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)

2 Acompactspacepowersystem, ,long-lastingsourceofelectricitythatisre lativelyinsensitivetothechillingcoldofsp aceandvirtuallyinvulnerabletohighradiati onfields,suchasEarth'sVanAllenbeltsandJu piter' :Thediscipline, :DESCRIPTIONTheThermalControlSystem(TCS) ,longtermsurvival, 'sexternalthermalenvironment,itsinternal heatgeneration( ,wasteheatfromtheoperationofelectricaleq uipment), ; ,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_.

3 D-otherplanetsinthesolar[system,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)]]

4 , , ,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( ).

5 ForaspacecraftinLEOtheorbitalaveragealbe dovariesfromabout24%to42%,dependingonorb italinclination, ,butisgenerallyabout251 :r_,c_1 i _JL.,,' ,..,',,,,'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.

6 Specificallydesignedtocontrolheatflows, ,suchasvacuum, , ,thermaldesignrequiresexpendituresinmass ,volume, ,sincepropertiessuchasthermalcapacitance , , ,gradientandrateofchangerequirements,but alsoestablishallocationsforpower,mass, , ,forexample,limitthematerialselectiontoo nlythosematerialsthatwillpreventElectroS taticDischarge(ESD),orthosethatcansurviv ealltemperaturesthespacecraftwillencount erthroughoutitslifecycle,ormaterialswhic hmeetstringentout- , ,butcriticallyimportantinput, ,resources,andinputsareinplaceforapartic ularphaseofthethermaldesign, , , ,designchangesinmanyofthespacecraftsubsy stemswilloccurduetoscience,technicaland/ orcostconsiderations, ,inherentinthethermaldesignactivity, ,preferablyroomtemperature, ,optics,andsensorsmayrequirecryogenictem peratureswellbelow-100 C, ,suchasthepresenceorlackofdirectsolarrad iation,solarradiationreflectedoffaplanet ,andthermalradiationfromaplanet.

7 Thateveninputasimmutableasthesolarconsta ntvariesinrealitybetween1318W/m2and1418W /m2foraspacecraftnearearth,andspacecraft 10orbitingearthoranotherplanetcangoinand outofitsshadow,itbecomessoonapparentthat thereisnosingleanswertothefrequentlyaske dquestion"'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)

8 WhereQ=heatradiatedtospacefromtheradiato rortherestofthespacecraftD=StefanBoltzma nnconstant, []=ThermalemittanceA=Area,m2T=Temperatur e,KSubscript'r'referstoradiator,"SC'refe rstothespacecraft,and'S' ,whichservesasaneffective, , (MLI),whichconsistsofupto30highlyreflect ivelayersofMylarorKapton,separatedbythin layersofnettingmadefromlowconductivityma terialssuchasDacron,toavoidcontactbetwee ntheradiationlayers( ,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).

9 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).

10 ,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).