NASA Technical Reports Server (NTRS) 20170000375: Experimental Characterization of a Composite Morphing Radiator Prototype in a Relevant Thermal Environment

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by  NASA Technical Reports Server (NTRS)

Experimental Characterization of a Composite Morphing
Radiator Prototype in a Relevant Thermal Environment
ChristopherL.Bertagne ,JorgeB.Chong†,JohnD.Whitcomb‡,DarrenJ.Hartl§
⇤
TexasA&MUniversity,CollegeStation,Texas,77843
LisaR.Erickson¶
NASAJohnsonSpaceCenter,Houston,Texas,77058
Abstract
Forfuturelongdurationspacemissions,crewedvehicleswillrequireadvancedthermalcontrolsystemstomaintaina
desiredinternalenvironmenttemperatureinspiteofalargerangeofinternalandexternalheatloads. Currentradiators
areonlyabletoachieveturndownratios(i.e. theratiobetweentheradiator’smaximumandminimumheatrejection
rates)ofapproximately3:1. Upcomingmissionswillrequireradiatorscapableof12:1turndownratios. Aradiatorwith
theabilitytoaltershapecouldsignificantlyincreaseturndowncapacity. Shapememoryalloys(SMAs)offerpromising
qualitiesforthisendeavor, namelytheirtemperature-dependentphasechangeandcapacityforwork. In2015, the
firstevermorphingradiatorprototypewasconstructedinwhichSMAactuatorspassivelyalteredtheradiatorshapein
responsetoathermalload. Thisworkdescribesafollow-onendeavortodemonstrateasimilarconceptusinghighly
thermallyconductivecompositematerials. Numerousversionsofthisnewconceptweretestedinathermalvacuum
environmentandsuccessfullydemonstratedmorphingbehaviorandvariableheatrejection,achievingaturndownratio
of4.84:1. Asummaryofthesethermalexperimentsandtheirresultsareprovidedherein.
I. Introduction
ForfuturecrewedspacecrafttargetingexplorationbeyondLowEarthOrbit(LEO)difficultthermalcontrolrequirements
willneedtobesatisfied. Thethermalcontrolsystem(TCS)willbeexpectedtomaintainpreciseinternaltemperatures
despite large large variations in the external thermal environment and internal system heat loads. In many cases
the TCS will be required to reject a high heat load to a warm orbital environment and a low heat load to colder
transitenvironments.1 Thisinverserelationshipbetweenthethermalenvironmentandtheheatrejectionneedsofthe
mission/spacecraftrequiresthermalcontroldeviceswithahighturndownratio. Theturndownratio,definedasthe
ratiobetweenthemaximumandminimumheatrejectioncapabilitiesoftheTCS,isaprimarymeasurementofTCS
performance.2 Moreprecisely,thisratioinvolvesthemaximumheatrejectionrateinthehottestenvironmentandthe
miminimumheatrejectionrateinthecoldestenvironment. TheturndownratioofaTCSisthereforemeasuredrelative
totheparticularthermalenvironmentsinwhichitisdesignedtooperate. FuturemissionsbeyondLEOarepredictedto
requireturndownratiosbetween6:1and12:1.3 Thehighestturndownratioastate-of-the-artTCSachievesis3:1(in
LEO).4Here,theTCSrequiresaheattransferfluidwithalowfreezingpoint.5 Suchfluids,however,aretypicallytoxic
(e.g. ammoniaontheInternationalSpaceStation),whichposeahazardtothecrew. ThislimitstheTCSarchitectureto
atwo-loop(multi-fluid)designthatisolatesthecrewfrompossibleexposuretotheharmfulfluid.
Several efforts have been undertaken to improve TCS performance through the development of variable heat
rejectionradiators.5 Examplesincluderoll-outfinradiators,6 freezableradiators,2 digitalradiators,7 andvariable-
emissivityradiators.8–11 Thisworksupportsthedevelopmentofanoveltypeofradiator,knownasavariable-geometry
ormorphingradiator,12 andprovidesdetailsregardingthethermalcharacterizationofvariousprototypesthatwere
built.
⇤NASASpaceTechnologyResearchFellow,DepartmentofAerospaceEngineering,3141TAMU,AIAAStudentMember
†UndergraduateResearcher,DepartmentofAerospaceEngineering,3141TAMU
‡Professor,DepartmentofAerospaceEngineering,3141TAMU,AIAAMember.
§AssistantProfessor,DepartmentofAerospaceEngineering,3141TAMU,AIAAMember.
¶ThermalEngineer,CrewandThermalSystemsDivision,2101NASAParkway.
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Shapememoryalloysareideallysuitedforthedesignofmorphingradiators. SMAsundergochangesinshapein
responsetochangesintemperature;areversibleprocessduetothestress-andtemperature-dependenttransformation
between two solid material phases: austenite and martensite.13 Applied as mechanical actuators on a morphable
radiatortheywouldexploitenergytransferredbetweentheenvironmentandworkingfluidtochangetheradiatorshape
withnoneedforexternalpower,control,orsensinginstrumentation. Sucharadiatorhasthepotentialtoachievethe
highturndownratiosnecessarytoenablesingle-loopthermalcontrolofavehicleusinganon-toxic,high-freezing-point
workingfluid,suchasapropyleneglycol/watersolution(PGW).Tradestudieshaveshownthatasingle-loopTCSwith
amorphingradiatorsystemwouldreducetheTCSmassbyapproximately25%comparedwithastandardtwo-loop
design.3 Thus, the morphing SMA radiator concept has the potential to revolutionize current TCS technology by
decreasingsystemmassandcomplexity,whileincreasingversatility.
II. Description of Morphing Radiator Concept
Theprimarygoalofthisworkwastodevelopandconductexperimentalstudiesonastructurallyandthermallyoptimized
compositemorphingradiatorpanel,orfacesheet. Thispaperfocusesprimarilyonthethermalanalysisconsiderations;
the mechanical design is discussed separately.14 Figure 1 shows a morphing radiator design which combines the
transformationresponseofshapememoryalloyswithathermallyconductiveandlinearlyelasticbiasingstructure,
creatingaradiatorpanelthatreconfigurespassivelyinresponsetochangesintemperature.12 Theradiatorconsists
ofacircularcompositepanelfixedalongthepanellineofsymmetryattheroot. Hereaflowtubeisattached,which
conductsheatfromtheTCSworkingfluidintotheradiator. Ahigh-emissivitycoatingisappliedontheinner(concave)
surface,shownwithdarkshading,andalow-emissivitycoatingisappliedontheouter(convex)surface,shownwith
lightshading. SMAsattachedtotheoutermostsurfaceofthepanelcausetheradiatortomorphbetweenvariousshapes,
alteringitsviewfactortospace. Thepaneltemperaturedrivesthisprocess. Whensufficientlycold,theradiatortakes
onthefullyclosedcircularshapeshowninFig.1a. Astheradiatortemperatureincreasesduetoawarmerambient
environmentand/orincreaseintheheatload(fromthefluidintheflowtube),theSMAscontract,whichopensthe
radiatortoawiderconfigurationasshowninFig.1b;themaximumheatrejectionshapeisdepictedinFigure1c. This
morphingbehaviorisintendedtobefullyreversible. Together,thevariableviewfactorandselectivesurfaceemissivity
increasetheturndownratiooftheradiator,aswillbeshown.
(a)Closedshapeforminimum (b)Semi-openshapeforintermediateheat (c)Openshapeformaximum
heatrejection. rejection. heatrejection.
Figure1: Schematicrepresentationofaflexiblemorphingradiatorpanel. Lightanddarkshadingrepresentslow-and
high-emissivitycoatings,respectively. 14
AnarrayofmorphingradiatorpanelsisshowninFig.2,illustratingtheirapplicationinaparallel-flowconfiguration.
Hotfluidenterstheradiatormanifoldviatheinletheader,whichdistributestheworkingfluidamongmultipleparallel
flow tubes. A series of individual morphing radiator panels are attached along each flow tube. The working fluid
temperaturedecreasesalongthelengthofeachtubeasheatisrejectedviaradiation. Inawarmenvironment(e.g. LEO),
theworkingfluidandpaneltemperaturesremainabovethemartensitestarttemperature,suchthattheSMAsarefully
contractedandallpanelsarefullyopen. Thus,heatisrejectedthroughthegreatestpossibleradiatorareaviathehigh
emissivitysurface. Inacoolerenvironment,thetemperaturenearthedownstreamendoftheflowtubedecreasesbelow
themartensitetransformationtemperature,causingtheSMAstoexpand. Thisallowsthepanelstoclose,minimizing
heatrejection. Asaresultofsuchbehavior,thisradiatordesignwilltendtomaintainaminimumfluidtemperatureinthe
rangeofthetransformationtemperaturesoftheSMA(i.e.,betweentheausteniteandmartensitefinishtemperatures15),
whichcanvastlysimplifytheoveralldesignoftheTCS.
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Figure2: Illustrationofanarrayofradiatorpanelsinaparallelflowconfiguration. Arrowsindicatefluidflowdirection.
This concept featured a number of design challenges. In particular the face sheet required three contradictory
material characteristics: adequate thermal conductivity to transport heat out of the fluid loop, ample flexibility to
deformtoitsopenhotshape,andsufficientstiffnesstoprovidespringforceforreturningtheradiatortoitsclosedcold
shape. Figure3showstheprototypemorphingradiatorusedintheexperimentalstudiesin2015.16 Thisprototypewas
designedtousewire-typeSMAstoachievethetemperature-inducedmorphingbehaviordepictedabove. (Notethat
Fig.3acorrespondstoFig.1c,withtheSMAsintheiraustenitic(high-temperature)state.) Theprimarycomponentin
thisprototypeisacompliantandthermallyconductivecopperpanel(7inlong,3inwide,0.007inthick),whichwas
rolledalongitslengthtoformthesemicircularshapeshowninFig.3. Tenshapememoryalloywiresrestagainstthe
outersurfaceofthepanelandarefixedateachendtothestraightedgesofthepanelwithapairofterminalblocks
fabricatedfrom0.25insquarealuminumstock. Thewiresareotherwiseunconstrained,allowingthemtoslidealongthe
panelastheytransformlocally. Benchtoptestsindicatedtheneedforanadditionalbiasingforcebeyondthatprovided
bythecoppertodrivethepaneltowardstheclosedcircularshapeundercooling. A1inwide, 0.007inthick1095
steelclosingspringwasattachedtotheconvexsideofthepanelforthispurpose. Inordertoincreasetherateofheat
rejectionviaradiationintheopenshape,theinsidesurfaceofthecopperpanelwaspaintedwithAeroglazeZ307®,a
flexible,high-emissivitypolyurethanecoating. Theoutsideofthepanelremainedunpainted. Theemissivitiesofthe
AeroglazeZ307paint,unpaintedcopper,andunpaintedsteelweremeasuredtobe0.943,0.047,and0.143respectively,
withaSurfaceOpticsCorp.ET-100®emissometer. Thecopperpanelwasattachedtoa0.375indiameterstainlesssteel
flowtubeusingathermallyconductiveepoxywhichallowedtheradiatortobeintegratedintoapumpedfluidloop
fortheexperiment. Theprototypewastestedinathermalvacuumenvironmentwhereitdemonstratedthemorphing
behaviorunderarangeofheatloadsandachievedaturndownratioof6.4:1.16
Thegoaloftheeffortsdescribedhere(andintheaccompanyingwork14)wastoimprovebeyondthepreliminary
prototypeandstudyof2015, producinganewdevicecomprisedofmoreadvancedcompositematerialsandSMA
componentswithtransitiontemperaturesbettertunedtothethermalvacuumenvironment. Acarbon-fiberfacesheetwas
developedforthispurpose. Studiesweredonetooptimizethedesign,takingintoconsiderationfiberorientationand
SMAintegration. PrototypefacesheetswerefabricatedusingmultiplepliesofCSWT40-800/5320-1,aunidirectional
prepregtapedesignedforout-of-autoclavecuring.17 Thefullprocessofmaterialselection,manufacturing,andfatigue
testingoftheseadvancedmorphingpanelsisdescribedintheaccompanyingpaper.14 Thefollowingsectionsdescribea
benchtopdemonstrationandthermalvacuumtestingoftheseprototypes.
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Steel closing spring
Flow tube
SMA wires
33
High-emissivity 22
coating 11
Copper
panel
99
10 88 66
77 55
Flow tube 44
Set screws
Aluminum
terminal block
(b) Inside of prototype showing high-emissivity
(a)Outsideofprototypeshowingprimarycomponents.
polyurethanecoating.
Figure 3: Prototype morphing radiator test article from previous 2015 studies.16 Panel is at room temperature
demonstratingopenshape. Opencirclesshowlocationswherethermocoupleswereattachedfortesting.
III. Benchtop Prototype
Afterfabricationandpriortothermalvacuumtesting,abenchtopprototypewasfabricatedasaproof-of-concept. This
prototype(showninFig.4)featuredanumberofSMAwiresinthemartensitephaseatroomtemperature(i.e. thepanel
wasintheclosedconfigurationatroomtemperature(Fig.4a)). Thepanelwasheatedusingtwoheatgunstosimulate
theheatloadfromthefluidlooponthespacecraft. Theincreaseintemperaturecausedthephasetransformationinthe
SMAwires,drivingthepaneltoitsopenconfiguration(Fig.4b). Whentheheatwasremovedthecompositecooled
convectivelyinthelaboratoryenvironmentandreturnedtotheinitialclosedconfiguration. Inthismanner,theshapeof
theradiatorwascontrolledbythermalloadingexclusively,andthemorphingbehaviorwasdemonstratedinanambient
roomtemperaturesetting.
(a)Closedconfigurationcorrespondingtominimumtemper- (b)Openconfigurationcorrespondingtomaximumtemper-
ature. ature,resultingfromheatprovidedbytwoheatguns.
Figure4: Ademonstrationofthebenchtopprototype.
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IV. Thermal Vacuum Chamber Tests
A. TestArticles
Followingthebenchtopproof-of-concept,threedifferenttestarticleswerefabricatedfortestinginathermalvacuum
environment. Table1givesanoverviewofthecharacteristicsofeachtestarticle. Thefirstrowdescribesthecarbonfiber
plylayering(i.e. thelayup)asasequenceofplyorientationangleswithineachcompositepanel. Theplyorientation
anglesweredefinedwithrespecttothecircumferentialdirection(i.e. thefibersofa0 plyarealongthecircumference
�
ofthepanelandthefibersofa90 plyareorientedparalleltotheflowtube). Thekeymechanicalandthermalproperties
�
ofthepanel,includingbendingstiffnessandeffectiveconductioncoefficientperunitlength(denotedbyK),arealso
t
listedforeachpanel,alongwiththeprimarydimensions. Moredetailsoftheseaspectsofpaneldesignareprovidedin
theaccompanyingwork.14
Table1: Overviewofthethreetestarticlesfabricatedforthethermalvacuumchambertests.
TestArticleA TestArticleB TestArticleC
Layup [90/45/0/45/90] [90/45/0/0/45/90] [90/45/0/0/0/45/90]
Bendingstiffness 0.44Pam3 0.82Pam3 1.42Pam3
· · ·
Cond. coeff. perunitwidth(K) 0.091W/K 0.121W/K 0.152W/K
t
Closeddiameter 3.0in 3.0in 3.5in
Width 3.0in 3.0in 4.0in
SMAtype&qty. Wires(x18) Wires(x36) Strips(x8)
SMAphaseatroomtemp. Austenite Austenite Martensite
Thefirsttwotestarticles,AandB,werenearlyidenticalapartfromthelayup. TestArticleAincorporatedalayup
withasingle0 ply,whilethelayupforTestArticleBincludedtwo0 plies. BothofthesetestarticlesutilizedSMA
� �
wiresintheaustenite(high-temperature)phaseatroomtemperature,requiringTestArticlesAandBtobeassembled
intheiropenconfigurations. ForTestArticleA,asingleSMAwirewasthreadedthrougheachholeintheterminal
blocksforatotalof18wires. SinceTestArticleBfeaturedtwo0 pliesthebendingstiffnesswasapproximatelytwice
�
thatofArticleA(seeTable1),thusrequiringapproximatelytwicetheforcetoopentothesameradius. Tomaintaina
comparablelevelofstressinthewiresbetweenthetwotestarticles,twoSMAwireswerethreadedthrougheachholein
theterminalblockonTestArticleB.Figure5showsTestArticleApriortothermalvacuumtesting.
Afterattachingthewirestothepanels,theflowtubeswereattachedusingthermaladhesiveandsecuredwitha
thermallyinsulatedtape. Thermocoupleswereattachedtothefacesheetatvariouslocationstorecordtemperature
acrossthepanel.
Figure5: TestArticleAfeaturingSMAwiresinstalledinastressed(open)configuration.
Test Article C was slightly larger than Test Articles A and B and used SMA strips instead of wires. Whereas
theSMAwireswereintheaustenitephaseatroomtemperature,theSMAstripsusedinTestArticleCwereinthe
martensite(low-temperature)phaseatroomtemperature. Thus,ArticleCwasassembledintheclosedconfiguration.
Priortoassembly,eachstripwasdetwinnedat300MPatogeneratethetransformationstrainneededforausteniteshape
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recoveryduringheating. Thisprocessinvolvedsubmergingthestripsinliquidnitrogentoreachthemartensitephase
throughoutthematerial,andthenapplying300MPaofaxialtensiontoeachstrip. Figure6showsTestArticleCfully
assembled.
Figure6: TestArticleCfeaturingSMAstripsinstalledinadetwinnedandstressfree(closed)configuration.
B. TestSetup
Thevacuumchamberusedintheexperimentswasasmall,high-vacuumthermalenvironmentchamberlocatedatNASA
JohnsonSpaceCenter. Figure7showsaschematicdiagramoftheflowloopusedinthethermalvacuumchambertests.
TheprimarycomponentwasanSPScientificRC211®,arecirculatingchillershownontheleftsideofFigure7. The
chillercontainsapumpandintegratedheaterandchillermodules,allowingthetemperatureoftheworkingfluidtobe
controlledbetween-45 Cand100 C.ThefluidloopusedDynaleneHC-50® —anontoxicwater-basedcoolantwitha
� �
freezingpointbelow-50 C—astheworkingfluid. Thetemperatureboundsforthetestwereselectedtoensurethe
�
fluiddidnotfreezeduringtesting.
Beginningatthechiller,thefluidfirstpassedthroughtwoparallelflowlines,onewithaninlineheater,andtheother
thatbypassedtheheater. Theselineswereopenedandclosedviaballvalvesduringthedifferentphasesofexperiment
toadjusttheflowtemperatureasdesired. DownstreamofthesevalvesthefluidpassedthroughaCoriolisflowmeter
manufacturedbyMicroMotion®,whichwasusedtomeasuretheinstantaneousflowratethroughouttheexperiments.
Thefluidthenenteredthevacuumchamberandpassedthroughtheflowtubecontactingthetestarticle. Twoimmersion
thermocouples,shownwithnumberedcirclesinFig.7,wereusedtomeasurethefluidtemperaturesattheinletand
outletoftheradiatorflowtube. Finally,thefluidexitedthevacuumchamber,passedthroughacheckvalve,andreturned
tothechiller.
Figure8showsthethermalvacuumchamber,chillercart,andfluidloop. Figure9showsaclose-upofthetest
sectioninsidethechamberwiththeradiatorinstalledintothefluidloop.
Figure7: Schematicdiagramshowingexperimentalsetupforthethermalvacuumchambertest.
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(a)Testsetupshowingthermalvacuumchamber,chiller (b)Closerviewofthermalvacuumchambertestsection
cart,andexteriorfluidlines. showinginteriorfluidlinesandLEDlamps.
Figure8: Photographsofthevacuumchambertestsetup.
Figure9: Close-upofmorphingradiatortestarticleafterbeinginstalledinthefluidloop(TestArticleBshown).
C. TestProcedure
Priortothestartofeachtest,theappropriatetestarticlewasmountedtotheflowlineinsidethechamber(seeFig.7).
Thechamberwasthensealed,avacuumestablished,andthefluidlinesopened. Thefluidtemperaturewascycled
overa 3hourperiod,duringwhichtimethemorphingbehaviorwasobserved. Temperaturedatawasrecordedat
⇠
variouslocationsonthepanelandflowtubebythethermocouplesthroughoutthetest. Additionally,acameramounted
outsideawindowonthevacuumchambercapturedimagesofthepanelat2secondintervalsthroughouttheheatcycling
process. Figures 10and 11showTestArticleAinsidethethermalvacuumchamberduringtesting.
(a) Open configuration corresponding to maximum fluid (b)Closedconfigurationcorrespondingtominimumfluid
temperature. temperature.
Figure10: ImagesofTestArticleAinthevacuumchamberphotographedbythecameraoutsidethechamber.
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(a) Open configuration corresponding to maximum fluid (b)Closedconfigurationcorrespondingtominimumfluid
temperature. temperature.
Figure11: ImagesofTestArticleAinthevacuumchambercapturedbythein-chambercamera.
V. Analysis of Experimental Data
AcustomMATLAB® scriptprocessedtheimagesofthetestarticlescapturedduringthetest,computingthepanel
radiusovertimeviaaleastsquaresregressiontechnique. TestArticlesAandBachievedarangeinpanelradiusof1
inchand1.1inchesrespectively. TestArticleConlyachievedaradiusrangeof0.4inches.
As the radius increased with fluid temperature, the view factor of the internal panel surface increased, thereby
increasingthetotalradiativeheatrejectionofthepanel. Thus,therangeofradiidirectlydrovetherangeofachievable
heatrejectionrates,establishingaturndownratioforthepanel. Tothisend,theanalysisofheatrejectioncentered
aroundTestArticleB,whichachievedthewidestrangeinradius. Thetemperaturehistoriesandpanelradiusovertime
areshownforTestArticleBinFigure 12.
AthermalfiniteelementmodelofTestArticleBwasdevelopedinABAQUS®. Byspecifyingthepanelradiusand
boundaryconditionsknownatvariousinstancesinthethermalvacuumtest,athermalanalysiswasperformedwhich
providedthesteady-statetemperaturedistributionandheatrejectionrateofArticleBateachinstance. Fromthese
simulationsthemaximumandminimumheatrejectionsofTestArticleBweredetermined;thusallowingcalculationof
theturndownratio. Thesimulationalsodirectlycalculatedtheminimumtemperatureatthefreeedgesofthepanel(i.e.
attheterminalblocks)tocomparewiththeminimumpaneltemperaturerecordedinthethermalchambertest.
The time history of heat rejection throughout the testing process for Test Article B is shown in Fig. 13a. The
comparison of minimum panel temperature between these predictions and the actual data from Test 2 is shown in
Fig. 13b. The overall accuracy of the minimum temperature predictions serves as a meaningful validation of the
thermalmodelandtherelatedresultsthatfollow. Intheopenconfiguration(hotphase,maximumradius)themodel
calculatedaheatrejectionrateof9.97W,withadiscrepancyinminimumtemperatureresultsofonly 7 C.Inthe
⇠ �
closedconfiguration(coldphase,minimumradius)aheatrejectionrateof2.06Wwascalculated,withatemperature
discrepancyof 3 C.Thus,theoverallturndownratiowascalculatedat4.84:1. Thelargertemperaturediscrepancy
⇠ �
forthehotphasewaslikelyduetothelargetemperaturegradientexistingbetweentheradiatorroot(pointofcontact
withtheflowtube)andtheterminalblocks( 26 C).Thisgradientismuchlesssignificantinthecoldphase,witha
⇠ �
temperaturevariationof 4 C,explainingthesmallertemperaturediscrepancyinthecoldphase.
⇠ �
Asacomparison,theturndownratiowascomputedforArticleBwithoutincludingchangesduetomorphing. The
radiatormodelwasgiventhesameradiusandsinktemperatureforwhichthemaximumheatrejectionwascomputedin
Fig.13a,whiletheroottemperature(thepaneltemperatureboundarycondition)wasthatoftheminimumheatrejection
case. Fromthissimulationtheturndownratiowithoutmorphingwasfoundtobe4.32:1. Thisdemonstratedanincrease
inturndowncapacitythroughthemorphingbehavior.
Earlierinthispaperitwasstatedthatthehighestturndownratioofastate-of-the-artTCSwas3:1. Itisimportant
toreiteratethatthisisrelativetotheparticularthermalenvironmentinwhichsaidthermalcontrolsystemoperates
(indicated above to beLow Earth Orbit). The turndown ratiosachieved by themorphing radiator areparticular to
thethermalenvironmentsexperiencedinthethermalvacuumchamber. Thoughsimilar,theenvironmentsofthetwo
systemsarenotidentical,whichshouldbenotedwhencomparingtheirturndownratios.
Regardingtheturndownratioachievedbythemorphingradiator,4.84islowerthanrequiredforanactualproduction
system(targetedbetween6:1and12:1asmentionedearlier). Thisturndownratiowaslargelyimpactedbyheatthat
escapedthroughthecircularopenendsoftheradiatorduringthetestingprocess;asourceofuncontrolledheatrejection.
Previousdesignstudieshaveshownthattheadditionoflow-emissivitysidecapswouldblockheatrejectionthroughthe
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openendsandsignificantlyreduceheatlosses,andthatlongcylindricalradiatorswithsidecapsareinfactneededto
ensureoptimalperformance.18 Futureworkwillinvolvemodelingtheradiatorwiththesefeaturestoexploretheeffects
onturndowncapacity.
(a)TimehistoryofTestArticleBpaneltemperatureduringtesting.Thermocoupleswereattachedtotheradiator
atvariouslocations,resultingintherangeoftemperaturesshown.
(b)TimehistoryofTestArticleBpanelradiusduringtesting.Theradiusofcurvatureofeachsideofthepanelis
shown.
Figure12: ExperimentalresultsfromthetestofArticleB(theprototypewiththestrongestmorphingresponse).
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(a)RateofheatrejectionofTestArticleBthroughoutthermalvacuumtest.
(b)ComparisonofminimumpaneltemperaturecalculatedinAbaqussimulationwiththatmeasured
inthethermalvacuumexperiment.
Figure13: ResultsfromthermalanalysisofTestArticleBmodel.
VI. Summary & Future Work
With the increasing need for advanced thermal control systems a variable heat rejection radiator offers a unique
solution. Acompositemorphingradiatoractuatedwithshapememoryalloyswasdesignedtoachievethispurpose.
Multipleprototypesweredevelopedandtestedinathermalvacuumenvironmentwheretheyexhibitedthedesired
temperature-inducedactuationbehavior. TestArticleB,featuringtwocircumferentiallyorientedcarbonfiberpliesand
36SMAwiresachievedthewidestrangeinradiusduringthetestingprocessandofferedaturndownratioof4.84:1.
Themorphingbehaviorwasshowntoaidtheturndowncapacity,whichwas4.32:1withoutmorphing.
TheseeffortshaveshownthatSMAmaterialsarecapableofmorphingacompositeradiatorpanelthrougharange
ofshapeconfigurations,allowingforvariableheatrejectionabovethatachievedbymodernthermalcontrolsystems.
Movingforward,theprojectwillfocusondesignimprovementsinvolvingSMAandflowtubeintegrationtoimprove
heattransferbetweentheworkingfluidandthepanel,thusimprovingthemorphingresponse. Furthermore,theradiator
willbemodeledwithsidecapstoreduceuncontrolledheatrejectionandincreasetheoverallturndowncapacityofthe
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