Spacecraft Thermal Control - NASA

1 EncyclopediaofPhysicalScienceandTechnolo gy,ThirdEditionbyAcademicPress(Finaldraf tdated,March30,2001)SpacecraftThermalCon trol/' :DescriptionSpacecraftThermalRequirement sandSpaceThermalEnvironmentsDesign,Analy sis,andTestingThermalControlHardwareLaun chandFlightOperationsAdvancedTechnologie sforFutureSpacecraftGlossaryAlbedoThefra ctionofincidentsolarenergythatisreflecte dofaplanetarybodyAstronomicalUnit(AU) ' ,asteroids, (LEO)Orbitswhosemaximumaltitudesarelesst hanapproximately1, (GEO)Anorbitintheplaneoftheequatorwhosep eriodmatchestheEarth'srotation,thus,thes pacecraftremainsoverthesamelocationofthe EarthatalltimesMultilayerinsulation(MLI) (RItU) , (RTG)Acompactspacepowersystem, ,long-lastingsourceofelectricitythatisre lativelyinsensitivetothechillingcoldofsp aceandvirtuallyinvulnerabletohighradiati onfields,suchasEarth'sVanAllenbeltsandJu piter' :Thediscipline, :DESCRIPTIONTheThermalControlSystem(TCS) ,longtermsurvival, 'sexternalthermalenvironment,itsinternal heatgeneration( ,wasteheatfromtheoperationofelectricaleq uipment).

2 ,spacecraftinorbitaroundEarthoranotherpl anetwillhaveavariableexternalenvironment iftheirorbitpassesthroughtheplanet' ,MultiLayerInsulation(MLI)blankets,two-p hasedevices(suchasheatpipes, ),mechanicallouvers,thermalstraps,heater s,RadioisotopeHeaterUnits(RHUs),thermost ats,temperaturesensors,mechanicalpumpsto circulateheattransferliquids, ,shownwithitsthermalcontrolsysteminFigur e2, ,designedforamissiontotheplanetSaturn, 'sraysareincreasinglyweakInsteadtheyhave RTGs,anuclearenergysource, ,alsocalledsatellites, ,meteorology, ,_tar,_a_:d-otherplanetsinthesolar[syste m,andstars,galaxies, ,Jupiter,Saturn,NeptuneandPlutoandspecia llocations[ (LEO),GeosynchronousEarthorbit(GEO), ,military, , , , (1to10kW)andthethermalcontrolsystemneeds toremovetheheattokeepthetransmitters,ele ctronicequipment, ( ,theyareveryelliptical) , ,andtheremaybefreemolecularheatingcaused byarapidpassagethroughtheearth'stenuous, butnotnegligible, ,theactuallaunchenvironmentandtransferor bitalmaneuvers,fromLEOtoGEOforexample, ,flybys,landers,rovers, ,whichorbitsJupiter,andtheCassinispacecr aft, , ,thethermalenvironmentofthespacecraftisc ontinuallychangingfromlaunchtofinaldesti nation, , (RTG)]]

3 , , ,launch, , ,propulsion,telecorn,mechanical, Thermal , avionics, , ,thehydrazinepropellantusedonthePathfind erspacecrafthadalowerallowablelimitof10C , , ,butitisnotunusualthattemperaturerequire mentsarealsoaccompaniedbyrequirementsadd ressingtemporaltemperaturestability,spat ialgradients,heatflow, , , ' ,albedo,planetaryinfraredthermalradiatio n, ; (alsocalledthesolarirradianee) ,r= (Watts/m2)/(AU)2(1)Foraspacecraftintherm alequilibriumreceivingheatonlyfi'omtheSu nandlosingheattospacebyradiationandwithn ointernalheatgeneration,theheatbalanceeq uationreducestoAbsorbedsunlight=Heatradi atedtodeepspaceApqa=AasTsc4(2)Andthetemp eratureofthespacecraft,Tsc,becomesTsc:(q Apct/(aAs))'_(3)6 WhereAp=ProjectedareatoSun(m2)A=Totalexp osedarea(m2)qSolarirradiance(W/m')ct=Sur faceabsorptanceinthesolarwavebande=Therm alemittanceo=Stefan-Boltzmannconstant, ,t_/e, ,correspondingtovariousplanets, , ( )torelativelyhigh( ).ForaspacecraftinLEOtheorbitalaverageal bedovariesfromabout24%to42%,dependingono rbitalinclination, ,butisgenerallyabout251 :r_,c_1 i _JL.

4 ,,' ,..,',,,,'t,.Jvlar:, ,..,.c,-or_fo" ,4_=c__;o_,;..,..;'"Table-2 Note; , , ,suchasanRTG,mustalsobeconsideredintheth ermaldesignasthiselectricity(lessanyradi atedawaybyradiotransmitters,lasers,oroth ersuchdevices) ,ANALYSIS, ,electricalpower,costandlabortoimplement thedesignonthespacecraft, , , , , , , ,butoftenoverlooked, , , ,thethermalsubsystemwillinvariablyimpose requirementsuponothersubsystemssuchasmec hanical,power,andscience, ,showninFigure8,iscontrolled,requiresres ources, , , ,thethermalengineerprogressesfromsimpleh eatbalancecalculationsandcoarsecomputera nalysis,topreliminarydesigns,development altesting,increasinglymoredetailedcomput ersimulations,detaileddesigns,therealiza tionofthedesigninhardware,integrationoft hehardware,thermaltestingtovalidatethede sign, , , , , ,specificallydesignedtocontrolheatflows, ,suchasvacuum, , ,thermaldesignrequiresexpendituresinmass ,volume, ,sincepropertiessuchasthermalcapacitance , , ,gradientandrateofchangerequirements,but alsoestablishallocationsforpower,mass, , ,forexample,limitthematerialselectiontoo nlythosematerialsthatwillpreventElectroS taticDischarge(ESD)

5 ,orthosethatcansurvivealltemperaturesthe spacecraftwillencounterthroughoutitslife cycle,ormaterialswhichmeetstringentout- , ,butcriticallyimportantinput, ,resources,andinputsareinplaceforapartic ularphaseofthethermaldesign, , , ,designchangesinmanyofthespacecraftsubsy stemswilloccurduetoscience,technicaland/ orcostconsiderations, ,inherentinthethermaldesignactivity, ,preferablyroomtemperature, ,optics,andsensorsmayrequirecryogenictem peratureswellbelow-100 C, ,suchasthepresenceorlackofdirectsolarrad iation,solarradiationreflectedoffaplanet ,andthermalradiationfromaplanet, ,thateveninputasimmutableasthesolarconst antvariesinrealitybetween1318W/m2and1418 W/m2foraspacecraftnearearth,andspacecraf t10orbitingearthoranotherplanetcangoinan doutofitsshadow,itbecomessoonapparenttha tthereisnosingleanswertothefrequentlyask edquestion"'Howhotdoesitget?"Instead, , "worst-casescenarios",ataminimum, ,ifapplicable, ; "cold" , ,launch,orbitinsertion, ,ifthemissionistoadifferentplanet,thelau nchisfollowedbyacruisephasebetweenplanet s,andpossiblyaerobraking,landing, , , +OAlbedo+PEarthlR4rPinternal=QRadiatorq" QSpacecraflwhere(4)QsolarQEa_hlRQAlbedoQ _d_to_Qspacecraft=heatabsorbedbythespace craftfromincidentsolar=heatabsorbedbythe spacecraftfromplanetaryIR=heatabsorbedby thespacecraftfromalbedo=heatrejectedfrom specifiedradiatingsurfaces=heatrejectedf romtheentirespacecraftexceptfromtheradia torsOncelaunched, , Cr*4*O'*(_r--_s)__Spacecraft_OCsc*Asc_gO '_g(_csc-Ts4s)(5)(6)whereQ=heatradiatedt ospacefromtheradiatorortherestofthespace craftD=StefanBoltzmannconstant, []=ThermalemittanceA=Area,m2T=Temperatur e,KSubscript'r'referstoradiator,"SC'refe rstothespacecraft,and'S' ,whichservesasaneffective, , (MLI)

6 ,whichconsistsofupto30highlyreflectivela yersofMylarorKapton,separatedbythinlayer sofnettingmadefromlowconductivitymateria lssuchasDacron,toavoidcontactbetweenther adiationlayers( ,athermalshort).. , , ,itmaybedesirablethattemperaturesnotchan gemuchwhenpartsofthespacecraftareturnedo ff,Inthesecases, , (andthusthesolararraysarenotpointedatthe sunandonlybatterypowerisavailable), ,thethermaldesignersmaychosetouseotherte chniques, , , :13L(7)whereQ=heatconducted,WA=crosssect ionofconductionpath,m:L=lengthofconducti onpath,mk=conductivityofmaterial,W/mKT=t emperature, , (amountofheatperunitarea).Therefore, , ,structural, , ,sinceaspacecraftcannotbeservicedorrepai redonceitislaunched,andtherearenumerousu ncertaintiesinthedesign,testing,integrat ion, , ' , , , ,,T4___Solarq-OAlbedo' '-OC_radiatorl_adiatorO"(radiatoT'Z_spac )(8)Forthenadirfacingsurfaceofa1meterbla ckcube, ,inordertosimplifytheproblem, (W/m2) :Qsotar=ot,qso_r*AreaQA_ao=a*qA_ao*AreaQ eta,,aR=_*qPta,_R*AreaLetusfurtherassume thattheinternalpowerdissipationis150 WandthattheSpacetemperatureis0 Kelvin(note:0 Kelvinisanidealcase;aD'picalspacecraftin LEOmightseesinksontheorderof200to230 KelvinonitsEarthfacingside).

7 ,sothattheircontributiontotheoverallheat exchangeisnegligible,thenweobtain:(a*( + )+c* )*A+150=-o,'*A*o-*T4(9)15 Thisequationshowsthatforagivenenvironmen tandinternaldissipation, "off"modetheinternaldissipationneedstobe madeupbyheatersconsumingscarcepower, , + ,forthenadir-facingsurface,onewouldobtai n:(10) + (11)Foradesiredtemperatureof27 C(300K), ,ontheotherhand,thenadir-facingsurfaceha dbeenselected, ,whenthequestionisasked:Whatistheheaterp owerrequirementforthetwodesignoptionsifa minimumtemperatureof0 Cisdesiredinthe"ofF'mode?Usingtheequatio nsabove,itcanbedeterminedthatthenadir-fa cingradiator,althoughlarger, , , , ,power, , , , ,theusercanprogramanylogicintothesesolve rsthatmayberequiredtoemulateuniquespacec raftthermalbehavior, "conductances" , ,thermalengineerscontrolthelevelofdetail thatisbeingsimulatedjudiciously, ,preandpostprocessingofdata, (asopposedtoalocallyoptimizeddesign). , , , , , , , ,oftenarraysofquartzlampsandreflectors, ' , , 'slifecycle, "systemlevelprotoflightorqualificationte st".

8 ,but, ,andthedegreeofexcessabove/belowthedesig ntemperatures, , , , , , ,correlationwithin1 Cto5 , , , ,coating, ,radiatorswithhighemissivecoatings,heate rs, , ,heatpipes,capillarypumpedloopsandloophe atpipes,thermalstraps,specialthermalinsu lationneededforMartiansurfaceenvironment ,mechanicalpumpscirculatingcoolingliquid s, , ,Nomex,silk, (overasquaremeter) ,suchasMartiansurfacewherea8to10ton-CO2i spresent, ,afoaminsulationwasused,whereas,forthemi crorover, ,capillarypumpedloops, , ,whentheareaislarge, , (EOS-MLS) , , (16-baldesor8bladesonJPLspacecratt) , ,thuswhentheyareinaclosedposition, ,capillarypumpedloops, (generatedeitherpassivelyoractively),the sedevicescanvarytheireffectivethermalcon ductancefromtheequipmenttotheradiatorsur face, , ,however, ,capillarypumpedloops, , , , ,ifnecessary, (EOS-TERRAin2000) ,heatswitches,sunshades, , ,tlowever,ifitisnecessa_tomakechanges(du etoequipmentfailure,unexpectedconditions ,etc.)itmaybepossibletomodifytheoperatio nofthethermalcontrolsystemifthespacecraf tisprovidedwiththerequisitetelemet_" ,thethermalenvironmentduringthegroundtes tingandlaunchperiodis,inevitably, , ,temperaturecontrolledairisoftenblownint othespacecraftforthispurpose,Theearlylau nchphaseisusuallynotaproblemthermallyasi tisve_-shortandthespacecraft' , , ,changesinthermostatsetpoints,useofredun dantthermalcontroldevices, ,informationgleanedfromevaluationofthefl ightdatacanconstitutevaluable"lessonslea ned" , ,thepurposeofnewNASA missionsistoachievenewsciencewhichinevit ablyrequiresmoresensitivemeasurements,gr eaterpointingaccuracy,operationinmorecha llengingenvironments, ,suchasheaters,MLI,heatpipes,louvers,spe cializedradiatorcoatings,etc.

9 , ,numerousrecentlylaunchedspacecraft( ,TERRA,Marspathfinder,etc.)andothersinth edevelopmentstage(ICESAT,Swift,MarsExplo rationRover,etc.)utilize"new"technology, suchastwo-phaseheattransportdevicesandlo nglifemechanicalpumps, , , ,thermaldesignismoreandmoreintimatelytie dtootherpartsofthespacecraftandaffects,a ndisaffectedby, : tightertemperaturecontrolrequirements(+/ -1 +/-perhaps20 Cofearlierequipment tighttemperaturecontroloververylargearea sinordertomaintaindimensionalstabilityfo rlargemirrors,opticalbenches,antennas,or similardevices temperaturecontrolatdeepcryogenictempera tures(<4K)foropticsandinstruments extremelychallengingthermalenvironments( nearthesunorverydeepspace) advancedthermalcontrolwithminimalresourc es( ,heaterpower,controlcircuitry,massandvol umeallowances,etc.)fi'omthespacecratt highlyintegratedspacecrattandinstrumentd esignswhichrestrictuseofconventionalappr oachestothermaldesign minimalgroundtestingandqualificationeffo rts(duetoschedule,cost,orotherconstraint s) miniaturizationofspacecrattresultinginhi ghpowerdensitycomponentsinsideneedingeff icientheatremovaldevicesAccordingly.)

10 ,polymerdevicesutilizingtheelectrochromi ceffect,andthinflapsofinsulationthatcanb eheldcloseto,oroffof,asurfacebyutilizing anelectrostaticeffect two-phaseheattransportdevicescapableofop eratingatdeepcryogenictemperaturesforsen sors,optics,andinstruments devicesutilizingphasechangematerialstogr eatlyincreasetheeffective"thermalcapacit ance"ofadevice, heatpumpstoallowradiativeheatrejectionwh enthetemperatureofthethermalsinkisnearor abovethedesiredcontroltemperature advancedcryocoolers(basedontheStirlingcy cle,reverseBraytoncycle,adiabaticdemagne tization,pulsetubeeffect,orotherthermalc ycle)forcoolingsensors advancedthermalinsulationmaterialforenvi ronmentswithalow,butnon-negligibleatmosp here( ,Marsandplanetaryenvironments,highaltitu deballoons,etc.) improvedanalyticalmodelsforsimulatingthe performanceofthermalcontroldesignsunderg roundtest,launch,andflightconditions longlifepumps(mechanical,EHD,etc.)forcir culatingacoolant25 miniaturizedthermalcontroldevicessuitabl eformicrosatsandnanosats materialswithveryhighthermalconductivity ( ,variousformsofcarbonandartificialdiamon d) two-phaseheattransportdevicescapableofis othermalizingverylargestructures devicescapableofabsorbingveryhighheatflu xes(hundredsofW/cm2)fromlasers,electroni cchips,powerconverters,andsimilarhighene rgydevices coolingtechnologiesembeddedwithsensors,e lectronicchips,orotherdevices improvedtemperaturemeasurementdevicesint egratedwithon-boardcontrollersandsoftwar eImprovedthermalcontroltechnologyisultim atelydrivenbytheneedforsuchcapability, , ' ,CaliforniaInstituteofTechnology, :RayBeckerwhoreviewedthearticle;SiinaHaa rpanenwhoputtogetherseveralofthetablesan dpictures;andGlennTsuyuki,ArtAvila,GaryK insella, ,B.