Prospectsfor
In-SpaceRe-Fueling
KenYoung
Booz-AllenHamilton
and
JeromeBell
BioDriTechnologies/SpaceLegacyLLC
©KenYoungandJeromeBell,2010.AllRightsReserved.
Introduction
Conceptsforrelievingtheterriblemass-fractionpenaltiesoflaunchingout
ofEarth’sgravitywellbyre-fuelingspacecrafton-orbithavebeen
proposedfordecades(Reference1).Certainlyitmakestheoreticalsenseto
avoidcarryingallneededmissionpropellants(aswellasothervitalfluids
and/orgases)fromliftofftothefinalorbitby‘toppingoff’atanin-space
‘fillingstation.’whichisclearlyanalogoustothere-fuelingofcarsonan
Interstatehighwayonalongautotrip.
Indeed,analogieshavebeendrawntothePonyExpressinthe1880s
andtosteamenginetrainsbeforethat.Inthosecases,the‘fuel’(feedand
waterfortheponiesandwoodandwaterfortheengines)wasgenerally
availableinsituatthewaystation;later,forcoal-drivenandthendiesel
202SpaceCommerce
trains,andfinally,automobiles,thefuelwasproducedelsewhereand
deliveredtore-fuelinglocations.
Thisanalogyisusuallyappliedtoconceptsforon-orbitdepots,
whereincryogenicorstorablepropellantsareproducedontheground,
transportedbyrocket-tankerstoanorbitalstoragedepot,whereitisthen
availablefortransferintoupperstagesand/ororbitalmaneuveringvehicles
(OMV).Twootherconcepts,‘in-situproduction’(ideally,also,‘on-
demand’)ofcryogenicsatthedepot,and‘in-spacere-fuelingfromthe
ground’(analogousto‘mid-airre-fuelingofaircraft’)havealsobeen
proposed.
Thischapterwilladdresstheconsiderabletechnological,operational
andeconomicchallengesforalloftheseconceptsataveryhigh,qualitative
level,andalsoassessestheprospectsofcommercializationofsuch
depot/re-fuelingsystems.Inaddition,wewilldiscusspotentialtechnology
demonstrationsorstudyprojectsthatmighthelptofocusthelogicaland
cost-effectivewayforwardtosuchan‘in-spacerefuelingsystem’aspartof
futurespaceinfrastructures.
ConceptsandtheirCharacteristics
ConceptScenarios(forillustrativepurposesonly)
ConceptAScenario:Cryogenics
Acryogenicpropellantstorageandtransferfacilityislaunched(or
assembledafterseverallaunches)inaLowEarthOrbit(LEO)atan
inclinationofapproximately29degandanoperationalaltitude(mightbe
assembledlower)ofabout200to220nauticalmiles. Actualaltitude
dependsondragcharacteristicsandtheoperatingperiodinthesolarcycle.
ThefacilitywouldconsistprimarilyofcryogenictanksforstorageofLH2
(orMethane,CH4,oranothersuchfuel)andLOXthatarewellinsulated
andshielded,aswellasalsopossiblyactivelycooledtopreventany
significantboil-offoveraperiodofmonths.
Aphotovoltaicenergysystemofmediumcapability(30-60KW)
drivenbyarticulatingsolarpanels(200-400m2)shouldsufficeforpower.
Thestructurehasanaccessibledockingportwithcryofluidtransfer
plumbingtosupplypropellantstovisitingcustomervehicles,suchasUpper
Stages(Centaur,PAMs,etc.)withpayloads(e.g.,comsats),Orbital
TransferVehicles(OTV)orEarthDepartureStages(EDS).TheseStages,
withAutomatedRendezvous,ProximityOperations,Dockingand
Undocking(ARPODU)capabilities,wouldarrivefromlaterlaunchesinto
lowerinsertionorbits,thenrendezvousanddocktemporarilyatthedepot
forre-fuelingand/or‘topping-off’propellants.
Reference2hasaninterestingproposedapproachtosuchaconcept,
andavariationthatavoidstheconsiderablechallengeofcryogenic‘boil-
TheInsideStory203
off’wouldbetousemorestorablepropellants.However,the‘market’for
suchstorablesinpotential‘customer’vehicleswouldhavetobeexamined
andjustifiedashavinganeconomicallyviablecustomerbase. The
preferenceforcryogenicpropellants,particularlyLOXandLH2,isthatthe
performanceefficiency,asmeasuredbytheengineIsp(specificimpulse)is
about25-30%greaterthanstorables.Table1summarizesConceptsA,B,
andCastotheirkeycharacteristics.
ConceptBScenario:Water-Based
ThisscenarioissimilartoConceptAintermsoforbitandbasic
function,butthedepotresourcewouldbe‘water-based.’ Thatis,large
quantitiesofproperly-treatedwater(probablycontainingchemicalcatalysts
suchaspotassium,sodiumorchlorine)wouldbeperiodicallydeliveredto
thefacility,whichwouldbecapableofelectrolyzingthewatertoproduce
H2andO2gases,thenliquefyingthemintoLH2andLOX.
Ideallythesecryopropellantswouldbecreated‘on-demand’and
transferred(withinafewhours)tothecustomervehicle.Obviously,there
wouldalsobeaneedforsomecryostoragecapability,ifonlytouseforthe
depot’sownorbitmaintenancepropulsionsystem. Thelargewatertank
storagerequirementandtheenergysourceneededtoprovidethehuge
(>500KW)electricaldemandforelectrolysisandliquefaction(Ref.3)
woulddrivethesizeandmassofthedepot,whichmighthavetobemuch
largerthanthatinConceptA.
ConceptCScenario:AGround-BasedSystem
Aground-basedre-fuelingsystemwouldbefundamentallydifferent
fromaspacedepot.Thisconceptpostulatesalaunchbooster/upperstage-
tanker(orfleetofthem)thatdeliverspropellants(cryosorstorables)tothe
alreadyon-orbitcustomervehicle(s)loiteringinLEO,waitingtobere-
fueled.Ideallymorethanoneofthesewaitingcustomervehicleswouldbe
waitinginarendezvous-compatibleorbitforcost-efficiencyreasons.
Thetankervehiclewouldrendezvouswiththem(oneatatime,in
probablysignificantlydifferent,butcompatible,orbits)andfacilitatethere-
fueling.Thetankerwouldthenreturntothegroundandberecoveredand
recycledforfuturereuse.Anearlyversionofthisscenariowouldbeto
discardtheemptytankerwithasafe,controlledre-entryandocean
disposal.
Therelativelylowenergypowerrequirement(<20KW)would
probablyallowforuseofbatteriesand/orfuelcells(possiblyevenusing
thecryoventgasesinthecryopropellantscase)and/orasmallphotovoltaic
array.Perhapsthekeydrivingrequirementonthere-fuelingtankerdesign
wouldbethecapabilitytoperformAutomatedRendezvous,Proximity
Operations,Docking/Undocking(ARPODU),withsophisticatedguidance,
navigation,andcontrol,avionic,attitudecontrolandpropulsionsystems,as
Concept/Type / Propellant / EnergySource/KWRequirement / Mass/Area
(approx.)
A-Cryogenic/
Storable
PropellantDepot
B-Water-
based/Cryo
Producedby
Electrolysis
Depot
C-Ground-based
In-Space
RefuelingTanker
S/C / LH2or
CH4/LOX
GH2/GO
to
LH2/LOX
LH2/LOX
or
Storables / Photovoltaic/30-60
Photovoltaicor
Nuclear/500-1000
Batteries+PV/<20KW / ~50,000kg/200m2
<50,000kg/1000m2
PV
or<50m2Nuclear
<15,000kg/<50m2
204SpaceCommerce
wellasasophisticatedstandard(common)dockingorberthingmechanism,
andstandardpropellanttransferinterfaces. Thetechnologicalonusison
thetankersystemratherthanonthecustomers’vehicles,althoughthey
wouldhavetobeadaptedwithcommondockingandfueltransfer
ConceptsandKeyCharacteristics
MajorTechnologyChallengesforeachconcept
AndCriticaltechnologydevelopment
CryogenicTransferinaMicro-gEnvironment
ConceptsA,BandC:Ifcryogenic(typicallyLH2andLOX)
propellantsinlargequantitiesaretoberapidlytransferredtothetanksofa
customer’svehiclesuchasanorbitaltransferstage,thenthatcapabilityina
micro-genvironmentmustfirstbesuccessfullydemonstrated. Asofthis
writing,thathasnotbeenaccomplished,althoughseveralexperimentsare
beingplannedaspartofthenewNASA’sFlagshipTechnology
Demonstrationapproach(seeSection4ofRef.4).Indicationsarethatsome
dockedthrustingorpossiblytetheredrotationmayberequiredtofacilitate
rapidtransfer,whichwouldfurthercomplicatetheprocess(Ref.5).
Storablepropellanttransferhasbeenaccomplished,andindeed,ithas
beendoneforyearsontheISS,andbeforethat,ontheRussianspace
stationsSalyutandMIR,althoughverylargequantitiesandhighrateshave
apparentlynotbeenachieved.Theattitudecontrolaspectsoftransferring
largemassesofcryogenicliquidsrapidlycouldalsopresentsignificant
challengestoavionics,duetothelargecenterofgravityshiftbetweenthe
dockedvehicles.Anapproachthathasbeensuggestedasanalternativeto
TheInsideStory205
actualfluid-flowtransferistosomehowexchangeentirepropellanttanks,a
fulltankforanear-emptyone. Thevehicledesignandtheoperational
challengesofsuchaschemeseemquitedaunting,andwillbetouchedon
later.
Long-term(weeksormonths)On-orbitCryogenic
Storage,ConceptA(cryogenics)
Ifcryosaredeliveredtoanon-orbitdepottobekeptreadyfor
customerre-fueling,thenobviouslyprovisionmustbemadeforlong-term
storagebeforeeventualre-fuelingofacustomervehicle. Duetothe
dynamicthermalenvironment(Earthshineaswellassolarradiation)cryo
‘boil-off’hasproventobeaverysignificantchallenge(References.1,2
and5).
Thelatestin-orbitCentaurfiring(usingcryopropellants,ofcourse)
wasamereninehoursafterinsertion. Alleviationtechniquesmaywell
involveothertechnologicalchallenges,suchasimprovedtankinsulation
materials,complexshieldingstructures,power-hungryactivecooling
systems,andevensophisticateddepotorientationcontrolavionicsand
propulsion.Aswiththecryotransfertechnology,noneofthesolutionsfor
thesechallenges(iftherearesome)hasyetbeendemonstratedon-orbit.
References1and2aswellastherecentlyannouncedplans(Reference4)
forNASA’sFlagshipTechnologyDemonstrationmissionsspecifically
addresscryotransferandstoragetests.
Althoughtheseproposedtestsarepredicatedontechnologiesthat
havebeenjudgedtobe‘mature’(i.e.,attheTechnologyReadinessLevel
(TRL)ofaround6or7),itistheopinionoftheauthorsthatcryotransfer
andlong-termstorageinspacetechnologyisactuallyatalowerTRL,
perhaps4or5.Testingsuchtechnologiesinasimulatedspace(micro-g)
environment(normallyrequiredforTRL6)hasprovenunfeasible,soan
exceptionmustbemade;hence,theproposedmission,FTD#2(Ref.4),in
the2015timeframe.Thisfirsttestwouldonlydemonstratecryo(LOXand
CH4)transferbetweentwotanksinthesamevehicle,orintra-vehicle.
Whethertheresultsofsuchatestcanbeextrapolatedtopredictinter-
vehiculartransfersuccesswillremaintobeseen.
On-orbitlargequantitywaterelectrolysisand
gaseousH2andO2liquefaction,ConceptB
(Water-based)
IfwateristobeelectrolyzedfirstintoGH2andGO2,andthen
liquefiedintoLH2andLOXcryogenicpropellantsinlargequantities,and
thenrapidlytransferred(‘on-demand’)tothetanksofacustomervehicle
suchasanorbitaltransferstage,thatcapabilityinamicro-genvironment
mustfirstbesuccessfullydemonstrated.
Asofthiswriting,largequantityelectrolysishasnotbeen
accomplished,althoughsmallamounts(5-10kg/day)ofGO2havebeen
206SpaceCommerce
producedbytheOxygenGenerationAssembly(OGA)forcrewbreathing
ontheISSsince2007(Reference6).Unfortunatelytheenergyrequirement
forsuchchemicalphasechangesystemsisextremely large: the
approximatepowerrequiredfortheOGAtoproduceabout10lbsofO2per
dayisabout2.6KW.Althoughitisprobablynotatotallylinear
conversion,thatwouldsuggestthat2000lbsofO2wouldrequiremore
than500KW.
AndthentheGO2mustbesuper-cooledandliquefied!Experiments
insuchmicro-gelectrolysisandliquefactiontechnologymaybeconsidered
aspartofthenewNASA’sTechnologyapproach,althoughthatis
uncertainatthiswriting.
EnergySource
ConceptA(cryogenics)
Becauseadepotthatonlytakesondeliveredcryopropellants,stores
themforpossiblylongperiods(weeksormonths),andthenhastofacilitate
theirtransfertocustomerspacecraft,isnotaparticularlyenergyintensive
operation,theprimaryenergysourcecouldprobablybephotovoltaicsolar
panelsof‘reasonable’size.Articulatingpanelsapproximatingthepairson
theISSthatcanproducebetween50-60KWshouldsuffice.Theneedfor
considerableshieldingand/oractivecoolingtopreventexcessive‘boil-off,’
however,couldcomplicatethesizeandlocationofsuchrotatingarrays.
Fuelcells,makinguseoftheavailable,ventedGH2andGO2couldbeused
incombinationwithsmallersolarpanels.
ConceptB(water-based)
Becauseofthisdepot’srequirementtoelectrolyzewatertogaseous
H2andO2andthenliquefythemtocryogenics,averylargeamountof
energy(>500KW)isneeded.Solarpanelscapableofreliablyproducing
thatmuchenergy,roughly10timesmorethantheISSsystemisactually
capableof,haveneverbeenbuiltandassembledinorbit,muchless
operatedandmaintained.Evenifitisphysicallypossible,assembly,
maintenance(bothontherotatingjointsandpanelsandonthedepot’sorbit
tocounterlargeaerodynamicdrageffects)andoperationofthevast,
rotatingarraysmaketheconceptunfeasibleintheauthors’opinion.
Hence,analternatelargeenergysource,suchasasmall,safenuclear
reactorispostulated.NASAGlennResearchCenterandtheAtomic
EnergyCommissionhavedevelopedsuchaprototypereactorinajoint
project,andonethesizeofa50-gallonkegissaidtobecapableof
producing40KW.ANASATechnologyDemonstrationProjectisplanned
inthe2012timeframe.(Ref.7)Thecostandenvironmental/safetyissues
involved,however,maymakenuclearanon-option.
TheInsideStory207
ConceptC(groundbased)
Theground-basedtanker/re-fuelerspacecraft,launchedintoorbitand
thenmaneuveredtorendezvouswithtwoormore(forefficiency)customer
vehiclestore-fuelthem,wouldlikelyberequiredtostayon-orbitforonlya
fewdaysoraweekorso,thusit’sownenergyneedscouldprobablybe
providedbyacombinationofbatteriesandasmallsolarpanelarrayset
similartothecurrentProgress.Fuelcellsdrivenfromavailable(venting)
GH2andGO2areanotherpossibility.
OperationalChallengesforeachconcept
InitialConstruction/Assembly
ConceptsAandB(depot)–Duetothelargeandcomplexnatureofa
cryogenic-tank–farmdepot,especiallyforConceptB,whereprocessing
mechanismsforelectrolysisandliquefactionarerequired,therefueling
facilitywillprobablyhavetobeassembledinorbit.
Ontheotherhand,ifasufficientlycapableHeavyLiftVehicle
becomesavailableaspartofafuturespaceinfrastructure,thenoneortwo
launchesmaybesufficienttodeliveracompletefacility. ConceptA
requiresalong-termstoragecapability,butthefacilityitselfwould
probablybesmalleroverallthantheConceptBstructure.Itwillnodoubt
havetodeployathermalshield,andiftheenergysourcewerearticulating
solarpanelsthenitwouldlikelyrequireseveraltemporarycrew/robotic
assemblymissions.
TheISSexperiencesinassemblyoperations/disciplineswillbe
extremelyvaluable,butnonethelesstheoperationalcoststobothtrainfor
andexecutetheassemblywilladdgreatlytothedepot’sLifeCycleCosts
(LCC).InthecaseofConceptB,itislikelythatseveralassemblymissions
willberequiredtointegrateandtestoutthecomplexfacility.Thisis
certainlythecaseifanattemptweremadetouseahugeacreageofsolar
panelstoprovidethemassiveenergy(>500KW)requiredforelectrolysis
andliquefaction.Theauthorsbelievethatsuchadesignisunfeasible,as
notedabove,indicatingthatsomeotherenergysourcewouldbeneeded.
Thesizeandtankagecapacityofapropellantdepotwouldbe
determinedbyperformingadetailedanalysisofthepotentialcustomer
missionbaseandtheiranticipatedpropellantrequirements,bothasa
functionoftheneededenergy(DeltaV)andthemission
frequency/scheduletiming.InthecaseofConceptA,alimitingfactormay
wellbethemaximumtimethatcryopropellantscanbeefficientlystoredon
orbit,whichispresentlynotknown. Inaddition,thelaunchvehicle
capabilitiesforbothinitialdepotassemblyandre-supply(tankers)of
propellantscouldalsobelimitingfactors.
208SpaceCommerce
SinceConceptCemploysEarth-basedtankerspacecraft,itwouldnot
haveanorbitalassemblychallenge,butinsteadwouldrequirefairly
complexlaunchwindowtiming,alongwithrendezvousandproximity
operationsrequirements,whicharealsofactorsunderConceptsAandB.
Thus,ConceptCwouldbeclearlylesscostintensiveforinitialoperations.
Ifthere-fuelingtankersarenotrecoverableandre-usable,however,then
ongoingcostsfornewtankervehicles,dependingonthecustomerdemand,
couldquicklyescalate.
Ground/LaunchOperations
Basedonextensivepreviousgroundoperationsexperienceitisalready
knownthatthehandling,loadingandsafeingofvolatilecryogenic
propellants(especiallyLH2)aredifficultandhazardous. Inaddition,
duringtheactuallaunch,ifthespacecraftpayloadconsistsprimarilyof
tanksofthesecryos,therearetheaddedrangesafetyconcerns(evenfor
non-crewedlaunches)becauseoflargerblastenvelopesandoff-nominal
trajectoryimpactzonesdownrange.
Infact,thesedangersconstituteamajorargumentinfavorof
ConceptB,inwhichthepayloadwouldbeessentiallywater,asitsground
handlingrequirements,evenwhentreatedwithcatalystchemicals,isboth
farlesshazardousandlesscostly.
AutomatedRendezvous,ProximityOperations,
Docking/Undocking(ARPODU)
AsConceptsAandBaredepotsinaspecificorbitallocation,itwouldbe
necessaryforcustomerspacecrafttocometothatlocationtore-fuel,much
likeautomobilescomingtothefillingstationoffthehighway.Thebulkof
therequirementsforARPODU,then,fallswithcustomerspacecraft,
althoughthedepotfacilityitselfwouldprovidefortrafficcontroland
communication,aswellasthenecessarydockingport(orgrapplingfor
berthing)systemstofacilitatere-fuelingrequirements.
Itwouldthereforebenecessarytoassurecompatibleand
interoperableARPODUinterfacesbetweenthedepotandthevisiting
spacecraft.Thesewillbediscussedbelow.
InthecaseofConceptC,thelaunchedtanker/re-fuelspacecraft
wouldbetheactiveARPODUvehicle,sothereverseonusistrue,but
interoperable,compatiblesystemssuchasautomatedrendezvousand
proximityoperationsbeacontargetsanddocking/berthingmechanismswill
benecessaryonthecustomerspacecraft.Perhapsthemosttaxingsystem
requirementsonthere-fuelingtankersarethoseofautomatedARPODU
capabilities,includingrobustpropulsionandattitudecontrolsystemsto
TheInsideStory209
enablemultiplerendezvousandrefuelingsindifferingcustomerspacecraft
orbits,whichmayvaryinbothaltitudeandorbitalplanes.
Whilesinglere-fuelingmissionsfromthegroundwouldlikelyprove
tobeeconomicallyinefficient,multiplerendezvousandproximity
operationsmissionshavealreadybeensuccessfullyaccomplishedwiththe
SpaceShuttle,soasimilarversatiletankercapabilityisreasonableto
postulate.
Orbitallocation,orbitmaintenance,launch
window,trafficmanagement/collisionavoidance
Theorbitallocation(inclination)ofthedepotsinConceptsAandBwould
bemarketdriven,astheywouldbebasedwherethemostprojected
customervehicletrafficwouldtransit. Itwouldbelikely,therefore,that
depotsservingforUS-launchedmissionswouldbelocatednearthedue
Eastinclination(approximately28.5deg).
PotentialtrafficatthisinclinationforLEO-startingmissionswould
consistofupperstages,suchasmodified/re-designedCentaurs,orPAMs
(PayloadAssistModules,butwithliquidpropellantsratherthanthenow-
commonsolids)carryingpayloadssuchascommunicationsatellites
destinedforGEO.OtherEarthDepartureStages(EDS)boundforBeyond
EarthOrbit(BEO)mightprovidetheoccasionalcustomer.Thebulkofthis
trafficwouldbecommercialComsats,estimatedatabout24peryear(Ref.
8),aswellasgovernmentfundedmissions(NASA,DOD,NOAA,etc.).
Whiletheidealorbitalaltitudeforadepotwouldbeverylow,inthe
rangeof100-150nauticalmiles,orbitmaintenancenecessarytocounter
aerodynamicdrag,aswellasrendezvousphasingconsiderationstoincrease
customervehiclecatch-upcapabilityandbroadenlaunchwindowdurations
wouldprobablyrequirethatthedepotorbitbeinanaltituderangeofat
least190-220nauticalmiles.Atthis‘popular’combinationofinclination
andaltituderange,theriskoforbitaldebriscollisionswouldberelatively
high,sodepotmaneuverabilityandguidance,navigation,andcontrol
systems(GN&C)requirementsforbothdragcompensationandcollision
avoidance/trafficmanagementwouldbeimportantandpossiblyvery
costly.
Andevenat190-220n.mi.,thepropellantcostsforaltitude
maintenancewillbesignificantandwouldprobablybecomparabletothe
costsexperiencedwiththeISS.DuringtheSolarCycleMaxperiods,for
abouttwoyearsofevery11years,ahigherorbit,suchas220n.mi.to250
n.mi.,mayberequired.Attitudemaintenancemustalsobeconsidered,
sincethathasproventobeasignificantpropellantcosttotheISSand
similarstructures.
Ifdesignedwell,useofControlMomentGyros(CMGs)formost
attitudeoperationsmightsuffice,butsmallthrusterswithlongmoment
210SpaceCommerce
arms(suchasusedontheISSforrollcontrol)cansignificantlyreduce
attitudecontrolpropellantcosts.
Long-termmaintenanceofthedepots,ascriticalon-orbitsystems
fail,loseefficiencyorbecomeobsolete,couldproveextremelycostlysince
repairmissionsarelikelytorequirehumanskillsand/orroboticcapabilities
andtheappropriatesupportingmissioninfrastructure(launchvehicles,
crewedspacecraft,EVAsystems,specializedtraining,etc.).Anotherlong-
termmaintenancecostforConceptsAandB,ofcourse,istheresupplyof
cryogenicsandwater,respectively.Hence,aresupplytankersystem,with
itslaunchboosters,mustalsobeanintegralpartofthelifecycle
infrastructure.Thisrequirementhasledtheauthorstosuggestthatan
evolutionaryapproach,startingwithConceptCtankers,shouldbeseriously
considered.
AlaterdevelopmentcouldbeasmallerAorBdepotinthesun-
synchronous(nearpolar)inclinations,around97-99deg,tosupportre-
fuelingforstagestakingEarth-observationpayloadsouttotheoperational
altituderangearound500n.mi.Whilefuelcoststoreachthataltitudeare
notexcessive,re-fuelingataloweraltitudewouldpermitmuchlarger
payloadssuchasbiggerobservationplatformstobedeliveredefficientlyto
thesun-synchronousorbitaltitudes.Itismorelikely,however,thatsucha
depotwouldserveasaroboticservicingplatformforrepairandfluid/gas
resupplyinconjunctionwithanOrbitalManeuveringVehicle(OMV)that
wouldcomeandgowhileservicingthesun-synchronoussatellites.
Interoperabilityandadaptabilitychallenges
Achallengetore-fuelingconceptsAandBisissueof
interoperabilityandadaptabilityofpotentialcustomervehiclessuchas
existingupperstageCentaurandsolid-fuelPAMs. Modificationsfor
ARPODUcouldinclude,forexample,aninternationalstandarddockingor
berthinginterface,aswellasforpropellantandpropulsionsystems(solids
orstorablesystemsconvertedtocryos)andthenecessaryfueltransfer
plumbinghavealreadybeenmentioned.
Arelatedissueistheactualphysicallocation(s)oncustomervehicles
ofpayload/payloadadapterand/ortherequireddocking/berthinginterface,
plustheroutingofthetransferplumbing.Typically,mostcurrentupper
stageslocatethepayload(normallyasatelliteoraLEOorBEOspacecraft)
onthefrontend,usuallyprotectedinsideashroudoradapteratlaunchuntil
deployedatthemissiondestination.Therearendoftheupperstagehasthe
propellanttanks,engineandenginebell,and,insomecases(likeCentaur)
itinitiallyfirestoplaceitselfandthepayloadintoorbit.
Ifthisupperstageistoapproachandtemporarilyattachatthedepot
forfluidtransfer,whereshouldthedockingorberthingportandthe
transferplumbingbelocated,withallthenecessaryquickdisconnect
valvesandumbilicals,etc.?Theobviousplacewouldseemtobealonga
TheInsideStory211
sideofthespacecraft,butnotonlydoesthatimposeaddedproximityops
navigationandattitude/translationcontrolrequirements,butsuchre-
designs,alongwithstructuralside-loadconstraints,mightactuallypreclude
dockingasacosteffectivepossibility.
Anotherconsequenceofaddingportsandplumbingtocurrentupper
stagerocketswouldbethesignificantshiftofthecenterofgravity,which
wouldthusaffectavionicsandGN&CfornotonlytheARPODUbutalso
possiblythemainenginethrustvectorcontrol.
Hence,aberthingsystemmaybemorefeasible. Berthingis
accomplishedbyadepot-grapplingdevice,suchasaroboticarmsimilarto
thoseontheShuttleandISS. Ifthedepotwerenotcrewed(or‘man-
tended’),thentheentireoperationwouldhavetobecompletely
autonomousorremotelycontrolledfromtheground.Whilethisisnotan
impossiblesolution,itcertainlyaddssignificantLifeCycleCosts(for
addedgroundandcommunicationsinterfacesandoperations).
Alesscostlysolutionwouldthereforebetolocatethedepotatthe
ISS,wherecommondockingandgrappling/berthingcapabilitiesalready
existandhavebeensuccessfullydemonstrated,buttheISSisnotatthe
idealinclination,asitislocatedat51.6deg.Thismightnotbea‘show-
stopper’forre-fuelingofBEOvehicles,althougheventhosemissions
wouldstillincurneedlessweight-to-initial-orbitandARPODU
performancepenalties,butGEOandsomeLEOmissionswouldcertainly
incurverysignificantperformancelosses.Safetyissues,atleastforcryo
depotsattheISS,wouldalsobeafactor.
Intermsofadaptability,thesignificantmodificationsthatwouldbe
requiredforcurrentupperstagevehicles,asnotedabove,leadstheauthors
toconcludethatforConceptsAandB,entirelynewupperstages(possibly
alsohavingOMVand/orOTVcapabilities)mustbedesignedintegrally
withanysuchre-fuelingsysteminfrastructure.[Conceptually,theOMV
hasbeenproposedseveraltimesinthepastasashortrangerobotic'space
tug'thatcouldmovepayloadsaboutinthevicinityoftheSpaceShuttle
and/orSpaceStation.TheOMVwoulduseaseparatepropellant/
propulsionmodulethatwouldbereturnedtoEarthbytheShuttlefor
refueling.Similarly,theOrbitalTransferVehiclewouldbeareusablespace
tug,poweredbyLox/LH2enginesandequippedwithanaerobrake
allowingittobereturnedforrefuelingandreuseatanorbitingspace
stationorpropellantdepot.]
Thatwouldcertainlybethecaseifthemodelwastoswap-out
propellanttank‘modules’usingaroboticmechanismtoreplaceanear-
emptytankwithafullone. Completere-designofconventionalupper
stagevehicleswouldobviouslybenecessary,aswellasdevelopmentand
demonstrationoftheroboticmechanism(s)andthecomplexoperations
required.
212SpaceCommerce
Whilethisisnotimpossible,butconsideringacurrenttechnology
readinesslevelofonly2or3,andtheaddedweightofthenecessary
structures,suchaperformanceoverheadwouldseemtonegatethedesired
objectiveofusingcryopropellantsinthefirstplace,atleastinthecoming
decades.
ForConceptC,asnotedabove,thetechnologicalinteroperabilityand
adaptabilityonusisonthetanker/re-fuelervehicle,whichwouldobviously
beacompletelynewdesign.However,customervehicleswouldstill
requirerelocationoftheirpayload-carryingdesign,aswellasberthing
fixturesandfluidtransferplumbingmodifications.Althoughthecustomer
vehiclewouldbethe‘passive’partnerinthisARPODUscenario,andthus
notrequirecomplexnewGN&Candavionics,thedocking/berthing
interfaceandtransferplumbingaccommodationchangeswouldstillbe
major.Thus,eventheground-basedtankerconceptwouldnecessitatenew
customervehicledesigns,ascontrastedwithmerelyadaptingcurrentupper
stages.
Economicconsiderationsandchallenges
PotentialMarkets(bothgovernmentaland
private/commercial)
AcursorylookatpotentialmarketsinLEO/GEOleadstheauthorsto
suggestthatGEOpayloadssuchascommunicationsatellitesand
observation(intelligenceandweather)satellites,bothcommercialand
government(military,NASAandNOAA),mayofferthemostpromisein
termsoffuturetrafficvolume.Currentprojections(Ref.8)areforabout24
suchpayloadmissionsperyear,orabout2everymonth.Thus,adepotor
ground-basedtankerservicethatcouldre-fueltheupperstagesofthe
Centaur/PAMclasswouldenablemuchlargersuchpayloads(withbigger
antennaeandpowerarrays,moreredundantsystems,etc.)tobeplacedin
GEO.Noquantitativeorqualitativeassessmentofsuchfuture
traffic/marketpossibilitieshasbeendoneasofthiswriting,althougha
hypotheticalscenariowillbeexaminedbelow.AlesslikelyLEOmarket,
butonestillworthyofconsiderationandanalysis,mayexistforSun-
synchronous-typeEarthobservation(commercial,NASA/NOAA,and
military)missions.
HypotheticalScenariotoestimateaRoughOrderof
Magnitude(ROM)MarketPriceandReturnon
Investment(ROI)forConceptC
AccordingtoaEuroconsultReport(Ref.8)ontheprojectedGEO
satellitemarketforthenexttenyears,about24comsatsperyearmaybe
launched.Theaveragecosttobuildonesatellitewillbeabout$100M,and
thelaunchcostwillbeabout$50M.Theaverageweightofthesatellite,
TheInsideStory213
includingitsGEOstationkeepingfuel,willbeabout8000lbs,althoughthe
trendistowardheavierpayloadsof12,000to15,000lbs.(‘Stationkeeping’
referstothefuelrequiredtomaintainthesatellite’sdesiredorbit.)
Heaviercomsatsarepreferredforatleastthreereasons:
1.SincetheGEOlongitudinal‘slots’arehighlyvalueditisbetter
tohaveonelargersatversustwoormore‘neighboring’smaller
sats,becauseofradio/TVfrequencyinterferenceandcollision
concerns.Inaddition,long-termoperationsandmaintenance
costsarelower.
2.Largersatscancontainmorebroadcastcapabilities,including
biggerarrays,largerantennae,broaderbandwidth,etc.
3.Moreredundantsystemsandmorestationkeepingfuelmeannot
onlymorereliablebutlongerusefulsatellitelives,thus
increasingandprolongingrevenuestreams.
Hypothetically,then,ifaConceptC,ground-basedtankerre-fueling
servicewerepostulatedtocaptureonethirdoftheprojectedannualmarket,
or8satellitesperyear,aROMpricingandROIcanbeestimated.
Assumingthatthe8customerswerewillingtobescheduledinpairs(a
weekorsoapart)forlaunch,theirupperstages(sayamodifiedCentaur)
couldbere-fueledinLEObythesametankervehicle,andthentheirlarger
satellitescouldbeboostedtotheirequatorial,GEOorbit. Ifthese
customerswerewillingtopayabout15%morethantheircurrentcostsfor
there-fueling,thenapositiveROIcanbepostulatedforthetankerservice.
Forexample,assumingthetwoGEOcustomerswishtobuildlarger
sats,increasingfrom8,000lbsto14,000lbs,theywouldprobablyonly
havemarginallyhigherbuildingcostsofaround$105M. Theycouldbe
launchedwiththeirupperstages(off-loadedtoabout1/3fullofcryo)anda
6,000lbsheaviercomsatpayloadforroughlythesame$50Meach.Ifthe
tankervehiclesystem,weighingatitslaunchonlyabout15,000lbs,with
about12,000lbsoftransferablefuel,couldbelaunchedforabout$20Mon
asmaller,cheaperlaunchsystem,performthetwore-fuelingsinLEOand
thenbedeorbitedintoasafeoceanentry,thechargeforeachre-fueling
couldbeabout$13M,returninga$5Mprofit,assuminga$1Mmissioncost
forthetankerservice.
TheGEOcustomerswouldbepayingabout$18Mextraeach,but
theywouldhavealmosttwicethecomsatcapabilityinGEO.Sincetheir
currentlaunchsystemcostsabout$6,250perpound,itwouldcostabout
$37.5Mfortheadditionalweight,(ifinfactthelaunchvehiclecouldeven
liftthatmuchextraweight),thenetsavingstothecomsatoperatorisnearly
$20M.
Thishypotheticalcase,ofcourse,doesnotconsiderthecapital
investments,estimatedintherangeof $400M,requiredfortheinitial
tankersystemandthemodifiedupperstagesystemsinfrastructures.
214SpaceCommerce
Amortizedat$5Mpermissionandat4missionsperyear,and
assumingnogovernmentsubsidies,itwouldtake20to25yearstoamortize
thedevelopmentcostofthetankersystem.Andwhilethereisnoguarantee
thatasmalllauncherforthetankercouldbefoundfor$20M,some
fledglingcommerciallaunchservicestodayhaveprojectedsuchprices.
Havingworkedthroughallthreescenarios,theauthorstherefore
concludethatareasonableROIisnotcurrentlypossibleforanyofthe
postulatedconcepts,asbothofthetwodepotinfrastructuresandtheir
LCCswouldlikelybeevenmorecostly.
RecommendedStudyProject/Technological
DemonstrationApproach
Whilethischapterhastakenonlyaverytop-levelandmostlyqualitative
lookatthedauntingchallengesfacingarealisticapproachtoan
economicallyviableOn-orbitRe-fuelinginfrastructure,otherconceptsmay
alsoexistandbeworthyofdetailedexamination.
Perhapsotherpropellants,suchasstorablesorsolids,orevenion-
engineresourcesshouldcontinuetobestudied.RecentNASAplansfora
FlagshipTechnologyDemonstrationProgramincludemissionsto
demonstratetransferandlong-termstorageonorbitofcryogenic
propellants(first,LOXandmethane,thenLH2),aswellasother
technologiesinvolved,suchasARPODUandpowersystems.
Theauthorsbelievesuchdemonstrationsareworththeconsiderable
investment,aslongas,inparallel,theentirecryo(orotherpropellant)re-
fuelingconceptsandtheirlong-rangepossibilitiesarestudiedin-depthfor
atleast6to12months.Particularemphasisshouldbeplacednotonlyon
thetechnicalandoperationalchallenges,butalsoontheLCCand
associatedscheduleandcostrisks.Realisticfuturemarket/trafficanalyzes,
particularlywithrespecttoprivateorcommercialmarkets,seemcriticalto
anychanceforprojectingabelievableROI.
Thestudyteamshouldbecomposedofknowledgeableexpertsnot
onlyfromNASAandothergovernmentagenciessuchasDoD,NOAAand
theDOE,butalsoacademiaandprivateindustries,suchaschemical/energy
companiesandspacecraft/launchproviders.
Conclusions
ThischapterhasdiscussedthreeconceptsforRe-fuelingSpacecraftOn
Orbit,andtheirconsiderabletechnological,operationalandeconomic
challenges.Themajorchallengesaresummarizedhere.
TheInsideStory215
Technology
Theon-orbittransferandlong-termstorageofcryogenicpropellants
isthebiggestchallenge.Thistechnologymaturitylevelisnohigherthan
TRL5,andperhapslower.Fortunately,considerablerealflightexperience
anddatafromthedecadesthattheCentaurhasbeensuccessfullyflying
exists,butverylittlehasbeendonetoextendsuchmissionstogain
knowledgetoextrapolateorapplyittothischallenge.
Solvingthetransferandstoragechallengesiskey,ofcourse,for
ConceptA.Itisalsokey,butpossiblytoalesserextent,onConceptBif
‘on-demand’on-orbitproductionofcryosfromwatercanbereliably
achieved.Eventhen,thechallengeremainstoprovideadequatepowerfor
therequiredelectrolysisofwaterandtheliquefactionofgaseousO2and
H2.Nuclearpower,evenwithitshighcostandattendantenvironmental
andsafetyissues,seemstobetheonlypossiblesolutiontothatenormous
powerrequirement.ConceptC,withitsrelativelyshort-termedon-orbit
missionsofaweekortwo,mayalsohavecryostorage‘boil-off’problems,
leadingtocostlyactive-coolingandrelativelyhighpowerrequirements.
DockingorBerthing
Thesecondtechnologicalhurdleforallthreeconceptsisnotsomuch
therequirementforARPO,butthatofdockingorberthingitself,withits
challengingimpactsonthecostumerupper-stagevehicles’structural
designsandtheirGN&Csystems(forConceptsAandB,particularly)and
onthetankerservicevehicleforConceptC.Thebiggestchallengewould
seemtobeproximityoperationsandthesubsequentactualdockingor
berthing,asthatmustbeeithercompletelyautonomousorremotely-
assistedbygroundcontrol.Similartechnologyhasbeenoperationalfor
yearsontheISS.Arelatedchallenge,ofcourse,isthatofsystems
interoperability,particularlywithrespecttoassuringinternationalstandards
forproximityoperations‘homing’systemsandrelatednavigationaidesand
sensors,aswellasforcommondocking/berthingmechanismsandfuel
transferplumbing.
Economics
Thebiggestproblemsseemtobetheapparentlackofasubstantial
customermarket,alongwiththehighcapitalinvestmentrequiredtobuilda
re-fuelingsysteminfrastructure.Acustomertrafficrateofonly8to12
missionsperyear(mostlytoGEO,allowingforafewgovernmentmissions
also)istoofewtorealizeapositiveROI.
Andwhileourhypotheticalscenariowascalculatedwithfranklyvery
optimisticassumptionsaboutlaunchpricesandvehicleandcustomer
satellitere-designcoststoshowapositiveROIof$5Mperyear,this
illustrateshowdifficultitmaybetorecovercapitalinvestmentsovera20-
25yearperiod.
216SpaceCommerce
Inaddition,theprobablehighoperationsandsustainingmaintenance
costs,especiallyforthedepotconcepts,wouldseemtomakerecoveryof
theirLCCahighriskventure.Abigproblemwiththeground-basedtanker
approachisobviouslytherecurringlaunchandnewtankerreplacement
costsintheeventthattankerscannotbedesignedandbuiltcheaplyfor
entry,recoveryandre-use.
Givenallthesemajorchallenges,aswellasmanylesserbut
significantonestoucheduponherein,it’sclearthatanin-depth,
cooperativestudyprojectinvolvinggovernment,internationalpartners,
privateenterprise(bothpotentialprovidersandcustomers),andacademic
participantsbeconductedinparallelwiththeupcomingNASAFlagship
TechnologyDemonstrationProgram.Indeed,thatProgramshould
probablyleadthestudy.Lifecyclecosts,technicalandschedulerisksand
realisticspacetraffic/marketanalysesshouldbeemphasized.
Andforthedepotapproach,particularly,theurgetofollowa‘buildit
andtheywillcome’credoshouldbestronglyresisted.Sinceadepotmust
alwayshaveatankerre-supplysystemasanessentialpartofits
infrastructure,prudentthinkingwouldseriouslyconsideranevolutionary
‘ground-basedtanker’approachinitially.Theultimatesuccessofsuchan
endeavorwouldclearlyhingeontheLEO/GEOcommercialspacemarket
customers.
Insummary,then,theauthorsbelievethattheprospectsforacost-
effective,commercial,in-spacere-fuelingsystemareatpresentverylow,
andentailextremelyhighrisks.
•••
TheInsideStory217
KenYoung
KenwasborninAustin,Texas.HereceivedanAero-
SpaceEngineeringdegreefromtheUniversityofTexasin
1962.HewasemployedbytheNASAMannedSpacecraft
Center(nowJohnsonSpaceCenter)inJune,1962and
subsequentlyworkedonallU.S.humanspaceflight
programs(fromMercurytoConstellation).DuringGemini,
Apollo,SkylabandASTP,Kenwasatrajectoryexpert,
missionplannerandarendezvousspecialist.Kenserved
asareal-timeflightdynamicsadvisorintheMission
ControlStaffSupportRoomsthroughoutGemini,Apollo
andSkylab.KenbecameChiefoftheFlightPlanningBranchpriortothefirst
SpaceShuttlelaunchin1981.
HeretiredfromNASAinlate1987andwentontoworkorconsultfor
severalNASA-JSCcontractors,includingGrumman,Northrop,Loral,
LockheedMartin,SAIC,BigelowandBooz-AllenHamilton,wherehenowis
aconsultantontheConstellationProgram.
Jerome(Jerry)Bell
Jerome(Jerry)Bell,anativeHoustonian,earnedhis
BachelorofScienceDegreeinAerospaceEngineering
fromtheUniversityofTexas,Austinin1963.Following
9monthsofEmploymentwiththeMcDonnellAircraft
CompanyinSt.Louisworkinginaerodynamics
supportingthePhantom2AircraftProgram,heaccepteda
positionwithNASAMannedSpaceCraftCenter(now
JohnsonSpaceCenter)whereheworkedasacivilservant
forover42yearsuntilretirementin2006.
HehasworkedoneverymannedspaceprogramfromProjectGeminiin
varioustechnicalandProgramManagementcapacitiesandProgramPhases
includingProgramformulation,requirementandconceptdevelopment,
operations,andtechnologyinselectedareas. JerryhasservedasaJSC
representativetoSeveralAgencywideactivitiesincludingtheNASASpace
StationTaskForceConceptDevelopmentGroupatNASAHQ,NonAdvocacy
reviewteams,ProposalreviewandSourceBoardtechnicalsupport,DOD’s
TechnologyReinvestmentProgram,andOperationsRequirementDevelopment
ManagerwithinJSCExplorationProgramOffice(SpaceExplorationInitiative
proposedunderGeorgeH.W.Bush).
Sinceretirement,hehasjoinedBioDriTechnologies/SpaceLegacy
LLCastechnicaladvisorprovidingcoordinationandindependent
advice/recommendationswithregardtoactivitiesbetweenBioDriandNASA.
Inthiscapacity,Mr.Bellhasacquiredadegreeofunderstandingabout
antimicrobialsandtheirpotentialspaceapplicationsandissues.
218SpaceCommerce
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