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Potential operating orbits for fission electric propulsion systems driven by the SAFE400

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Safety must be ensured during all phases of space fission system design, development, fabrication, launch, operation, and shutdown. One potential space fission system application is fission electric propulsion (FEP), in which fission energy is
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  Potential OperatingOrbits forFissionElectricPropulsionSystemsDrivenbytheSAFE-400 MikeHouts, Larry Kos,David Poston NASAMSFC,TD40,MarshallSpaceFlightCenter,Alabama,35812 256)544-7143/Fax: 256)544-5926michaelhouts@msfc,nasa.gov Abstract. Safetymustbeensuredduringallphasesofspacefissionsystemdesign,development,fabrication,launch,operation,andshutdown.Onepotentialspacefissionsystemapplicationisfissionelectricpropulsion(FEP),inwhichfissionenergyisconvertedintoelectricityandusedtopowerhighefficiency(Isp>3000s)electricthrusters.Forthesetypesofsystemsitisimportanttodeterminewhichoperationalscenariosensuresafetywhileallowingmaximummissionperformanceandflexibility.Spacefissionsystemsareessentiallynon-radioactiveatlaunch,priortoextendedoperationathighpower.Oncehighpoweroperationbegins,systemradiologicalinventorysteadilyincreasesas fission productsbuildup.Foragiven fission productisotope,themaximumradiologicalinventoryistypicallyachievedoncethesystemhasoperatedforalengthoftimeequivalenttoseveralhalf-lives.Afterthattime,theisotopedecaysatthesamerateitisproduced,andnofurtherinventorybuildsin.ForanFEPmissionbeginninginEarthorbit,altitudeandorbitallifetimeincreaseasthepropulsionsystemoperates.Twosimultaneouseffectsoffissionpropulsionsystemoperationarethus(1)increasingfissionproductinventoryand(2)increasingorbitallifetime.Phraseddifferently,asfissionproductsbuildup,moretimeisrequiredforthefissionproductstonaturallyconvertbackintonon-radioactiveisotopes.Simultaneously,asfissionproductsbuildup,orbitallifetimeincreases,providingmoretimeforthefissionproductstonaturallyconvertbackintonon-radioactiveisotopes.Operationalconstraintsrequiredtoensuresafetycanthusbequantified. INTRODUCTION The fission processwasfirstreportedin 1939, andin1942theworld's first man-madeself-sustainingfissionreactionwasachieved.Creatingaself-sustainingfissionchainreactionisconceptuallyquitesimple. All thatisrequiredisfortherightmaterialstobeplacedintherightgeometry-noextremetemperaturesorpressuresrequired-andthesystemwilloperate.Since1942fissionsystemshavebeenusedextensivelybygovernments,industryanduniversities.Fissionsystemsoperateindependentlyofsolarproximityororientation,andarethuswellsuitedfordeepspaceorplanetarysurfacemissions.Inaddition,thefuelforfissionsystems(highlyenricheduranium)isessentiallynon-radioactive,containing0.064curies/kg.Thiscomparesquitefavorablytocurrentnuclearsystems(Pu-238inradioisotopesystemscontains17,000curies/kg)andcertainhighlyfuturisticpropulsionsystems(tritiuminD-Tfusionsystemswouldcontain10,000,000curies/kg).Anadditionalcomparisonisthatat:launchatypicalspacefissionpropulsionsystemwouldcontainanorderofmagnitudelessonboardradioactivitythandidMarsPathfinder'sSojournerRover,whichusedradioisotopesforthermalcontrol.Theprimarysafetyissuewithfissionsystemsisavoidinginadvertentsystemstart-addressingthisissue through propersystemdesignisstraightforward.Theenergydensityoffissionissevenordersofmagnitudegreaterthanthatofthebestchemicalfuels,andifproperlyutilizedismorethanadequateforenablingrapid,affordableaccesstoanypointinthesolarsystem.One potential spacefissionsystemapplicationisfissionelectricpropulsion(FEP),inwhichfissionenergyisconvertedintoelectricityandusedtopowerhighefficiency(Isp>3000s)electricthrusters.Forthesetypesofsystemsitwillbeimportanttodeterminewhichoperationalscenariosensuresafetywhileallowingmaximummissionperformanceandflexibility.Spacefissionsystemsareessentiallynon-radioactiveatlaunch,priortoextendedoperationathighpower.Oncehighpoweroperationbegins,systemradiologicalinventorysteadilyincreasesasfissionproductsbuildup.Foragivenfissionproductisotope,themaximumradiologicalinventoryis  typicallyachievedncehesystemasoperatedoralengthoftimeequivalentoseveralalf-lives.Afterthattime, theisotopeecaystthesameatetisproduced,ndnofurthernventoryuildsn.ForanFEPmissioneginninginEarthorbit,altitudeandorbitalifetimencreasesthepropulsionystemperates.wosimultaneousffectsffissionpropulsionystemperationrethusincreasingissionproductnventoryndincreasingrbitallifetime.Phrasedifferently,sfissionproductsuildup,moreimeisrequiredorthefissionproductsonaturallyonvertbackintonon-radioactivesotopes.Simultaneously,sfissionproductsuildup,orbitallifetimeincreases,providingmoreimeforthefissionproductsonaturallyonvertackntonon-radioactivesotopes.Operationalconstraintsequiredoensureafetycanhusbequantified. THE 400KILOWATT THERMALSAFEAFFORDABLE FISSIONENGINE (SAFE-400) NASA'sbaselinePhase1spacefissionelectricpropulsionsystemisbasedonthe400kWtSafeAffordableFissionEngine(SAFE-400)reactor.Hardware-basedresearchanddevelopmentrelatedtoPhase1spacefissionsystemdevelopmenthasbeenongoingsince1996(Houts,1997).Inadditiontoearlymoduletests,a30kWtSAFEcorehasbeenfabricatedandtested,anda100kWtcoreiscurrentlyinfabrication.Thenextstepafterfabricationandtestingofthe100kWtcoreisfabricationandtestingofa400kWtcorethatisnearlyflight-prototypic.Designdetailsarebeingaddedtothe400kWtcore,andfabricationshouldbeginin2003.WhileSAFEperformancemustbeadequatetoenablemissionsofinterest,theemphasisofPhase1systemdesignanddevelopmentisonsafety,affordability,andschedule.KeyfeaturesofPhase1systemsincludeahighleveloftestability,utilizationofestablishedtechnology,andutilizationofexisting / operationalfacilities.DetailsofrecenteffortsatNASAMSFCrelatedtotheSAFEaregiveninVanDyke,2002.DetailsofthereferencePhase1400kWtdesign(SAFE-400)aregiveninPoston,2002.AcomparisonofPhase1systemoptionsisgiveninHouts,2002.ApictureofacoupledSAFE-30 / StirlingenginetestisshowninFigure1. FIGURE1. SAFE-30ProvidingThermalPowertoaStirlingEngine,ResultinginElectricityProduction.  ORBITAL LIFETIMEASA FUNCTION OFALTITUDE Orbitallifetime asa functionof altitude wascalculatedforthreespacecraftmass-to-frontalarearatios:82kg/m2,250kg/m2,and1000kg/m2.Spacecraftaltitudesof550to700kmwereexamined.TherangewaschosenbasedonSpaceShuttlecapabilitiesandatmosphericdrag.Thepayloadcapability(includingattachmenthardware)ofthespaceshuttletoa550kmorbitexceeds13,500kg.ThispayloadcapabilityissignificantlygreaterthanthatrequiredbyallbutthemostambitiousPhase1FEPmissions.At700kmatmosphericdragisextremelylow(effectivelyzeroabove800km);however,thespaceshuttleisnotcapableofreachingthatorbit.Thecode LIFTIM (Alford,1974).< 70O 675 65O 6256OO57555O5255OO47545O425 4OO 37535O 325 30027525O2252OO1750365 730 10951460182521902555292032853650 Days from12/31/2011 FIGURE   OrbitalLifetime asa FunctionofAltitudefor Three SpacecraftMass-to-AreaRatios.wasusedtoperformthecalculations.Aninitialorbitinsertion date of31December2011was assumed, andpredictedfluctuationsoftheEarth'satmosphereweretakenintoaccount.ResultsofthecalculationsareshowninFigure2.Thecalculationsareconservativeinthatthelaunchdateischosentobejustpriortopeakatmosphericdensityataltitude.Atmosphericdensityvaluestwostandarddeviationsabovethepredictedvalue(+2s)andtwostandarddeviationsbelowthepredictedvalue(-2s)areplotted.AsshowninFigure2,an82kg/m2spacecraft  e.g. an8200kgFEPvehiclewith100m2offrontalarea)placedina550kmorbitisestimatedtorequire20monthstore-enter,assuminga12/31/2011launchanda+2s(worst-case)atmosphere.Thesamespacecraftplacedina600kmorbitwouldrequireovernineyearstore-enter,giventhesameassumptions.Figure2alsoshowsthatat250kg/m2(e.g.10,000kg,40m2area)orbitallifetimeformissionsstartingat550kmexceedstenyears.Orbitsof700kmorhigherallhavelifetimesmuchgreaterthan10years(forthe82,250,and1000kg/m2spacecraftanalyzed).Givenareasonablesystemspecificpowerandspecificimpulse,increasingtheorbitalaltitudeofanFEPsystemfrom550to600kmwillrequireaboutonedayoffull-poweroperatingtime.Iftheexpelledpropellantmassis  considerednegligible,thenforthe82kg/m2caseonedayofoperationeffectivelyincreasestheorbitallifetimebyoversevenyears.Forspacecraftconfigurationsthatresultingreaterthan82kg/m2,theorbitallifetimewouldbeincreasedevenmore. RADIOLOGICALINVENTORY ASA FUNCTIONOFLIFETIME RadiologicalinventoryoftheSAFE-400atlaunchisontheorderof10Curies(primarilyduetoU-234).RadiologicalinventoryasafunctionoftimeaftershutdownwascalculatedfortheSAFE-400andforspacecraftpreviouslylaunchedbyNASAusingthecode MONTEBURNS (Poston,1999).RadiologicalinventoryasafunctionoftimeaftershutdownforvariousSAFE-400operatingtimesandforvariousspacecraftcurrentlyinEarthorbitisshowninFigure3.AsshowninFigure3,SAFE-400radiologicalinventorydecreasesrapidlyaftershutdown,andmorerapidlythantheinventoryinPu-238poweredspacecraft.SevendaysoffullpowerFEPoperatingtimeistypicallysufficienttoraisetheorbitalaltitudeofaspacecraftbyseveralhundredkilometers,placingthespacecraftabovethepointwhereatmosphericdragissignificantformostreasonablestartingorbits.AsshowninFigure3,followingoneweekoffull-poweroperationSAFE-400radiologicalinventorydropsbelow100Curieswithin3years.Forfull-poweroperationof1day,radiologicalinventorydropsbelow100Curiesinaboutayear.Theactualpotentialradiologicalhazardfromareentrywoulddependnotonlyonradiologicalinventory,butalsoonradiationtypeandsystemgeometryfollowingreentry. 1.0E+061.0E+05 _ 1.0E+04 .=.- 30 g1.0E+03 1.0E+021.0E+010.010.1 @ IM /MImJMME1/ W 10100IO X 10000IO00 X daysafter shutdown  3 yr 30yr 300yr SAFE4001day ofoperation--=- SAFE400 7 days ofoperation SAFE400 1monthofoperation --x--- SAFE4003 months ofoperationSAFE4001yearofoperation -- Transit 5 @ variusactivities -- Nimbus 5 @ 37,000Cieach -- LES 2 @159,000 Ci e ch FIGURE 3. RadiologicalActivityofCurrentlyOrbitingSpacecraftandtheSAFE-400as a FurictionofTime. OBSERVATIONS AllpotentialcivilianFEP missions currentlyunder discussion wouldnotreturntolowEarthorbitfollowingextendedoperation.Forthesemissions,preliminarycalculationsindicatethatstrictmissionsafetyrequirementsaremetifFEPoperationbeginsaboveanorbitalaltitudeofabout500km.Forthesescenarios,systemradiologicalinventoryduringaworst-casefailure / re-entryiscomparabletothatatsystemlaunch.Ifmoredetailedcalculationssustainthisobservation,thendeployingFEPsystemsdirectlyoutofthespaceshuttlecargobay(noadditionalstagesrequired)maybeaviableoption.Deploymentdirectlyfromthespaceshuttlehasseveralpotentialadvantages,includingtheextremelyhighreliabilityofthespaceshuttle(>99.8%),thelargevolumeoftheshuttlecargobay,the    shuttle's delivered payload mass capabil ity, the presence of astronauts to help ensure proper FEP system deployment and start, and the ability to return the FEP system to earth if desired. If a decision is made against using t he space shuttle to deploy the FEP vehicle, it will still be important to determine the des ir ed range of initial operating orbits to ensure mission safety while optimizing mission performance and flexibility. RECOMMENDATIONS FOR FUTURE RESEARCH Miss ion options for potential NASA Phase I FEP systems should continue to be explored. More detailed mi ssion design and analysis should be performed to define missions that ensure safety while providing maximum mission performance and flexibility. The benefits and issues associated with using the space shuttle to deploy FEP systems should be further investigated. Increased FEP system definition wi ll add fidelity to mission analyses. ACKNOWLEDGMENTS Unless otherwise referenced, the research reported in this paper was funded by and performed at NASA's Marsha ll Space Flight Center or at Los Alamos National Laborator y. REFERENCES Alford. R.L. and Liu, J.J. (1974) The Orbital Decay and Lifeti me (L1FTIM) Prediction Progra m. M-240-1278, Northrop Services, In c . Huntsville, AL. Houts, M.G., Poston, D.l .. Emrich. W.J. (1997) Heatpipe Power System a nd Heatpipe Bimodal System Design an d Development Op ti ons, in Space Nuclear Power nd opulsion. edited by Mohamed S. EI-Genk and Mark D. Hoover, DOE Con f970 1 1 5. American Institute of Ph ysics. New York, pp. 1317- 13 22. Houts, M.G. e1 al. (2002) Phase I Space Fission Propuls io n System Design Cons id erations, r. to be published in Space Nuclear Power and Propulsi on , edited by Mohamed S. EI -Genk, American In stitute of Phys ic s, New York. 2002, with in the se proceedings. Poston, D. 1. and Trellue, H.R . User's Manual, Version 2.0 for MONTEBURNS. Version 58 , LA-UR-99-4999, September 1999, Los Alamos National Laboratory. Poston, D. 1 et a1. (2002) -Design and Analysis of the SAFE-400 Reactor, to be publish ed in Space Nuclear Power and Propulsion. ed ited by Mohamed S. EI -Genk. American Institute of Physics, New York, 2002, within these proceedings. Vandyke, M.K. et a l. (2002) Safe Affo rd able Fi ssion Engine (SAFE) Testing a nd Development Progress, to be published in Space Nuclear Power and Propulsion. edited by Mohamed S. EI -Genk, American Institute of Physics, New York, 2002. within these proceeding s. 721 I I I ,
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