Thepackage couldbelowered asclosetotheMoonas desired. Oneproposed configuration wouldtetheraninstrument package 50kmabovethe lunarsurface fromasatellite inastable300kmorbit.Byusingapolarorbit,complete coverage ofthelunarsurface couldbeobtained. Occasional adjns_ents tothetether lengthmayberequired tokeepthepackage atasafealtitude. Sensitive measurements of lunarmagnetic fieldandgravitational anomalies couldbeperformed. CONTACTS: • PietroMedina • PaulPenzo REFERENCES: Colombo G.,etal.,"Dumbbell Gravity Gradient Sensor: ANewApplication of Orbiting LongTethers, SAOReport inGeoastronomy No.2,June1976 Medina P,"PROTEUS-PRObe Tethered forExploration ofUncovered Satellites: TheProteus LunarMission, ESAWPP-081, 1994,pp.512-527 89 --SCIENCE - ScienceApplications TetheredPlatform APPLICATION: Provides aremote platform to theSpaceStation forspaceandEarthobservation purposes. DESCRIPTION: Aplatform, attached tothe SpaceStation byamultifunction tether(power link, datalink),provides anewmeanstoallowhigh precision pointing performance bythecombination ofdisturbance attenuation viatetherandactive control ofamovable attachment point. CHARACTERISTICS: •Length: 10lan •Mass: 10,000 kg •Powerrequired: Upto 15kWbyTether •LinkDataRate: •Pointing Accuracy:PowerLineLink Upto20Mb/sbyTether Optical FibersLink Upto10ArcsecondsPotential For Technology Demonstration: Mid-Term CRITICAL ISSUES: • Space Station impacts • Dynamic noiseinduced ontether • Movable attachment pointcontrol • Powerlinktechnology • Optical fiberslinktechnology • Tether impact protection technology STATUS: ASI/Aeritalia SATPDefinition Studyininitialdesignassessment phase, mid-term report issuedinMarch 1986.Finalreportforthecurrent study phaseissuedinMay1987 BallAerospace, Selected TetherApplications StudyPhaseIII DISCUSSION: Atethered pointing platform wouldtakeadvantage ofthefacilities ofthe station formaintenance andrepairwhilebeingisolated fromcontamination andmechanical disturbances. Asaninitialstep,amedium sizepointing platform seemsthemostsuitable facility foraclassofobservational applications. Infact,ifambitious astrophysical projects justifythedesignofadedicated complex free-flyer, medium observational applications of relatively shortduration couldtakeadvantage ofastandard pointing facility abletoarrange atdifferent timesseveral observational instruments. Thispointing facility couldallow reduction ofcosts,avoiding thecostofseparate service functions foreachapplication. 90 CONTACTS: • Franco Bevilaequa • PietroMerlina • JamesK.Harrison REFERENCES: Applications ofTethers inSpace) NASA CP-2422, March 1986. SATPDefinition Study,Mid-Term Report, Aeritalia, TA-RP-AI-002, March21, 1986. SATPDefinition andPreliminary Design, FinalReport, Aeritalia, TA-RP-AI-006, 1987. Proc.ofFourth International Conference onTethers inSpace)Washington DC, 10-14April1995 91 -SCIENCE - Shuttle Science Applications Platform APPLICATION: Provides aremote platform totheSpaceShuttle forvarious science andapplications purposes. DESCRIPTION: Aplatform, attached to theSpaceShuttle byatether,provides aunique meansbywhichremote applications maybe performed. CHARACTERISTICS: • Physical Characteristics: TBD • Potential For Technology Demonstration: Near-Term CRITICAL ISSUES: • Dynamic noiseinduced ontether • Micrometeoroid damage STATUS: Various investigators (listedbelow) haveexamined preliminary concepts DISCUSSION: Possible usesforaremote platform include stereoscopic sensing, magnetometry, atmosphere science experiments, andchemical release experiments. CONTACTS: • Franco Angrilli • Franco Bevilacqua • Franco Mariani • Antonio Moccia • SergioVetrella REFERENCES: Applications ofTethers inSpacerNASA CP-2422, March 1986. Proc.ofFourth International Conference onTethers inSpace_Washington DC, 10-14April1995 92 -SCIENCE - Tethered Satellite forCosmic DustCollection APPLICATION: Tocollect micrometeoric material fromtheupper atmosphere. DESCRIPTION: Asatellite tethered totheSpaceShuttle islowered intotheupperatmosphere. Thesurface ofthesatellite contains numerous small collecting elements whichwould document theimpact ofcosmic dustor actually retaintheparticles foranalysis backonEarth.\\\ Ii_v.'.Z_- _0_0 -- CHARACTERISTICS: • Tether Length: 100km • Operating Altitude: 120km • Tether Diameter: 1meter • PowerRequirements: Minimal, enough tooperate solenoid activated irises//// o "I" -.... Potential For Technology Demonstration: Near-Term CRITICAL ISSUES: • Efficient analysis oflargecollector surface areastodetectmicron-sized particles andimpact craters STATUS: Preliminary concept designinvestigated atIndiana University Northwest DISCUSSION: Thisconcept proposes tocollect intactcosmic dustparticles smaller than2 microns whichimpact thecollector surface atvelocities lessthan3km/sec, andthestudy ofimpact cratersandimpact debriswhichresultfromimpacts ofallsizedparticles at velocities greater than3km/sec. Itisestimated thatata120kmaltitude, between 1x103 and1x104particles willsurvice collection intactpersquaremeterperday,andbetween 2 x104and2x105impact craterswillberecorded persquaremeterperday.Thefigurein theillustration aboverepresents the"survivable" impact conesforparticles striking a tethered satellite. Foramaximum impact velocity of3km/sec, aisapproximately 22 degrees. CONTACTS: • George J.Corso REFERENCES: G.J.Corso, "AProposal toUseanUpperAtmosphere Satellite Tethered tothe SpaceShuttle fortheCollection ofMicro-meteoric Material," Journal oftheBritish Interplanetary SocietTT Vol.36,pp.403-408, 1983. 93 --SPACESTATION -- Microgravity Laboratory APPLICATION: Provide areadilyaccessible laboratory inEarthorbitwiththeminimum gravity levelpossible. DESCRIPTION: Alaboratory facility onboardtheSpaceStation atits vertical centerofgravity. Twoopposing tethers withendmasses aredeployed vertically fromtheSpaceStation (one aboveandonebelow). Theirlengths are variedtocontrol theSpaceStation center ofgravity, placing itonthemicrogravity modules tominimize theirgravity gradient acceleration (artificial gravity level). CHARACTERISTICS: • Physical Characteristics: TBDA_AA CRITICAL ISSUES: • Evaluation oftheoverall impacts totheSpaceStation • Determination ofjusthowgoodthelab'smicrogravity wouldbe • Identification oftheprocess andtechnologies tobestudied inmicrogravity, andthelaboratory facilities andcapabilities theywillrequire • Development ofthenecessary gravity-measuring instrumentation • Evaluation ofthetethersystem's costeffectiveness STATUS: AJSCtethered gravity laboratory study(addressing theissuesofactive center-of-gravity control, identification oflow-gravity processes tobe studied, andevaluation ofthelaboratory g-level quality) SEDS-1 and-2missions abdTSS-1andTSS-1R haveprovided measurements oftheacceleration fieldsandassociated noise duringtetherandpayload deployment DISCUSSION: Toallowtheperformance ofexperiments undermicrogravity conditions (10-4gandless)forextended periods oftime,amicrogravity laboratory facility couldbe incorporated intotheSpaceStation. Thelaboratory modules wouldbelocated ontheSpace Station proper, atitscenterofgravity. Twoopposing TSS-type tethers withendmasses wouldbedeployed vertically fromtheSpaceStation (oneaboveandonebelow), toassure thatthestation centerofgravity ismaintained withinthelabmodules. Itsexactlocation wouldbecontrolled byvarying theupperandlowertetherlengths, allowing prolonged and careful control oftheresidual microgravity magnitude anddirection insidethelab.A nearlyconstant microgravity couldbemaintained. Thesetethers wouldlowerthegravity- gradient disturbances transmitted totheexperiments beingperformed whileenhancing 94 stationattitudecontrol. Although peoplewouldbeamajorsourceofdisturbances, human accesstomicrogravity experiments ispreferred (atleastinitially) overremote access. This configuration wouldeasilyaccommodate thispreference. Onecandidate microgravity labcurrently understudyfortheSpaceStation, isthe Materials Technology Lab(NIl'L). Itisprojected tobeacommon module, equipped asa lab,toperform avariety ofexperiments related tomaterials technology. Biological experiments mayalsobeperformed inmicrogravity inanother module. Although thisisthepreferred microgravity labconfiguration, twoalternatives are alsopossible. Onewouldbetohavethelabconnected byacrawler toasingletetherfrom theSpaceStation. Thecrawler wouldposition thelabonthestation=tether system center ofgravity. Theotherconfiguration wouldbetofixthelabtoasingletetherfromthe station. Thelabwouldbepositioned atthesystem centerofgravity byvarying thetether length. Bothalternatives havetheadvantage ofisolating thelabfromdisturbances, but theyhavethedisadvantages ofreducing humanaccessandprobably precluding theuseof themicrogravity modules planned fortheinitialSpaceStation. CONTACTS: • Franco Bevilacqua • MarioCosmo • PietroMedina • Enrico Lorenzini REFERENCES: Applications ofTethers inSpac%NASA CP-2422, March 1986.(pp.223-238) G.VonTiesenlaausen, ed.,"TheRolesofTethers onSpaceStation," NASA TM- 86519, Marshall SpaceFlightCenter, October 1985. Lorenzini, E.C.,"AThree-Mass Tethered System forMicro-g/Variable-g Applications," Journal ofGuidance_ Control? andDynamics, Vol.10,No.3,May- June1987,pp.242-249 95 --SPACESTATION- ShuttleDeorbitfromSpaceStation APPLICATION: Allows theShuttle Orbiter tobedeboosted toEarthwhilethe SpaceStation isboosted toahigherorbit DESCRIPTION: Uponcompletion ofa Shuttle re-supply operation totheSpace Station, theShuttle isdeployed onatether toward theEarth. TheSpaceStation, accordingly, israisedintoahigherorbit, causing excessmomentum tobetransferred fromtheShuttle orbittotheSpaceStation orbit.Afterdeployment, theShuttle is released causing theShuttle todeorbit. CHARACTERISTICS: • InitialSpaceStation/Shuttle Orbit: • Tether Length: • FinalSpaceStation Orbit: • FinalShuttle Orbit: • Estimated Mass:Tlemt After O_turatlatinrumoemtum _ Tatw •/ Telil' _"i) SlI,IIll /amm.mtJmmhm_ _x31D_ 500km 65km 518x629km 185x453km 250,000 kg (Space Station) 100,000 kg(Shuttle)Potential For Technology Demonstration: Mid-Term CRITICAL ISSUES: • Excess angular momentum scavenged bySpaceStation mustbeusedinorder tobeneficially usethisapplication • Dynamic noiseinduced bytetherdeployment andseparation • Alignment oftethertoSpaceStation toeliminate torques STATUS: Martin Marietta, Selected TetherApplications Study,PhaseIII NASA-MSFC System study DISCUSSION: Thisapplication potentially couldbeoneofthemostcosteffective usesof atether. Themaindisadvantage isthattheexcessmomentum transferred totheSpace Station mustbeefficiently used,otherwise thestationwillbeinanorbittoohighfor subsequent Shuttle re-supply missions. Several ideasonuseofthisexcessmomentum have beenstudied, suchasaltering STVboostsbytheSpaceStation withShuttle re-supply missions (seeApplication "Tethered STVLaunch"). Another method isusingan electrodynamic tether(seeApplication "Electrodynamic PowerGenerator") togenerate powerattheexpense oforbitalenergy todeboost theSpaceStation. CONTACTS: • JamesK.Harrison • LesJohnson 96 REFERENCES: G.VonTiesenhausen, ed.,"TheRolesofTethers onSpaceStation," NASA TM- 86519, Marshall SpaceFlightCenter, October 1985. 97 --SPACE STATION - Tethered STVLaunch APPLICATION: Allows anSTVtobe boosted toahigherorbitattheexpense ofSpace Station angular momentum. DESCRIPTION: AnSTVwouldbedeployed fromtheSpaceStation onatetherawayfrom Earth,inpreparation forlaunch. Uponseparation fromthetether,orbital angular momentum is transferred fromtheSpaceStation totheSTV, causing theSpaceStation Altitude tobelowered whilethatoftheSTVisraised. CHARACTERISTICS: • InitialSpaceStation/ STVOrbit: • Tether Length: • FinalSpaceStation Orbit: • FinalSTVOrbit: • Estimated Masses:500km 150km 377x483km 633x1482km 250,000 kg (Space Station) 35,000 kg(STV)Potential For Technology Demonstration: Far-Term CRITICAL ISSUES: • Angular momentum takenawayfromtheSpaceStation mustberesupplied inordertobeneficially usethisapplication • Dynamic noiseinduced bytetherdeployment andseparation • Alignment oftethertoSpaceStation toeliminate torques STATUS: Martin Marietta, Selected TetherApplications StudyPhaseIII DISCUSSION: Martin Marietta hasstudied theapplication oftethered deployment ofthe STVaswellasShuttle fromtheSpaceStation. Eitheroftheseapplications alonewould causeanunacceptable change inaltitude oftheSpaceStation. Whencombined, properly sequencing STVlaunches andShuttle deorbits, theorbitalangular momentum oftheSpace Station maybepreserved whileproviding alargenetpropellant savings fortheShuttle, STVandSpaceStation. 98 CONTACTS: • JamesK.Harrison * LesJohnson REFERENCES: Applications ofTethers inSpace7NASA CP-2422, March 1986. G.VonTiesenhausen, ed.,"TheRolesofTethers onSpaceStation," NASA TM- 86519, Marshall SpaceFlightCenter, October 1985. Application "Shuttle Deorbit FromSpaceStation" Proc.ofFourth International Conference onTethers inSpacerWashington DC, 10-14April1995 99 --SPACE STATION - Variable/Low Gravity Laboratory APPLICATION: Provide areadily accessible laboratory inEarthorbitwitha variable, low-gravity level. DESCRIPTION: Alaboratory facility, attached byacrawler toatetherdeployed vertically fromtheSpaceStation. Thegravity gradient between thestation-tether system centerofgravity andthelaboratory produces an artificial-gravity forcethroughout thelab.The labgravity level,withaconstant vertical direction, isvariedbychanging thelaband crawler distance fromthesystem's centerof gravity. Thelabcanattainmicrogravity levels ifitcanmovetothecenterofgravity. CHARACTERISTICS: • Physical Characteristics: TBD • g-Level: Upto10"1Potential For Technology Demonstration: Far-Term CRITICAL ISSUES: • Evaluation oftheoverall impacts totheSpaceStation • Determination ofjusthowgoodthelab'slowgravity wouldbe • Identification oftheprocesses andtechnologies tobestudied inlowgravity, andthelaboratory facilities andcapabilities theywillrequire • Development ofthenecessary gravity-measuring instrumentation • Evaluation ofthetethersystem's costeffectiveness • Determination ofhowgravity-level medical experiments shouldbe performed inaSpaceStation system • Design ofatethercrawler andlabmodule - Development ofsystems fortheremote control ofthelabexperiments STATUS: AstudybyAlenia-SAO-Padua U.forNASA-JSC ontethered gravity laboratory study(addressing theissuesofactivecenter-of-gravity control, identification oflow-gravity processes tobestudied, andevaluation ofthe laboratory g-level quality) AstudybySAOforNASA-MSFC ontethered variable gravity elevators. TSS-1and-llLSEDS-1 and-2haveprovided measurements ofthe acceleration fieldchange andassociated noiseduringtetherandpayload deployment 100 DISCUSSION: Toallowtheperformance ofexperiments underconditions ofconstant or variable lowgravity (upto10-1g)forextended periods oftime,avariable/low gravity lab couldbeattached toacrawler onatetherdeployed vertically fromtheSpaceStation. The artificial gravity atanypointalongthetetherisproduced bythegravity gradient between thatpointandthestation/tether system centerofgravity, andisproportional tothe distance between them.Thelabcouldvaryitsgravity level,withaconstant direction, by varying itsdistance fromthesystem centerofgravity. Aconstant gravity levelcouldbe maintained byadjusting thelabposition tocompensate fororbital variations inthesystem gravity level.Thelabcouldalsoattainmicrogravity levelsifitcouldmovetothecenter ofgravity. Thislabcouldstudyprocesses withbothgravity andtimeasvariables. Ithas beencalculated thethelabcouldattaing-levels of10-6,10-4,10-2,and10-1atdistances abovethecenterofgravity ofabout2m,200m,20kin,and200kin,respectively. Inaddition toeasygravity control, theuseofatethersystem foralowgravity lab wouldhaveotheradvantages. Itwouldreducedisturbances transmitted tothelab(toabout 10-8g),minimize thegravity gradient acceleration insidethelab,andenhance overall system attitude control. Itwouldhavethedisadvantage ofreducing humanaccesstolab experiments, requiring theincreased useofremote controls. Also,itcouldonlyprovide a gravity levelofupto10-1g. Thislabcouldbeusedtoexamine theeffectsoflowgravity onbothphysical and biological processes. Somebiological processes ofinterest wouldbeplantandanimal growth, andhuman performance andmedical processes (suchasthoserelated tothe cardiovascular, skeletal, andvestibular systems). Suchphysical processes ascrystal growth, fluidscience, andchemical reactions couldbestudied. Conditions onlowgravity bodies (suchasasteroids) couldbesimulated toexamine natural processes (suchasmeteor impacts). Ofparticular interest wouldbethedetermination ofthegravity threshold for various processes. CONTACTS: • ChrisRupp • SilvioBergamaschi • Franco Bevilacqua • MarioCosmo • Enrico Lorenzini • PietroMerlina REFERENCES: Applications ofTethers inSpace,NASA CP-2422, March 1986. G.VonTiesenhausen, ed.,"TheRolesofTethers onSpaceStation," NASA TM- 86519, Marshall SpaceFlightCenter, October 1985. F.Bevilacqua andP.Merlina, "TheTethered SpaceElevator System," Second International Conference onTethers InSpace, Venice, Italy,1987. E.C.Lorenzini etal.,"Dynamics andControl oftheTether Elevator Crawler System", Journal ofGuidance, Control, andDynamics, Vol.12,No.3,pp.404-411 1989 101 -SPACE STATION - Attitude Stabilization andControl APPLICATION: Provides theSpaceStation with restoring torques around pitchandrollaxes DESCRIPTION: Atethered ballast could be deployed toserveasanattitude stabilizer. This feature couldbeusedonatemporary basisduring theconstruction oftheSpaceStation orona permanent basis toalleviate theCMG's requirements aswellasfunction asabackup facility incaseofACSfailure. CHARACTERISTICS: •Mission Duration: uptosome days •Masses: Deployer -650Kg; Tether ~400Kg; Ballast _1400Kg *Tether Length: 6000m •Potential For Technology Demonstration: Mid-Term_STABUZER CRITICAL ISSUES: •Attitude dynamics ofthetether-stabilized Station during-deployment ofballast. •Assessment ofmass,propellant, CMG's sizing,redundancy philosophy and contingencyreboost scenario. STATUS: •Feasibility analysis performed byAleniaandSAOforNASA/JSC DISCUSSION: Thetypical configuration oftheSpaceStation results inaspacecraft thatrequires acomplex andcareful designoftheAttitude Control System. CMG's sizingandRCSpropellant allocated depend on several nominal andemergency operations thatneedtobemanaged. Theattitude tetherstabilizer concept seemstohavethepotential forbeinganeffective wayofovercoming someoftheabovedifficulties The advantages include: system simplicity, relatively lowcostsandreusability. CONTACTS: •PielroMedina •Enrico Lorenzini REFERENCES: "Tethered Gravity Laboratories Study". Performed byALENIA Spazio andSAOunderNASA-JSC Contract NAS9-17877. 102 -TRANSPORTATION- Generalized Momentum Scavenging fromSpent Stages APPLICATION: Scavenge angular momentum fromaspentstageforthe benefit ofthepayload. DESCRIPTION: Afterthe injection ofanupperstageandits payload intoanelliptical parkorbit,the payload istethered abovethespentstage. Attheproper time,thepayload is released whichcausesapayload boostand spentstagedeboost. CHARACTERISTICS: •Physical Characteristics: TBD •Potential For Technology Demonstration: Mid-TermE CRITICAL ISSUES: • Massoftetherandreelequipment versuspayload performance gain • Integration impact onsystems STATUS: Preliminar7 evaluation completed byMIT,Michoud andTether Applications Detailed analysis inprogress atSAOincollaboration withTether Unlimited DISCUSSION: Thisconcept appears tobeimpractical duetomassrelationships and integration costs.Themostimmediate application isfornewlydeveloped upper stage/payload combinations andthosehaving ahighratioofspentupperstagetopayload mass. CONTACTS: • Manual Martinez-Sanchez • JoeCarroll • LesJohnson • Enrico Lorenzini 103 REFERENCES: J.A.Carroll "Guidebook forAnalysis ofTetherApplications," Contract RH4- 394049, Martin Marietta Corporation, March 1985.Available fromtheauthor M.Martinez-Sanchez, "TheUseofLargeTethers forPayload Orbital Transfer," Massachusetts Institute ofTechnology, 1983. G.Colombo, "TheUseofTethers forPayload Orbital Transfer," NASA Contract NAS8-33691, SAO,Vol.II,March 1982. 104 -TRANSPORTATION - Internal Forces forOrbital Modification (Orbital Pumping) APPLICATION: Tochange the orbitaleccentricity ofaSpaceStation orplatform without theuseof propulsion systems. DESCRIPTION: Theinternal mechanical energy ofaSpaceStation (intheformofexcess electrical energy transferred toamotor) isused tovarythelengthofatether attached toanendmass.Thelength ischanged inphasewiththenatural libration ofthetether,whichis known aslibration pumping. Proper timing oftetherdeployment and retrieval doneinthisfashion canbe usedtochange theorbital eccentricity.IL it 1 s*_'J os/ 4 CHARACTERISTICS: • Physical Characteristics: • Potential For Technology Demonstration:Undetermined Mid-Term CRITICAL ISSUES: • Internal vs.external energy trade-off • Powerrequired andheatgenerated bytheoperation • Change inorbitsisrelatively slow STATUS: Preliminary feasibility shownbyMartin Marietta Denver DISCUSSION: Orbiteccentricity canbeincreased bylibration pumping asisshowninthe illustration. At(1)themassisfullyextended, andlibration starts.At(2),withthemassin aprograde swing,theretrieval motorpulisthespacecraft toward themass,adding energy to theorbit.At(3),whichisthenewapogee oftheorbit,thetetherlengthisataminimum. At(4),withthemassinaretrograde swing,thetetherisre-deployed andtheretrieval brakesareusedtodissipate orbitalenergy intheformofexcessheat.At(5),thenew perigee, themassisagainfullydeployed. Thisprocedure isrepeated untilthedesired eccentricity isreached. CONTACTS: • Manual Martinez-Sanchez • JoeCarroll 105 REFERENCES: G.VonTiesenhausen, ed.,"TheRolesofTethers onSpaceStation," NASA TM- 86519, Marshall SpaceFlightCenter, October 1985. Breakwell, J.V.,Gearhart, J.W.,"Pumping aTethered Configuration toBoostits OrbitAround anOblatePlanet," AAS86-217, Int.Conf.1986. 106 -TRANSPORTATION - Satellite BoostfromOrbiter APPLICATION: Boostasatellite payload intoacircular orelliptical orbit higherthantheOrbiter orbit. DESCRIPTION: Asatellite isdeployed alongatether"upward" (awayfromthe Earth)fromtheShuttle Orbiter. Libration beginsandmomentum istransferred fromthe Shuttle orbittothesatellite. Thesatellite is released andplacedintoahigherorbitwhileat thesametimegivingtheShuttle adeboost to returntoEarth. Lessfuelisrequired forboth thesatellite andtheOrbiter. ATSS-derived deployer couldbeused. CHARACTERISTICS: • Length: Dependent ondesired orbit(see"Discussion" below) • Tether System: Eitherpermanent orremovable fromOrbiter • Potential For Technology Demonstration: Near-Term CRITICAL ISSUES: • Release mechanism forpayload • Airborne support equipment forOrbiter • Micrometeorite damage STATUS: Energy Science Labdevelopment contract completed March 1987 MIT,Martin Marietta-Denver havecompleted preliminary assessment BallAerospace, Selected TetherApplications Study,PhaseIII SAOanalysis for"SEDSAT" mission DISCUSSION: Thisapplication hasbeenstudied invarious formsbyseveral contractors as notedabove. Oneexample studied isthetethered deployment oftheAXAF (Advanced X- RayAstrophysics Facility) intoitsoperational orbit.Forthisexample, theAXAF is assumed tohaveamassof9,070kgandtheShuttle (afterdeployment) amassof93,000 kg.WiththeShuttle andAXAF ataninitialelliptical orbitof537x219km,theAXAF is deployed alonga61kmtether. Asmomentum istransferred fromShuttle toAXAF, the Shuttle orbitdescends toanew531x213kmandtheAXAF orbitascends toanew593x 274kmorbit.Aftertetherseparation, theAXAF isdirectly inserted intoa593km circular orbit.Simultaneously, theShuttle takesonanelliptical 531x185kmorbit,from whichitwillmakeafmalOMSbumbeforeitsreentry. 107 CONTACTS: • JamesK.Harrison • JoeCarroll • LesJohnson • Enrico Lorenzini • Manual Martinez-Sanchez REFERENCES: Applications "Upper StageBoostfromOrbiter" and"SmallExpendable Deployer System" Carroll, J.A.,"Guidebook forAnalysis ofTetherApplications," Contract RH4- 394049, Martin Marietta Corporation, Feb.1985. Proe.ofFourth International Conference onTethers inSpace_Washington DC, 10-14April1995 108 --TRANSPORTATION - Shuttle Docking byTether APPLICATION: Enables Shuttle Orbiter to docktootherstructures suchastheSpaceStation. DESCRIPTION: Atether deployed bythe SpaceStation isattached toadocking module. Thismodule wouldcapture andretrieve theShuttle, allowing aremote rendezvous. CHARACTERISTICS: •TetherLength: 40-100 Kin •Potential For Technology Demonstration: Mid-Term CRITICAL ISSUES: •Accurate guidance system, suchas GPSneeded •Rendezvous andcapture technique definition required •Post-rendezvous tetherdynamics •Alignment oftethertension with Station centerofmassi --T_J_ramt k._ STATUS: Martin Marietta, Selected TetherApplications Study,PhaseHI DISCUSSION: Atether,attached toadocking module, wouldbedeployed toward theEarth fromtheSpaceStation. Thelengthofdeployment isadjusted sothatthevelocity ofthe docking module matches thevelocity atapogee ofanelliptical orbitoftheShuttle. This wouldcauseincreased OMSpropellant available totheShuttle. Thisapplication would probably becombined withApplication "Shuttle Deorbit fromSpaceStation". CONTACTS: • JamesK.Harrison • ChrisRupp REFERENCES: Applications ofTethers inSpace,NASA CP-2422, March 1986. 109 --TRANSPORTATION - Tether Reboosting ofDecaying Satellites APPLICATION: Toretrieve, repair,andreboost a defective ordecaying satellite. DESCRIPTION: Apermanent tetherattached to theSpaceShuttle isusedtorendezvous withadecaying satellite. Itcantheneitherberepaired byShuttle crewmen and/orreboosted intoahigherorbit.Thiswould eliminate theneedtolaunch areplacement forthe defective ordecaying satellite. CHARACTERISTICS: • Physical Characteristics: • Potential For Technology Demonstration:Undetermined Near-Term CRITICAL ISSUES: • Mechanisms andrendezvous techniques tocapture satellite • Compatibility withexisting satellite systems • Trade-off ofthemission andreboost requirements STATUS: Preliminary analysis indicates feasible concept Nodefined mission requirement DISCUSSION: Integration ofthissystem maybecostly. Theconcept appears tobe feasible, butthepracticality hasnotbeenestablished. Nomission drivers haveyetbeen determined. CONTACTS: • JoeCarroll REFERENCES: G.VonTiesenhausen, ed.,TetherApplications Concept Sheets, June28,1984. 110 -TRANSPORTATION - Tether Rendezvous System APPLICATION: Usedtosupplement theoperations of theSpaceStation andOMV. DESCRIPTION: TheTetherRendezvous System wouldbe usedtocapture andretrieve payloads, OTVsortheSpaceShuttle totheSpaceStation. Thesystem wouldconsist ofa"smart" hookwhichwouldbeabletorendezvous andattachtoapayload withorwithout human intervention. CHARACTERISTICS: • Physical Characteristics: Undetermined • Potential For Technology Demonstration: Mid-Term CRITICAL ISSUES: • Extentofsystem capabilities needstobedetermined • Dynamics inthetetherandontheSpaceStation afterrendezvous • System design • Rendezvous andcapture techniques • Hardware required STATUS: Concept understudybyAeritalia Preliminary evaluations havebeenpositive DISCUSSION: TheTether Rendezvous System cansupplement theoperations ofthe SpaceStation oranyspaceplatform byaccomplishing remote rendezvous, increasing flexibility, decreasing riskandsavingagreatamount ofpropellant forincoming vehicles (STV,OMV,ortheShuttle Orbiter). CONTACTS: • ChrisRupp • JoeCarroll • Franco Bevilacqua REFERENCES: G.VonTiesenhansen, ed.,TetherApplications Concept Sheets, June28,1984. Stuart,D.G.,"Guidance andControl forCooperative Tether-Mediated Orbital Rendezvous," Journal ofSpacecraft andRockets, 1988. 111 -TRANSPORTATION - Upper StageBoostfromOrbiter APPLICATION: Boostanupperstage payload intoahigherorbit. DESCRIPTION: Anupperstageis deployed alongatether"upward" (awayfrom theEarth)fromtheShuttle Orbiter. Libration begins andmomentum istransferred fromthe Shuttle totheupperstage,enhancing the performance envelope oftheupperstage motor. ASEDS-derived (e.g.noretrieval capability) deployer system couldbeused. TheOrbiter couldbedeboosted alongwiththe upperstageboost. Spinup capability forsome upperstagesmayberequired. CHARACTERISTICS: • Length: • Tether Deployment System: • Potential For Technology Demonstration:Dependent ondesired finalorbit Permanent orremovable fromOrbiter, TSS-derived Near-Term CRITICAL ISSUES: • Requirement forspinupcapability maybedifficult STATUS: BallBrothers, Selected TetherApplications Study,Phase HI SEDSAT project atUniversity ofAlabama inHuntsville SEDSAT deployment study atSAO DISCUSSION: Thisapplication couldbetailored totheSpaceTransfer Vehicle (STV). An expendable tethersystem orTSS-derived system couldeliminate amajorportion ofthe STVpropellant required andincrease payload capability foraspecific mission withafixed STV.TheSEDSAT project (currently cancelled) wassupposed tobethefirstspacemission toboostasatellite intohigherorbitwithatether. Theboosting effectwasobserved at TSS-1R tetherbreakup 112 CONTACTS: • JamesK.Harrison • LesJohnson • Enrico Lorenzini • Mauro Pecehioli REFERENCES: "Study ofOrbiting Constellations inSpace," Contract RH4-394019, Martin Marietta, Smithsonian Astrophysical Observatory, December 1984. Pecchioli, M.,andGraziani, F.,"AThrusted SlinginSpace: ATether-assist Maneuver forOrbitTransfer," Second International Conference onTethers In Space,Venice, Italy,1987. Proc.ofFourth International Conference onTethers inSpaee_Washington DC, 10-14April1995 Applications "Satellite BoostfromOrbiter" and"SmallExpendable Deployer System" 113 -TRANSPORTATION- Tether Assisted Transportation System (TATS) APPLICATION: TATS isatether-based system thatprovides theSpaceStation Alpha withtransport capability not dependant onconventional propulsion DESCRIPTION: Theneed andthe feasibility oftheadditional Tether Assisted Transportation System havebeen evaluated inthecontext ofthe International Space Station Alpha. A preliminary cargo's traffic analysis indicated thatlargebenefits intermsof massandcostsaving areexpected by tetherdeorbit ofdisposable cargoes. The tetherusewasdiscovered topresent also additional benefits increasing thesafetyof theStation andsimplifying theexecution ofsomeoperations. CHARACTERISTICS: *Mission Duration :uptosome hours •Altitude :400-450 Km •Activephase: <lday •Returntoground :Re-entry Capsules •Accommodation :SpaceStation •MassDeployer :300Kg(typical) •TetherMass:40Kg(typical)) •Capsule Mass:150Kg(typical) •Tetherlength:about37Km •Potential ForTechnology Demonstration: Near-Term.....1:-| 1Structure oftheTATS-module 2Storage system forthere-entry capsules 3Manipulator onrails 4Airlocktothespace 5Docking mechanism andairlockto station 6Tether system inoperating position 7Tethersystem inposition during preparation 8Storage system forreplaceable tether units 9Re-entry capsule instartposition 10Pathlraeking system CRITICAL ISSUES_ •System configuration analysis, trade-off anddesign •Re-entry capsule architecture definition •SpaceStation-based Operations definition •Station storage systemforcapsules andwastecontainers design •Station robotic forTATSelements handling def'mition •Tethersystem deployment timingforproper prograde swing •Dynamics oftetherafterpayload release 114 DISCUSSION: Apotential utilization scenario ofanadditional Tether Assisted Transportation System hasbeendevised toshowtheextent ofitscapabilities. Asan example, thefollowing evolution couldbeconsidered: InitialCapability •Frequent Sample Return •Raduga-type Capsule Deorbit WasteDisposal SmallPayloads Disposal FullCapability •Frequent Sample Return •Raduga-type Capsule Deorbit •WasteDisposal •Cargoes Deorbit (PROGRESS, ATV) •LargeModules andPayloads Disposal TATS consists ofasetofre-entry capsules inastorage compartment, toolstoallowthe loading oftheprocessed samples, aseparation system (springs), andatether deployer to perform properly capsule deployment andrelease. Theanalysis ofpossible waysto accommodate theTATSsystem ontheStation hasbeenfocused onthetwomainoptions for accommodation: External andInternal Accommodation. Several possible options havebeen envisaged forpossible accommodation ofthesystem bothattheISSAUSsection andatthe ISSARSsection. CONTACTS: •PietroMerlina REFERENCES: "Tether Assisted Transportation System (TATS)", 11439/95/NL/VK, AleniaSpazio/RSC Energia/DASA, 1995.ESA/ESTEC contract No. 115 -TRANSPORTATION - Faiisafe Multiline Tethers forLongTether Lifetimes (Hoytether) APPLICATION: Long-life, damage resistant tether system for extended-duration, high-value, andcrew-rated missions. Applications include low-drag, longlifetethers foratmospheric andionospheric science, electrodynamic tethers forin-orbit powerandpropulsion, and high-strength tethers forLEO-GEO-Lunar transport systems. DESCRIPTION: Thelifetimes ofconventional single-line tethers are limited bydamage duetometeorite andorbital debrisimpactors to periods ontheorderofweeks. Although single-line tetherlifetimes canbeimproved byincreasing thediameter ofthetether, thisincursa prohibitive masspenalty. TheHoytether, showninthefigure, isa tetherstructure composed ofmultiple lineswithredundant interlinking thatisabletowithstand manyimpacts. Hoytether Section CHARACTERISTICS: •Canbedesigned tohavesurvival probabilities of>99%forperiods ofmonths toyears. CRITICAL ISSUES: Development ofmethods tofabricate anddeploy many-kilometer longmultiline tethers. STATUS: •1/2kmlongsamples ofbi-andtri-lineHoytethers werefabricated duringaPhaseISBIReffort. •A1/2kmbi-lineHoytether wassuccessfully deployed fromaSEDSdeployer ground tests. •Development ofmethods forfabricating anddeploying multi-kilometer conducting andnon- conducting Hoytethers continues underaPhaseIISBIRcontract. DISCUSSION: Analytical modeling, numerical simulation, andground-based experimental testing ofthis designindicate thatthistetherstructure canachieve lifetimes oftensofyearswithout incurring amass penalty. Moreover, whilesingle-line tethersurvival probability dropsexponentially withtime,redundant linkage infailsafe multiline tetherskeepsthetethersurvival probability veryhighuntilthetetherlifetime is reached. Thesurvival probability ofafailsafe multiline tetheriscompared tothatofanequal-mass single linetetherinnextfigure. 116 0.8 0.6 2 Q,. >m0.4 P 0.2failsafemultiline _\ tether ,_n tethergleline _,,, I,,, I,,,],,,I,,, 40 80120 160 201 Time,Years Lifetime comparison ofequal-weight singleline andfailsafe multiline tethersforalow-load mission. CONTACTS: •Robert P.Hoyt •Robert L.Forward REFERENCES: Proceeding oftheFourth International Conference onTethers inSpace, Washington, DC,10-14 April 1995. R.L.Forward, R.P.Hoyt, Failsafe Multistrand Tether SEDS Technology_ Demonstmtiom Final Report onNAS8-40545 SBIR94-1Phase IResearch Study. R.L.Forward, Failsafe Multistrand Tethers forSpace Propulsion, Forward Unlimited, FinalReport onNAS8-39318 SBIR91-1Phase IResearch Study. 117 SECTION 4.0TETHER FUNDAMENTALS 118 4.1 GRAVITY GRADIENT 4.1.1 General Gravity-gradient forcesarefundamental tothegeneral tetherapplications ofcontrolled gravity, and thestab'dization oftethered platforms andconstellations. Thebasicphysical principles behind gravity- gradient forces willbedescribed inthissection. Thisdescription willbeinthreeparts. Thefastwill discuss theprinciples behind thegeneral concept ofgravity-gradient forces. Thesecond willcontinue the discussion, addressing thespecific roleoftheseforcesincontrolled-gravity applications. Thethirdwill address theirroleinthestabilization oftethered platforms andconstellations. Forthepurposes ofthisdiscussion, itwillbesufficient todescribe themotion ofthesimple "dumbbell" configuration, composed oftwomasses connected byatether. Figure 4.1shows theforces actingonthissystem atorbitalvelocity. Whenitisoriented suchthatthereisavertical separation between thetwomasses, theuppermassexperiences alargercentrifugal thangravitational force,andthelower massexperiences alargergravitational thancentrifugal force.(Thereasonforthisisdescribed laterinthe discussion.) Theresultofthisisaforcecouple applied tothesystem, forcing itintoavertical orientation. Thisorientation isstablewithequalmasses, andwithunequal masses eitheraboveorbelowthecenterof gravity. Displacing thesystem fromthelocalvertical produces restoring forcesateachmass,whichactto returnthesystem toavertical orientation. Therestoring forcesactingonthesystem areshown inFigure 4.2(seeRef.1). j. fGravitational Force=GMM1#12 CenterofCentrifugal Force=M1r1o3(_ TetherTension Gravity _ CenterofMass "_""""""""""""" ""____..Orbit Centrifugal erTension Force=M2r2_2 2 Gravitational \ _Ir Force=GMM2#22 I I I LocalVertical EARTH Figure4.1Forces onTethered Satellites 119 Sincethegravitational acceleration changes nonlinearly withdistance fromthecenteroftheEarth, thecenter ofgravity ofthetethered system willnotcoincide exactly withitscenter ofmass. The separation becomes morepronounced asthetetherlength increases. However, theseparation isnot dramatic forsystems usinglessthanverylargelonglengths. Therefore, forthepurpose ofthisdiscussion itwillbeassumed thatthecenterofmasscoincides withthecenterofgravity. Furthermore, tofacilitate an "uncluttered" discussion, thetwomasses willbeassumed tobeequal,andthetethermasswillbeignored. Centrifugal- Gravitational Force Resultant ;/_. Restoring Force C°mp°ie _ _l"Tether .""_'''"" Tether / "_'_'_, , sy Resultant RestoringForce Co ponen I Centrifugal- ILocal Gravitational Force IVertical EARTH Figure 4.2Restoring Forces onTethered Satellites Thegravitational andcentrifugal forces(accelerations) areequalandbalanced atonlyoneplace: thesystem's centerofgravity (C.G.). Thecenterofgravity (ormass),located atthemidpoint ofthetether whentheendmasses areequal,isinfreefallasitorbitstheEarth, butthetwoendmasses arenot.They areconstrained bythetethertoorbitwiththesameangular velocity asthecenterofgravity. Forthecenter ofgravity inaKeplerian circular orbit,equating thegravitational andcentrifugal force, GMMo _Moroo)2 r2oand 2= GM (°o 3 ;where ro G=universal gravitational constant (6.673x10-11Nm2/kg2), M=massoftheEarth(5.979 x1024kg), Mo=totaltethersystem mass(kg), r=radiusofthesystem's centerofgravity fromthecenterofthe Earth(m),and 120 Sincetoo=orbital angular velocity ofthecenterofgravity (s-l). Vo coo- and ro where Vo=orbital speedofthecenterofgravity, (m/s),and To=orbitalperiod ofthecenterofgravity (s), Vo GM- and ro 4re2ro3T2= o GM Notethattheorbital speed,period, andangular velocity depend ontheorbital radius, andareindependent ofthetethersystem mass. Ifthetwoendmasses wereinKeplerian circular orbitsattheirrespective altitudes andwerenot connected byatether, theirorbital speeds would bedifferent fromthetethered configuration. Forthe uppermass,applying equations (1)and(2), 2 GM C°l= (ro+L)3and GMV_= ;where 1(ro+L) L=tetherlengthfromthecenterofgravity tothemass(m). Similarly, forthelowermass, co22=GM (ro-L)3and V2 GM (ro-L) Itcanbeseenthatwithout thetether,theuppermasswouldmoveataslower speedandthelower masswouldmoveatahigher speed. Thetether,therefore, speeds uptheuppermassandslowsdownthe lowermass. Thisiswhytheuppermassexperiences alargercentrifugal thangravitational acceleration, andwhythelowermassexperiences alargergravitational thancentrifugal acceleration. Theresulting upward acceleration oftheuppermassanddownward acceleration ofthelowermassgiverisetothe 121 balancing tethertension. Theyalsoproduce therestoring forces whenthesystem isdeflected froma vertical orientation. Themasses experience thistension asartificial gravity. Theartificial-gravity forceandtethertension areequaltothegravity-gradient force. Thegravity- gradient forceonamass,m,attached tothetetheratadistance, L,fromthesystem's centerofgravity is equaltothedifference between thecentrifugal andgravitational forcesonit.Anapproximate valueforthis forceisgivenby, FGG=3Lm0302 Formassmbelowthecenterofgravity, thegravity-gradient forceissimply FGG---3Lm0302, indicating thatthegravity-gradient forceactsupward abovethecenterofgravity anddownward belowit. Theforceactsalongthetetherandawayfromthecenterofgravity. Furthermore, thegravity-gradient acceleration andforceincrease asthedistance fromthecenterofgravity increases andastheorbital radius ofthecenterofgravity decreases. (Amorerigorous derivation ofthisequation ispresented inRef.2,and alsoinRef.3).Figures 4.3and4.4showthetethertension (artificial-gravity force)andartificial-gravity acceleration asafunction oftetherlengthfromthecenterofgravity forvarious system masses inLEO(see 1.0 10 10 100 1.000 TETHERLENGTHfKM_ Ref.4).Figure 4.5shows thetethermassandg-level asafunction oftetherlengthforatethermadeof Kevlar 29.Thisfigureincludes tapered tethers whicharediscussed below. Figure 4.3TetherTension DuetoGravity Gradient Versus Tether Length From CenterofGravity andEffective Satellite MassInLEO 122 10.000- Figure4.4"Artificial Gravity" atTethered Masses inLEO 20 10- N,B ! I 0.1, 1 0 600g-LEVEL .050.10.15 0.2r'J'/ 300 LENGTH OFTETHER (kin)MATERIAL: KEVLAR 29 SAFETY FACTOR--' 3.5 WORKING STRESS-" 0.7x1O_nm-z 0ENSffY: 1450kg m-_ •ALTITUDE "-5001ore STEADY STATE Figure 4.5TetherMassandg-Level Versus TetherLength forKevlar 29Tethers 123 Sincethegravity-gradient forceandacceleration inorbitvarywithGM/ro3(where Misthe planetary mass), theyareindependent oftheplanet's size,andlinearly dependent onitsdensity. The acceleration islargest around theinnerplanets andtheMoon(0.3-0.4 x10-3g/kin forloworbits, whereg isEarthgravity), andabout60-80% lessaround theouterplanets. Thegravity-gradient acceleration decreases rapidly astheorbitalradiusincreases (to1.6x10-6g/kininGEO). Although thevertical orientation ofthetethersystem isastableone,thereareforces whichcauseit tolibrate(oscillate) aboutthevertical. Theseweakbutpersistent forcesinclude atmospheric dragdueto thedifferent airdensities encountered inthenorthward andsouthward passes ofnon-equatorial orbitsand duetosolarheating andelectrodynamic forces(forconducting tethers). Station-keeping andotherrocket maneuvers wouldalsocontribute todriving (ordamping) libration. Thenatural frequency forin-plane (in theorbitplane)librations is_/3o_o=1.732too,and2cooforout-of-plane librations (adetailed derivation is contained inRef.2). Sinceboththedisplacement andrestoring forces increase linearly withtetherlength, libration frequencies axeindependent oftetherlength. Therefore, thetethersystem willlibrateasasoliddumbbell (except forverylongtethers, wherethegravity gradient itselfvaries). Libration periods, however, do increase atlargeamplitudes. Sincethetetherconstrains themotion ofthemasses, thesensed acceleration isalways alongthetether. Furthermore, thetethercangoslackifthein-plane libration angleexceeds 65°, oriftheout-of-plane libration angleexceeds 60°.Theslackness canbeovercome byreeling orunreeling thetetheratanappropriate rate.Additional information ontetherlibration ispresented inRef.5andalso Section 5.0. Libration canbedamped outbyvarying thetetherlength. Itwouldbedeployed whenthetension wastoohighandretracted whenthetension wastoolow.Sincethein-plane andout-of-plane librations havedifferent periods, theycouldbedamped simultaneously. Shorter-period, higher-order tether vibrations couldalsobedamped inthisway. Sincetheportion ofthetetheratthecenterofgravity mustsupport thetetheraswellasthemasses, themassoflongtethers mustbetakenintoaccount. Tominimize thetether's masswhilemaintaining its required strength, itscross-sectional areacouldbesizedforaconstant stressatallpointsalongitslength. Theoptimum design forveryhightethertensions would beanexponentially tapered tetherwitha maximum areaatthecenterofgravity andminima attheendmasses. Tethers ofconstant cross-section havelimited length, asindicated inFigure4.5,whereas tapered tethers canhaveunlimited length; butthen, itsmasswillincrease exponentially alongwithitscross-section. Adetailed discussion oftapered tether design isprovided inRef.6. Inaddition tothegeneral areasofcontrolled gravity andtethered-platform andconstellation stabilization, gravity-gradient effects playafundamental roleinapplications related tomomentum exchange andtethered-satellite deployment. These aspects arediscussed inSection 4.3,entitled "Momentum Exchange." 4.1.2 Controlled Gravity Asafirststepindiscussing theroleofgravity-gradient effects incontrolled-gravity applications, a fewdefinitions willbeestablished. Thedefinitions usedinthisbookwillbethoserecommended bythe controlled gravity panelatthetetherapplications conference inVenice, ItalyinOctober 1985(Ref.4).The term"controlled gravity" means theintentional establishment andcontrol ofthemagnitude, vector properties, timedependence, andassociated "noise" (uncertainty) oftheacceleration fieldwithin a designated volume ofspace. Inaddition, thefollowing definitions arealsoprovided: g=theacceleration ontheequator atmeansealevelontheEarth's surface (9.81m/s2); microgravity =10-4gandsmaller; lowgravity =10-1gto10-4g; Earthgravity --1g; 124 hypergravity =greater than1g; reduced gravity =microgravityandlowgravity;and enhanced gravity =hypergravity. Therearetwobasictetherconfigurations whichcanbeusedtoprovide controlled-acceleration fields:gravity-gradient-stabilized configurations (rotating onceperorbitinaninertial flame), androtating configurations (rotating morerapidly thanonceperorbit). Thissection willcovergravity-gradient- stabilized configurations. Rotating configurations arediscussed laterinSection 4.2. Inanorbiting, vertically-oriented, gravity-gradient-stabilized tethersystem composed oftwoend masses connected byatether, allportions ofeachendmassexperience thesameacceleration, caused by thetethertension pulling ontheendmass. Thisforceisperceived asartificial gravity. Asdescribed before, itsmagnitude isproportional tothetetherlengthfromthesystem's centerofgravity, andmaybe heldconstant orvariedbydeploying andretracting thetether. (ForLEO,thegravity gradient isabout4x 10-4g/kin.) Itsdirection isalongthetetherandawayfromthecenterofgravity. Thissameprinciple canbeusedinmorecomplex configurations (constellations) ofthreeormore bodies. Forexample, consider athree-body system stabilized alongthegravity gradient. Inthissystem, a thirdbodyisattached toacrawler mechanism ("elevator") onthetetherbetween thetwoprimary end masses. Thecrawler mechanism allows thethirdbodytobemoved easilytoanypointalongthetether between theendmasses. Theacceleration field(artificial gravity) inthethirdbodycanbecontrolled easily bymoving itupordownthetether. Itsdistance fromthesystem's centerofgravity determines the magnitude oftheartificial gravity withinit.Thisartificial gravity actsinthedirection alongthetetherand awayfromthecenterofgravity. Thetwoendmasses experience theartificial gravity determined bytheir distances fromthecenterofgravity, asinthetwo-body system. Theartificial gravity thattheyexperience canalsobeheldconstant orvariedbyincreasing ordecreasing thetetherlength. Whenpositioned atthecenterofgravity, thethirdbodycouldexperience anacceleration fieldas lowasabout10-8gatthecenterofgravity, and10.7gand10-6gatdistances fromthecenterofgravity of 20cmand2m,respectively. Using appropriate control laws,thethirdbody's position couldbe automatically adjusted toproduce adesired g-level timeprofile ortominimize transient disturbing effects. Gravity-gradient effects canalsobeusedtocontrol thelocation ofthesystem's centerofgravity. Thiswould beaveryuseful capability fortheSpaceStation ifmicrogravity experiments weretobe performed on-board. Twotethered masses would bedeployed vertically fromtheSpaceStation -one aboveandonebelow. Bycontrolling thetetherlengths, theposition ofthecenter ofgravity couldbe maintained ataparticular pointinthesystem ormoved totheotherpointsasdesired. Thismeans thatthe artificial gravity atallpoints inthesystem would becorrespondingly controlled toafreedegree of resolution. Forexample, thecenterofgravity couldbeadjusted tocoincide withtheminimum possible acceleration field. Allofthesesystem configurations allowthegeneration andfinecontrol ofawiderangeofg- levels. Usingappropriate control laws,tetherlengths andtherelative positions ofsystem components can bevariedtoproduce desired gravity fieldsandtheirtimeprofiles, tominimize transient disturbances tothe gravity field,andtocarefully control thelocation ofthesystem's centerofgravity. Inaddition toallof this,tethers alsoprovide two-axis stabilization ofthesystem. Gravity-gradient systems haveseveral advantages overrotating systems. Theycanprovide artificial gravity forlarge-volume structures moreeasily. Also,thegravity gradient andCoriolis accelerations withinthesevolumes aremuchlessthanthoseproduced inrotating systems. Oneresultof thisisaloweroccurrence ofmotion sickness. However, onedisadvantage ofgravity-gradient systems is thattheywould require verylongtethers toachieve g-levels approaching 1gormore. Infact,current tethermaterials arenotstrong enough tosupport theirownweight atsuchtetherlengths. However, by usingmoderate lengths andarelatively smallrotation rateabouttheC.G,g-levels of1gormorecanbe achieved, withsomeincrease intheCoriolis acceleration andgravity gradient. Figure 4.6provides additional information concerning theacceptable valuesofartificial-gravity parameters (Ref.4). 125 ARTIRCIAL GRAVrrY PARAMETERS •UNAIDED TRACTION REQUIRES 0.1G •ANGULAR VELOCITY SHOULD BELESSTHAN3.0RPMTO AVOIDMOTION SICKNESS •MAXIMAL CENTRIPETAL ACCELERATION NEEDNOT EXCEED EARTH GRAVITY •CORIOLIS ACCELERATION SHOULD NOTEXCEED 0.25 CENTRIPETAL ACCELERATION FORALINEAR VELOCITY OF3FEET/SECOND INARADIAL DIRECTION •GRADIENT SHOULD NOTEXCEED 0.01G/FOOT INRADIAL DIRECTION TETHER MASSMIGHT BELIMITED TO10,000TO20,000 POUNDS ARTIFICIAL GRAVITY PARAMETERS /'31// / /t=,=-=,y-/===_ _/_/3( _/ .rr/ .., /./\., Ac=rrA=_/ i/,-2;,-.,_._ o._1J-J__.,_,¢Z<'_..,,,l '''',,,,=! oiI o.I t.Q 10 IC[_1_IPETAI. ACCF.I.IAATi_I I_ ¢ORIOUS &¢CILII_TION •0:IS¢INTRIPITAL J_..CI[I,IAATIOk *TITHGllMASSMI_: r,Om]_SlC"_'l_vltOCmr II _=I==At_I_. RIVI,,_LCYl,,mo_r._ _ Figure4.6Acceptable Values ofArtificial-Gravity Parameters Tether technology suggests anumber ofexciting application possibilities. Forexample, sincea tethercanbeusedtoattainagravity fieldsimply bydeploying acounterweight alongthegravity gradient, theestablishment ofadesirable low-level gravity on-board theSpaceStation appears practical. Theuseof 0.01-0.1gon-board theSpaceStation mightpermit simpler andmorereliable crew-support systems (suchaseatingaids,showers, toilets, etc.),operational advantages (nofloating objects, easiertoolusage, andpanelsandcontrols whichareoperated asinground training), andperhaps somelong-term biological advantages. Thetethermasswould beasignificant partofthestation masstoproduce 0.1g(using a tapered 450kmtether), butwouldberelatively smallfor0.05gorless.However, careful consideration willhavetobegiventothedisadvantages oftethersystem massandcomplexity, andtoassurance of 126 survival incaseoftethersevering bymeteoroid ordebris impact. Suchasystem would alsoaffecta microgravity laboratory, requiring ittobemoved fromtheSpaceStation totheC.G.location. Avariable/low gravity laboratory module couldbeattached byacrawler mechanism toatether deployed alongthegravity gradient fromtheSpaceStation. Amicrogravity laboratory couldalsobebuilt aspartoftheSpaceStation atitscenterofgravity. Theselabscouldbeusedtoexamine theeffects of microgravity andlowgravity onbothphysical andbiological processes. Somebiological processes of interest wouldbeplantandanimal growth, andhuman performance andmedical processes (suchasthose related tothecardiovascular, skeletal, andvestibular systems). Thegravity-threshold values forvarious biological phenomena couldalsobestudied. Suchphysical processes ascrystal growth, fluidscience, and chemical reactions couldbestudied. Manyexperiments inmaterials science andmanufacturing couldbe performed inthesegravity ranges. Liquid propellant storage andrefueling facilities couldbetethered to theSpaceStation. Theartificial gravity produced bythetetherwould assistinpropellant handling and transfer. Figure 4.7showsthetetherlengths necessary toallowpropellant settling fortheproper transfer ofvarious propellants. Thesearebutafewofthepossible applications oftheartificial-gravity environments produced by gravity-gradient effects. Detailed descriptions ofapplications utilizing thesegravity-gradient effects are contained inthe"Tether Applications" (Section 3.0)ofthishandbook. Notethat,duetothewidevariety ofpossible system configurations, alloftheseapplications arecontained inonecategory. There are applications whichoverlap twoormorecategories andwhichcouldbelogically listedunderanyoneof them. Inthesecases,ajudgment hasbeenmadeastowhichcategory isthemostappropriate forthe particular application anditislistedinthatcategory. Theapplications related totheartificial gravity produced bygravity-gradient effects appears inthe"Controlled Gravity" and"Space Station" categories of the"Applications" section, asappropriate. FluidSettling •SETTLING REQUIREMENT -GRAVITY DOMINATE SURFACE TENSION •FLUIDSETTLING PARAMETER ISBONDNUMBER (Bo) P*_-=D2 p=FLUIDDENSITY Bo=-_-- (_=SURFACE TENSION COEFFICIENT D=TANKDIAMETER •FLUIDSETTLES IFBo>10 -Bo=50CHOSEN TOBECONSERVATIVE PROPELLANT SETTLING ONASTATIC TETHER (Bo=50) $I 4 lm {, =* 1 lee 0 OXYGEN HY0110GIEN NITIOGEN NONG- HYO_t,ZUlE TETI_OXIOE UETH_. NYO.qAZlNIE Figure 4.7HuidSettling Properties ofVarious Liquid Propellants UnderConditions ofArtificial Gravity -Required TetherLength Versus Propellant 127 4.1.3 Constellations Gravity-gradient forcesalsoplayacritical roleinthestabilization oftethered constellations. A tethered constellation isdefined asageneric distribution ofmorethantwomasses inspaceconnected by tethers inastableconfiguration. Theycanbeconfigured ineitherone,two,orthreedimensions. Allof thenon-negligible forcesorgradients available inloworbitcomeintoplaytostabilize thesevarious configurations. Thevertical gravity gradient hasthestrongest influences, butdifferential airdrag, electrodynamic forces, theJ22gravity component (anharmonic oftheEarth's gravitational potential), and centrifugal forcesalsocontribute. Different configurations utilizedifferent combinations: 1-Dvertical and horizontal, drag-and gravity-gradient-stabilized andelectromagnetically stabilized (2-D). Tethered constellations aredivided intothetwobasiccategories shown inFigure 4.8(Ref.4,p. 296).Theseaxe"static" and"dynamic" constellations. Staticconstellations aredefined asconstellations whichdonotrotaterelative totheorbiting reference flame(theydorotateattheorbital ratewhenreferred toaninertial frame). Dynamic constellations, ontheotherhand,aredefined asconstellations which do rotatewithrespect totheorbiting reference frame. Thesetwobasiccategories aresubdivided further. Staticconstellations include gravity-gradient-stabilized (one-dimensional, vertical), drag-stabilized (one- andgravity-gradient-stabilized (two-dimensional) constellations. Dynamic constellations include centrifugally stabilized twodimensional andthree-dimensional constellations. Thissection willaddress onlythestaticconstellations. Static Figure 4.8TypesofTethered Constellations 128 Fromthestandpoint ofstability andcomplexity, agravity-gradient-stabilized, one-dimensional, vertical constellation isthemostdesirable configuration. Adiagram showing threebodies tethered inthis configuration isshown inFigure 4.9. Examples included thethree-body configurations usedfor variable/low gravity andtm'crogravity labs,andfortheposition control ofthesystem centerofgravity. Earlier discussion ofvertical configurations included descriptions oftheirdynamics (including libration). Thedominant influence ontheseconstellations isthevertical gravity gradient. FLIGHT -. DIRECTION" Figure4.9Example Configuration ofl-D,Gravity-Gradient-Stabilized, Vertical Constellation Stability inone-dimensional, horizontal constellations isprovided bytensioning thetethers. (Such aconstellation isdepicted inFigure 4.10.) Bydesigning suchaconstellation sothattheballistic coefficient ofeachofitselements islowerthanthatoftheelement leading itandhigher thanthatofthe element trailing it,atension ismaintained inthetethers connecting themalongthevelocity vector. The resulting differential dragonitselements prevents theconstellation fromcompressing, andthetension in itstethers prevents itfromdrifting apart.Inprinciple, thereisnolimittothenumber ofplatforms which canbeconnected inthismanner. However, itshould benotedthatdragtakesorbital energy outofthe constellation, shortening itsorbital lifetime unlesscompensated bysomeformofpropulsion. FLIGHT DIRECTIC Figure 4.10Example Configuration ofl-D,Drag-Stabilized, Horizontal Constellation Thefundamental parameter forone-dimensional, horizontal constellations isthedifferential ballistic coefficient ofthetwoendbodies. Inthecaseofamassive frontbodyandavoluminous rearbody (balloon), itisequaltotheballi._tic coefficient ofthelatter. Tether lengths andorbital lifetimes are competing requirements andareneversufficiently satisfied inthealtitude rangeofinterest. Sincethe vertical gravity gradient dominates overthedifferential airdragattheSpaceStation altitude andabove, the maximum horizontal tetherlengthmustbeshortforstability. Atloweraltitudes (150-200 kin)wherethe 129 differential airdragbecomes relatively strong, tetherlengthmaybelonger, buttheorbital lifetime willbe limited. The"fish-bone" configuration wasthefirstproposed two-dimensional constellation anditutilizes bothgravity-gradient andair-drag forcesinordertoattainitsstability. Asimple "fish-bone" constellation isdepicted inFigure 4.11.Foranalytical purposes, thisconstellation canbereduced toanequivalent one- dimensional, horizontal constellation bylumping theoverall ballistic coefficient oftherearleg(balloons plustethers) andthefrontlegattheendsofthehorizontal tether. Additional information onthestability analysis oftheoriginal "fish-bone" configuration shown inFigure4.11ispresented inRef.4(p.171-172) andcontains calculated values ofitsstability limitsversus altitude. Analysis hasrevealed thatthis configuration islessstablethanacomparable one-dimensional, horizontal constellation. Thenecessity of amassive deployer atthecenterofthedownstream vertical tethersubsystem greatly reduces thearea-to- massratioofthatsubsystem. Flight Directionm11, I1 I mll,A1m12,A2 C) 0 m22, A3c.g. ,,.--..__..._ Local VerticalI 12 ()- m12,A2 ¢ Figure 4.11Example Configuration of2-D,"Fish-Bone" Twoadditional designs foratwo-dimensional constellation, utilizing gravity-gradient andair-drag forcesforstability, havebeenproposed. Thesedrag-stabilized constellation (DSC) designs aredepicted in Figure 4.12.Withthistypeofconfiguration, thegravity gradient isexploited foroverall attitude stability (theconstellation's minimum axisofinertiamustbealongthelocalvertical), anddifferential air-drag forcesareusedtostretch theconstellation horizontally forshapestability. Thedragforceisfullyexploited toassuretheminimum tension inthehorizontal tethers, andnottocounteract thegravity-gradient forceas itdoesinthe"fish-bone" configuration. Design parameters forDSCsystems arepresented inRef.4(p. 175-178). Twodesigns foratwo-dimensional constellation u"ttlizing gravity-gradient andelectromagnetic forcesforstability havebeenproposed. Theseelectromagnetically stabilized constellation (ESC) designs areshowninFigure 4.13. Intheseconfigurations, thegravity gradient isagainusedforoverall attitude stability (theminimum axisofinertia isvertical) andelectromagnetic forces areusedtostretch the constellation horizontally forshapestability. (These electromagnetic forcesarediscussed indetailinRef. 7andsection 4.4). 130 tFm ml,A1 | _ m_._, mI,A1 _,LocalVertical Fw GUaDRtlmUUm Figure 4.12TwoDesigns of2-DDSCConstellations Horizontally Inthequadrangular configuration, current flowsintheouter-loop tethers, interacting withthe Earth's magnetic field,togenerate electromagnetic forcesintheouterloop.Thecurrent direction ischosen suchthattheseforcespushthetethers outward, tensioning them(likeairinsideaballoon). Although the shapeisdifferent inthepseudo_lliptical constellation (PEC)design, thesameprinciple ofelectromagnetic tensioning oftheouter-loop tethers isapplied. Thetwolumped masses provide extraattitude stability without affecting theconstellation shape. Moreover, sincetheresultant forceiszero,theorbitaldecayrate isprovided byairdragonly.Design parameters forESCsystems arepresented inRef.4(p.176-177). ]/ VmilcalF.m7 m r Figure 4.13TwoDesigns ofESC2-DConstellations Where ShapeStability is Provided byElectromagnetic forces 131 Preliminary conclusions onthedesign oftwo-dimensional constellations havebeenreached. The "fish-bone" constellations arelessstablethantheone-dimensional, horizontal constellations. "Fish-bone" constellations arestablewithveryshorthorizontal tethers (lessthan100mat500kmaltitude). The alternative quadrangular DSCandESCconstellations (andPECsforspecial applications) extfibit abetter staticstability. Suitable design parameters canprovide goodstability withareasonably lowpower requirement forESCsandfeasible balloons forDSCs. Typical dimensions fortheseconstellations are10km(horizontal) by20km(vertical) withballoon diameters ofabout100mforDSCs, apower consumption ofabout5.5kWforESCsand2kWfor PECs. TheESCconstellations havegreater tension inthehorizontal tethers thantheDSCconstellations andanorbital decaywhichissmaller byanorderofmagnitude. ESCsaresuitable forlowinclination orbits. Moreover, sincetheytendtoorienttheirlongitudinal planeperpendicular totheEarth's magnetic field(Bvector), asmalloscillation aboutthevertical axisattheorbitalfrequency isunavoidable evenat loworbital inclinations. DSCs, ontheotherhand,aresuitable foranyorbital inclination. IntheDSCs, theyawoscillation occurs athighinclinations onlyduetotheEarth's rotating atmosphere. Thereareseveral proposed applications forone-dimensional, vertical constellations. Athree-body configuration couldbeusedformicrogravity/variable-gravity laboratories attached totheSpaceStation or theShuttle. Athree-body system couldbeusedontheSpaceStation tocontrol thelocation ofthecenterof gravity. Asystem of3ormorebodies attached totheShuttle orSpaceStation couldbeusedasamulti- probelabforthemeasurement ofthegradients ofgeophysical quantities. A3-body system couldalso function asanELF/ULF antenna byallowing acurrent toflowalternatively intheupperandlowertether toinjectanelectromagnetic wavewithasquare waveform intotheionosphere. Aspaceelevator (or crawler) fortheSpaceStation isyetanother application. There areseveral proposed applications fortwo-dimensional constellations. An electromagnetically stab'flized constellation couldprovide anexternal stable frame forgiantorbiting reflectors. Multi-mass constellations ingeneral allowaseparation ofdifferent activities whilekeeping themphysically connected, suchasforpowerdistribution, etc.Detailed analysis ofthesetwo-dimensional structures maybefoundinRef.7. 4.2 ROTATION OFTETHER SYSTEMS 4.2.1 General Tethers willalmost always beinvolved insomeformofrotational configuration. Anyplanet- orbiting tethersystem, bynature, willrotateabouttheplanetattheorbitangular velocity. Thecombination ofthecentrifugal forcesduetorotation andgravity gradient actingonthetetherendmasses causes ittobe stabilized inavertical position abouttheplanetcenterofmass. Inmanyinterplanetary applications, rotation willbedesired tocauseanartificial-gravity environment ortocreateacentrifugally stabilized configuration. 4.2.2 Controlled Gravity Atether-mass system maydesirecontrolled gravity foranumber ofapplications. Thesemayrange fromanartificial-gravity environment formanned interplanetary missions toacontrolled-gravity platform forindustrial spaceapplications. Thecalculation oftheacceleration atapointforpurely circular motion is presented here.Withreference toFigure4.14,weassume thatpointP(which would represent themass) isataconstant radius, r(thetether), fromthecenterofourrotation system. 132 where,P X Figure 4.14Circular Motion ofaPoint. Theacceleration canthenbefoundbytheexpression: -_=(-rc02)"_+(r_0)e--_ ; -_= r %= r= 0)=acceleration atthepointP(m/s2), unitvectorinradialdirection, unitvectorintangential (velocity) direction, radius(length oftether) (m), angularvelocity(tad/s), _o=angularacceleration (md/s2). Notice thatiftheangular velocity isconstant theacceleration simplifies to a-g)=(-rm2)e-_r ; wherethenegative signindicates thattheacceleration actstoward thecenterofrotation (seeReL8). Asanexample, suppose itisdesired tocalculate thegravity levelatamanned module rotating aboutanother similar module withangular velocity of2.0rpm,attached byatetheroflength200meters. Thecenterofmasswillbeexactly between them,and,withthisastheorigin, thedistance toeachmodule is100meters. Then,thecalculation is, a=rO_2 =(lOOm)If-_tflmin'_ (.2*trad_l =4.38m/s22 133 Tocalculate thegravity level(ascompared toEarth's): 4.38rigs2 a-- 9.8m/s2 =0.45g. 4.3 MOMENTUM EXCHANGE 4.3.1 General-Conservation ofAngular Momentum Tethers canhaveusefulspaceapplications byredistributing theorbital angular momentum ofa system. Atethercanneither createnordestroy system angular momentum, onlytransfer itfromonebody toanother. Angular momentum isdef'med (forarotating system, Figure4.15)as, where m= r- v= =--_--_mr2_-_mrxv= angular momentum ofsystem (kgm2s"1), massofsystem (kg) radiusvectorfromcenterofrotating coordinate system (usually theEarth)tosystem centerofmass(m), velocity ofsystem centerofmassnormal tor(ms"1),and system angular velocity (s-1). Figure4.15Angular Momentum inaRotating System 134 Ingeneral, momentum exchange canbeusedforvarious tether applications usingdifferent momentum exchange techniques. Thesetechniques willbedescribed first,followed byexamples oftheir application. Ausefulchartispresented inSubsection 5.4.4ofSection 5.0,"Tether Data". 4.3.2 Tether Payload Deployment Consider asystem composed oftwobodies connected byavariable-length tetherasinFigure 4.16 (seeRef.9). Inordertoinitiate atethered deployment, suchasdeploying apayload (M2)downward fromthe Shuttle (M1),itisfirstnecessary toprovide aninitialimpulse tothepayload tostartseparation. Aftera certain length oftetherhasbeendeployed, themasses areinsufficiently different orbitssothatgravity- gradient andcentrifugal forcescontinue theseparation. Ifthetwomasses werenotconstrained byatether, massM1would acquire alowerorbital circular velocity andM2would obtain ahigher orbital circular velocity intherneworbits. Thisisbecause asM1moves further awayfromtheEarth's gravitational field, itspotential energy israisedanditskinetic energy islowered. ForM2theexactopposite istrue.Sincethe masses areconstrained byatether,theyalsomustmoveatthesameorbital velocity. MassM2,therefore, will"drag"massM1alonguntillibration occurs. Libration (pendulum motion) willcontinue duetothe centrifugal, gravitational, andtethertension restoring forces. Domlmnnt Resullam Fon:e _ng M1 Ir_i I(e°_cr_°)Y ''..''" I_ I I IToh-T-- r"-...I Ii/...... _R.,o.,.J _Ln......... X/ .-" Cirvler_Orbit T_lhotTerrdonA.,.o''- OfTeqhemd S_tsm'n I_(Deoa_Foreel F<m:e Figure 4.16Tethered Deployment Inthiscase,massM1gained angular momentum equaltoanidentical amount lostbyM2.Thisamount of angular momentum transferred isequalto: Ah=M 1VAR I=M2VAR 2. Themomentum istransferred fromM1toM2through thehorizontal component ofthetethertension. This tension iscaused bytheCoriolis termoftheacceleration expression ofthelibrating masses. Ifthetetherisnowcut,theuppermass,M1,isboosted intoanelliptical orbithaving higher energy thanitwouldhavehadduetoitsgreater velocity. Thepointintheorbitwherethetetherissevered will correspond totheperigee ofM1.Thesituation isexactly reversed forM2,whichwillbeatitsapogee at thispoint. 135 Thepreceding discussion explains thebasicmechanics ofmomentum transfer intethers. Thereare manyvariations oftethered deployment, manyofwhicharebeyond thescopeofthistext.Onlysomeof themorebasiconeswillbedescribed here. Staticanddynamic tetherdeployment arebasically thesame,except thatstaticdeployment occurs withthetetherremaining undersmallangular displacements fromthevertical, anddynamic deployments utilizelargeangular displacements. Forcertain dynamic deployments, itispossible toimpart additional energy toonemassattheexpense oftheother. Inordertoimplement thisexchange, thedeployment begins withalargeangular displacement, tethertension ispurposely keptlowuntiladesired length is reached. Whenbrakes areapplied, alargeangleprograde swingoccurs. Whentheuppermass(payload) leadsthelowermass,thetetherissevered. Inthisway,anaddedboostduetotheadditional velocity of theprograde swingisaccomplished. Another method oftethered deployment islibration pumping. Thetetherisinitially deployed then alternately extended andretrieved inresonance withtethertension variations during libmtion. (In-plane libration causes thesetension variations duetoCoriolis effects.) Spinpumping isyetanother method, whereby libration pumping iscarried further tothepointthatthetethersystem iscaused tospin.Inboth cases,theaddedenergy increases thedeparture velocity ofthepayload, justasinthedynamic tethered deployment case. 4.3.3 Orbit Variations Ifthepayload deployment described previously iscarefully done,theorbitsofbothmasses canbe changed foroneorbothoftheirbenefits. TheShuttle, forexample, canboostapayload intoahigher orbit andatthesametimedeboost itselfbacktoEarth. Conversely, theShuttle couldperform atethered deployment ofitsexternal tanks,whereby thetanksaredeboosted backtoEarthandtheShuttle isboosted toahigherorbit.Applications suchasthesearetermed "momentum scavenging" sinceexcess momentum isutilized forabeneficial purpose. Thelrickwiththisapproach isthatexcess momentum mustbe available. Onemajorapplication whichisdescribed intheapplications section ofthehandbook isthe Space Station-Shuttle deboost operation. Thisisanexcellent example where bothmasses benefit. Resupply missions oftheSpaceStation bytheShuttle arefinalized byatethered deployment ofthe Shuttle. Inthisway,theSpaceStation isboosted toahigher orbitandtheShuttle isde-boosted backto Earth. Inordertoutilizetheadditional momentum oftheSpaceStation, tethered deployments ofanSTV arealternated withthoseoftheShuttle. Fuelsavings canbeobtained bybothShuttle andSTVinthis example. Tethers canalsobeusedtochange orbiteccentricity. Thisisdonebylibration pumping of tethered mass,phased asinFigure4.17(Ref.9). 2t st 1 ./ AS,_,J S_' 4 Figure4.17OrbitEccentricity Change 136 At(1)themassisfullyextended, andlibration commences. At(2),withthemassinaprograde swing, theretrieval motorpullsthespacecraft toward themass,adding energy totheorbit(through theuse ofexcess electrical energy transferred tothemotor). At(3),whichisthenewapogee oftheorbit,the tetherlengthisataminimum. At(4),withthemassinaretrograde swing, thetetherisre-<teployed and theretrieval brakes areusedtodissipate orbitalenergy intheformofexcessheat.At(5),thenewperigee, themassisagainfullydeployed. 4.4 ELECTRODYNAMICS 4.4.1 General Electrodynarnic tethersystems canbedesigned toproduce several usefuleffects byinteracting with magnetic fields. Theycanbedesigned toproduce eitherelectrical power orthrust(either apropulsive thrustoradrag). Theycanalsobedesigned toalternately produce electrical power andthrust. In addition, theycanbedesigned toproduce ULF/ELF/VLF electromagnetic signals intheupperatmosphere, andshape-stability fororbiting satellite constellations. Electrodynamic systems canbedesigned to produce electrical power. 4.4.2 Electric Power Generators Thediscussion ofelectric power generation bytethersystems willbeginwithelectrodynamic systems inlowEarthorbit. Consider avertical, gravity-gradient-stabilized, insulated, conducting tether, whichisterminated atbothendsbyplasma contactors. Atypical configuration isshown inFigure 4.18 (Ref.9,10).Asthissystem orbitstheEarth,itcutsacrossthegeomagnetic fieldfromwesttoeastat about8km/s.Anelectromotive force(emf)isinduced acrossthelengthofthetether. Thisemfisgiven bytheequation: whereV ,=. V _., .--.> V- dl=x3). alonglengthoftether induced emfacrossthetetherlength(volts), tethervelocity relative tothegeomagnetic field(m/s) magnetic fieldstrength (webers/rn 2),and differential dement oftetherlength-avectorpointing inthe direction ofpositive current flow(m). Forthespecial casewherethetetherisstraight andperpendicular tothemagnetic fieldlineseverywhere alongitslength, theequation fortheemfsimplifies to: v=(7x ; where -.y L=tetherlength-avectorpointing inthedirection ofpositive current flow(m). Theequation fortheinduced emfacrossthetetherinthisspecial casecanalsobewritten as: V=LvBsin0 ; 137 where .-.> ---> 0=anglebetween vandB. (From theseequations, itcanbeseenthatequatorial andlow-inclination orbitswillproduce thelargest ernfs, sincethemaximum emfisproduced whenthetether velocity andthemagnetic fieldare perpendicular toeachother.) pt_s_cmr_ Cllll)tgllm Figure 4.18PowerGeneration WithanElectrodynamic Tether Theemfactstocreateapotential difference acrossthetetherbymaking theupperendofthetether positive withrespect tothelowerend.Inordertoproduce acurrent fromthispotential difference, the tetherendsmustmakeelectrical contact withtheEarth's plasma environment. Plasma contactors atthe tetherendsprovide thiscontact, establishing acurrent loop(aso-called "phantom loop") through the tether,external plasma, andionosphere. Although processes intheplasma andionosphere arenotclearly understood atthistime,itisbelieved thatthecurrent pathislikethatshown inFigure 4.19.Thecollection ofelectrons fromtheplasma atthetopendofthetetherandtheiremission fromthebottom endcreates a net-positive charge cloud(orregion) atthetopend,andanet-negative charge cloudatthebottom. The excess freecharges areconstrained tomovealongthegeomagnetic fieldlinesintercepted bythetetherends untiltheyreachthevicinity oftheEregion ofthelowerionosphere where therearesufficient collisions withneutral particles toallowtheelectrons tomigrate acrossthefieldlinesandcomplete thecircuit. Tooptimize theionosphere's abilitytosustain atethercurrent, thetethercurrent density ateachend mustnotexceed theexternal ionospheric current density. Plasma contactors musteffectively spread the tethercurrent overalargeenough areatoreduce thecurrent densities tothenecessary levels. Threebasic tethersystem configurations, usingthreetypesofplasma contactors, havebeenconsidered. Theyare:(1) apassive large-area conductor atbothtetherends;(2)apassive large-area conductor attheupperendand anelectron gunatthelowerend;and,(3)aplasma-generating hollow cathode atbothends. 138 Figure4.19TheCtm'ent PathExternal ToAnOrbiting Electrodynamic TetherSystem Inthefirstconfiguration, theupperconductor (probably aconducting balloon) collects electrons. The lowerplasma contactor inthisconfiguration (perhaps aconductive surface oftheattached spacecraft) utilizes itslargesurface areainasimilar waytocollect ions. Toachieve higher currents, itispossible toreplace thepassive large-area conductor atthelower endwithanelectron gun,providing theequivalent ofcollecting apositive ioncurrent byejecting anegative electron current. Ejecting theseelectrons atahighenergy distributes themoveraneffectively largecontact region. Unfortunately, electron gunsareactiveplasma contactors, requiring on-board electrical power to drivethem. Thethirdconfiguration isquitedifferent fromthefirsttwo.Based uponresearch results and performance modeling uptothispoint,itisprojected tobethemostpromising ofthethreesystems. Instead ofrelying onapassive andphysically largeconducting surface tocollect currents, ahollow cathode ateachtetherendgenerates anexpanding cloudofhighlyconductive plasma. Theplasma density isveryhighatthetipofthetetherandfallsofftoionospheric densities atalargedistance fromthetip. Thisplasma provides asufficient thermal electron density tocarrythefulltethercurrent ineitherdirection atanydistance fromthetetherend,untilitismerged intotheambient ionospheric plasma currents. This caseofcurrent reversibility allows thesystem tofunction alternately aseitheragenerator orathruster, withgreater easethaneitheroftheothertwoconfigurations (aswillbediscussed inmoredetailinthenext section). Hollow cathodes arealsoactiveplasma contactors, requiring on-board electrical powerandagas supply tooperate. However, theyrequire muchlesspower thananelectron gun,andthegassupply should notimpose asevereweight penalty. Twodiagrams ofahollow cathode plasma source areshown inFigure 4.20. Additional diagrams andinformation relating totheconstruction andoperation ofthe PMGhollow cathode plasma contactor aregiveninFigures 4.21,4.22and4.23. Typical characteristics ofahollow cathode andanelectron gunarecompared inFigures 4.24and4.25.Moreinformation onthe TSSandPMGflights results aregiveninSection 1. TSS-1R flightshowed thatlargertethercurrents canbegenerated andatmuchlowersatellite potentials thanweretheoretically predicted. Afinalassessment ontheperformance ofhollow cathodes flownonPMG compared tootherconfigurations cannot begivenaslongasTSS-1R dataanalysis is completed andmoreflights testareverified. Inaddition, theremaybeparticular applications forwhich passive contactors orelectron gunsaredesirable. Ontheotherhand,usinghollow cathode plasma contactors should alsobesaferforspacecraft systems, sincetheyestablish aknown vehicle ground reference potential withrespect tothelocalplasma. 139 mMOSSSEC110N.O*0*0.O.o_e•.o. e'.'" _"• o.°.°,-,..oO. *.._*o_ HiDensity n..10,7 "_ _,ne--1015 4-(lo-_oo)_T,d -.--._100ton_n.,,T,.._.,_=0 n.--10a n.."e"101 T\i---./ I II":(10_-10"*)v,=10-1000arnp Can*,cm,e.,1)e_.u_,,He,. J --i!v 2)tl_)41_Alfv ___izabon (insheath?)Modelw/oB ExNnd_ Spear,s)wu_Vo |fl_|n,RIa=n2R22(R2)=Const. @Equilibrium nio"10_*---*-niex-10" wF_•RetardElectrons |AcceL Ioels:"'Bohm Condition") _T_L @.,,vn, j.-n,.t[k___T__ m. CanSupport •Cunm_t I--4/r Rzj,I4R" (RZn)e if I fir1e Figure 4.20Diagrams ofaHollow Cathode Plasma Contactor ® ® ® _'"DIATlmlllum @ Figure 4.21Diagram ofthePlasma Motor/Generator (PMG) Hollow Cathode Assembly 140 CowO_ Co_nless n,.,T,,,_.._0 n,-10I n_.n,"10I n.-1017 ........Modelw/oIiFiekl exlmnding Sph*m(s! wu_Vo @F._ n,..,..,10'_,.--._n,_:.lOS wF@•Rmalrd EleceroM IAccel. Ions:"Bohm Conditio.'3 VI:_E/"-_V--'-- @anyn.j,-n_4[k-'--TL- Tme CanSupport •Cu_mm I-4.,..j.-4:qm.,I,,t/_T---T-.-- _rme @Ro-l-lOom .,).10_2/cc ll_110-4-lO'_)ve= 101.000amp}l CanIncrease w1)Electron Heating 2l@)_Alhnm :::>Ionization (msheeth?) Figure4.22PlasmaCloudExpansion forPMGHoUow Cathode Plasma Contactor If_ Confinemem l--_llk_," =eO.Frd4frR_ lo,/,,@1.ooo,, 1.-"---"-_x :s_lo,,_r."_ / / \ / / \ / _106/c¢ @100mI i / _nn:10'-lO*/cc) I I lOe/cc @10m , I i _ / \ lo,./=@1m/ X \\ // \ x / _l_(x(mo/ma)x{ 2st((lOOF r4s,(loop __,,_.._ _ I_* II=0¢Ewibrium| Figure4.23Electron Current FlowTo/From theIonosphere forPMG Hollow Cathode PlasmaContactor 141 I (A) HOLLOW CATHODE I (A) 1 ELECTRON GUN{10+30)LOWIMPEDANCE (VOLT) V \ / 0HIGHIMPEDANCE V( V (Sk) (VOLT) Figure 4.24Comparison oftheIVCharacteristics ofaHollow Cathode andElectron Gun Electron Gun Hollow Cathode •Current Range I.<1A I,>10A(Nominal) •Power Consumption -1KW -10KW •LifeTime Similar Similar •Automatic Switching No Yes •Main Applications Basic Science LowImpedanceExp.andPower Coupling Dissipation Thrusting and Power Generation Figure 4.25Comparative Characteristics ofanElectron GunandaHollow Cathode 142 Thecurrent passing through thetethercanbecontrolled byanyoneofseveral methods, depending uponthetypeofplasma contactors used.Forsystems withpassive conductors atbothends,control isby variable resistance, inserted between thetetherandoneoftheplasma contactors. Forsystems usingan electron gunasaplasma contactor, tethercurrent iscontrolled bythecurrent emitted bytheelectron gun. Unfortunately, thesemethods areveryinefficient. TheynotonlywastealloftheI2Rpower lostinthe resistors, plasma sheaths (around theplasma contactors), andelectron gunimpedance, buttheyalso transfer mostofitasheatbackintothespacecraft, whereitmaycausesignificant thermal control andheat rejection problems. Thebasicequation ofthecurrent loop(circuit) is: VIND=IR+AVLo w+AVup +AVIoN+AVLOAD ; where VIND I R AVLow AVup AVIoN AVLOAD=emfinduced acrossthetether(volts), =tethercurrent (amps), =resistance ofthetether(ohms), =voltage dropacrossthespacecharge regionaround thelowerplasma contactor (volts), =voltage dropacrossthespacecharge regionaround theupperplasma contactor (volts), =voltage dropacrosstheionosphere (volts), and =voltage dropacrossaload(volts), Thisequation simply statesthattheemfinduced acrossthetetherbyitsmotion through themagnetic field isequaltothesumofallofthevoltage dropsinthecircuit. TheIRtermintheequation isthevoltage drop acrossthetetherduetoitsresistance (according toOhm'sLaw). Toprovide anexpression fortheworking voltage available todriveaload,thisequation canbe rewritten as: AVLOAD =VIND-IR-AVLo w-AVup -AVIoN Thevoltage dropacross thespacecharge region (sheath, electron gun,orplasma cloud) ateach tetherendiscaused bytheimpedance ofthatregion. Thevoltage dropacrosstheionosphere islikewise duetoitsimpedance. Theproblem withtheseequations isthattheimpedances ofthecharge regions around thetetherendsarecomplex, nonlinear, andunknown functions ofthetethercurrent. The impedance oftheionosphere hasnotbeenclearly determined. Although somelaboratory studies have beenperformed, andestimates made,detailed flighttestmeasurements willhavetobeperformed before thesequantifies canbeclearly determined. Ithasbeencalculated thattheionospheric impedance should beontheorderof1-20ohms(Ref. 11).Thehighest impedance ofthetethersystem areencountered atthespacecharge sheathregions around theupperandlowerplasma contactors. Reducing theseimpedances willgreatly increase theefficiency of thetethersystem inproviding largecurrents. PMGdataindicate thatplasmas released fromhollow cathode plasma contactors greatly reduce thesheath impedance between thecontactors andtheambient plasma surrounding them. Although processes intheseplasmas andintheionosphere arenotwell understood andrequire continued studyandevaluation through testing, preliminary indications arethat feasible tetherandplasma-contactor systems should beabletoprovide largeinduced currents. Asindicated earlier, theelectric currents induced insuchtethersystems canbeusedtopower loads onboardthespacecraft equipped withthem.Theycanalsobeusedasprimary power forthespacecraft. Ithasbeencalculated thatelectrodynamic tethersystems should becapable ofproducing electrical powerin themultikilowatt topossibly themegawatt range(Ref.4,p.161-184). Calculations a200KWsystem is giveninfigure4.26. 143 Thereisapricetobepaidforthiselectrical power, however. Itisgenerated attheexpense of spacecraft/tether orbitalenergy. Thiseffectisdescribed indetailinthenextsection. Inprinciple, electrodynamic tethersystems cangenerate electrical power notonlyinEarthorbit, butalsowhentheymovethrough themagnetic fieldsofotherplanets andinterplanetary space. The magnetic fieldininterplanetary spaceisprovided bythesolarwind,which isamagnetized plasma spiralling outward fromthesun. References 1(p.1-22through 1-24,3-49through 3-65),2,4(p.153-184, 547-594), 10,11, and datafromDr.JamesMcCoy ('NASA/Johnson SpaceCenter) aretheprimary references forthissection. 144 PMG -200 KWREF_,RKNCE SYSTEM TETHER LENGTH 20KM(!0UP+!0 DN)WORKING TENSION NOMINAL VOLTAGE 4KV WORKING ANGLE BATEDPOWER 200 KW RATED THRUST PEAK POWER 500 KW PEAK THRUST42N 17DEG 25N >I00 N CONDUCTOR #00AWGALUMINUM WIRE DIAMETER9.3_4@20°C RESISTANCE 8.40HNS@20°C 7.7OHMS @0°C 7.1OHMS @-20@C INSULATION 0.5 MMTEFLON (I00 VOLTS/MIL) FAREND MASS 50AMPHOLLOW CATHODEASS'Y (INCLUDINGELECTRONICS &CONTROL) TETHER CONTROLLERELECTRONICS aMISC. HDWR. (POWER DISSIPATION LOSSES @1%-2KW) ARGON SUPPLYaCONTINGENCY. RESERVE TOTAL3640 KG 278 KG 25KG 94KG 4,200 KG TETHER DYNAMICS CONTROL TETHERCURRENT/POWER CONTROL TETHER OUTSIDEDIAMETER TETHER BALLISTIC DRAGAREAPASSIVE, IXB PHASING DCIMPEDANCEMATCHING 10.3MM 206 SQMETERS DRAG FORCE @I0-11KG/M3 (300 KM1976 USSA-400 _4SOLAR MAX) I2R LOSSES @200KW HOLLOW CATHODE POWER IONOSPHERIC LOSS @50AMP12N .96KW 19.25 KW 2.50 KW 1.25 KW TOTAL PRIMARY LOSSES 23.96KW EFFICIENCY ELECTRIC (177 KWNET @50AMP/200KW)88.5% OVERALL (201 MECH. TO177F/,EC. KW)88.1% INCLUDINGCONTROLLER/POWER PRDCESSOR LOSSES @I% 2.00 K'W TOTAL (NET POWER OUT 175.0 KW) 25.96 KW FINALEFFICIENCY ELECTRIC-87.5% OV_/_-87.1% Figure4.26Calculated Performance ofanElectromagnetic Tether System 145 4.4.3 Thrusters Asmentioned intheprevious twosections, electrodynamic tethersystems canbeusedtogenerate tlanastordrag. Consider thegravity-gradient-stabilized system inEarthorbit,forexample. Itsmotion through thegeomagnetic fieldinduces anemfacrossthetether. Whenthecurrent generated bythisemfis allowed toflowthrough thetether, aforceisexerted onthecurrent (onthetether) bythegeomagnetic field (seeFigure 4.27). Thisforceisgivenby: whereF--(:dl)xB =I_x_ ; alonglengthoftether alonglengthoftether forceexerted onthetetherbythemagnetic field(newtons), tethercurrent (amps), differential element oftetherlength-avectorpointing inthedirection ofpositive current flow(m),and magnetic fieldstrength (webers/m 2) /..t"<_,-. -/'-'F'_//:l//? y×/"-.. /"THRUSTliaOTORI Figure 4.27ThrustGeneration WithAnElectrodynamic TetherSystem 146 Forthespecial caseofastraight tether,thisequation simplifies to: ---) ----> F=ILxB ; where ---) L=tetherlength-avectorpointing inthedirection ofpositive current flow(m). Thisequation fortheelectromagnetic forceonastraight tethercanalsobewritten as: F=ILBsin0 ; where ...-> ..-.) 0=anglebetween LandB. Itsmaximum valueoccurs whenthetetherisperpendicular tothemagnetic field. Depending ontherelative orientation ofthemagnetic fieldtothetethervelocity, thisforcecanhave acomponent parallel tothevelocity andoneperpendicular tothevelocity. Considering theparallel (inplane) component, whenever thecurrent induced inthetetherbythemagnetic fieldisallowed toflow, thiscomponent oftheforcealways actstoreduce therelative velocity between thetethersystem. Inlow Earthorbit,wheretheorbital velocity ofthetetherisgreater thantherotational velocity ofthegeomagnetic fieldandtheyarerotating inthesamedirection, thisforceisadragonthetether. Thismeans thatwhen electric powerisgenerated bythesystem foron-board use,itisgenerated attheexpense oforbital energy. Ifthesystem istomaintain itsaltitude, thislossmustbecompensated byrockets orotherpropulsive means. Whencurrent fromanon-board power supply isfedintothetetheragainst theinduced emf,the direction ofthisforceisreversed. Thisforcefollows thesameequation asbefore, butnowthesignofthe crossproduct isreversed, andtheforcebecomes propulsive. Inthisway,thetethercanbeusedasa thruster. Therefore, thesametethersystem canbeusedreversibly, aseitheranelectric generator orasa thruster (motor). Asalways, however, thereisapricetobepaid.Thepropulsive forceisgenerated atthe expense ofon-board electrical power. Itisnecessary todistinguish between tethersystems orbiting atsubsynchronous altitudes, and thoseorbiting ataltitudes greater thanthesynchronous altitude, where thesenseoftherelative velocity between thesatellite andthemagnetic fieldrestframeisreversed (oftenthought ofintermsofaconcept known asthe"co-rotating field"). Ananalogous situation existsinorbitsaround Jupiter foraltitudes greater than2.2Jovian radiifromitscenter(theJoviansynchronous altitude: i.e.,thealtitude atwhichthe rotational angular velocity ofanorbiting satellite equalstherotational velocity ofJupiter anditsmagnetic field). Another analogous situation existsininterplanetary spaceifaspacecraft moves outward ataspeed of400kin/s). Insuchcases,dissipation oftheinduced electrical current would produce athrust(nota drag)onthetether. Again, theforceactstobringtherelative velocity between thetetherandthemagnetic fieldrestframetozero. Insuchcases, feeding current intothetetheragainst theinduced emfwould produce adrag.Whenmoving inadirection opposite tothedirection ofmotion ofthemagnetic field,the effects wouldbereversed. Systems havebeenproposed tooperate reversibly aspowerandthrustgenerators (Ref.4and10). Suchsystems couldprovide anumber ofcapabilities. Calculations oftheperformance ofa200KW system isgiveninfigure4.26. Inaddition tothein-plane component, theelectromagnetic forceonthetethercurrent generally also hasanout-of-plane component (perpendicular tothetethervelocity). Foranorbiting tethersystem, the out-of-plane forcecomponent actstochange theorbital inclination, whiledoingnoin-plane mechanical 147 workonthetetherandinducing noemftooppose theflowofcurrent inthetether. Thismakes electrodynamic tethers potentially idealfororbitalplanechanges. Unlike rockets, theyconserve energy during orbital planechanges. Ifthecurrent isconstant overacomplete orbit,theneteffectofthisforceis zero(sincereversals intheforcedirection duringtheorbitcanceleachotherout).Ontheotherhand,ifa netorbital inclination change isdesired, itcanbeproduced bysimply reversing thetethercurrent atpoints intheorbitwheretheout-of-plane forcereverses itsdirection, orbyallowing atethercurrent toflowfor onlypartofanorbit.Attention mustbepaidtothisout-of-plane forcewhenoperating atetheralternately asagenerator andthruster, andwhenoperating atethersystem which alternately generates andstores electrical energy. Strategies forusingelectrodynamic tethers tochange orbitsareshown inSection 5.0. Electromagnetic forces alsocausethetethertobowandproduce torques onthetethersystem. Thesetorques causethesystem totiltawayfromthevertical untilthetorques arebalanced bygravity- gradient restoring torques. Thesetorques produce in-pl,ane andout-of-plane librations. Thenatural frequencies ofin-plane andout-of-plane librations are,/3times theorbital frequency andtwicethe orbitalfrequency, respectively. Selective timephasing oftheILxBloading, ormodulation ofthetether current, willdamptheselibrations. Theout-of-plane librations aremoredifficult todampbecause their frequency istwicetheorbitalfrequency. Unless careistaken,day/night power generation/storage cycles (50/50 power cycles) canactively stimulate theselibrations. Careful timing oftetheractivities willbe required tocontrol alltetherlibrations. Additional information onelectromagnetic libration control issuesis shown alsoinSection 5.0. 4.4.4 ULF/ELF/VLF Antennas Asdiscussed inSection 4.4.2, themovement ofanEarth-orbiting electrodynamic tethersystem through thegeomagnetic fieldgivesrisetoaninduced current inthetether. Onesideeffectofthiscurrent isthatastheelectrons areemitted fromthetetherbackintotheplasma, ULF,ELF,VLFelectromagnetic wavesareproduced intheionosphere (seeRef.11). Inthecurrent loopexternal tothetether,electrons spiralalongthegeomagnetic fieldlinesandclose atalowerlayeroftheionosphere (seeFigure 4.28).Thiscurrent loop(orso-called "phantom loop") acts asalargeULF, ELF,andVLFantenna. (Thephantom loopisshown inFigure 4.29). The electromagnetic wavesgenerated bythisloopshouldpropagate totheEarth's surface, asshown inFigure 4.30. Thecurrent flowgenerating thesewaves canbethatinduced bythegeomagnetic fieldorcanbe provided byatransmitter onboardthespacecraft sothatthetetherisinpartanantenna. Messages canbetransmitted fromthetether(antenna) bymodulating thewaves generated bythe current loop.Iftheinduced current isusedtogenerate thesewaves, itismodulated byvarying aseries impedance orbyturning anelectron gunorhollow cathode onthelowertetherendonandoffatthe desired frequency. Ifatransmitter isused,current isinjected intothetetheratthedesired frequency. TheULF,ELF,VLFwaves produced intheionosphere willbeinjected intothemagnetosphere moreefficiently thanthosefromexisting ground-based, man-made sources. Itisbelieved thatthe ionospheric boundary mayactasawaveguide, extending theareaofeffective signalreception farbeyond the"hotspot"(areaofhighest intensity reception, withanestimated diameter ofabout5000kin)shown in Figure 4.30. Ifthisturnsouttobethecase,thesewaves mayprovide essentially instant worldwide communications, spreading overtheEarthbyducting. Calculations havebeenperformed, predicting that power levelsoftheorderof1Wbynightand0.1Wbydaycanbeinjected intotheEarth-ionosphere transmission linebya20-10kmtetherwithacurrent oftheorderof10A.Suchtethersystems would produce wavefrequencies throughout theULF(3-30Hz)andELFbands(30-300 H_z),andevenintothe VLFband(about3000Hz). 148 1 Figure 4.28Electron PathsintheElectrodynamic TetherGenerator LowerBoundry OfIonosphere Figure 4.29The"Phantom Loop"oftheULF/ELF Tether Antenna 149 III I |#|t!e_t Figure 4.30Propagation ofULF/ELF/VLF Waves ToTheEarth's Surface FromAnOrbiting TetherAntenna Itshould benotedthatiftheinduced tethercurrent isusedtopowertheantenna, orbital energy will becorrespondingly decreased. Ameans ofrestoring thisorbital energy (suchasrocket thrust) willbe required forlongmissions. 4.4.5 Constellations Asmentioned earlier, electromagnetic forcesexerted bythegeomagnetic fieldonthecurrent in orbiting tethers canbeusedinconjunction withgravity-gradient forces tostabilize two-dimensional constellations (seeFigure 4.13).Theforceexerted onacurrent inatetherisexactly theforcedescribed in Section 4.4.3. Thetethercurrents usedintheseconstellations canbethoseinduced bythegeomagnetic fieldorthoseprovided byon-board powersupplies. Thebasicconcept isthatgravity-gradient forceswillprovide vertical andoverall attitude stability fortheconstellation, andelectromagnetic forceswillprovide horizontal andshapestability (seeRef.1, p.1-29,and4,p.150-203). Thisisaccomplished inthequadrangular configuration byestablishing the current direction ineachofthevertical tethers suchthattheelectromagnetic forces onthempushtheside arcshorizontally awayfromeachother. Eachsidearcmaybecomposed ofanumber ofsatellites connected inseriesbytethers. Thecurrent directions forthetethers oneachsidearcwillbethesame, providing aconsistent outward force.Largemasses areplaced atthetopandbottom juncture pointswhere thetwosidesjointogether. Thisprovides additional stability fortheconstellation. 150 .REFERENCES Applications ofTethers inSpace, Workshop, Williamsburg, Virginia, 15-17June1983, Workshop Proceedings, NASA CP-2364 (Vol.1),NASA CP-2365 (Vol.2),March 1985. Beletskii, V.V.andLevin, E.M.,"Dynamics ofSpaceTether Systems," Advances inthe Astronautical Sciences, Vol.83. .Arnold, D.A.,"TheBehavior ofLongTethers inSpace," Journal oftheAstronuatical Sciences, Vol.35,No.1,p.3-18,January-March, 1987. . . . . . . 10. 11.Applications ofTethers inSpace, Workshop, Venice, Italy,15-17October 1985,Workshop Proceedings, NASA CP-2422 (Executive Summary, Vol.1,Vol.2),1986. Carroll, J.A.,Guidebook forAnalysis ofTetherApplication_, Contract RH4-394049, Martin Marietta Corporation, Feb.1985. Pearson, J.,"Anchored LunarSatellites forCislunar Transportation andCommunication," Journal oftheAstronautical Science_, Vol.27,No.1,p.39-62, Jan.-Mar. 1979. Lorenzini, E.C.,"Novel Tether-Connected Two-Dimensional Structures forLowEarth Orbits," Journal oftheAstronautical Sciences, Vol.36,No.4,p.389-405, Oct.-Dec. 1988. Greenwood, D.T.,Principles ofDynamics, Prentice Hall,Inc.,Englewood Cliffs,NewJersey, 1965. Tiesenhausen, G.yon,ed.,"TheRolesofTethers onSpaceStation," NASA-TM-86519, NASA/MSFC, Oct.1985. McCoy, J.E.,"Plasma Motor/Generator Reference System Designs forPower andPropulsion," AAS86-229, Int.Conf.1986. Grossi, M.D.,"Spaceborne LongVertical WireasaSelf-Powered ULF/ELF Radiator," IEEE Journal ofOceanic Engineering, Vol.OE-9,No.3,p.211-213, July1984. 151 SECTION 5.TETHER DATA 152 5.1General Thishandbook wouldnotbecomplete without providing theuserwithspecific dataandother information relevant totheanalysis oftetherapplications. Totheauthors' knowledge, thebestsummarization ofthisdataiscontained inJ.A.Carroll's Guidebook forAnalysis ofTether Applications_ published in1985 undercontract totheMartin Marietta Corporation. Itprovides aconcise review ofthosetechnical areas whichareessential totetheranalyses. Fortheuninitiated, itisthefirstexposure theyshouldhavetoensure thattheyunderstand thebroadimplications ofanyapplication theymightconsider. Fromhere,theycan explore themanyreferences givenintheBibliography. TheGuidebook isreproduced hereinfull,except foritsbibliography whichwouldberedundant. J.A. Carroll's introductory remarks andcreditsarepresented below: ThisGuidebook isintended asatooltofacilitate initialanalyses ofproposed tetherapplications inspace.Theguiding philosophy isthatatthebeginning of astudyeffort, abriefanalysis ofallthecommon problem areasisfarmore usefulthanadetailed studyinanyonearea.Suchanalyses canminimize the wasteofresources onelegant butfatallyflawedconcepts, andcanidentify the areaswheremoreeffortisneeded onconcepts whichdosurvive theinitial analyses. Inareasinwhichharddecisions havehadtobemade,theGuidebook is: Broad, ratherthandeep Simple, ratherthanprecise Brief,ratherthancomprehensive Illustrative, ratherthandefinitive Hencethesimplified formulas, approximations, andanalytical toolsincluded in theGuidebook should beusedonlyforpreliminary analyses. Fordetailed analyses, thereferences witheachtopicandinthebibliography maybeuseful. Notethattopicswhichareimportant ingeneral butnotparticularly relevant to tethered system analysis (e.g.,radiation dosages) arenotcovered. ThisGuidebook waspresented bytheauthor undersubcontract RH4394049 withtheMartin Marietta Corporation, aspartoftheircontract NAS8-35499 (Phase IIStudyofSelected Tether Applications inSpace) withtheNASA Marshall Space Flight Center. Someofthematerial wasadapted from references listedwiththevarious topics, andthisassisted thepreparation greatly. Muchoftheothermaterial evolved orwasclarified indiscussions withoneormoreofthefollowing: DaveArnold, JamesArnold, IvanBekey, Guiseppe Colombo, MiltContella, DaveCrisweU, DonCrouch, Andrew Cutler, MarkHenley, DonKessler, HarrisMayer, JimMcCoy, BillNobles, TomO'Neil, PaulPenzo, JackSlowey, Georg vonTiesenhausen, andBill Thompson. Theauthor isofcourse responsible forallerrors, andwould appreciate beingnotified ofanythatarefound. 153 5.2Generic Issues MAJOR CONSTRAINTS INMOMENTUM-TRANSFER APPLICATIONS CONSTRAINT: Alltypes Librating Spinning Winching Rendezvous Multi-stage lvte_Jur,d.ui,_JHighdeltaVORBIT TETHER TETHER TETHER BASICS DYNAMICS PROPERTIES OPERATIONS Apside Forces onBmeteoroid Tether recoil location endmasses sensitivity atrelease Orbitplanes mustmatch Dif.nodal regressionTether can goslack Highloads onpayload Highloads onpayload Gravity Control ofTethermass losses dynamics &lifetimeFacility attitude &"g"svariable Retrieval can bedifficult Extremely high powerneeded Shortlaunch& capture windows Waiting time between stages Retrieval energy_ Facility aair. r_eur_1_'wIidr_r_vl_ t_r_1_x-t_1"L_ xr_.o•t_irmi_$ II CONSTRAINTS: Alltypes Electrodynamic Aerodynamic Beanstalk (Earth)ORBIT BASICS |l Aero.dragTETHER TETHER TETHER DYNAMICS PROPERTIES OPERATIONS Libration Degradation, _tmeteoroids & debrisimpact Miscchanges Plasma High-voltage inorbit disturbances insulation Gravity Use: Hanging SpinningTether drag &heating Tether mass; debrisimpact Libration- SensitiveRecoil &orbit changes after tetherbreak Consequences offailure <0.1geeonly, Docking awkward 154 5.3OrbitEquations andData 5.3.1Orbits andOrbital Perturbations KEYPOINTS Basicorbitnomenclature &equations areneeded frequently infollowing pages.Comparison of tether&rocketoperations requires orbittransfer equations. NOTESThefigures andequations atrightareasummary oftheaspects oforbital mechanics most relevant totetherapplications analysis. Formorecomplete anddetailed treatments and manyofthederivations, consult refs.1-3. Thef_stequation intheboxisknown astheVisVivaformulation, andtotherightofitis theequation forthemeanorbitalangular rate,n.Muchoftheanalysis oforbittransfer AVs andtetherbehavior follows fromthosetwosimpleequations. Someanalyses require aclose attention tospecific angular momentum, h,soanexpression forh(forcompact objects) is alsogivenhere. Ingeneral, sixparameters areneeded tocompletely specify anorbit.Various parameter sets canbeused(e.g.,3position coordinates &3velocity vectors). Thesixparameters listedat rightarecommonly usedinorbital mechanics. Notethatwheni=0,f2becomes indeterminate (andunnecessary); similarly withcowhene--0.Also,i&Darehere referenced tothecentral body'sequator, asisusually doneforLowEarthOrbit(LEO). For highorbits,theecliptic orotherplanesareoftenused.Thissimplifies calculation of3rd bodyeffects. TheeffectsofsmallAVsonnear-circular orbitsareshownatright.Therelative effects are showntoscale:aAValongthevelocity vector hasamaximum periodic effect4times largerthanthatofthesame,AVperpendicular toit(plusasecular effectin0whichthe others don'thave). Effects ofoblique orconsecutive AVsaresimply thesumofthe component effects. Notethatout-of-plane AVsatapointotherthananodealsoaffectD. ForlargeAVs,thecalculations aremoreinvolved. Theperigee andapogee velocities ofthe transfer orbitarefirstcalculated fromtheVisVivaformulation andtheconstancy ofh. Thentheoptimum distribution ofplanechange between thetwoAVscanbecomputed iteratively, andtherequired totalAVfound. Typically about90%oftheplanechange is doneatGEO. Tofindhowmuchagivenin-plane tetherboostreduces therequired rocket AV,thefull calculation shouldbedoneforboththeunassisted andthetether-assisted rocket. Thisis necessary because thetetheraffects notonlytheperigee velocity, butalsothegravity lossesandtheLEO/GEO planechange split.Eachm/softetherboosttypically reduces the required rocket boostby0.89m/s(forhanging release) to0.93m/s(forwidely librating release). Notethatforlargeplanechanges, andlargeradius-ratio changes evenwithout plane changes, 3-impulse "bi-elliptic" maneuvers mayhavethelowesttotalAV.Suchmaneuvers involve aboosttonear-escape, asmallplaneand/orperigee-adjusting AVatapogee, andan apogee adjustment (byrocket oraerobrake) atthenextperigee. Inparticular, thismaybe thebestwaytoreturnaerobraking OTVsfromGEOtoLEO,ifadequate timeisavailable. REFERENCES 1.A.E.Roy,Orbital Motion, AdamHilgerLtd.,Bristol 1978: 2.Bate,Mueller, &White,Fundamentals ofAstrodynamics, DoverPub.,1971. 3.M.H.Kaplan, Modem Spacecraft Dynamics &Control, JohnWiley&Sons,1976. 155 Orbit&OrbitTransfer Equations rapo=a(l+e) rper=a(1 -e) p=a(1-e 2) V2=21 V_2irc=p/r Ve2sc=2_/rn=_=_7_ h=_=r2_=rVcos¢ Peacth=398601 ]_3/sec2 Px=G*Massofx BASICORBIT EQUATIONSM=M.+ntSatellite Position __at t=O //_'_-"><_ MoPeriapsis _in_e°f ORBITAL ELEMENTS a=semi-majoraxis e=eccentricity i=inclination =long. ofasc. node w=argument ofperiapsis Mo:positionatepoch __r'_sinSAVv/n /"_t_t2(1-c4)sS)AVh/n L_2 (_s_l)x/_Vv/rn /)_""_l- s_:'_Ai_dV/Vcirc EffectsofSmall _VsonNear-Circular Orbits _J..yGEO •AVi.,¢:_LEO Total _Visminimized when sinYLP.Q=rLEO sinYGEO rGEO Large Orbit Transfers(e.g., LEO--GEO) 156 5.3.2Orbital KEYPOINTSPerturbations Differential nodalregression severely limitscoplanar rendezvous windows. Apsidal recession affects STSdeboost requirements fromelliptical orbits.Thirdbodiescanchange theorbit planeofhigh-orbit facilities. NOTESThegeoid(earth's shape)isroughly thatofahydrostatic-equilibrium oblateellipsoid, witha 296:297 polar:equatorial radiusratio.Therearedepartures fromthisshape, buttheyare muchsmaller thanthe1:297oblateness effect andhavenoticeable effects onlyon geosynchronous andotherresonant orbits. Thefocushereisonoblateness, because itisquitelargeandbecause ithaslargesecular effects onflando3fornearlyallorbits.(Oblateness alsoaffects n,butthiscanusually be ignored inpreliminary analyses.) Asshownatright,satellites orbiting anoblate bodyare attracted notonlytoitscenterbutalsotowards itsequator. Thisforcecomponent imposes atorque onallorbitsthatcrosstheequator atanangle,andcausesthedirection ofthe orbitalangular momentum vectortoregress asshown. islargest wheniissmall,buttheplanechange associated withagivenA,Ovarieswith sin(i).Hencetheactualplanechange ratevarieswithsin(i)eos(i), orsin(2i), andishighest near45°.Fornear-coplanar rendezvous inLEO,therequired out-of-plane AVchanges by 78sin(2i) m/sforeachphasing "lap".Thisisindependent ofthealtitude difference (tofirst order), sincephasing &differential nodalregression ratesbothscalewithAa.Hence evenat bestarendezvous mayrequire anout-of-plane AVof39m/s.Atothertimes,out-of-plane AVsof2sin(i)sin(AD./2)V_-t (=upto2Ve_-¢I)areneeded. Thelinkage between phasing andnodalregression ratesisbeneficial insomecases:ifan objectisboosted slightly andthenallowed todecayuntilitpassesbelowtheboosting object, thetotalA_isnearlyidentical forboth.Hencerecapture neednotinvolve anysignificant planechange. Apsidal recession generally hasamuchlessdominant effectonoperations, sinceapsidal adjustments (particularly oflow-eorbits) involve muchlowerAVsthannodaladjustments. However, tetherpayload boostsmayoftenbedonefromelliptical STSorbits, andperigee drif_maybeanissue.Forexample, OMSdeboost requirements fromanelliptical S'ISorbit aretonnes lower(andpayload capability muchhigher) ifperigee isnearthelanding site latitude attheendofthemission. Perigee motion relative today/night variations isalso important fordetailed dragcalculations, andforelectrodynamic day-night energy storage (where itsmears outandlimitstheeccentricity-pumping effectofasustained day-night motor-generator cycle). Justastorques occurwhenthecentralbodyisnon-spherical, therearcalsotorques whenthe satellite isnon-spherical. TheseaffectthesateUite's spinaxisandcauseittoprecess around theorbital planeataratethatdepends onthesatellite's massdistribution andspinrate. Inhighorbits,central-body perturbations become lessimportant and3rd-body effects more important. InGEO,themainperturbations (--47m/sdyr) arecausedbythemoonandsun. Thefigureatrightshowshowtoestimate theseeffects, usingthe3rdbodyorbital planeas thereference plane. REFERENCES 1.A.E.Roy,Orbital Motion, AdamHilgerLtd.,Bristol, 1978. 2.Bate,MueUer, &White, Fundamentals ofAstrodynamics, DoverPub.,1971. 157 Orbital Perturbations OBLATENESSCAUSESLARGE SECULARCHANGESIN/_&O0: _:uptoIra_/week inLEO O_:upto2rad/week inLEO Nodal Re6ression inLEO: _=-6_.6 cosirad/yr (a/re)3 -5(I-e2) 2 (re=6378km)i<90° i>90° Forsun-synchronous orbits:(i=I00°±_) cosi_-.0988(a/re)3"5(1-e2) 2 _" .2__ Forcoplanarlow-AV rendezvous between2objects (e,=e,_0s iI=i_), nodal coincidence intervalsare: 180 (_/re) 4"5 _tnc= _a[cos _km.yrsApsidal recession inLEO: _m 63.6(2-2._ sinai) rad/yr (alre)3.5 (I-e2) 2 i<63.4° i=63._ i>63._ Motion ofthelongitude of perigee with respect tothe sun's direction ("noon") is: (_$:f_+._--271/yr x /_3 :-75 cosis,_3Ins_/_ "Smeared out" 3rdbody\.. /__ Third-Body Perturbations (non-resonant orbits) 158 5.3.3Aerodynamic Drag KEYPOINTS Tetherdragaffects tethershape&orbital life;atomic oxygen degrades tethers. Out-of-plane dragcomponent caninduceout-of-plane tetherlibration. Themainvalueofpayload boosting bytetheristheincreased orbital life.Unboosted orbital lifeofspacefacilities isaffected by tetheroperations. NOTESThefigureatrightshowstheorbiter trolling asatellite intheatmosphere, asisplanned for the2ndTSSmission inthelate1980s.Thetetherdraggreatly exceeds thatontheend-masses andshould beestimated accurately. Thedragincludes asmallout-of-plane component that cancause¢-libration. Tetherdragisexperienced overarangeofaltitudes, overwhichmostofthetermsinthedrag equation vary:theairdensity p,theairspeed V_a,andthetetherwidth&angleofattack. In free-molecular flow,CLissmall,andCD(ifbasedonADisnearlyconstant at2.2.(CDrises neargrazing incidence, butthenA_islow.) OnlyPvariesrapidly, butitvariesinawaywhichlendsitselftosimple approximations. Empirical formulae havebeendeveloped bytheauthor andareshown atright.Theygive valuesthataxeusually within25%ofref.1,whichisstillregarded asrepresentative forair density asafunction ofaltitude &exosphere temperature. Theseestimates holdonlyfor p>lE-14, beyond whichhelium &hydrogen dominate &thedensity scaleheightHincreases rapidly. NotethatovermuchofLEO,atomic oxygen isthedominant species. Hyperthermal impact ofatomic oxygen onexposed surfaces cancauserapiddegradation, andisaproblem inlow- altitude applications oforganic-polymer tethers. Thespaceagebeganin1957ata200-yrhighinsunspot count.Anewestimate ofmeansolar cycletemperatures (atright,fromref.2),ismuchlowerthanearlier estimates. Mission planning requires bothhigh&meanestimates forproper analysis. Ref.2&papers inthe samevolume discuss models nowinuse. IfthetetherlengthLis<<H,thetotaltethered system dragcanbeestimated fromthetotal A.&themidpoint V&p.IfL>>H, thetopendcanbeneglected, thebottom calculated normally, andthetetherdragestimated from1.1p_,t_om *tetherdiameter *H*V2_,withH& V_evaluated oneHabovethebottom ofthetether. ForLbetween thesecases,thedragis bounded bythesecases. Asshown atright,theorbital lifeofmorecompact objects (suchasmightbeboosted or deboosted bytether)canbeestimated analytically ifTexisknown. Forcircular orbitswiththe samer,V,,a&P"bothvarywithi,butthesevariations tendtocompensate &canbothbe ignored infirst-cut calculations. Theconversion ofelliptical to"equal-life" circular orbitsisanempirical fittoanunpublished parametric studydonebytheauthor. Itapplies whenapsidal motions relative totheequator andrelative tothediurnal bulgearelargeovertheorbitallife;thisusually holdsinbothlow& high-iorbits.Foradetailed studyofatmospheric drageffects, ref.3isstilluseful. REFERENCES 1.U.S.Standard Atmosphere Supplements, 1966.ESSA/NASA/USAF, 1966. 2.K.S.WChampion, "Properties oftheMesosphere andThermosphere andComparison withCIRA72",inTheTerrestrial UpperAtmosphere, Champion andRoemer, ed.;Vol3,#1 ofAdvances inSpaceResearch, Pergamon, 1983. 3.D.G.King-Hele, Theory ofSatellite OrbitsinanAtmosphere, Butterworths, London, 1964. 159 ir-r, b-'air Vorb "" "5[nCD-dr V2 Width _rFdrag : rel I"/-'5" /-/// I...-_Aerodynamic Drag CL<'2 / Flift_ Fdra_ _ CD=2.2j thermal Vre1 / (Tsurface) Lift &Drag inFree-Molecular Flow (__Dtether; _= 10-Tkg/m_)P "10-4, 10-6. 10-8. Kg p,m-_ | I0-10 10"12 I 1o-14! 04 ! i-- I I I, ! 4 ,I, __1.02Tex "3%_-' SUN-_ .;-%:_"""_-_- "/11l_USStandard Arm --',;.' D-- i , 1%1.12Tex,i,._.,.88Tex ._,,1111111 : _i_"_:-__- _JlllllllllH i _ Predicted Global e_ for ]_t_'-__.eanSolarCycleJ • m _L & i_ J J JL xMain gas species _nmass): N__2,02 N__2,o O,N2 O,He 1oo 2o0300 400500 600 700 800 900 100o He_O Altitude inKm Air DensityasFunction ofAltitude&Exosphere Temperature 70<Alt<118: _11exp(-Alt/6) -#o//_ H6(km) 118<Alt<200:/O == _(Alt-95)'_/2600 . H (Alt-95)/3 1200<Aft. _1.47E-16 Tex(3000-Tex) H_(Alt-200)+Tex/29 lJo>iE-14&" P (I+2.9(Aft-200) ITex)m Circular Orbit Life (-14<Lo_ <-io) Equal-Life Circ. Alt..15m2yr M (I+2.9(r-6578)/Tex) 11 kg CDA 3000 -Tex _-Perigee +Apo-Per 2+.154(Apo-Per)IHpe r 160 5.3.4Thermal Balance KEYPOINTS Aerothermal heating oftethers issevere atlowaltitudes (<120kin).Tether temperature affects strength, toughness, &electrical conductivity. Extreme thermal cycling maydegrade pultruded composite tethers. "Viewfactors" arealsousedinrefined micrometeoroid risk calculations. NOTESPreliminary heattransfer calculations inspaceareoftenfarsimpler thantypical heat transfer calculations ontheground, sincethecomplications introduced byconvection are absent. However theabsence ofthe"clamping" effectoflargeconvective couplings toairor liquidsallows veryhighorlowtemperatures tobereached, andmakes thermal design important. Ataltitudes belowabout140kminLEO,aerodynamic heating isthedominant heatinputon surfaces facingtheramdirection. Theheating scaleswithpaslongasthemeanfreepath_.is muchlargerthantheobject's radius.Itisaboutequaltotheenergy dissipated instopping incident airmolecules. Indenserair,shock&boundary layersdevelop. Theyshieldthe surface fromtheincident flowandmakeQrisesloweraspincreases further. (Seeref1.) Because tethers arenarrow, theycanbeinfreemolecular flowevenat100kin,andmay experience moresevereheating thanthe(larger) lowerendmasses do.Underintense heating hightemperature gradients mayoccuracrossnon-metallic tethers. Thesegradients maycause eitheroverstress orstressreliefonthehotside,depending onthesignoftheaxialthermal expansion coefficient. Athigher altitudes theenvironment ismuchmorebenign, butbaremetal(low-emittance) tethers canstillreachhightemperatures whenresistively heated orinthesun,sincethey radiate heatpoorly. Silica,alumina, ororganic coatings >1gmthickcanincrease emittance andhencereduce temperatures. Thetemperature ofelectrodynamic tethers isimportant sincetheirresistance losses(which maybethemajorsystem losses) scaleroughly withTabs. Foragooddiscussion ofsolar,albedo, andlongwaveradiation, seeref.2.Thesolidgeometry whichdetermines thegainsfromthesesources issimple butsubtle, andshould bedone carefully. Averaged around atether, earthview-factors change onlyslowly withaltitude & attitude, andarenear0.3inLEO. Surface property changes canbeanissueinlong-term applications, duetotheeffects of atomic oxygen, UV&high-energy radiation, vacuum, deposition ofcondensable volatiles fromnearby surfaces, thermal cycling, etc.Hyperthermal atomic oxygen hasreceived attention onlyrecently, andisnowbeingstudied infill,fiber,andcoating degradation experiments ontheSTS&LDEF. Continued thermal cycling overawiderange(suchasshownatbottom right)maydegrade composite tethers byintroducing amazeofmicro-cracks. Also,temperature canaffectthe strength, stiffness, shapememory, andtoughness oftethermaterials, andhencemayaffect tetheroperations andreliability. REFERENCES 1.R.N.Cox&L.F.Crabtree, Elements ofHypersonic Aerodynamics, TheEnglish Universities PressLtd,London, 1965.Seeesp.Ch9,"LowDensity Effects" 2.F.S.Johnson, ed.,Satellite Environment Handbook, Second Edition, Stanford University Press,1965.Seechapters onsolar&earththermal radiation. 3.H.C.Hottel, "Radiant HeatTransmission," Chapter 4ofW.H.McAdams, HEAT TRANSMISSION, 3rdedition, McGraw-Hill, NewYork,1954,pp.55-125. 161 (_=5.68E-8 W/mgK) _ inter, '%Teq,:'A-_--) _4------_aerod: I_ _/Qalbed, N_/_Qear th__.02- .2(metals) _.9 (most non-metals)_N/Qalbedo=.37(±.3) *1368 (±40) W/m2 •gsAFCos(SunZenithAngle) (if>0) =215 (_I00) AEFW/m2 "F"=Shaded fraction ofsphereorits projection onto/_/ __ro_r_o c_ro___,/ ]/VIEW FACTORCALCULATIONS _"1.00____ i ;- 0.50_e=_ o.oo_.__.o,,.up" 0200 400 600 800 .1000 AltitudeinKm Earth Viewfactors inLEO F_ :F_ F_=_ (F_+ F_'+ 2F_)/4 Maxi _/Fractionof Earth Viewfac--------to--r_for Tethers Albedo// __-_time insun _x_=sun--_ "_:9oO g2_o v_ti=i, \\I 5oo.m_t\\1 Inelination _Range K200 =:._,E=.9 _j oo0-23.50 I_-: _ "I 28.5° 0-5_.o 0 90 180 270 360 >66.5 ° 0-90 ° Deg. past "Midnight" (_=Sunout-of-plane angle) Tether Temperature Over IOrbit 162 5.3.5Micrometeoroids andDebris KEYPOINTS Micrometeoroids canseverthintethers &damage tetherprotection/insulation. Orbiting debris cansevertethers ofanydiameter. NOTESAtthestartofthespaceage,estimates ofmeteoroid fluxesvariedwidely. Earthwasthought to haveadustcloudaround it,duetomisinterpretation ofdatasuchasmicrophone noisecaused bythermal cycling inspacecraft. Bythelate1960s mostmeteoroids nearearthwere recognized tobeinheliocentric ratherthangeocentric orbit.Thetime-averaged fluxismostly sporadic, butmeteor showers canbedominant duringtheiroccurrence. Thereisasmalldifference between LEOanddeep-space fluxes, duetothefocusing effectof theearth'sgravity (which increases thevelocity &flux),andthepartial shielding provided by theearth&"sensible" atmosphere. Foratypical meteoroid velocity of20km/sec, these effects combine tomaketheriskvaryasshownatrightinLEO,GEO,andbeyond. The picture ofametalplateafterhypervelocity impact isadapted fromref.3. Theestimated frequency ofsporadic meteoroids overtherangeofinterest formosttether applications isshownbythestraight lineplotatright,whichisadapted fromref.4&basedon ref.1.(Ref1isstillrecommended fordesignpurposes.) Formasses <IE-6gm(<0.15mmdiam. atanassumed density of0.5),thefrequency islowerthananextension ofthatline,since several effects clearverysmallobjects fromheliocentric orbitsingeologically shorttimes. Overanincreasing rangeofaltitudes andparticle sizesinLEO,themainimpact hazard isdue nottonatural meteoroids butrathertoman-made objects. Theplotsatright,adapted from refs4&5,showtheriskspresented bythe5,000orsoobjects tracked byNORAD radars(see ref.6).Asteep"tail"inthe1995distribution ispredicted sinceitislikelythatseveral debris- generating impacts willhaveoccurred inLEObefore 1995.Suchimpacts areexpected to involve a4-40cmobject striking oneofthefewhundred largest objects andgenerating millions ofsmalldebrisfragments. Recent optical detection studies whichhaveasizethreshold ofabout 1cmindicate a population ofabout40,000 objects inLEO.Thismakes itlikelythatdebris-generating collisions havealready occurred. Studies ofresidue insmallsurface pitsontheshuttleandother objects recovered fromLEOindicate thattheyappear tobeduetotitanium, aluminum, and paintfragments (perhaps flakedoffsatellites bymicrometeoroid hits).Recovery oftheLong Duration Exposure Facility (LDEF) laterthisyearshouldimprove thisdatabase greatly, and willprovide dataforLEOexposure area-time products comparable tothoseinpotential long- duration tetherapplications. REFERENCES 1.Meteoroid Environment Model--1969 [NearEarthtoLunarSurface], NASA SP-8013, March 1969. 2.Meteoroid Environment Model--1970 [Interplanetary andPlanetary], NASA SP-8038, October 1970. 3.Meteoroid Damage Assessment, NASA SP-8042, May1970.Shows impact effects. 4.D.J.Kessler, "Sources ofOrbital DebrisandtheProjected Environment forFuture Spacecraft", inJ.ofSpacecraft &Rockets, Vol1864,Jul-Aug 1981. 5.D.J.Kessler, Orbital Debris Environment forSpaceStation, JSC-20001, 1984. 6.CLASSY Satellite Catalog Compilations. Issuedmonthly byNORAD/JSYS, Peterson Air ForceBase,CO80914. 163 Micrometeoroids &Debris • > ¢_10"_ vl !0-l o_ 10°OI00200 400 6OO800IOO0 Altitude inKm Relative _mRisksinLEO [RelRisk _(I-Fearth)(.57+.43re/r)] PopulationCorrected mLimitingSize I I ' ' I I I I Altitude,km Observed Debris Flux (corrected to_-om limiting size)Cumulative Flux in1995 (600-1100 km) 0.5 0 l::: _)0.4 :m 0.3 0 ,-.( 0.2 0 4) •,-i0.1 _S q)Altitude =500kan Inclination: 30°- - 6_..... i% o24•,,o,,,,,,,, km/sec DebrisImpactVelocity_-,.,...v!::-.-_.':-;_ .•. ._:_"_""""n ."O_bitalMotto"' 0i.....03 -uenc"ofSpace "_'0:1''/+._'.'_'.._ '-.Relativee+req Y/"+_-:_'_..+__< _+':_:_-_'C:'"_ _DebrisFluxasaFunction L i--_._._!"_::-"_'_:.._ofDirectionofApproach--_!___ o_0o_,___0o_-_____._."_-... 164 5.4Tether Dynamics andControl 5.4.1Gravity Gradient Effects KEYPOINTS "Microgee" environments arepossible onlyinsmallregions (_5m)ofaLEO Milligee-level gravity iseasytoget&adequate forpropellant settling, etc.facility. NOTESThefigureatfightshowsthereasonforgravity-gradient effects. Thelongtank-like objectis keptaligned withthelocalvertical, sothatthesameendalways facestheearthasitorbits around it.Ifoneclimbsfromthebottom tothetop,theforceofgravity gradually decreases andthecentrifugal forceduetoorbitalmotion increases. Thoseforcescancel outonlyatone altitude, whichis(nearly butnotexactly) thealtitude ofthevehicle's centerofmass. Atotherlocations anobjectwillexperience anetforcevertically awayfromthecenter of mass(oranetacceleration, iftheobjectisallowed tofall).Thisnetforceisreferred toasthe "gravity-gradient force." (Butnotethat1/3ofthenetforceisactually duetoacentrifugal forcegradient!) Exactandapproximate formulas forfinding theforceonanobjectaregiven atright. Theforceoccurswhether ornotatetherispresent, andwhether ornotitisdesirable. Very- low-acceleration environments, whichareneeded forsometypesofmaterials processing and perhaps forassembling massive structures, areonlyavailable overaverylimited vertical extent, asshownatright.Putting avehicle intoaslowretrograde spincanincrease the "height" ofthislow-gee region, butthatthenlimitsthelow-gee region's otherin-plane dimension. Sincegravity gradients inloworbitsaround various bodiesvarywithB/r3,thegradients are independent ofthesizeofthebody,andlinearly dependent onitsdensity. Hencethegradients arehighest (.3-.4milligee/km) around theinnerplanets andEarth's moon, and60-80% lower around theouterplanets. Inhigherorbits,theeffectdecreases rapidly (to1.6microgee/km in GEO). Therelative importance ofsurface tension andgravity determines howliquidsbehave ina tank,andisquantified withtheBondnumber, Bo=par/¢. IfBo>10, liquidswillsettle,buthigher values(Bo=50) areproposed asaconservative designcriterion. Ontheotherhand,combining a smallgravity gradient effect03o<10) withminimal surface-tension fluid-management hardware maybemorepractical thaneitheroptionbyitself.Locating apropellant depotattheendofa power towerstructure mightprovide anadequate gravity-gradient contribution. Ifhigher gravity isdesired, butwithout deploying thedepot,another option istodeploy an"anchor" massonatether,asshownatright. Manynominally "zero-gee" operations suchaselectrophoresis mayactually becompatible withusefullevelsofgravity (i.e.,usefulforpropellant settling, simplifying hygiene activities, keeping objects inplaceatworkstations, etc.).Thisneedstobestudied indetailtoseewhat activities aretrulycompatible. REFERENCES1.D.Arnold, "General Equations ofMotion," Appendix AofInvestigation ofElectrodynamic Stabilization andControl ofLongOrbiting Tethers, Interim Report forSep1979--Feb 1981, Smithsonian Astrophysical Observatory., March1981. 2.K.RKroll,"Tethered Propellant Resupply Technique forSpaceStations," LAF-84-442, presented atthe35thLAFCongress, Lausanne Switzerland, 1984. 165 Fgg O L crN- -L O FggGravity Gradient Effects Feentrifugal =F_=Mn2r _"G ravity-gradient" l.t.t t:tit, $:t:t,I_>F4,,:,:,: t:Cl_ t:t:tl t:Ctl t:t:$1 t:t:tl t_l:tl t:t[tl t:t:tl t:t:t: t:t:tl t.t.t $.t.L t:Ctl t:C,: CCt:t:Ctl ,:CI: t:t:tl t:t:tl t:t:tl t:t:tl t:t:tl t:t:t. t:t:t, t:t:tl t:t:t,(2/3gravity &1/3eentrif.) Fgg=I_-F_= 3LMn 2 L=r-rein__Mr ', rcm _M "Gravity-gradient" Fgravity =F_,=M]a/r2 Origin of"Gravity-Gradient" Forces "_<I0-3gee over5km Magnitude of GravityGradient Effects inLEO 0TV Tethered DepotA Attached & _ Tethered __ "Anchor"(any mass) _% Two Propellant-Settling Options"._---- Electrophoresis _!I?+----- LSSAssembly _;_-:,,-- Microgravity 10.3GCa/cium Retention Walking "Desktop" work Eating, Hygiene luid Settling FullQravity PotentialOverlap ofRegions for Low-Gee &Gee-Dependent Operations 166 5.4.2Dumbbell Libration inCircular Orbit KEYPOINTS Libration periods areindependent oflength, butincrease atlargeamplitude. Out-of-plane libration canbedrivenbyweakforcesthathavea2ncomponent. Tethers cangoslackif 0m_,>65 °or_m_,>60 °. NOTESThetwofigures atrightshowtheforcesonadumbbell incircular orbitwhichhasbeen displaced fromthevertical, andshowthenettorqueonthedumbbell, returning ittowards thevertical. Themaindifference between thetwocasesisthatthecentrifugal forcevectors areradialinthein-plane case,andparallel intheout-of-plane case.Thiscausesthenet forceintheout-of-plane casetohaveasmaller axialcomponent andalargerrestoring component, andiswhy¢-libration hasahigherfrequency than0-1ibration. Fouraspects ofthislibration behavior deserve notice. First,therestoring forcesgrowwith thetetherlength, solibration frequencies areindependent ofthetetherlength. Thustether systems tendtolibrate"solidly", likeadumbbell, ratherthanwiththetethertryingtoswing fasterthantheend-masses ascanbeseeninthechainofachild'sswing.(Thisdoesnothold forverylongtethers, sincethegravity gradient itselfvaries.) Forloworbitsaround anyof theinnerplanets orthemoon,libration periods areroughly anhour. Second, tethered masses wouldbeinfree-fall except forthetether, sothesensed acceleration isalways alongthetether(asshownbythestick-figures). Third,theaxialforce canbecome negative, for_>60°orneartheendsofretrograde in-plane [ibrations >65.9°. Thismaycauseproblems unlessthetetherisreleased, orretrieved atanadequate rateto prevent slackness. Andfourth, although 0-1ibration isnotclosetoresonance withanysignificant driving force, _-libration isinresonance withseveral, suchasout-of-plane components ofaerodynamic forces(innon-equatorial orbitsthatseedifferent airdensity innorthward andsouthward passes) orelectrodynamic forces(iftethercurrents varying attheorbital frequency are used).Thefrequency droopatlargeamplitudes (shown atright)setsafinitelimittothe effects ofweakbutpersistent forces,butthislimitisquitehighinmostcases. Theequations givenatrightareforanessentially one-dimensional structure, withone principal moment ofinertiafarsmaller thantheothertwo:A<<B<C. IfAiscomparable to B&C,thenthe0-restoring forceshrinks with(B-A)/C, andthe0-1ibration frequency by Sqrt((B-A)/C). Another limitation isthatacoupling between {}&0behavior (seeref.1)has beenleftout.Thiscoupling iscausedbythevariation ofend-mass altitudes twiceineachO- libration. Thisinduces Coriolis accelerations thataffect 0.Thiscoupling isoften unimportant, since4nisfarfromresonance with1.73n. Libration isreferenced tothelocalvertical, andwhenadumbbell isinaneccentric orbit, variations intheorbital ratecauselibrations whichinturnexertperiodic torques onan initially uniformly-rotating object. Inhighlyeccentric orbitsthiscansooninduce tumbling. 2 REFERENCES 1.D.Arnold, "General Equations ofMotion," Appendix AofInvestigation of Electrodynamic Stabilization andControl ofLongOrbiting Tethers, Interim Report forSep 1979--Feb 1981,Smithsonian Astrophysical. Observatory., March 1981. 2.P.A.Swan,"Dynamics &Control ofTethers inElliptical Orbits," IAF-84-361, presented atthe35thIAFCongress, Lausanne, Switzerland, October 1984. 167 Dumbbell Libration inCircular Orbit Fnet Faxial__IFcentrifugal Frest°re/ _ 1 _0Fnet I__,_|t Feentrifugal _"_-3n2sin8 cos8 =-I.5n2sin(28) _-_+_ n_sin28max-sin20 (8=±_ nsinSmax when 8=0) no_nV3cosSmax_-4n2sin_ cos_=-2n2sin(2_) _±2n_in2_max-sin2_ (_=_+2n sin_max when _:0) n__2n_8os_max 3.501 )_" ),_I I I R /...&T=3LMn_y [ :/.....% 2.50_. ._"'. _._, F-sin28max-sin28" n1.5_"........... "...OF-.'.. (30-second _- 5°_'_ __."'..".. 4,intervalsl,1oo_-_?_'%25°:?•u._,._.___--..',,_¢ .=.... 0.5_ ":::'.o -_ "_"_ _".__"- ...._._ -05G , IiTether goes,slack 0 1530 45 60 75 90 8,Degfrom Vertical TensionVariations inLibrating Dumbbells (compared totension inhanging dumbbells)2.00 ne....._,_ n.1.0_ _m_ate>65°I/ 0.5__ _ or,>60)/o/ 1.6cycles/orbit _[ for8max_30" 0.00 ii ) 01530 45 60 75 Deg. Amplitude,8or_ LibrationFreq. vsAmplitude 168 5.4.3Tether KEYPOINTSControl Strategies Open-loop control isadequate fordeployment; fullretrieval requires feedback Tension laws cancontrol 0&_-libration plustetheroscillations. Manyotheroptions existforlibration, oscillation, &finalretrieval control. NOTESThetableatrightshowshalfadozendistinct waysinwhichoneormoreaspects oftethered system behavior canbecontrolled. Ingeneral, anything whichcanaffectsystem behavior (andpossibly causecontrol problems) canbepartofthesolution, ifititselfcanbe controlled without introducing otherproblems. Thus,forexample, stifftethers havesometimes beenconsidered undesirable, because the stiffness competes withtheweakgravity-gradient forcesneartheendofretrieval. However, ifthefinalsection oftetherisstiffANDnearlystraight whenstress-free (rather thanpig- tailshaped), then"springy beam"control lawsusingasteerable boomtipmightsupplement orreplace otherlawsneartheendofretrieval. Amovable boomhasmuchthesameeffect asastifftether&steerable boomtip,sinceitallowstheforcevectortobeadjusted. Thebasicconcepts behindtension-control lawsareshown atright.Libration damping is donebypayingouttetherwhenthetension isgreater thanusualandretrieving itatother times.Thisabsorbs energy fromthelibration. Asshownontheprevious page,in-plane libration causeslargevariations intension (duetotheCoriolis effect), so"yoyo" maneuvers candampin-plane librations quickly. Suchyoyomaneuvers canbesuperimposed on deployment andretrieval, toallowlargelengthchanges (>4:1)pluslargein-plane libration damping (orinitiation) inlessthanoneorbit,asproposed bySwet.l Retrieval lawsdeveloped fortheTSSrequire moretimethanRef.1,because theyalso include damping ofout-of-plane libration builtupduringstation keeping. Ruppdeveloped thefirstTSScontrol lawin1975;2muchoftheworksincethenisreviewed in(3).Recent TSScontrol concepts combine tension andthrustcontrol laws,withpuretension control serving asabackup incaseofthruster failure.4Axialthrusters raisetethertension whenthe tether isshort,whileotherscontrol yaw&dampout-of-plane libration toallowfaster retrieval. Anovelconcept whichinessence eliminates thefinallow-tension phaseofretrieval isto havetheendmassclimbupthetether.5Sincethetether itselfremains deployed, its contribution togravity-gradient forcesandstabilization remains. Thepracticality ofthis willvarywiththeapplication. REFERENCES1.C.J.Swet,"Method forDeploying andStabilizing Orbiting Structures",U.S. Patent #3,532,298, October 6,1970. 2.C.C.Rupp,ATetherTension Control LawforTetherSubsatellites Deployed Along LocalVertical, NASA TMX-64963, MSFC, September 1,1975. 3.V.J.Modi,GengChang-Fu, A.RMisra,andDaMingXu,"OntheControl oftheSpace Shuttle BasedTethered Systems," ActaAstronautica, Vol.9,No.6-7,pp.437-443, 1982. 4.A.K.Banerjee andT.R.Kane,"Tethered Satellite Retrieval withThruster Augmented Control," AIAA82-1-21, presented attheAIAA/AAS Astrodynamics Conference, San Diego, Calif.,1982. 5.T.R.Kane,"ANewMethod fortheRetrieval oftheShuttle-Based Tethered Satellite," J. oftheAstronaut. Sci.,Vol32,No.3,July-Sept. 1984. 169 Tether Control Strategies EFFECTIVENESS OFVARIOUSCONTROL CONCEPTS _APPLICATION CONTROL OUTPU_ TensionLibration in-planeOut-of-plane Longitudinal Transverse Pitch &Roll Strong Weak Strong Strong Strong (Note: tension control isweakwhentetherisshort) El.Thrust Only ifMI#M2 Thruster Strong, butcostly ifprolonged None ifprolonged MovablemassTether Oscillations Endmass Attitude Osc. Yaw None Only odd Noneharmonics NoneNone Strong, butcostly Stiff tether, Movable boomGood w/shorttether [IPossible butawkward None I,None Strong iftether isvery short; weak otherwise Aerodynamic High drag--use only iflowaltitude needed for other reasons, • '- ,i l//FIll/k" ) Swing Deploying &retrieving tether atdifferent tensions absorbs energy anddamps libration.Orbital motion T s'&b_ %." StretchF2/F2P C o c ¢ C Damping Tension =kl(L-Lc) +k2L (kl&k2arecontrol gains; LlLcaretheactualand thecommandedtether length.)Deployment aths oftip -Full retrievaltakes _6hours with thrusters &_24 without. deployed in4.6hours TENSION CONTROL FORLIBRATION DAMPING... ANDDEPLOYMENT/RETRIEVAL 170 5.4.4Momentum Transfer Without Release KEYPOINTS Tethers merely redistribute angular momentum; theydonotcreateit.Changes intether length, libration, andspinallredistribute momentum. Momentum transfer out-of-plane or indeepspaceispossible butawkward. NOTESThetwofigures atfightshowtwodifferent tetherdeployment (andretrieval) techniques. In bothcases,theinitialdeployment (which isnotshown) isdonewithRCSbumsoralong boom. Inthecaseatleft,thetether ispaidoutundertension slightly lessthanthe equilibrium tension levelforthattetherlength. Thetetherisslightly tiltedawayfromthe vertical duringdeployment, andlibrates slightly afterdeployment iscomplete. Intheothercase,aftertheinitialnear-vertical separation (toabout2%ofthefulltether length), thetwoendmasses areallowed todriftapartinnear-free-fall, withverylowbut controlled tension onthetether.Justunderoneorbitlater,thetetherisalmost alldeployed andtherangeratedecreases toaminimum (duetoorbitalmechanics). RCSbumsortether braking areusedtocushion theendofdeployment andprevent endmassrecoil.Thenthe tethersystem beginsalarge-amplitude prograde swingtowards thevertical. Inbothcases,theangular momentum transferred fromonemasstotheotherissimply, as statedinthebox,theintegral overtimeoftheradiustimesthehorizontal component of tethertension. Inonecase,transfer occursmainly during deployment; intheother,mainly during thelibration afterdeployment. Ineachcase,momentum transfer isgreatest whenthe tetherisvertical, sincethehorizontal component oftethertension changes signthen. Anintermediate strategy---deployment undermoderate tension--has alsobeeninvestigated. However, thistechnique results inveryhighdeployment velocities andlargerotating masses. Italsorequires powerful brakesandamoremassive tetherthanrequired withthe othertwotechniques. Asdiscussed underTether Control Strategies, changing atether's lengthinresonance with variations intethertension allowspumping ordamping oflibration orevenspin.Dueto Coriolis forces, in-plane libration andspincausefarlargertension variations thanout-of- planelibration orspin,soin-plane behavior isfareasiertoadjustthanout-of-plane behavior. Neglecting anyparasitic lossesintetherhysteresis &thereelmotor, thenet energyneeded toinduceagivenlibration orspinissimply thesystem's spinkinetic energy relative tothelocalvertical, whenthesystem passesthrough thevertical. Twomomentum transfer techniques whichappear applicable forin-plane, out-of-plane, or deep-space useareshownatright.Thewinching operation canuselighter tethers than othertethered-momentum-transfer techniques, butrequires averypowerful deployer motor. Thetangential AVsimply prevents acollision. Thespin-up operation (proposed byHarrisMayer) issimilar tothewinching operation. It usesalargertangential AV,atetherwithstraight andtapered sections, andasmallmotor. Retrieval speedsupthespinbyafactorof1/L2.Surprisingly, thelongtapered section of tethercanbelessthanhalfasmassive astheshortstraight section thatremains deployed afterspin-up. REFERENCES 1.J.Tschirgi, "Tether-Deployed SSUS-A, Report onNASA Contract NAS8-32842, McDonnell Douglas, April1984. 171 Momentum Transfer Momentum Transfer During Deployment&RetrievalMomentum Transfer During Libration (after low-tension deployment) ,I,_ Straight Tether Small_I Ii I_ __I_ISecti°n Tapered_Vs Small_'_'_ _r-Tether aVs _IJ /Section I2 '" Deployment Followed byWinching (inorbit orindeep space) OneSpin-Up Technique ForUse inDeep Space 172 5.4.5OrbitTransfer byRelease orCapture KEYPOINTS Theachievable orbitchange scaleswiththetetherlength(aslongasAt'<<r).Retrograde- libration releases areinefficient, butallowconcentric orbits.Apogee &perigee boostshave different valuesindifferent applications. Tethered capture canbeseenasatime-reversal of atetherrelease operation. NOTESThefigures totherightshowthesizeoftheorbitchanges caused byvarious tether operations. Whenreleased fromavertical tether, theendmasses areobviously onetether lengthapartinaltitude. Thealtitude difference 1/2orbitlater,Ar,,varieswiththeoperation butisusually farlarger. Thelinearrelationship shown becomes inaccurate whenAr approaches r.Tethered planechanges aregenerally limited toafewdegrees andarenot covered here. Tether release leavesthecenter-of-mass radiusateachphaseangleroughly unchanged: if theuppermassisheavier, thenitwillriselessthanthelowermassfalls,andvice-versa. Notethatthelibration amplitude, 0=_._,istakenaspositive duringprograde libration and negative during retrograde libration. Hence retrograde libration results inAr<7L.In particular, thepre-release &post-release orbitswillallbeconcentric if0ma_=-60°.But sincemethods ofcausing -60°librations usually involve +60°librations (which allowmuch largerboosts bythesametether), prograde releases mayusually bepreferable unless concentric orbitsareneeded orotherconstraints enterin. Therelative tetherlength, mass,peaktension, andenergy absorbed bythedeployer brake duringdeployment asafunction of(prograde) libration angleareallshownintheplotat right.Libration hasalargeeffectonbrakeenergy. Thismaybeimportant whenretrieval of alongtetherisrequired, afterrelease ofapayload oraftertethered-capture ofafree-flying payload. Thedouble boost-to-escape operation atrightwasproposed byA.Cutler. Itisshownsimply asanexample thateventhough momentum U-ansfer isstrictly a"zerosumgame", a tethered release operation canbea"WIN-win game"(alargewin&asmallone).Thesmall winonthedeboost-end ofthetetherisduetothereduced gravity losses1/2orbitafter release, whichmorethancompensate forthedeboost itself.Another example isthat deboosting theshuttlefi'omaspacestation canreduce bothSTS-deboost &station-reboost requirements. Rendezvous ofaspacecraft withtheendofatethermayappear ambitious, butwithprecise relative-navigation datafromGPS(theGlobal Positioning System) itmaynotbedifficulty Therelative trajectories required aresimply atime-reversal ofrelative trajectories that occuraftertetherrelease. Approach toahanging-tether rendezvous isshown atright. Prompt capture isneeded withthistechnique: ifcapture isnotachieved within afew minutes, oneshouldshifttonormal free-fall techniques. Tethered capture haslargebenefits insafety(remoteness) andoperations (noplumeimpingement; largefuelsavings). The mainhazardiscollision, duetoundetected navigation ortetherfailure. REFERENCES1.G.Colombo, "Orbital Transfer &Release ofTethered Payloads," SAOreportonNASA Contract NAS8-33691, March 1983. 2.W.D.Kelly,"Delivery andDisposal ofaSpaceShuttle External TanktoLowEarth Orbit," J.oftheAstronaut. Sci.,Vol.32,No.3,July-Sept 1984. 3.J.A.Carroll, "Tether-Mediated Rendezvous," report toMartin Marietta onTask3of contract R.H3-393855, March 1984. 4.J.A.Carroll, "Tether Applications inSpaceTransportation, IAF84-438, atthe35thIAF Congress, Oct1984.Tobepublished inACTA ASTRONAUTICA. 173 OrbitTransfer byTethered Release orCapture _--l:J L _ Aro= L M,rz+Mzr_=Mr;r+; (Hl>>M2)_Z_r_=13L (+60) Effect ofLibration onBoost Effects ofTether Deployment andRelease (release atmiddle ofswing) ocRelTension/Length c_Length,MaxTension c_I/(I+.866 Sinemax)1.00_ _(Length *Cosemax) 2 6@ 9_ Amplitude Effects of.tibration (for equal-Aa boosts) Phasingorbit(s) _-12_L 6LL{2)I tTrajectory forTethered Capture from Above (intether-centered LV-LH reference frame)%\ ,<--._.._ Ifdone right, atether boost/deboost operation canreduce AV-to-escape forbot__.hh endmasses! /J t 9_ STS hovers till captured-or-+ ¢I OMV "chases" passivetarget 174 5.4.6Energy KEYPOINTSandAngular Momentum Balance Tether operations causehigher-order repartitions ofenergy &angular momentum. First- orderapproximations thatneglect theseeffects maycauselargeerrors. Extremely long systems havestrange properties suchaspositive orbitalenergy. NOTESThequestion andanswer atfightaredeceptively simple. Theextenttowhichthisisso,and thebizarre effects whichoccurinextreme cases,canbeseeninthe3graphs atright.At top,deploying &retrieving twomasses onaverylongmassless tetherchanges notonlythe top&bottom orbitalradiibutalsothatoftheCM.Inaddition, thefree-fall location drops belowtheCM.Otherkeyparameter changes underthesameconditions areplotted underneath. Notethatwhenthetetherlengthexceeds about30%oftheoriginal orbitalradius, theentire system liesbelowtheoriginal altitude. Also,ataradiusrationear1.95:1, themaximum tetherlengthcompatible withacircular orbitisreached. Atgreater lengths (andtheinitial amount ofangular momentum), nocircular orbitispossible atanyaltitude. Tether retrieval atthemaximum-length pointcancausethesystem toeitherriseordrop, depending onthesystem stateatthattime.Ifitcontinues todrop,thereisarapidrisein tethertension, andthetotalworkdonebythedeployer quickly becomes positive. This energy inputeventually becomes largeenough (at2.89:1) toevenmakethetotalsystem energy positive. Thesystem isunstable beyond thispoint:anysmalldisturbance willgrow andcancausethetethersystem toescapefromthebodyitwasorbiting. (Seeref.2.) Thecaseshownisratherextreme: except fororbitsaround smallbodiessuchasasteroids, tethers eitherwillbefarshorter thantheorbital radius, orwillgreatly outweigh theend masses. Eitherchange greatly reduces thesizeoftheeffectsshown. Theeffectsonarbitrary structures canbecalculated usingtheequations listedatright,which arebasedona generalization oftheconcept of"moments" ofthevertical massdistribution. Changes in tetherlengthormassdistribution leavehunchanged, sootherparameters (including rm,n, andE)mustchange. (Forshorttethers, thechanges scaleroughly withthesquareofthe system's radiusofgyration.) Inmanycasesdifferent conditions aremosteasilycompared by firstfinding theorbitalradiusthatthesystem wouldhaveifitslengthwerereduced to0,rLt =0. Themechanism thatrepartitions energy andangular momentum isthatlength changes causetemporary system displacements fromthevertical. Thiscausesbothtorques andnet tangential forcesonthesystem, whichcanbeseenbycalculating theexactnetforcesand couples foranon-vertical dumbbell. Thesameeffectoccurs onaperiodic basiswith librating dumbbells, causing theorbitaltrajectory todepartslightly fromanelliptical shape. Othertopicswhicharebeyond thescopeofthisguidebook butwhoseexistence shouldbe notedare:eccentricity changes duetodeployment, orbitchanges duetoresonant spin/orbit coupling, andeffectsof2-&3-dimensional structures. REFERENCES 1.G.Colombo, M.Grossi, D.Arnold, &M.Martinez-Sanchez, "Orbital Transfer and Release ofTethered Payloads," continuation ofNAS8-33691, finalreport fortheperiod Sep1979--Feb 1983,Smithsonian Astrophysical Observatory, March 1983.(Inparticular, seethetableonpage21.) 2.D.Arnold, "Study ofanOrbiting Tethered Dumbbell System Having Positive Orbital Energy," addendum tofinalreportonNAS8-35497, SAO,Feb1985. 175 Question: Answer:Energy &Momentum Balance What arethesources ofthedumbbellspin angular momentumand deployer brakeenergy? Orbit changes which repartition h&E. Forarbitrary nearly-one-dimensional vertical structuresincircular orbit, analysiscanbebased on5"moments": IN=ZMiriN (for N:-2..2) Each ofthese hasphysical meaning: Fgrav=pI-2 Epot=-pI_I Mass=I0 Feen_'n211 htot=n12 Ekin=.Sn2124-P*@Mi r_ paMi Some other useful equations include: rcm =I1/I0 n2=pI-2/I I E=p_.5I_2"12/I1)- I_i) _Lt= _=1_2(12)2/(11"(I0 )2)VERY-LONG-TETHER EFFECTS: [email protected] Ra0.60- u0.40 s 0.20. Fr. ,,-, ., location __•(F{=F#] 0.0(_ 1.0 2.0 3.0 4.0 Radius Ratio (r2/rI) Equal-Angular-Momentum Orbits (hto_l =1) 1.00_o.I 0.60_ MaxlenEth _o? h°rbit !_o_[_;V,T _.T J °.4°i:_°_ i / .0 2.0 _.0 4.0 Radius Ratio (r2/rI) Angular Momentum Repartitioning, Tether Length, &Deployer Work 2 0 -2 -4 -6 -8 -108 _: Unstable 6-..,., _@..: orbit (E>O)"-%6_e.. ,,_"__ t I. I I I 1.0 2.03.0 4.0 Radius Ratio (r2/r I) Changes inEnergy,Tension,&Period 176 5.5Tether Material Consideration 5.5.1Tether Strength andMass KEYPOINTS Tether strength/weight ratioconstrains performance inambitious tethermassiseasilyderivable fromdeltaVandpayload mass.operations. Required NOTESUsable specific strength canbeexpressed invarious ways.Threewaysareshown atfight.V_, Lc,andLlsareheredeemed intermsofatypical designstress(new/m') rather thanthe (higher) ultimate stress. Including thesafety factor herestreamlines thesubsequent performance calculationsv Higher safetyfactors areneeded withnon-metals thanwith metals sincenon-metals areoftenmorevariable intheirproperties, brittle, abrasion- sensitive, and/orcreep-sensitive. Asafetyfactorof4(basedonshort-term fiberstrength) is typical forKevlar, butthemostappropriate safetyfactorwillvarywiththeapplicatiol; The"characteristic velocity," V_,isthemostusefulparameter intetherboost calculations, because thetethermasscanbecalculated directly fromAV/Vo independently oftheorbit, andnearly independently oftheoperation. Thetableatthebottom, which lists tether/rocket combinations thathavethelowest lifesycle massrequirements, holds whenever kv¢=lkin/see &Isp=350 see. Thecharacteristic length Lcisusefulinhanging-tether calculations. Itvaries withthe orbital raten.(Thesimple calculation givenassumes L<<r,ifthisisnottrue,l/reffects enterin,andcalculations suchasthoseusedinrefs3-5mustbeused.)Thesafe1-geelength Llgismainly usefulinterrestrial applications, butisincluded sincespecific strength isoften quoted thisway.(NotethatVcandLcvarywithSqrt(strength), andLlgdirectly with strength). Thespecific modulus isofinterest because itdetermines thespeedofsoundinthetether (C--the speedoflongitudinal waves), thestrainunderdesignload(AL/L={V_2/C)2), &the recoilspeedafterfailureunderdesignload(=V_2/C). Tether masscalculations arebestdonebyconsidering eachendofthetetherseparately. If Mpl>>Mp2, thenMtxcanbeneglected inpreliminary calculations. DuPonesKevlar isthehighest-specifiestrength fibercommercially available. Current RND efforts onhigh-pe_ormance polymers indicate thatpolyester canexhibit nearly twicethe strength ofKevlar. Twofiberproducers havealready announced planstoproduce polymers withtwicethespecific strength ofKevlar. Inthelongrun,thepotential maybegreater withinorganic fiberslikeSiC&graphite. Refs. 3-5focusontherequirements of"spaceelevators." Theydiscuss laboratory testsofsingle- crystal fibersandsuggest that10-fold improvements inspecific strength (or3-foldinV¢& Lc)areconceivable. REFERENCES 1."Characteristics andUsesofKevlar 49Aramid HighModulus Organic Fiber"available fromDuPonesTextile FibersDepartment, 1978. 2.G.Graft,"Superstrong Plastics Challenge Metals," HighTechnology magazine, February 1985,pp.62-63. 3J.Isaacs, H.Bradner, G.Baekus, andA.Vine, "Satellite Elongation intoaTrue "Skyhook"; alettertoScience, Vol.151,pp.682-683, Feb11,1966. 4.J.Pearson, "TheOrbital Tower: aSpacecraft Launcher UsingtheEarth's Rotational Energy," ActaAstronautica, Vol.2,pp.785-799, Pergamon, 1975. 5.H.Moravec, "ANon-Synchronous Orbital Skyhook," J.oftheAstronautical Sciences, Vol.YXV,No.4,pp.307-322, Oct-Dec 1977. 177 Specific Strength andRequired Tether Mass C=20km/s C=speed ofsound =L/modulhs ¥density10km/s STETHER STRENGTH PARAMETERS t,/design stress' 5km/s ¥ d-_si--_---=Char.vel Vc: Vc/_n=Char. length Lc: VC_/E=Safe Iglength LIE:SPECIFIC STRENGTH---) I I.... Graphite, /491 _Advanced. Kevlar polymers _29@@=commercial @=potential T SPECIFIC MODULUS Steel Ti _J . •25 .501.0 2.0km/s (Anywhere) 125 2505001000 km (LEO) 6 25 100 400km(Ground) ,Design stress isassumed tobeI/2theultimate strength formetals IandI/4theshort-term individual fiber strength forother materials. I SPECIFIC STRENGTH&MODULUS OFSEVERAL TETHERMATERIALS Gaussian"normal" IT] bell-shaped curve -if 1.00L<<r) 0.8(>(ifLc L_ iI _ Mt0.6@_.M.tf Mt+Mp L1<Lc 0.2_ L2>Lc Mtl<(Mpl O.OC L<LeL=Lc L>Lc_ Mr2>>Mp2 Ht<¢MpMt=MpHt>>Mp" -..<_.........-.._._- >t-/._.>;/:>_/ Tether Length &Required Mass -q.... / %1/\ ii} \,,I I....I ,-/-_apered aM::t_/ forX>I /l,N_x2forX<I IIco 18 4 •5Mt •67Mp .25 .00 0.00 0.501.001.50 2.00 X(=AV/kVc, orL/Lc) " Required Tether Mass (Mt) I '1.00 for spinning operationsl*k--_1"15forhanging -_1.21±.01 forswinging , U•41forwinching AV=Sumofperigee +apogee boosts Expected #ofusesI101001000 !Best tether AV,km/s .14 .9 1.8 2.6 !Required"Mt/Mpl "02 1 11 95 (For kVc=Ikm/s androcket Isp=350seconds; marginal deployer &dryrocket masses neglected.) Best Tether _VforCombined Tether/Rocket Boosts 178 5.5.2Tether Impact Hazards KEYPOINTS Micrometeoroids canseverthintethers &damage tetherprotection/insulation. Orbiting debris(orothertethers) cansevertethers ofanydiameter. Debriscouldimpact anEarth- based"Space Elevator" overonceperyear. NOTESSporadic micrometeoroids areusually assumed tohaveantypical density ofabout.5anda typical impact velocity inLEOofapproximately 20kin/see) Atimpact speedsabovethe speedofsound, solidsbecome compressible andtheimpact shockwavehaseffects like thoseofanexplosion. Forthisreason, theriskcurveassumes thatiftheEDGE ofan adequately largemeteoroid comescloseenough tothecenterofthetether(within 45°or .35Dr),failurewillresult. Experiments donebyMartin Marietta onTSScandidate materials haveusedglassprojectiles firedat6.5kin/see, belowthe(axial) speedofsoundinKevlar. Twodamaged tethers from thosetestsareshown atright.Thescaling lawused(p°_V°67) indicates thatthisis representative oforbitalconditions, butthatlaw(usedforimpacts onsheetmetal) maynot applytobraided fibers. Fortethers muchthicker than10mmorso(depending onaltitude), theriskdoesnotgo downmuchasDtincreases, because eventhough themicrometeoroid riskstilldecreases, the debris risk(which INCREASES slightly withDr)begins todominate. Aswith micrometeoroids, thetetherisassumed tofailifanypartofthedebrispasseswithin0.35D, ofthecenterofthetether. Thedebrisriskatagivenaltitude varieswiththetotaldebriswidthatthataltitude. Thiswas estimated from1983CLASSY radarcross-section (RCS)data,bysimply assuming thatW= Sqrt(-RCS) andsumming Sqrt(RCS) overalltracked objects inLEO.6Thisunderestimates W forobjects withappendages, andover-estimates itfornon-librating elongated objects without appendages. CLASSY RCSdataareexpected tobeaccurate forRCS>7m2.The700objects withRCS> 7meaccount for3kmofthetotal5kmwidth,soerrorswithsmaller objects arenot critical. Smalluntracked objects maynotaddgreatly tothetotalrisk:40,000 objects averaging 2cmwidewouldincrease therisktoal-cmtetherbyonly20%.Wwasassumed independent ofaltitude, sothedistribution ofriskwithaltitude couldbeestimated bysimply sealing Figure1fromRef.4. Asshownatright,debrisimpact withaspaceelevator couldbeexpected morethanonce peryearatcurrent debrispopulations. Therelative density at0°latitude wasestimated from dataonpp.162-163 ofref.6. Similar calculations canbemadefortwotethers indifferent orbitsatthesamealtitude. Ifat leastoneisspinning orwidely-librating, themutual riskscanexceed 0.1cut/kinyr. This makes "tether trafficcontrol" essential. REFERENCES 1.Meteoroid Environment Model---1969 [NearEarthtoLunarSurface], NASA SP-8013, March 1969. 2.Meteoroid Environment Model--1970 [Interplanetary andPlanetary], NASA SP-8038, October 1970. 3.Meteoroid Damage Assessment, NASA SP-8042, May1970.(Shows impacteffects) 4.D.J.Kessler, "Sources ofOrbital DebrisandtheProjected Environment forFuture Spacecraft", inJ.ofSpacecraft &Rockets, Vol18#4,Jul-Aug 1981. 5.D.J.Kessler, Orbital DebrisEnvironment forSpaceStation, JSC-20001, 1984. 6.CLASSY Satellite Catalog Compilations asof1Jan1983,NORAD/J5YS, 1983. 179 Impact Hazards forTethers Braided Stainless Kevlar, steel wire, grazed direct hitCuts Km°Yr in1 LEOall) 0 I o ,20mm .25mm.30mm .35 Maxnon-fatal)am diameterDm _Meteoroid RiskstoaImmTether Fortethers withDt>1(mm), &Maxnon-fatalDm=.25Dt, _mcuts _Dr-2"6Km.Yr Effective Width, W ....(Any position between _the2extremes shown hetether.) J! ,7_ _2 I | I ! ! [ ! ' 1! IAssumptions: _W_5km i _3i ___ W>>Dt i 10[/-- IA_independent ofal_] 41e_ %__rel- 10km/sec .| ,o-,j ""--, n1 KmYr.::_: IR:/.skvarieswithib_laLa_l_'J 0 I000 2000 3000 4000 Altitude, km Debris Impact Rate onTethers inLEO/ mm@ Debris Risk totheLowest 4000 km ofanEarth-based SpaceElevator: Risk =_Width *_*RelDensityat_=0 Earth"Surface Area"at ~5km*_7.3 kmlsec *N.72 =4*pi*Sqr(_7378 km) =3.9E-8/sec =li.2 cuts/year I 180 5.6Electrodynamic Tethers 5.6.1Interactions withEarth's Magnetic FieldandPlasma KEY"POINTS Tether (&other) resistance canlimittheoutput ofelectrodynamic tethers. collection methods &effectiveness areimportant_and uncertain.Electron NOTESSincethepublication ofref.1,20yearsago,electrodynamic tetherproposals andconcepts havebeenafrequent sourceofcontroversy, mainly intheseareas: 1.Whatplasma instabilities canbeexcited bythecurrent? 2.Whatisthecurrent capacity oftheplasma returnloop? 3.Whatisthebestwaytocollect electrons fromtheplasma? ThefirstTethered Satellite mission maydomuchtoanswer thesequestions. Thediscussion belowandgraphics atrightmerely seektointroduce them. Thecurrent flowing through anelectrodynamic tetherisreturned inthesurrounding plasma. Thisinvolves electron emission, conduction alonggeomagnetic fieldlinesdowntothe lowerionosphere, cross-field conduction bycollision withneutral atoms, andreturnalong otherfieldlines. Thetethercurrent causes aforceonthetether(andonthefield)perpendicular toboththe fieldandthetether(horizontal, ifthetetherisvertical). Motion ofthetetherthrough the geomagnetic fieldcausesanEMFinthetether.Thisallowsthetethertoactasagenerator, motor, orself-powered ultra-low-frequency broadcast antenna? Themotion alsocauses eachregionofplasma toexperience onlyashortpulseofcurrent, muchasinacommutated motor. Basedonexperience withcharge neutralization ofspacecraft inhighorbit,ithasbeen proposed thatelectrons becollected byemitting aneutral plasma fromtheendofthe tether,toallow 3 localcross-field conduction. InGEO,thegeomagnetic fieldtrapsaplasma inthevicinity ofthespacecraft, and"escape" alongfieldlinesmaynotaffectitsutility. Thismayalsoholdinhigh-inclination orbitsinLEO.Butinlowinclinations inLEO,any emitted plasma mightbepromptly wipedawaybytherapidmotion acrossfieldlines. Apassive collector suchasaballoon hashighaerodynamic drag,butaend-on sailcanhave anorderofmagnitude lessdrag.Theelectron-collection sketchatbottom rightisbasedona preliminary analysis byW.Thompson. 5Thisanalysis suggests thatacurrent moderately higherthantheelectron thermal current (=Ne*-200km/sec) mightbecollected ona surface normal tothefield.Thisisbecause collecting electrons requires thatmostionsbe reflected awayfromthecollection regionasitmoves forward. Thispre-heats anddensities theplasma aheadofthecollector. Thevoltage required forcollection isjustthevoltage needed torepelmostoftheions,about12V. REFERENCES1.S.D.Drell,H.M.Foley,&M.A.Ruderman, "DragandPropulsion ofLargeSatellites in theIonosphere: AnAlfven Propulsion Engine inSpace," J.OfGeophys. Res.,Vol.70,No. 13,pp.3131-3145, July1965. 2.M.Grossi, "AULFDipole Antenna onaSpaceborne Platform ofthePPEPL Class," Report onNASA Contract NAS8-28203, May1973. 3.ILD.Moore, "TheGeomagnetic Thruster--A HighPerformance "Alfven Wave" Propulsion System Utilizing Plasma Contacts," ALGA PaperNo.66-257. 4.S.T.Wu,ed.,University ofAlabama atHuntsville/NASA Workshop onTheUsesofa Tethered Satellite System, Summary Papers, Huntsville AL,1978.SeepapersbyM.Grossi etal,R.Williamson etal.,andN.Stone. 5.W.Thompson, "Electrodynamic Properties ofaConducting Tether," FinalReport to Martin Marietta Corp.onTask4ofContract RH3-393855, Dec.1983. 181 <Electrodynamic TetherPrinciples PLASMA CONTACTOR OESELERA-- _.. / I/ A88_o[E_ATIN_ . ORBffAL Nf "-. 5COSi_,--'_ )Electron emitter _ Ioo0 500Collisional _cross-field conduction inlower ionosphere. o 8•,' :I\\- !l_\ -Sunspot %\\\ maximum- '\"%\ -a"; " --d ytxme, \\\ ....atni_h_,.\\\ •,\"\ \ -Sunspot ",._k"k \ -_nimuun: -.\kk_ \ ----daytime ".,\_\ ----- atnight .._l--& ) O,I I f I 9 lO II 12 Logao Nelm?_Arctan_ ___ _Efficiency :I.\,, Erag_ _ _ _....... _e _ -",'_\I® z-__'\I /Useful% i o/outpu_ 0 I_Max _ Electron Generator _--_/ collecting Performance ."sail"(+12V) ® Lowdensity _plasma region I°Geomagneticfield_ TopView ofElectronCollection 182 5.6.2Electrodynamic OrbitChanges KEYPOINTS Electrodynamic tetherusewillaffecttheorbitmwhether desired ornot.Station keeping and/orlargeorbitchanges without propellant usearepossible. NOTESTheoffsetdipole approximation shown atrightisonlyafirstapproximation tothe geomagnetic field:harmonic analyses ofthefieldgivehigher-order coefficients upto20% aslargeasthefundamental term.Ref.1contains computerized models suitable forusein detailed electrodynamic studies. Thegeomagnetic fieldweakens rapidly asonemoves intohigher orbits, andbecomes seriously distorted bysolarwindpressure beyond GEO.However, ohmic lossesinatether arealready significant inLEO,soelectrodynamic tethers aremainly usefulinloworbits wheresuchdistortions arenotsignificant. Astheearthrotates, thegeomagnetic fieldgenerated within itrotates also,andthe geomagnetic radiusandlatitude ofapointininertial spacevaryovertheday.Ifa maneuvering strategy whichrepeats itselfeachorbitisused(necessary unlessthespacecraft haslargediurnal powerstorage capacity), thentheaverage effect,asshownatright,willbe adueeastthrustvector. Variations ingeomagnetic latitude (andthusinBh)canceloutvariations inthecomponent offlightmotion perpendicular tothefield,sothesevariations donotcauselargevoltage variations inhigh-inclination orbits.(Notethattherelevant motion ismotion relative toa rotating earth.) Out-of-plane libration, variations ingeomagnetic radius, anddiurnal variation ofthe"geomagnetic inclination" ofanorbitcanallcausevoltage variations. Peak EM:Fs(which drivehardware design) mayapproach 400V/kin. However thesevariations neednotaffectthethrustmuchifaspacecraft hasavariable- voltage powersupply: neglecting variations inparasitic power, constant powerinvestment inacircular orbithastogiveconstant in-plane thrust. Theout-of-plane thrustisprovided "free"(whether desired ornot).Average voltage &thrustequations forvertical tethers are shownatright. Thetableshowshowtochange allsixorbital elements separately ortogether. Other strategies arealsopossible. Thefteffects canbecalculated fromtheintegrals listed.For orbitswithin11°ofpolarorequatorial, diurnally-varying strategies become moredesirable. Computing theireffects requires usingthevarying geomagnetic inclination instead ofi(& moving itinsidetheintegral). Notethatthe"DC"orbit-boosting strategy alsoaffects i. Thiscanbecanceled outbysuperimposing a-2Cos(2O) current ontheDCcurrent. Asdiscussed underElectrodynamic Libration Control Issues,eccentricity andapsidechanges canstrongly stimulate 0-1ibration unlessthespacecraft centerofmassisnearthecenterof thetether. Othermaneuvers should notdothis,butthisshould bechecked usinghigh- fidelity geomagnetic fieldmodels. REFERENCES1.E.G.Stassinopoulos &G.D.Mead,ALLMAG, GDALMG, LINMA:Computer Programs forGeomagnetic Field&Field-Line Calculations, Feb.1972,NASA Goddard. 2.R.D.Moore, "TheGeomagnetic Thruster--A HighPerformance "Alfven Wave" Propulsion System Utilizing Plasma Contacts," AIAAPaperNo.66-257. 3.H.Alfven, "Spacecraft Propulsion; NewMethods," Science, Vol.176,14Apr1972,pp.167-168. 183 Electrodynamic OrbitChanges _, • [Tilt _11°[EARTH'S I"SPIN AXIS GEOMAGNETIC NORTH POLE ¢-_= ']/\_.AVERAGE _THRUSTVECTORIS EASTWARD --re3 OFFSET DIPOLE APPROXIMATION TOGEOMAGNETIC FIED EFFECTOFEARTH'S SPIN ONTIME-AVERAGED FIELD Orbital traok_ -_ _/" _ n3am@_new" _o29cos#s_izLt(_) i -' __"_"__ R,_:029 OOSiILt(r_-_-)3n---_-__ _ "" HOWTOCHANGEORBITSUSINGANELECTRODYNAMIC TETHER Element SemlmaJor axis Phase Eeeentrlelty Cos(e) Lineof.apsfdes Sin(O) Inclination -Cos(2.tZ) Ascending nodeStrategy ThrustVector Sawtooth-v,,¢._--_.-- _..._ I--.I,,,.Ir'! Ljv,,,.j ! 0.•i-: -sln(=_l O=POSITIONOFVOOCLEWITH REFERENCE TOffSPERIGEE ,._=POSITIONWItHREFERENCE TOASCENOWG NODE k=,,-4TOHI_SPERAMPERE .AY_(r.lr),.r0o...... oEffect ,=eoz(l)--_-fl dt 1.5kln_eos(I)_ ._tdt ae.veos(J) k.__lflcos(,)dtma Aw_eos(I) kl_sin(D)dtmite al,__._sin(t)ens:_(_ dt2ran At')__2ma_s|n(/'D')eos(_}dt I=TETHERLENGTH m=TOTALVEHICLEM_S n=0r_rAL_ULARRATE 184 5.6.3Tether Shape andLibration Control KEYPOINTS Properly controlled ACcomponents canbeusedtocontrol 0and¢-libration. Solar-energy storage andeorcochanges strongly stimulate ¢-libration. ACcurrents otherthan1& 3/orbit shouldnotaffect¢-libration much. NOTESThemaneuvering strategies ontheprevious pagehaveassumed thatelectrodynamic tethers willstayvertical. However, asshown atright,thedistributed forceonthetethercauses bowing, andthatbowing iswhatallowsnetmomentum transfer totheattached masses. Notethatnetmomentum canbetransferred tothesystem evenifthewireisbowedthe wrongway(aswhenthecurrent issuddenly reversed); momentum transferred tothewire getstothemasses later. Thisfigurealsoillustrates twootherissues: 1.Bowing ofthetethercausesittocrossfewerfieldlines. 2.Unequal endmasses anduniform forcescauseoverall torques &tilting. Thebowing causesthetethertoprovide lessthrustwhiledissipating thesameparasitic power. Thenetforceonthesystem isthesameasifthetetherwerestraight butina slightly weaker magnetic field. Thetorque onthesystem causesittotiltawayfromthevertical, untilthetorque is balanced bygravity-gradient restoring torques. Foragivensystem massandpower input, disturbing torques varywithLandrestoring torques withL2,solonger systems cantolerate higherpower. Themassdistribution alsoaffects power-handling capability, asseeninthe sequence attopfight. Modulating thetethercurrent modulates anyelectrodynamic torques. Current modulation at 1.73ncanbeusedtocontrol in-plane libration. Out-of-plane torques canalsobemodulated, butanother control logicisrequired. Thisisbecause theonce-per-orbit variation inout-of- planethrustdirection makesacurrent withfrequency F(incyclesperorbit)causeout-of- planeforcesandtorques withfrequencies ofF-1andF+I,asshownintheFourier analysis at bottom right.Hence¢libration control (1:--2)requires properly phased F=IorF=3currents. Higher frequencies candampoddharmonics ofanytetherbowing oscillations. Control of bothin-&out-of-plane oscillations maybepossible sincetheyhavethesamefrequencies andthusrequire different currents. Applications thatrequire significant F=Icomponents forotherreasons cancauseproblems. Foursuchstrategies areshownatright.Sin&Coscontrols allowadjustment ofeorco.The two"Signof..."lawsallowconstant powerstorage over2/3ofeachorbitandrecovery the restoftheorbit.Theselawswouldbeusefulforstoring photovoltaic output foruseduring darkperiods. Thesestrategies driveout-of-plane libration (unless thecenterofmassisatthecenter of thetether). Thelibration frequency decreases atlargeamplitudes, soifthesystem isnot driven toostrongly, itshould settleintoafinite-but-large-amplitude phase-locked loop. Thismaybeunacceptable insomeapplications, duetoresulting variations ingravity or tetherEMF.Insomeeases,suchaseccentricity changes, adding aF=3component might canceltheundesired effectofanF=Icurrent whilekeeping thedesired effect. REFERENCE1.G.Colombo, M.Grossi, M.Dobrowolny, andD.Arnold, Investigation ofElectrodynamic Stabilization &Control ofLongOrbiting Tethers, Interim Report onContract NAS8- 33691, March 1981,Smithsonian Astrophysical Observatory. 185 Electrodynamic Libration Control Issues F=T_i_O •F.L (,No Tether Current:I=1.0 I:Sin_ 0 :Cos§0.=Sign of (.5+Sin_)0 :SignofO (.5+Cos _)INCREASING STABILITY > (for fixed totallength &mass &I)..) -.> -.> FORCONTROL OF: MODULATE IAT: Out-of-plane libration* In-plane libration* Tether oscillationsInor3n 1.73n >5n *Iormass distribution must belopsided _Latitude vs --) Eleetrodynamic I Side Forces: JFourier Analysis of Out-Of-Plane Forces: 0 02n 3n 0 0 .50Io .50Io cos290_ _..50 I I ,'I , 090180270 3601,drives _ ibration I DeE Past Ascending Node,_ I.65I.39 .51I.07 0.50 0.5O 186 SECTION 6.0SPACE SCIENCE ANDTETHERS 187 6.1Overview Somescientific applications oftethers havebeenpresented already inothersections ofthishandbook (seesection 3and4).Inthissection wewillillustrate therolethattethers canplayinthefutureadvancement ofspacescience.We hopethatthissection willgrowin thenexteditions. According totheNon-advocate Tether Systems Applications Review (1993), chaired byDr.M.Greenfield (see"contacts" Section), "...Space tetherteclmology hasthenear-term potential tomeetabroadrangeofscience andtechnological aspects. Theunique capabilities oftether technology enable theaquisition ofscience otherwise notachievable andcan provide concepts forspaceapplications...". Copies ofreportcanbeobtained eitherfromthe chairman orfromtheeditors. Spaceresearch withtethers hasemphasized twoparticular applications: 1)Reaching otherwise unaccess_le flight regions withdownward deployed tethers; 2)Active experimentation withthesurrounding plasma. Agoodexample oftheeffortcarried onbythescientific community isthethe workshop heldinAnnHarbor, Michigan inJuly1994.Copies oftheExecutive Summary can beobtained byProf.B.Gilchrist (see"contacts" Section). Thefocusofthisworkshop wason howIonospheric-Thermospheric-Mesospheric (ITM) Science canbenefit fromspaceborne tethers. NASA's sponsored TIMED mission promises toaddsubstantially totheknowledge of theglobalresponse oftheITMregion. Amulti-masstethersystem couldaddmany"in-situ" dataontheeffects ofsmall scalespatial structures anditsinteractions (see"Applications" Section),. Asthereports quotes "...Justastheadvancement ofremote sensing technology enabled theTIMED mission tobeconceived, theability oftethered payloads inspacewithspatial separations ranging for1Kmto100kmwillenableaprogram ofin-situmultiprobe diagnostics ofthe ITMregion tobeundertaken.". Theworkshop identified thefollowing areasthatwould benefit fromtethered spacecraft: •Magnetospheric-Ionospheric coupling: Energy dissipation andconfiguration ofthree dimensional highlatitude current systems. •Effects ofplasma structureson largeandsmallscaleelectrodynamics. •Ion-neutral momentum andenergyexchange atdifferent spatialscales. •Momentum andenergy transport processes bygravity waves. •Thermospheric cooling (energy loss)through radiative emissions. •Theroleofelectromagnetic andelectrostatic wavesinenergy transfer processes. •Thegeneration andflowofelectrical currents intheITMregion 188 Ataskgroupchaired byprof.Heelis followed uptheobjectives laidoutbythe Michigan Workshop. Thekeyscience questions tobeanswered fromaseriesof"in-situ" tether-aided observations inthelowerthermosphere, highlighted significant advances as: •Determination oftheeffective scalesoverwhichpolarization electric fieldsaregenerated andhowtheymapalongthemagnetic fieldlines. •Determination ofthewindeffectivness inproducing polarization fieldsanddriving field- aligned currents. •Identification ofthetypewindsresponsible forconductivity variations andthose responsible forelectric fieldgeneration. •Assessment ofgravity wavegenerators andofpossible seedmechanism forF-region plasma instabilities. Assessment oftherelecvance ofthermospheric cooling toglobalchange andimpovment ofprediction ofthefuturephysical characteristics inthethermosphere, mesosphere and stratosphere. •Identification oftheresponse ofthelowerionosphere-thermosphere tolargescale weather systems andtransient phenomena associated withlightning. Themeasurements Parameter Neutral Atmospheric comp. Neutral Wind Vector Ion Compositionthatcouldaddress I Dynamic Range 105_10ncm-3 -500to500m/s 1to105cm"3theabovequestions arelistedinthefollowing table. tllll Accuracy Resolution Sample <+10%and AM/M=1at smaller for M=30 <4Km majorspecies 5% +10% 1m/s <4Km +10% Dt i lnterval IonDrift -2to+2Km/s +10% Velocity vector Ion/Electron/ 300to3000K+I0% b Neutral Temp. Electric field -200to+200 +I0% mVector d.c. mV/m Current Density/ -65to+65KnT_+0.1% Magnetic field FUVImaging 10Rto50KR _0.5% Energetic 10eVto30KeV+5% Particles 107toI0l° - cm-2s-1sr-qeV-1 IREmissions 2x10-9to5xl0410% 13-17.5 gm Wcm"2sr-_zhM/M=1 <4KmComp. atM=16 <500mTotal 1% 1m/s <500m 50K <4Km 0.05mV/m <4Kin 0.01% <1Km N/A <1Km N/A <4Km 30degpitch angle ttR/R3% 120Krn 2x10"gto2x10-75%4,.17-6.25 gm .......ZkR/R0.4% 120Km iii i iiii ii 189 Moreinformation ontheinstrumentation andtheengineering aspects ofthismission canbefoundinthesection "Proposed Missions" (ATMMission). Areportentitled "Tether- basedInvestigation oftheIonosphere andLowerThermosphere (TIILT)" hasbeenprepared topresent thescientific rationale behind thistypeofmission aswellasthemeasurements and instrumentation. Copies ofthisreportcanbeobtained byProf.Heelis. Thereareothermissions, however, thatwouldbenefit fromtethers .Forexample, AKTIVE spacecraft, launched bytheformer USSRin1989,aimedatinvestigating VLF radiowave propagation andwave-particle interaction inthemagnetosphere usinga10KW VLFtransmitter withalargeloopantenna (20mdiameter). Electromagnetic effects occurring nearthespacecraft weremonitored byacoorbiting subsatellite, asshowninfigure 6.1. o subsatellite ,,"" s'j I AKTIVE ,,'"j/ Figure6.1Aktive spacecraft andsubsatellite 190 Theprimary objectives oftheAKTIVE program wereasfollows: I)Radiation Properties oftheloopantenna. 2)Spatial structure oftheelectromagnetic fieldsinthenearzone_10kin). 3)Nonlinear effects inthenearzone 4)Propagation ofwavesinthewhistler mode,andtheirreflection fromthe ionosphere 5)Non-linear effects inwhistler wavepropagation 6)Precipitation ofcharged particles formtheradiation beltsduetointeraction with VLFwaves. 7)VLFemissions triggered fromtheorbiting AKTIVE transmitter. 8)Comparison withemissions triggered byground basedVLFtransmitters. Alas,AKTIVE encountered several technical problems andtheprogram was terminated. Nevertheless, whentheera-radiating properties ofspacebome tethers willbe finallyassessed, someoftheaboveobjectives, namely 4,5,7and8willgreatly benefit. No further workhasbeendone,however, inthisdirection. SomeTSSinvestigations arecurrently addressing thesequestions. 6.2Synergy Someyearsago,Lockheed-Martin, thenMartin-Marietta, sponsored somestudies to lookintothesynergy oftethers withotherspacemissions, namely AFE(Aeroassist Flight Experiment), cancelled byNASA in1991, andTIMED (Thermosphere-Ionosphere- Mesosphere Energetic Dynamics). Prof.Hurlbut (see"Contacts" Section) performed the studyandtheresultsareshownintables1and2,respectively. AFEwasaresearch "pathfinder" forageosyncronous, lunarandplanetary earth return aerobraking spacecraft. Prof.Hurlbut indicated thatatethered system could accomplish almost fiftypercent ofAFEobjectives byexploring amuchgreater altitude rangeforalongerduration thanAFEwassupposed tofly. ThestudyonTIMED aimedatdeterminating whichofitsinstruments could potentially flyonapathfinder typetethered spacecraft. Notethatthestudyof Lockheed-Martin onTIMED focused ononeofitsearliest configurations. Themajorfinding ofthisstudywasthatatethered spacecraft couldpossibly validate instrttments whichwereoperated inthe130-140 Kmaltitude range. 191 Table1.AFEVS.Tethered System AFEFlight Experiment 1.Forebody-Aemthermal Characterization Experiment (FACE)Tethered System Appficabifity Heat-flux andskintemperature measurements atallaltitudes willprovide thermal accommodation coefficients andvalidations of models/codes. 2.Radiative Heating Experiment Possibly applicable -Needsfurther study. (P.I-m) 3.WallCatalysis Experiment Anextension of(1)toprovide valuable 0hrCE) catalytic vs.lowcatalytic gas/surface interaction data. 4.BaseFlowHeating Experiment (BFHE)Spherical afterbody datawilldifferfrom aerobrake geometry butwillbeveryvaluable withaddedAerostabilizer instrument data. Possibly applicable -Needsfurther study. 5.Afterbody Radiometry Experiment (ARE) 6.Alternate Thermal Protection Materials (ATPM)Possibly applicable -Needsfurther study. 7.HeatShield Performance (HSP) 8.Pressure Distribution/Air Data System (PD/ADS) 9.Aerodynamic Performance Experiment (APEX) 10.Rarefield-Flow Aerodynamics Measurement Experiment (RAME) (RAME) 11.Plasma, IonandElectron Concentration Experiment (PIECE) 12.Microwave Reflectometer Ionization Sensor (MPdS)Possibly applicable -Needsfurther study. Measurement ofstatic/dynamic pressures at multiple satellite locations extremely valuable. Satellite withAerostabilizer willacquire extremely important aerocharacterization data overawidealtitude range. Measurements ofmomentum transfer characteristics andaeroparameters (CD,CL, etc.)combined with(1)extremely valuable forvalidation ofexisting predictive analytical programs. Possibly applicable -Needsfurther study. Probably N/A 13.AftFlowIonization Sensor Probably N/A (MRIS) 14.IonMassSpectrometer Experiment (IMSE)Measurements ofspecies andtotaldensity extremely important foratmospheric modeling. 192 Table2.TIMED -Tethered Pathfmder Synergy Timed Flight Experiment 1.Fabry-Perot Interferometer 2.Neutral MassSpectrometer 3.IonMassSpectrometer 4.Langmuir Probe 5.IonDriftMeterandRetarding Potentiometer 6.UVSpectrometer 7.Imaging Photometer 8.Triaxial Accelerometer 9.Energetic Particle Analyzer 10.Global UVAirglow Ima_er 11.SolarEUVSpectrometer/UV Photometer 12.VectorMagnetometer 13.NearInfrared Spectrometer 14.Electric FieldDetector/Plasma WaveExperiment 15.Infrared LimbSounder 16.FastElectron Spectrometer 17.Energetic Particle SpectrometerTethered System Pathfinder Probably N/A-Requires morestudy. Applicable forgascomposition, temperatures andtransverse winds. Applicability although ioncomposition and driftvelocities ofsecondary importance. Applicable formeasurement ofelectron temperatures andion/electron densities. Applicable formeasurement ofion temperatures, velocities anddensities. Applicable formeasurement ofO3,NO temperatures, Noctilucent clouds, aerosols, andotherminorconstituents. Possibly N/A-Requires morestudy. Applicable asahighprioriW instrument. Probably N/A-Requires morestudy. Probably N/A-Requires morestudy. Probably N/A-Requires morestudy. Applicable for Probably N/A Probably N/Amagnetic fieldmeasurements -Requires morestudy. -Requires morestudy. Probably N/A-Requires morestudy. Probably N/A-Requires morestud),. Probably N/A-Requires morestudy. 193 SECTION 7.0REFERENCES 194 7.1General Duetothelargeproduction oftether-related paperswehavelimited oursearch toworkspublished inthescientific literature. Wehavealsoincluded thelistof papers presented atthelasttetherconference heldinWashington. Theproceedings ofpapers presented atthefourinternational conferences on Tethers inSpaceaswellasworkshops canbefoundin: •"Applications ofTethers inSpace" Workshop heldinWilliamsburg, VAJune 15-17, 1983.NASA Contract NAS8-35403. •"Applications ofTethers inSpace" Workshop heldinVenice, Italy,October 15-17,1985.NASA Conference Publication CP2422. "International Conference ofTethers InSpace", heldinArlington, VA, September 17-19, 1986.Proceedings published bytheAmerican Astronautical Society inAdvances inTheAstronautical Sciences, Vol.62,1987 •"Tether Dynamic Simulation Workshop", heldinArlington, VASept161986. NASA Conference Publication CP2458. "Space Tethers forScience intheSpaceStation Era",Conference heldin Venice, October 4-8,1987.Proceedings published bySocieta' Italiana diFisica, Bologna, Italy,1988(ISBN 88-7794-016-6). "Tethers InSpaceToward Flight", Conference heldinSanFrancisco, CA,May 17-19, 1989.Proceedings published bytheAmerican Institute ofAeronautics andAstronautics, 1989(ISBN0-930403-50-9). •International Round TableonTethers inSpace", heldinNoordwijk, The Netherlands, September 28-30, 1994.ESAWPP-081. "Fourth International Conference onTethers inSpace", heldinWashington, DC,April10-14, 1995.Published byScience andTechnology Corporation, Hampton, VA. 195 7.2TableofContents oftheFourth International Conference onTethers inSpace VOLUME I Manuscripts Unavailable atTimeofPublication General Information xxiiixix FUTURE MISSIONS Atmospheric Science withTethers; Heating, Cooling andChemistry inthe Lower Thermosphere 3 KateP.Kirby, Smithsonian Astrophysical Observatory FutureofTethers inSpace 11 MarioD.Grossi, Smithsonian Astrophysical Observatory DEMONSTRATED TECHNOLOGY TSS-1vs.TSS-1R 27 BrunoStrim,MarioPastaandEttoreAllais,AleniaSpazio S.p.A. TheFirstandSecond Flights oftheSmallExpendable Deployer System (SEDS) H.Frayne Smith,NASA, Marshall SpaceFlightCenter43 Plasma Motor-Generator (PMG) FlightExperiment Results 57 JamesE.McCoy, C.O2qeill, J.Stanley andT.Settecerri, NASA, Johnson SpaceCenter; MarioD.GrossiandRobert D.Estes,Smithsonian Astrophysical Observatory, M.Dobrowolny, G.Vannaroni, E.Melchioni, C.Bonifazi, C.Cosmovici andL.less,IFSI-CNR; R.JerryJost,System Planning Corporation; R.C.Olsen,NavalPostgraduate School, D.C.Ferguson, R.Tolbert, D.Rau, IraKatz(S-Cubed) andJ.Lilley,NASA, LewisResearch Center, JosephA.Carroll, TetherApplications, Inc.;G.Taconi, L.MinaandW.Goree, University ofGenoa APPROVED FUTURE MISSIONS SEDS/SEDSAT Project Overview 85 JamesK.Harrison, NASA, Marshall SpaceFlightCenter TheOEDIPUS-C Sounding Rocket Experiment 95 H.Gordon James, Communications Research Centre; J.GlenRumbold, Canadian SpaceAgency OVERVIEW TheNASA/ASI TSS-1Mission: Summary ofResults andReflight Plans 107 W.JohnRaittandD.C.Thompson, CASS, CenterforAtmospheric andSpace Sciences, UtahStateUniversity; N.Stone,NASA, Marshall SpaceFlight Center; M.Dobrowolny, Instituto diFisicadelloSpazio Interplanetario; C.Bonifazi, Agenzia Spaziale Italiana; B.Gilchrist andP.M.Banks, 196 SpacePhysics Research Lab,University ofMichigan; D.Hardyand M.Oberhardt_ Phillips Laboratory, GPSP;S.Williams andP.R.Williamson, STARLaboratory, Stanford University TSSMission 1FlightDynamic Anomalies 119 Donald D.Tomlin andDavidK.Mowery, NASA, Marshall SpaceFlight Center; BrunoMusetti andBrunaCibrario, Alenia Spazio, S.p.A. FlightDatafromtheFirstandSecond Flights oftheSmallExpendable Deployer System (SEDS) 133 Charles C.Rupp,NASA, Marshall SpaceFlightCenter Plasma MotorGenerator Mission Report 149 JerryJostandDeanChlouber, System Planning Corporation; Thomas L.Wilson, Christine A.O_NeilandJamesE.McCoy, NASA, Johnson SpaceCenter SEDSAT TetherDynamics Research 165 Cheryl D.Bankston, NASA, Marshall SpaceFlightCenter; Dennis RayWingo, Center forSpacePlasma andAeronomic Research, University ofAlabama inHuntsville; MarkA.Stedham, Defense Intelligence Agency, Missile and SpaceIntelligence Center ASpaceTether Experiment STEX 181 LarryL.Burgess, F.M.KustasandF.J.Jarossy, Lockheed MartinAstronautics FlightResults fromtheOEDIPUS-A Tethered Experiment 193 H.Gordon James, Communications Research Centre; George Tyc, Bristol Aerospace Limited Sub-Satellites forAerothermodynamic Studies intheLowerThermosphere FrankC.Hurlbut, University ofCalifornia atBerkeley211 SpaceTethers forIonospheric-Thermospheric-Mesospheric Science-Report onthe1994International Summer Workshop, AnnArbor, MI 221 BrianE.Gilchrist, L.M.Brace,andG.R.Carignan, SpacePhysics Research Laboratory, University ofMichigan; R.Heelis, University ofTexas-Dallas, CenterforSpaceScience; W.J.Raitt,UtahStateUniversity, Centerfor Atmospheric &SpaceScience; C.Rupp,NASA, Marshall SpaceFlightCenter; H.G.James, Communications Research Center; C.Bonifazi, Agenzia Spaziale Italiana; K-IOyama, ISAS;G.Wood,NASA, Langley Research Center Summary andConclusions fromtheInternational RoundTableonTethers inSpace-ESTEC, September 1994 227 Mauro Novara, ESA/ESTEC TSS-1ELECTRODYNAMICS TSS-1SETSandCOREExperiment Results andPlansforReflight 239 BrianE.Gilchrist, PeterM.Banks, SvenG.Bil6nandNestor R.Voronka, University ofMichigan, SpacePhysics Research Laboratory; CarloBonifazi, Agenzia Spaziale Italiana; DonC.Thompson andW.JohnRaitt,UtahState University, Center forAtmospheric andSpaceScience; VictorM.Ag_ero, ScottD.Williams andAnthony C.Fraser-Smith, Stanford University, EERA/STARLAB 197 Transient Response oftheTSSSystem intheIonosphere 253 SvenG.BildnandBrianE.Gilchrist, University ofMichigan, SpacePhysics Research Laboratory; Enrico Melehioni andMarino Dobrowolny, Consiglio Nazionale delleRicerche, Instituto diFisicadelloSpazioInterplanetario Thruster Pickup IonsDetected bySPREE During TSS1 267 Louise C.Gentile, Boston College Institute forSpaceResearch; W.J.Burke, J.S.Machtmak, DavidA.HardyandD.E.Htmton, Phillips Laboratory Electron BeamPropagation Observed Durign TSS1283 DavidA.Hardy, Marilyn R.Oberhardt andWilliam J.Burke,Phillips Laboratory/GPSG; Donald C.Thompson andW.JohnRaitt,Centerfor Almospheric andSpaceSciences, UtahStateUniversity; Louise C.Gentile, Boston College Institute forSpaceResearch Correlator Measurements ofMI-IzWave-Particle Interactions During TSS1 Electron BeamOperations 299 M.PaulGough, SpaceScience Centre, University ofSussex; DavidA.Hardy, Marilyn R.Oberhardt andWilliam J.Burke,Phillips Laboratory; Louise C.Gentile, Boston College Institute forSpaceResearch Observations ofIonosphere Heating intheTSS-1Subsatellite Presheath 315 IraKatzandMyron Mandell, S-Cubed Division ofMaxwell Laboratories; EnricoMelchioni, Istituto diFisicadelloSpazioInterplanetario; Marilyn Oberhardt, Phillips Laboratory, SpaceSciences Laboratory; DonThompson, UtahStateUniversity; Torsten Neubert andBrianGilchrist, University ofMichigan; CarloBonifazi, Agenzia Spaziale Italiana Induced Ionospheric Double ProbeTetherPotential Measurements andModels for TSS-1Electrodynamics 331 ScottD.Williams andV.M.Agiiero, Stanford University; Donald C.Thompson andW.JohnRaitt,CenterforAlmospheric andSpaceSciences, UtahState University; BrianE.Gilchrist, PeterM.BanksandNestorVoronka, SpacePhysics Research Laboratory, University ofMichigan Current-Voltage Characteristics oftheTethered Satellite 347 U.Guidoni, andM.Dobrowolny, Agenzia Spaziale Italiana; E.Melchioni and G.Vannaroni, Istituto diFisicadelloSpazioInterplanetario -CNR, J.P.Lebreton, ESA/ESTEC Identification ofCharge Carriers intheIonospheric Branch oftheTSS-1 TetherGenerated Current System 359 N.H.Stone,NASA, Marshall SpaceFlightCenter; K.Wright, TheUniversity ofAlabama inHuntsville; J.D.Wirmingham andC.Gurgiolo, Southwest Research Institute; U.Sam[r,TelAvivUniversity; C.Bonifazi, ItalianSpaceAgency; B.Gilchrist, TheUniversity ofMichigan; M.Dobrowolny, Instituto Fisiea Spazio Interplanetario/CNR TSS-1DYNAMIC OPERATIONS Deployer Performance Results fortheTSS-1Mission 375 Leland S.Marshall andRonald V.Geiger, Martin Marietta Astronautics LongPeriodTension Variations inTSS-1andSEDS-2 389 198 Gordon E.Gullahorn, MarioCosmo, Harvard-Smithsonian Center forAstrophysics; Robert G.Hohlfeld, Metropolitan College, Boston University TSS-1Mission: Sub-Satellite Attitude Reconstruction 399 BrunoMusetti, BrunaCibrario, MariaStellaDiRaimondo, andPaoloMartella, AleniaSpazio S.p.A. Simulation ofTSSPassive Skiprope Damper 409 JayN.Estes,NASA, Johnson SpaceCenter, DavidD.Lang,LangAssociates Active andPassive Damping ofTethered Systems 419 Francesco Angrilli, G.Bianchini andR.DaFomo,University ofPadova; G.Fanti,University ofParma TSS-1TetherDamping. HasAnything BeenLearnd? 433 S.Bergamaschi, G.Carletti andP.LionStoppato, University ofPadova AComplex, Frequency Domain Skiprope Observer forTethered Satellites JohnKGlaese, Control Dynamics441 Transform Domain BasedObservation andPrediction ofTether Skiprope Oscillations fortheTSS-1FlightExperiment 455 Stephen M.Rodrigue, Abolfazl M.Amini, George E.loupandJuliette W.Ioup, Department ofPhysics, University ofNewOrleans; StanN.Carroll, D.KeithMowery andDonD.Tomlin, NASA, Marshall SpaceFlightCenter AReview ofTSSThermomeehanieal Properties intheLightofPreliminary Experimental Results 469 F.An/villi, G.Bianchini andS.Debei,Dipartimento diIngegneria Meccanica, Universit_ diPadova; G.Fanti,Dipartimento diIngegneria Industriale, Universit_ diParma Assessment ofShuttle PlumeEffects ontheTethered Satellite System-I Palmer B.Chiu,NASA, Johnson SpaceCenter;, BillC.O'Donnell, LinCom Corporation489 SPONSORED PROJECTS Reflight oftheTethered Satellite System Mission 507 BeckyC.Soutullo, NASA, Marshall SpaceFlightCenter AnAcquisition System toReveal Ground E.M.Emissions fromTSS1-R 515 Cinthya Ottonello, Dipartimento diIngegneria Biofisica edElettronica - University ofGenoa; S.Pagnan, Istituto diAutomazione Navale - National Research Council ofItaly AGround-Based Receiving System fortheDetection ofSignals fromthe Tethered Satellite System 527 S.T.NobleandW.E.Gordon, RiceUniversity; R.D.Estesand M.D.Grossi, Harvard-Smithsonian Center forAstrophysics Breadboard TestandFlightPreparation oftheRAPUNZEL Mission 535 D.Sabath, Fachegebiet Raumfahrttechnik; C.Kessler andM.Krischke, Kayser-Trede GmbH; V.L.Balakin andV.Shaehmistov, Samara StateAerospace University 199 SEDS/SEDSAT Mission Plans543 Charles C.Rupp,NASA, Marshall SpaceFlightCenter; William J.Webster, Jr., NASA, Goddard SpaceFlightCenter Dynamics andControl ofSEDSAT Deployment 551 E.C.Lorenzini, Harvard-Smithsonian Center forAstrophysics; D.K.Mowery andC.C.Rupp,NASA, Marshall SpaceFlightCenter Sensitivity Analysis ofSEDSAT Orbital Injection 563 J.PelfiezAlvarez, E.T.S.I. Aeronfiuticos, Universidad Polit6cniea deMadrid; EnricoC.Lorenzini, Harvard-Smithsonian CenterforAstrophysics Design Considerations foraLong-Lifetime SpaceTether 577 F.M.Kustas, F.J.Jarossy andL.L.Burgess, Lockheed MartinAstronautics Company VOLUME II SMALL EXPENDABLE DEPLOYER SYSTEM (SEDS) SEDSDeployer Design andFlightPerformance Joseph A.Carroll, TetherApplications593 Development oftheSEDSEMPTensiometer andTetherAttachment Mechanism RayD.Rhew,NASA, Langley Research Center SEDSEndMassPayload Magnetometer Engineering Performance andRotational DataAnalysis Results 613 JohnH.Stadler, NASA, Langley Research Center Design oftheSignalandPowerConditioning Subsystems fortheSEDS EndMassPayloads 627 JohnI£Diamond, NASA, Langley Research Center, Wendy G.Nagumy, Lockheed Engineering andScience Company SEDSTetherDeployment Ground Tests 653 BruceKWallace, NASA, Marshall SpaceFlightCenter SEDS-II Deployment Control LawandMission Design669 Enrico C.Lorenzini, Harvard-Smithsonian CenterforAstrophysics; D.K.Mowery andC.C.Rupp,NASA, Marshall SpaceFlightCenter TheSmallExpendable Deployer System (SEDS) Missions 1and2Thermal Analysis andFlightDataComparison 685 SherylL.Kittredge, NASA, Marshall SpaceFlightCenter DataReduction andValidation inaSmallProject Environment: TheSEDS-1, PMGandSEDS-2 Cases 697 LeeR.Huynh andP.Chewning Toulmin, Hughes-STX Corporation; William J.Webster, Jr.,NASA, Goddard SpaceFlightCenter TheSEDSDataServer: Contents, Access Procedures, andMechanisms William J.Webster, Jr.,NASA, Goddard SpaceFlightCenter; BrianH.Kirouac, Hughes-STX Corporation705601 20O AComparison ofSEDS-2 FlightandDynamics Simulation Results 711 JohnKGlaese, Control Dynamics, Division ofbdSystems Three-Dimensional Modal Development withTimeandTetherLength asa Dynamics Diagnostic ToolforSEDS 721 Juliette W.Ioup,George E.IoupandStephen M.Rodrigue, University of NewOrleans; George M.Wood,NASA, Langley Research Center Restitution ofTethered Satellites Systems Motion 73I R.C16dassou andSdbastion Herbiniere, CentreNational d'l_tudes Spatiales Acceleration LevelsandDynamic NoiseonSEDSEnd-Mass 747 MarioL.Cosmo, E.C.Lorenzini, G.E.Gullahom, Smithsonian Astrophysical Observatory TheFinalDynamic StateofSEDS-2: ADetermination oftheTorsional Pendulum PeriodfromtheSubsatellite Magnetometer Data 761 Christopher D.BlunkandBrianD.Humphry, Eleanor Roosevelt HighSchool; Michael R.Douglass, University ofNorthCarolina atChapel Hill; William J.Webster, Jr.,NASA, Goddard SpaceFlightCenter; George H.Strachan, Eleanor Roosevelt HighSchool Visualization oftheMotion oftheSEDS1and2Subsatellites 767 Michael Douglass, University ofNorthCarolina atChapel Hill; P.Chewning Toulmin andLeeR.Huynh, Hughes-STX Corporation; William J.Webster, Jr.,NASA, Goddard SpaceFlightCenter Analysis ofVideoImages oftheRe-entry oftheSEDS-I EndMass 773 DavidL.Talent, Lockheed Engineering andSciences Company TheVisualBrightness oftheSEDS-2 Tether 795 William J.Webster, Jr.,JamesJ.Butler, JamesT.McLean, LockeM.Stuart,NASA, Goddard SpaceFlightCenter; G.Si,Swales andAssociates Ground RadarDataandOptical Measurements -SEDS,PMG,andSEDS-2 815 Thomas J.Settecerri, Lockheed Engineering andSciences Company Comparison ofPredicted andActualOrbital Lifetimes fortheSEDS-2 Mission Steven W.Evans, NASA, Marshall SpaceFlightCenter837 Micro-Meteoroid andOrbital DebrisDamage Analyses onSEDSTether KazuoBenHayashida, Jennifer H.Robinson andScottA.Hill, NASA, Marshall SpaceFlightCenter847 ELECTRODYNAMICS Characteristics ofPlasma Contactors forSpacecraft Charging Control 861 PaulJ.WilburandJ.Steven Snyder, Colorado StateUniversity Plasma Contactor Device Family forSpaceUseWorking Upto10A: Review oftheFunctional Testing Activity 873 M.Minucci, A.SeveriandM.Capacci, ProelTecnologie; C.Bonifazi andF.Svelto,ASI 201 Current Collection Enhancement byNeutral GasEmissions IntoaPlasma L.CondeandL.Le6n,E.T.S.I. Aeron_uticos, Universidad Polit6cnica deMadrid881 Ionization inAnElectron Collecting Contactor 889 Eduardo Ahedo, E.T.S.I. Aeron_uticos, Universidad Polit6cnica deMadrid; M.Martinez-Sfinchez, Department ofAeronautics andAstronautics, M.I.T. Stability ModelforPlasma Contactors 897 V.Lapuerta andE.Ahedo, E.T.S.I. Aeromiuticos, Universidad Polit6enica deMadrid Plasma Contactors forElectrodynamic Tether Sysetsm 903 Igor.4.Beznos andKonstantin N.Kozubsky, "Fakel" Enterprise; Alexei S.Isakov andVladimir Kim,Research Institute ofApplied Mechanics andElectrodynamics ofMoscow Aviation Institute Central Problem ofSpaceElectronics andElectrodynamic-TSS 909 N.A.Savich, Institute ofRadioEngineering andElectronics ofthe Russian Academy ofSciences RMHD-Simulation ofAnElectrodynamic Tethered Satellite System intheIonosphere 921 Andreas SchrOer, Ruhr-Universit_t Bochum Current Closure foraTethered Satellite System 935 Chia-Lie ChangandAdamDrobot, Science Applications International Corporation; Konstantinos Papadopoulos, University ofMaryland Campaigns ofGround Listening totheE.M.Emissions Expected 949 fromSpaceborne Electrodynamic Tether Systems G.Tacconi, S.Dellepiane andC.Ottonello, University ofGenoaDIBE; S.Pagnan andL.Minna, IAN/CNR National Research Council; M.D.Grossi, SAOHarvard-Smithsonian Astrophysical Observatory AHighPerveance Electron Generator withON/OFF Modulation Capabilities for Current Upto1A965 M.Minucci, G.NociandA.Severi, ProelTecnologie; C.Bonifazi and F.Svelto,ASI Alfven WaveSignature fromConstant-Current Tethers 973 Robert D.Estes,Harvard-Smithsonian CenterforAstrophysics; JaunR.Sanmartin, ETSIAeron_uticos, Universidad Polit6cnica deMadrid Nonreciprocal Magnetoplasma Sheath Waves onStructures inSpace 983 Andrea A.E.Lftttgen andKeithG.Balmain, University ofToronto TheSpace-Charge EffectonCurrent Collected byaSpherical Probeina Magnetized Plasma 993 Weiwei LiandJ.G.Lagramboise, Department ofPhysics andAstronomy, YorkUniversity PICSimulation ofSpherical-symmetric ActiveandPassive High Voltage Discharges 1009 Hartmut Marschall andF.M.Neubauer, Universitht zuK61n 202 OntheCreation ofSpaceCharges intheFluxTubesofElectrodynamic Tethers inSpace 1019 KL.Stenzel andJ.M.Urrutia, Department ofPhysics, University ofCalifornia TheMagnetic Presheath ofaPositively Charged Spacecraft: Estimating ItsPotential 1025 .LG.Laframboise, Physics andAstronomy Department, YorkUniversity; L.J.Sonmor, Institute forSpaceandAtmospheric Studies, University ofSaskatchewan; B.A.Whalen, Herzberg Institute ofAstrophysics, National Research Council TheUseofElectron BeamExperiments onOrbiting Tethered Platforms for Remote Sensing ofE_intheAuroral Acceleration Region: TheActive Magnetospheric Particle Acceleration Satellite (AMPAS) 1035 LindaA.Habash, Torsten Neubert andBrianE.Gilchrist, University ofMichigan; W.JohnRaitt,UtahStateUniversity AnExtended Electron BeamforAuroral Studies 1045 Manuel Martinez-Sanchez, Department ofAeronautics andAstronautics, MIT; JaunR.Sanmartin, ETSIAeronauticos, Universidad Politecnica deMadrid AnInvestigation ofForcesinaShielded Conductor 1067 C.H.Spenny, C.O'DellandW.F.Bailey, AirForceInstitute ofTechnology TETHER TECHNOLOGY SEDSCharacteristics andCapabilities 1079 Joseph A.Carroll andJohnC.Oldson, TetherApplications ESADevelopment onExpendable Tether Mechanism Technology 1091 Renato Licata, AleuiaSpazioS.p.A.; J.Mignel Gavira, ESA-ESTEC TetherSystem Applications inSupport ofLargeSpaceInfrastructures 1101 LuigiBussolino, PietroMedina andMariaAntonietta Perino, Alenia Spazio S.p.A. Tethered Re-Entry Application Deployment Control 1107 Renato Licata, AleniaSpazio S.p.A. Rotating SpaceStation forOut-Of-Plane Orbital Transfer UsingTethers BruceA.Mackenzie, SpaceStudies Institute1119 System Engineering andIntegration (SE&I) Aspects foraTether Application attheISSA 1133 PaulTetzlaff, DASA/RST Rostoek Raumfahrt undUmweltschutz Adaptive Attitude Control fortheBasePlatform ofaTethered System withFuzzyLogic 1143 W.Hallmann, E.Plescher andW.Zahnow, Faehhoehschule Aachen, SpaceDepartment Failsafe Multistrand TetherSEDSTechnology 1151 Robert P.HoytandRobertL.Forward, Tethers Unlimited Temperature Behavior ofTethers inOrbit 1161 W.Hallmann, B.JustenandE.Plescher, FHAachen, SpaceDepartment 203 Effects ofDebris Damage onNewLong-Life Tethers I171 F.Angrilli, G.Bianchini, R.DaForno,S.DebeiandB.Saggin, Center OfStudyandSpaceActivities "G.Columbo", CISAS, University ofPadova; G.Fanti,Department ofIndustrial Engineering, University ofParma StressDistribution Measurements inComposite SpaceRopes 1181 GiulioFanti,Dipartimento diIngegneria Industriale, Universit_ diParma Termination Methods forSynthetic FiberTethers 1195 Douglas P.Bentley, Cortland CableCompany TheProgress-M BasedSample &Material Re-Entry Demonstration 1203 PietroMerlina, AleniaSpazioS.p.A.;PeterSchwebke, DASA-RST; MauroNovara, ESA-ESTEC AnAnalytic Characterization oftheOptimal MassProblem forAerobraking Tethers 1217 StevenG.Tragesser andJamesM.Longuski, Purdue University; JordiPuig-Suari, Arizona StateUniversity TetherSurvivability: SEDS-2 AsADiagnostic Tool 1233 Martin Beech, TheUniversity ofWestern Ontario; P.Chewning Toulmin, Hughes-STX Corporation TETHER SCIENCE Expected Magnetic FieldResults fromaTetherMission 1245 Patrick T.Taylor, NASA Goddard SpaceFlightCenter; JoyA.Conrad, Hughes STXCorporation; JamesJ.Frawley, Herring BayGeophysics Tethered Systems intheMagnetospheric Studies 1259 Stanislav LKlimov, Anatoly A.Petrukovich, Mikhail L.Pivovarov, Andrey V.Prudkoglyad, Vjacheslav G.Rodin,Alexander A.Skalsky, SpaceResearch Institute (IKI)Russian Academy ofScience; Valery E.Korepanov, Special Design Division of National Academy ofSciences ofUkraine Spectroscopic Measurements fromaTethered Satellite Platform 1269 KellyV.Chance, Harvard-Smithsonian CenterforAstrophysics Gravity Gradiometry fromtheDownward Tethered Satellite System 1273 Franco Fuligni, V.IafollaandS.Nozzoli, Istituto diFisicadelloSpazio Interplanetario; M.Cosmo, M.D.Grossi, G.E.Gullahorn andE.Lorenzini, Harvard-Smithsonian Center forAstrophysics Analysis ofaDumbbell Sensor forSpaceGradiometry 1281 Simone B.Bortolami, M.G.H. Biomotion Lab-Harvard Medical School; F.Angrilli, University ofPadova; C.Jekeli,OhioStateUniversity; M.D.Grossi, Harvard-Smithsonian CenterforAstrophysics 204 VOLUME m DYNAMICS ANewFullNon-Linear ModeloftheTethered Satellite Systems BasedontheCharacteristics Method 1299 F.Angrilli, R.DaFornoandB.Saggin, University ofPadova Dynamics ofLow-Tension Spinning Tethers 1309 A.MisraandM.Keshmiri, McGill University; V.J.Modi,University ofB.C.; G.TycandR.Hart,University ofManitoba; F.Vigneron andA.Jablonski, Canadian SpaceAgency Resonant Conditions foraStretched Spinning Tether 1325 RayP.S.HamandAlbertC.J.Luo,University ofManitoba Analytical Solutions foraModelofaSpinning Tether1341 Thomas G.BerryandJosephJ.Williams, University ofManitoba Severed TetherDynamics andProbability 1353 Kenneth J.Welzyn andJennifer H.Robinson, NASA, Marshall SpaceFlightCenter Dynamics ofaSpaceTethered Satellite System withTwoRigidEndbodies W.Steiner, A.Steindl andH.Troger, Technical University Vienna1367 Attachment PointMotion forActive Damping ofVibrations inTethered Artificial Gravity Spacecraft 1381 Shannon L.Thornburg andJ.DavidPowell, Stanford University Simultaneous Attitude andVibration Control ofTethered Satellite Systems S.Pradhnn andV.J.Modi,TheUniversity ofBritish Columbia; A.K.Misra,McGill University1395 Deployment andRetraction ofaContinuous Tether: TheEquations Revisited F.L.Janssens, D.Poelaert andE.B.Crellin, European SpaceResearch and Technology Center (WMM) Attitude Maneuvers ofaFlexible SpaceStation byMeansofDeployable Tethers Franco Bernelli-Zazzera, Amalia Ercoli-Finzi andP.Mantegazza, Dipartimento diIngegneria Aerospaziale, Politecnico diMilano Optimal MassFlexible Tethers forAerobraking Maneuvers JordiPuig-Suari, Arizona StateUniversity1441 Atmospheric Research UsingTethered Satellite Systems inElliptic Orbits 1451 Howard A.Flanders andPennyL.Niles,Martin Marietta Astronautics Constrained Iterative Spectral Deconvolution forAnalysis ofClosely Spaced ModalPeaksintheFourier Transform ofTethered Satellite Dynamics Data1461 Abolfazl M.Amini, Southern University, BatonRougeandUniversity of NewOrleans; George E.IoupandJuliette W.Ioup, University ofNewOrleans AnExtended Kalman FilterforObserving theSkiprope Phenomenon ofthe Tethered Satellite System 14751415 1425 205 HaikBiglari, Sverdrup Technology, Inc.;Zachary J.Galaboff, NASA, Marshall SpaceFlightCenter Validation ofAnalytical Modeling ofOEDIPUS Tethers UsingExperimental Results fromTE-LAB 1483 F.R.Vigneron, A.M.Jablonski andR.Chandrashaker, Canadian SpaceAgency; B.McClure, J.Bergrnans andD.Stale),,Carleton University; G.Tyc,Bristol Aerospace Limited NEAR FUTURE AnUpdate toProposed SpaceTetherApplications forInternational SpaceStation Alpha 1501 Donald S.Crouch, A.Colton ParkandGilbert M.Kyrias, Martin Marietta Astronautics; BrunoStrim,Saverio Lioy,GeneCompton (NASA Rep),Alenia Spazio S.p.A.; Andrea Lorenzoni, Agenzia Spaziale Italiana; Carolynn Conley, MunizEngineering AProposed Shuttle-Tethered Satellite System Atmospheric Verification Mission Donald S.Crouch, Howard A.Flanders andGilbert M.Kyrias, Martin Marietta Astronautics1513 AIRSEDS-ITM: AProof-of-Concept TetherMission IntotheEarth's UpperAtmosphere 1527 Andrew D.Santangelo, TheMichigan Technic Corporation; Terrance G.Onsager andCraigA.Kletzing, University ofNewHampshire DualTethered Satellite Systems forSpacePhysics Research 1543 PennyL.Niles,Martin Marietta Astronautics; BrianE.Gilehrist, University ofMichigan; JayN.Estes,NASA, Johnson SpaceCenter AIRSATT-Atmospheric/Ionospheric Research Satellite UsingAdvanced Tether Technology 1559 B.E.Gilchrist, J.Dodds, B.C.Kennedy, University ofMichigan; P.L.Niles, Martin Marietta Astronautics; C.C.Rupp,NASA, Marshall SpaceFlightCenter Tethered Multi_Probe forThermospheric Research 1567 E.C.Lorenzini, M.L.Cosmo, M.D.Grossi, K.Chance andJ.L.Davis, Harvard-Smithsonian Center forAstrophysics Alenia Spazio LongTermCommitment toSpaceTethers: Past,Present andFuture 1577 PaoloPiantella andFrancesco Giani,AleniaSpazioS.p.A. SpaceResearch intheBICEPS Experiment 1585 1-1.Gordon James, Communications Research Centre; Andrew W.Yau, NRCC Herzberg Institute ofAstrophysics; George Tyc, Bristol Aerospace Limited AShuttle Deployed TetherTechnology Demonstration Mission toServe Canadian andUnited StatesNeeds 1599 G.Tyc,BristolAerospace Limited; C.C.Rupp,NASA, Marshall Space FlightCenter; A.M.Jablonski andF.R.Vigneron, Canadian SpaceAgency WhatIsNecessary toGetYourIdeastoNASA1611 Patricia M.Dory,NASA, Marshall SpaceFlightCenter 206 FARFUTURE Synthetic Aperture RadarInterferometry byMeansofTethered Antennas Antonio Moccia andMarcoD'Errico, Universit_ degliStudidiNapoli; SergioVetrella, Seconcla Universi_ degliStudidiNapoli1631 TheElectromagnetic CleanSubsatellite SPELIS forStudies onPlasma-Wave Phenomena Caused byOperations oftheElectrodynamical Tethered System inSpacePlasmas 1643 Stanislav 1.Klimov, YuriN.Agafonov, Alexander A.Skalsky and Vyacheslav G.Rodin, SpaceResearch Institute ofthe Russian Academy ofScience Methods ofDeployment ofthePolymodule TetherSystem withUtilization oftheConversed Ballistic Missile Separation Block 1653 Gennady V.Malyshev, L.M.Kalashnikov, V.M.Kulkov, N.N.Markin andA.P.Svotin, Research Institute ofApplied Mechanics andElectrodynamics ofMoscow Aviation Institute; V.M.Ivanov andV.I.Mironov, Central Research Institute ofMachine Building; N.A.Obukhov, Makeev Design Officeof Mechanical Engineering Optimization ofaLowAltitude Tethered ProbeforMartian Atmosphere DustCollection 1663 Moniea Pasea, Universit_ diRoma"LaSapienza"; Enrico Lorenzini, Harvard-Smithsonian Center forAstrophysics AIRSEDS-IIrM: ATSS-2Precursor Mission toTestandDemonstrate Tethered Systems intheEarth's UpperAtmosphere 1675 Andrew D.Santangelo, TheMichigan Technic Corporation Optimal De-spinning andRetrieval ofaTethered Artificial Gravity Spacecraft Selma1.SaeedandJ.DavidPowell, Stanford University1685 Tethered Stabilized Platform Attitude Control byMoving theAttachment PointInfluence ofTether Transverse Vibration 1697 Francesco Angrilli, R.DaForno,G.Bianehini, B.Saggin, University ofPadova; G.Fanti;University ofParma FutureTetherMissions inSpace:AEuropean Perspective 1705 LuigiBussolino, PietroMerlina andMariaA.Perino, Alenia SpazioS.p.A. ADigital Robust Controller foraTethered Reflector/Antenna System 1721 PeterM.Bainum andZhaozhi Tan,Howard University ASkyhook fromPhobos toMars 1737 William B.Thompson, University ofCalifornia atSanDiego; Martin O.Stern,California SpaceInstitute, Scripps Institution ofOceanography SOUNDING ROCKETS Design oftheOEDIPUS-C Suborbital Tethered Payload 1749 W.Eliuk,R.Rob,G.TycandI.Walkty, Bristol Aerospace Limited; G.Rumbold, Canadian SpaceAgency; H.G.James, Communications Research Centre 207 SpacePlasma Experiments withtheTethered OEDIPUS-C Payload 1765 H.Gordon James, Communications Research Centre; KeithG.Balmain, University ofToronto Tethered-Probe Measurements ofECHO 7Charging-Discharging William J.BurkeandGeorge P.Murphy, Phillips Laboratory; PerryR.Malcolm, USAFAcademy1781 Design, Qualification andCalibration oftheTetherForceSensor(TFS) fortheOEDIPUS-C Mission 1793 G.Tyc,W.R.Whitehead andJ.L.Phillips, Bristol Aerospace Limited; J.G.Pierson, Pierson Associates Incorporated; A.M.Jablonski and F.R.Vigneron, Canadian SpaceAgency Tether Laboratory Demonstration System (TE-LAB) AGround TestFacility fortheOEDIPUS Tether Missions 1809 Alexander M.Jablonski, FrankR.Vigneron andRajagopalan Chandrashaker, Canadian SpaceAgency; JohnL.Bergmans, BruceA.McClure and Douglas A.Staley, Carleton University; George Tyc,Bristol Aerospace Limited Experimental Investigation oftheDynamics ofSpinning Bodies 1823 V.£Modi,S.Pradhan andM.Chu,University ofBritishColumbia; G.Tyc, Bristol Aerospace Limited; A.K.Misra,McGill University Sounding Rocket Tethered Payload Experiments: TheNASA Charge Program 1837 IV.JohnRaitt,CASS, UtahStateUniversity PLASMA MOTOR GENERATOR (PMG) Comparison ofTheoretical Predictions withPlasma MotorGenerator (PMG) Experimental Data 1847 JohnR.Lilley,Jr.,AgnesGreb,lraKatzandVictoria A.Davis, S-Cubed Division ofMaxwell Laboratories; JamesE.McCoy, NASA, Johnson SpaceCenter; JoelGalofaro andDaleC.Ferguson, NASA, LewisResearch Center Correlation ofTetherCurrent withDay/Night Cycles During PMGMission DeanChlouber andR.JerryJost,System Planning Corporation; Thomas L.Wilson, NASA, Johnson SpaceCenter;1857 Comparison ofRemote-Sensor Determination ofPMGTwo-Body Dynamics withTelemetry-inferred Tether Dynamics 1873 DeanChlouber andR.JerryJost,System Planning Corporation; Thomas L.Wilson andJamesE.McCoy, NASA, Johnson SpaceCenter VHFRadarMeasurements ofPMGIonospheric Interactions 1875 KJerrydostandDeanChlouber, System Planning Corporation Ionosphere-Thermosphere Coupling withLarge-Amplitude, Nonlinear Ion-Acoustic Solutions Triggered byElectrified Spacecraft 1877 R.JerryJostandDeanChlouber, System Planning Corporation; Thomas L.Wilson, NASA, Johnson SpaceCenter Hawaii-Hilo Ground Observations ontheOccasion ofthePMGFlightof June23,1993:Further Spectral Analysis 1879 Cinthya Ottonello andGiorgio Tacconi, Dipartimento diIngegneria Biofisica 208 edElettronica-University ofGenoa; SergioPagnan, Istituto diAutomazione Navale -National Research Council ofItaly;LucaMina,Advanced Engineering Technology -TorteACortedeiLambruschini Eleetrodynamic Interactions Between thePMGTetherandtheMagneto-Ionic Medium oftheIonsphere 1891 MarioD.GrossiandRobertD.Estes,Harvard-Smithsonian Center forAstrophysics; JamesE.McCoy, NASA, Johnson SpaceCenter Tether Current-Voltage Characteristics 1899 R.C.Olsen,Chung-Jen ChangandChia-Hwa-Chi, NavalPostgraduate School APPENDIX A Author Index 1923 APPENDIX B Attendee List1929 209 7.3Bibliography Alfven, H.,"Spacecraft Propulsion: NewMethods," Science, Vol.176,p.167-168, 14Apr.1972. 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Wright, A.N.andSchwartz, S.J.,"TheEquilibrium ofa Conducting BodyEmbedded inaFlowing Plasma," Jo_al ofGeo0hvsical Research, Vol.95,p.4027-4038, April1, 1990. Yu,S.,"OntheDynamics andControl oftheRelative Motion Between TwoSpacecraft," ActaAstronautica, Vol. 35,No.6,p.403-409, March 1995. Zhu,R.,Misra, A.K.andModi, V.J.,"Dynamics and Control ofCoupled Orbital Motion ofTethered Satellite Systems," Journal oftheAstronautical Sciences, Vol.42, No.3,p.319-342, September 1994. 217 SECTION 8.0CONTACTS 218 A_ Dr.Eduardo Ahedo E.T.S.I. Aeronauticos PlazaCardenal Cisneros 3 28040Madrid, SPAIN 3413366310 Mr.A.J.Alfonzo Omitron, Inc. 6411IvyLane Suite600 Greenbelt, MD20770 301/474-1700 Mr.Andrew M.Allen NASA, Johnson SpaceCenter(CB) Houston, TX77058 713/244-8719 Prof.Yakov Alpen Harvard Smithsonian Center forAstrophysics 60Garden Street Cambridge, MA02138 617/495-7933 Dr.JesusPelaezAlvarez E.T.S.I. Aeronauticos Dpto.FisicaAplicada P1.Cardenal Cisneros 3 28040, Madrid, SPAIN 3413366306 Mr.JohnAnderson NASA Headquarters MailCodeCC Washington, DC20546 202/358-4665 Prof.Francesco Angrilli CISAS -University ofPadova Dept.ofMechanical Engineering VIAVenezia, 1 35131, Padova, ITALY 39498286790 Mr.DavidA.Arnold 75Woodbine Road Belmont, MA02178 617/484-7741B_ Prof.PeterM.Bainum Howard University Dept.ofMechanical Engineering 23006thStreet,N.W. 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Washington, DC20546 202/358-0899 Prof.Giovanni Carlomagno Dip.diEnergetica, Termofluidodinamica Applicata e Condizionamento Ambientale Universita' diNapoli P.leTecchio, 80 80125Napoli 39-81-7682178 Prof.RobertL.Carovillano NASA Headquarters CodeSS 300E.Street,S.W. Washington, DC20546 202/358-0894 Mr.JosephA.Carroll TetherApplications 1813Gotham Street ChulaVista,CA91913-2624 619/421-2100 Dr.KellyChance Harvard-Smithsonian Center forAstrophysics 60Garden Street MS50 Cambridge, MA02138 617/495-7389 Mr.Chia-Lie Chang Science Applications International Corp. 1710Goodridge Drive,T-2-3 McLean, VA22102 703/734-5588 Dr.Franklin R.Chang-Diaz NASA, Johnson SpaceCenter Houston, TX77058 713/244-8923 220 Mr.Maurizio Cheli NASA, Johnson SpaceCenter Houston, TX77058 713/244-8739 Mr.Aaron Chilbert Naval Research Laboratory Code 8210 4555Overlook Avenue, S.W. 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Prof.SergioVetrella Dip.diIngegneria Aerospaziale Seconda Universita' diNapoli ViaRoma,29 81031Aversa (CE) tel+39-81-5044035 Dr.FrankR.Vigneron Canadian SpaceAgency P.O.Box11490 Station H Ottawa, Ontario, K2H82SCANADA 613/998-2741 Mr.Giuseppe Viriglio Alenia Spazio S.p.A. CorsoMarche 41 10146Torino, ITALY W- Mr.BruceK.Wallace NASA, Marshall SpaceFlightCenter EL64 Marshall SpaceFlightCenter, AL35812 205/544-1306 Dr.William J.Webster, Jr. NASA, Goddard SpaceFlightCenter Code920.0 Greenbelt, MD20771 301/286-4506 Mr.Kenneth J.Welzyn NASA, Marshall SpaceFlightCenter FlightDynamics Branch ED-13 Marshall SpaceFlightCenter, AL35812 205/544-1731 Mr.ScottL.Wetzel AlliedSignalTechnical Services Corp. 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Code8233 Washington, DC20375-5355 202/404-8337 233 Acknowledgments Thisedition ofthehandbook is dedicated tothememory ofthepeople involved intheadvancement oftethers who arenolonger withus,among them Stanley Shawan, BillyNunley andSilvio Bergamaschi. 234