NASA Technical Reports Server (NTRS) 20110008470: High-Payoff Space Transportation Design Approach with a Technology Integration Strategy

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by  NASA Technical Reports Server (NTRS)

High-Payoff Space Transportation Design Approach with a
Technology Integration Strategy
C. M. McCleskeyl and R. E. Rhodes and2
NASA Kennedy Space Center, Florida, 32899
T. Chen3
NASA Marshall Space Flight Center, Huntsville, Alabama, 35812
and
J. Robinson4
The Boeing Company, Huntington Beach, California, 92647
A general architectural design sequence is described to create a highly efficient, operable,
and supportable design that achieves an affordable, repeatable, and sustainable
transportation function. The paper covers the following aspects of this approach in more
detail: (1) vehicle architectural concept considerations (including important strategies for
greater reusability); (2) vehicle element propulsion system packaging considerations; (3)
vehicle element functional definition; (4) external ground servicing and access
considerations; and, (5) simplified guidance, navigation, flight control and avionics
communications considerations. Additionally, a technology integration strategy is forwarded
that includes: (a) ground and flight test prior to production commitments; (b) parallel stage
propellant storage, such as concentric-nested tanks; (c) high thrust, LOX-rich, LOX-cooled
first stage earth-to-orbit main engine; (d) non-toxic, day-of-launch-loaded propellants for
upper stages and in-space propulsion; (e) electric propulsion and aero stage control.
1 Aerospace Technologist, Engineering Directorate, Kennedy Space CenterINE-D2, and AIAA Senior Member.
2 Aerospace Technologist, Engineering Directorate, Kennedy Space CenterlNE-D2, and AIAA Senior Member.
3 Insert Job Title, Department Name, AddresslMail Stop, and AIAA Member Grade for third author.
4 Insert Job Title, Department Name, AddresslMail Stop, and AIAA Member Grade for fourth author (etc).
1
American Institute of Aeronautics and Astronautics
092407
High-Payoff Space Transportation Design Approach with a
Technology Integration Strategy
C. M. McCleskeyl andR. E. Rhodes2
NASA KennedySpaceCenter, Florida, 32899
T.T. Chen3
NASA MarshallSpaceFlightCenter, Huntsville, Alabama, 35812
l.W. Robinson4
TheBoeingCompany (ret), HuntingtonBeach, Califorma, 92647
This paper describes a general approach for creating architectural concepts that are
highly efficient, operable, and supportable, in turn achieving affordable, repeatable, and
sustainable space transportation. The paper focuses on the following: (1) vehicle
architectural conceptconsiderations (including important strategies for greater reusability);
(2) vehicle element propulsion system packaging considerations, including integrated main
and auxiliary propulsion systems; (3) vehicle element functional integration; (4) ground
element functional integration; (5) simplified and automated electrical power and avionics
integration; and, (6) ground and flight test prior to production commitments. Additionally,
we have provided four essential technologies enabling the high payoffdesign approach: (a)
parallel stage propellant storage, such as concentric-nested tanks, for more efficient space
vehicledesign and operation; (b) high thrust, L02-rich,L02-cooled first stageearth-to-orbit
main engine; (c) non-toxic, day-of-Iaunch-Ioaded propellants for upper stages and in-space
propulsion; and(d)electricpropulsionandflight controls.
Nomenclature
ASTF =AftSkirtTestFacility MPS =MainPropulsionSystem
CH =Methane MR =MixtureRatio
4
EHA =Electro-HydrostaticActuator O&M =OperationsandMaintenance
EMA =Electro-MechanicalActuators OML =OuterMoldLine
ETO =Earth-to-Orbit RP-l =RocketPropellant- I
GRC =GlennResearchCenter SCAPE =Self-ContainedAtmosphericProtective
GSE =GroundSupportEquipment Ensemble
HMF =HypergolicMaintenanceFacility SPST =SpacePropulsionSynergyTeam
I =SpeclficImpulse SRB =SolidRocketBooster
sp
KSC =KennedySpaceCenter SSME =SpaceShuttleMainEngine
LID =Length-to-Diameterratio TPM =TechnicalPerformanceMeasures
LH2 =LiqUldHydrogen TPS =ThermalProtectionSystem
L02 =LiquidOxygen TVC =ThrustVectorControl
LRU =LineReplacementUnit
1 Aerospace Technologist, Engineering Directorate, NASA Kennedy Space Center, mail code NE-Dl, and AIAA
SeniorMember.
2 Aerospace Technologist, Engineering Directorate, NASA Kennedy Space Center, mail code NE-Dl, and AIAA
SeniorMember.
3LeadAerospaceEngineer, SpacecraftandVehicle SystemsDepartment, NASAMarshall SpaceFlightCenter, mail
codeEV-91, andAIAA SeniorMember.
4TheBoeingCompany(retired); chairman, SpacePropulsionSynergyTeam, andAIAAAssociateFellow.
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Introduction
A six-step sequential process is suggested to provide a general architectmal design sequence to create a highly
efficient, operable, and supportable design that achieves an affordable and economically sustainable transportation
function. Thesixcritical, sequentialstepsare·
Six CriticalStepstoanAffordable, EconomicallySustainableSystem Design
I. Simplifythevehicle/groundsystem architecture
2. Efficientlypackageeachvehicleelement'spropulsionsystem (i.e., tank, engineandcompartmentlayouts)
3. Integrate vehicle elementfunctions into the lowest number ofsubsystems/components with mmimum ground
supportrequirements
4. Integrategroundelementfunctions intothelowestnumberofworkstations,facilities, andsupportequipment
5. Simplifyavionicsandflightcontrol design intominimum components, then, powerandautomatewhat'sleft
6. Extensivelyflight test to demonstrate accomplishment ofallproduction andoperations needs andobjectivesfor
full operationalsystemcapabilitywiththeaffordabilityobjectivemet
These steps are importantto perform during early conceptual design to avoid the accumulation ofunaffordable
design, development, production, andgroundoperationswork. Throughadherencetothese steps, onecanalsoavoid
time delays that cause added expenditures, as well, and thus achieve the objectives oflife cycle affordability and
operationalsustainability.
Each of the steps discuss the specifics of the approach followed by a section identifying how each step
addresses technical criteria that influence space transportation affordability and sustainability. This paper uses the
list ofcriteria developed by the Space Propulsion Synergy Team (SPST), a NASA-sponsored volunteer group of
experiencedgovernment, industryandacademiacolleaguesinthespacepropulsioncommunity.1
Specifically, the SPST identified eighteen (18) highly influential criteria as design drivers of space
transportation affordability and sustainability. Selected from among 64 candidate design drivers, these eighteen
measurable criteria were previously determined by the SPST to be the most important to understand and control
during the design cycle for achieving life cycle affordability. An in-depth treatment of these criteria and their
influence on life cycle affordability and sustainability are provided in the referenced Joint Propulsion Conference
paper.2
Thispaperadds anothercriteriontotheseeighteen-reusability. Forthepurposesofthispaper,reusabilityisthe
abilityofasystem, orpartsofasystem,tobereusedformultipleflights.
A systemwith low reusability requires ahigh degree ofworkfor its nextflight. A systemwith highreusability
is assumed to be one that fully retains its functional integrity between flights, and thus little orno work is required
other than payload handling and propellant servicing. Highly reusable systems are more affordable by (1)
minimizing routine purchases of high-value flight equipment by its owner-operator, (2) reducing routine
maintenance expense, and (3) increasing system productivity through shorter processing durations. Therefore, we
can logically conclude that highly reusable system design techniques and advanced technologies are needed to
enableaffordableandsustainablespacetransportation.
The affordability benefits due to reusability become less clear when comparing low reusability systems (e.g.,
ocean-retrievedelements)withfully expendablesystems.
The total list ofnineteen (19) criteria (i.e., the 18 derived bythe SPST plus the reusability criterion explained
above), andtheircorrelationwithkey lifecycle affordabilityparametersderivedbythe authors, is shown inTable 1.
The paper concludes by identifying specific technologies and design approaches that supportthe steps described in
achievingthe overallobjectivesofaffordabilityandeconomicsustainability.
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Step 1. Simplifythe VehicleArchitecture
A. Discussion/Specifics
Simplify the vehiclebyminlmizmgthe number ofuniquevehicle elements andmaximizereusability m those that
remain intheconcept.
This keeps the number ofproduction and operations ground stations to a minimum, and therefore, holds the
marginal and fixed operating costs to a minimum. A high degree of reusability allows the owner-operator to
minimize the requirement for routinely purchasing high-value flight equipment and replacement parts. The
minimum number of elements minimizes unique annual fixed costs associated with the production, supply,
servicing, and maintenance of each unique stage or element. Finally, a simplified flight system made up of a
minimum number ofelements enables a simplified ground system. A simplified ground system not only reduces
fixed infrastructure supportcosts,butalso decreasesdirectoperatingcosts perflight andgreaterflightthroughputby
reducing work contentand eliminatingtime-consuming work. As aresult ofthe reduced work andtime, the system
utilization,affordability, andsustainabilityfortheequipmentowner-operatorare all improved.
B. SpecificEngineeringCriteriaAddressedforImprovement
Step I directly addresses about half of the nineteen (19) engineering criteria for achieving affordable and
sustainable spacetransportationsystems (referenceError! Reference source notfound. for specific correlationsto
affordability);
• Percentageofelementsandsystemsthatarehighlyreusable
• Totalnumberofseparateidentifiedvehiclepropulsionsystemsand/orseparatestages
• Totalnumberofflighttanks mthearchitecture
• Number ofmaintenanceactIOns unplannedbeforeorbetweenmissions
• Number ofmamtenanceactionsplannedbeforeorbetweenmissions
• Totalnumberoftraditionalgroundinterfqcefunctionsrequired
• Totalnumberofvehicleelement-to-elementsupportsystems
• Numberofflightvehicleservicmgmterfaces
• Number ofconfined/closedcompartments
• Number ofmechanicalelementmatmgoperations
Step2. EfficientlyPackageEachVehicleElement'sPropulsion System
A. Discussion/Specifics
Achieve a far more compactvehicle design whose cryogenic propulsion systems are far simpler to operate in
flight andonthegroundthancurrentdesign approaches
Thedesignstrategycontainsanumberofspecificsforachievingthis, andisenabledthrough:
1. Use ofminimum number ofmain and auxiliary propellant commodities. preferably confined to dav-of
launch-loaded oxygen andhydrogen only. The idea is to keep the accumulation ofdesign effort, recurring
production work, and ground processing time and expense to a minimum. Separate propulsion systems are
additivetotheses lifecyclecostelements.
2. Keep ground interface connections close to ground level to avoid a series of elevated. articulatmg
umbilrcals-particularly. lrfi-offumbilicals O.e.. thosethatremain connecteduntilvehicle first-motion or.
T-O umbilicals). Avoiding large structures keeps ground system development and maintenance costs down.
When such systems are outfitted with active mechanical, plumbingand electrical systems, the development,
maintenanceandoperationscostsgoup. Also, theresponsiveness ofthesystemtoproducealaunchgoesup.
3. Avoid production complexities. operational propellant loading complexities. andsafety risks ofspecifymg
common-bulkhead tandem tankarrangements andseparateauxiliarypropulsionsystems.
4. Use concentric-nested propellant tanks arranged such that complex feed systems and other external
subsystems are eliminated (note concentric-nested means one tank is concentrically arranged inside
another. but no common wal/). This approach eliminates long feed lines and a number ofactive support
subsystems for the propulsion function; e.g., long propellant feed lines for the pump feed system, L02
"pogo" suppression system, anti-geyseringand thermal conditioningsub-systems. Furthermore, concentric
nested tank concepts minimize the volume and weight ofthe total system. As such, the approach has wide
and far-reaching architectural implications across a number ofspace transportation applications and across
the spacetransportation life cycle. Italso allows the incorporationofadvancedconcepts such as integrating
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the turbo-pump into the main propellant tank sump, further avoiding the need for added sub-systems
previouslymentioned.
5 Use a minimum number ofmain engines with a minimum amount ofturbo-machinery and interconnecting
main propulsion system plumbmg. The more main engines (meaning devices incorporating feed turbo
machinery, combustion chambers, and exhaust nozzles) there are in the system, the more interconnecting
plumbing, avionics, softwareandopportunitiesforreliabilityproblemsaccumulate. Thisrapidlyexpandsthe
effort required to: design the main propulsion system, develop and test, continually manufacture
(particularlyifexpendedeachflight), processonthe ground, andcontrolriskinflight.
6. Use electricpropellantvalve actuation andeliminatedistributedhydrauhcandpneumaticprrmaryandback
up systems from all engine and propellant system controls (including thrust vector control. but only if
differential required). This approach avoids numerous support systems for the propellant valve controls
and/or gimbaled engines and reduces the number of support systems for the flight vehicle and ground
support equipment. It also means fewer parts to produce and less assembly work leading to more flights in
shortertimes withlesseffort.
7. In the longer-term. consider usmg L02-rich. L02-cooledmam engine combustion for first stage earth-to
orbitpropulsion thatadjusts the mixtureratio backtowardstoichiometricratio as the thrustrequirementis
reduced durmg ascent. This concept reduces the combined propellant tank volume, and hence reduces
vehicle length and dry weight. This allows a more efficient design for both flight and ground operations. It
also better enables the design criteria of minimum number of propellants since it avoids having to use
unique, highthrust propellant combinations applicable to lower-altitude, fIrst stage flight only. Use ofsolid
boosters orkerosene is generallyseen as benefIcial for low-altitude, fIrst stage flight where specifIc impulse
is traded for higher thrust. This concept uses a common set ofpropellants throughout the ascent phase and
variesthethrust/I characteristicsduring flight withaminimumnumberofdedicatedsystems.
sp
B. SpecificEngineeringCriteriaAddressed for Improvement
Step 2 enables or directly addresses at least twelve (12) ofthe nineteen (19) engineering criteria for achieving
affordableandsustainablespacetransportationsystems(referenceTable 1for specifIccorrelationsto affordability);
• Totalnumberofseparateidentifiedvehiclepropulsionsystemsand/orseparatestages
• Percentageofelementsandsystemsthatarehighlyreusable
• TotalnumberoftraditIOnalgroundmterfacefunctionsrequired
• Numberofflightvehicleservicinginterfaces
• Number ofdifferentfluids required
• Numberofsafetydrivenfunctionalrequirementstomamtamsafecontrolofsystemsdurrngfllghtandground
operations
• Number ofconfined/closedcompartments
• Number ofsafetydrivenlimitedaccesscontroloperations
• Number ofmaintenanceactIOnsunplannedbeforeorbetweenmiSSIOns
• Number ofmaintenanceactionsplannedbeforeorbetweenmissions
• Number ofcommoditiesusedthatrequiremedicalsupportoperations androutmetraining
• Number ofCriticality1system andfailureanalysismodes
Step3. IntegrateVehicleElementFunctions intothe LowestNumberofSubsystems/Components
A. Discussion/Specifics
Create a generrc functional systems breakdown structure for each conceivedvehicle element. and combine (or
mtegrate) as many functions mtosmgularsystemsto provideaminimum ofstandalone. dedicatedsubsystems.3This
will minimize the number of accumulated subsystems and supporting components for a given set of required
functions. For example, combme various propulsion and power functions with common propellants and fluid
commoditiesto avoidseparate fIll anddrain, storage and distribution subsystems, groundinterfaces, andGSE. Also,
usetechnicalapproachesthat inherentlyrequire fewer separate supportsubsystemstoperformthefunction (e.g., use
electric actuation to displace distributed hydraulics and its.support subsystems and GSE). In so doing, the
cumulative design and development effort for both flight and ground support systems is greatly reduced. The
recurring production effort is likewise reduced, along with the attendant number ofseparate suppliers requit:ed to
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sustain the system. The recurring ground operations work and processing time are reduced, as well as recurring
labor, materials, andotherdirectcosts.
B. SpecificEngineeringCriteriaAddressed for Improvement
Step 3 enables or directly addresses a large number of the nineteen (19) engineering criteria for achieving
affordableandsustainablespacetransportationsystems(referenceTable 1for specificcorrelationsto affordability);
• Totalnumberofflighttanks inthearchitecture
• Totalnumberofseparateidentifiedvehiclepropulsionsystemsand/orseparatestages
• TotalnumberoftraditionalgroundinterfacefunctIOns required
• Number offlightvehicleservicinginterfaces
• Number ofdifferentfluids required
• Number ofsafetydrivenfunctionalrequirementstomamtamsafecontrolofsystemsduringflight andground
operations
• Number ofconfined/closedcompartments
• Number ofsafetydrivenlimitedaccesscontroloperations
• Number ofmamtenanceactIOnsunplannedbeforeorbetweenmissIOns
• Number ofmaintenanceactionsplannedbeforeorbetweenmissions
• Numberofcommoditiesusedthatrequiremedicalsupportoperationsandroutinetraining
• NumberofCriticality I system andfailureanalysismodes
• Number ofsafingoperationsatlanding(forreusable elements)
Step 4. Integrate ground elementfunctions into the lowestnumberofworkstations,facilities, and
supportequipment
A. Discussion/Specifics
Keep routine external groundservicing and access reqUirements simple andto a minimum: e.g., strive for no
mastsortowers:
Thedesignstrategycontainsanumberofspecificsfor achievingthis, andisenabledthrough:
1. Avoid breakmg the functIOnal mtegritv ofany flight systems during ground turnaround operatIOns. For
highlyreusable systemdesigns, theobjectiveisto avoidelectricalandplumbingdisconnections, alongwith
mechanical system disconnections that would require re-rigging, re-calibration, re-alignment, and
functional checkout. Minimize any operator "closeout" activity by design. This will significantly reduce
routinegroundoperationsworkandtimerequiredtoproduceaflight.
2. Requirethe flightsystemsto bedesignedfor mamtamabilitvas ameans for achievingreliabilitvandsa(etv
objectives. Thisavoidsthe needto gointoamaintenancestationduringgroundturnaroundoperations.
3. Locate internal line replacement units (LRUs) near outer mold-line to avOid all mternal access for
installation and/or routme change-out. Doing so eliminates the need for internal access kits for routine
operations, conditionedair,controlledpersonnelentryanddrag-onlighting.
4. Strive to remove as many closedvehicle compartments as possible. This approach minimizes subsystem
hardware and routine ground operations required in such compartments, i.e. purges and hazardous gas
detection subsystems This will also support the objective to avoid the need to go into a dedicated
maintenancestationforroutine groundturnaroundoperations(seeFigure 1).
5. AVOId vehicle element deSign complexity when other passive system design techniques can achieve the
sameresults. potentwllvwith a netweight reduction Examples: (1) open truss vs. enclosed compartment,
or (2) radiation-cooled vs. forced cooling, or (3) MPS head start vs. ground-supplied turbine spin system.
Retainingexistingfunctional andhardwarecomplexityalsoreducesthe overallsystemreliabilityandsafety
offlight.
6. Integrate and use main propellant tanks for auxiliary propulsion andpower services wherever practical
Avoid dedicated tank sets. These require separate subsystem designs and added design, integration, and
ground operations and associated logistics chain that accumulate both production and ground operations
work content. The design objective is to keep the cumulative amount of plumbing systems that the
production line and the operator encounter to an absolute minimum. Provide enough margins to fully
accountforplumbinginterconnects.
7. Minimize the number ofdedicated fluids and gases as well as the number ofseparate speciahzed fluid
pUritiesrequired. Allsubsystemsmustbecoordmatedonthis duringdesign definition.
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8 Locate the remaining vehicle groundservicing points near ground level. and avoid elevated fluid system
servicingandpurgepointsrequirmgtowerswithdedicatedservicearms andaccess platforms.
9. Avoid T-O umbiilcals. However, all unavoidable day-o{-launch propellants should have their flight-to
ground interfaces at ground level to avoid system complexitles assoczated with elevated services.
umbilicals. etc.
lO.Load all stage propellants (including spacecraft modules> on day oflaunch Avoid assembly operations
involvingstages andmodules with loadedandfully pressurizedflUidsystems which are consideredan un
safe practice for personnel safetv. Non-propellant fluids. e.g.. life support fluids and gases, should be
loadedpriortoday oflaunch.
B. SpecificEngineeringCriteriaAddressed for Improvement
Step 4 enables or directly addresses a large number of the nineteen (19) engineering criteria for achieving
affordableandsustainablespacetransportationsystems(referenceTable I forspecificcorrelatiOnstoaffordability);
• Percentageofelementsandsystemsthatarehighlyreusable
• NumberofmaintenanceactionsunplannedbeforeorbetweenmiSSIOns
• Numberofmaintenanceactionsplannedbeforeorbetweenmissions
• Totalnumberofflighttanks inthearchitecture
• Totalnumberoftraditionalgroundinterfacefunctions required
• Number offlightvehicleservicinginterfaces
• Number ofdifferentfluids required {
• Number ofsafetydrivenfunctionalreqUirements to mamtainsafecontrolofsystems durmgflightandground
operations
• Numberofconjined/closedcompartments
• Number ofsafetydrivenlimitedaccesscontroloperations
• Number ofcommoditiesusedthatrequiremedicalsupportoperatIOnsandroutinetraining
• Number ofCriticality1system andfailureanalySISmodes
• Number ofsajingoperatIOnsatlanding(forreusable elements)
Step 5. Simplifyavionicsand flight control design into minimum components; then, powerand
automatewhat's left
A. Discussion/Specifics
Steps I through 4 have setthe stage for avehicle design thatmaximizes functionality with aminimum ofhardware
systems. The next stepistobringthose systemsto life bypoweringthem, enablingthemto communicatewithother
flight systemsand/orflight crews, andwithgroundsystems and/orgroundcrews. Thisstepalso requires attentionto
the principles ofminimizing complexity. Finally, automate what is left by applying savings amortized in the first
four(4)stepsofthisprocess.
Thedesignstrategycontainsanumberofspecificsfor achievingthis, andisenabledthrough:
1. Use simple anddependable flight controlmechanisms that do notrequire routine flUid and/or gas servicing
during ground operations. Similar to approach in Step 3, but applies to electric power support offlight
controlsthatare less expensive to design, produce, operate, and sustainthan existing fluid and gas powered
systemsfor flightcontrolactuation.
2. Keep the number ofdedicated avionics boxes to an absolute minimum (achievable Ifall ofthe above is
adheredto> to the pointwhere no dedicated active avionics coolingsubsystems are required. The thermal
threshold that drives passive cooling to active cooling for a suite of avionics components must be
understood, managed, and constantlymonitoredand controlledthroughoutthe designprocess. Attentionand
visibility mustinvolve all subsystem designersandbemade ahardandfast designrequirement. Onewayto
do this is to keep the number ofdedicated avionics boxes to an absolute minimum (achievable ifall ofthe
above is adheredto) to the point where no dedicated active avionics cooling subsystems are required. In so
doing, an avionics system not requiring added active thermal control systems can help keep the cumulative
DDT&E,production, andoperations costsdownto aminimum. Withoutsuchattention, avionicssystemsare
likely to require the use ofcomplex cold plates and supporting working fluids, flight and ground support
equipment, andworkinvolvedwiththeirdesignandupkeep.
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3. Build-in enough mass margins to account for avionics cable lengths and interconnectivity hardware. Not
doingsorequires addedeffortandhighrisk ofexpensive design, productionand groundoperationsre-work.
This candelay operationaldeploymentand create less flexibility forthe owner-operatorto leavethe cabling
alone for a variety ofcustomers. With little or no margin, the operator is often forced to spend time and
moneyoptimizingthe cablingtomaintainpropulsionmarginforaparticularpayload.
4. Avoidspecializedground power. Standardize the ground powerto a single interface and build in, from the
beginning, enough mass margins to convert power on board. The time and expense involved in ground
systems design and developmentcan bereduced, along withadded hook-ups to ground services (see Figure
2).
5. Buildin remoteautonomous aviOniCS functIOnal verificationeverytime thesystems are powered-up. This can
dramaticallyincreasegroundoperations efficiencies for launchvehicles and spacecraft. Withoutthis built-in
design feature, the ground operators are left to playa game of "twenty questions", so to speak, just to
determine whetherfunctional integrity is available to continue with critical servicingand launch operations.
This provides autonomous verification that the vehicle's active systems have retained functional integrity:
i.e., that full electrical power, electronic signal, and software integrity exists upon power-up prior to
continuing critical operations. Modem computer peripherals and automobiles do this to a greater degree
upon power-up for their operators, for instance, than seen in most modem launch vehicles and spacecraft.
The objective is to automatically determine that there are no electrical shorts or opens, and that no
inadvertent software errors have been introduced. A greaterdegree ofvehicle availability, achievedthrough
more responsive flight system power-up times, increases affordability by achieving greater flight rate
capabilityforagivensetoffixed andvariableoperatingcosts.
B. SpecificEngineeringCriteriaAddressed for Improvement
,Step 5 enables or directly addresses a large number of the nineteen (19) engineering criteria for achieving
affordableandsustainablespacetransportationsystems(referenceTable I for specificcorrelationstoaffordability);
• Number ofseparateelectricalsupplyinterfacefunctions required
• Percentageofelementsandsystemsthatarehighlyreusable
• Percentofallsystemsnotautomated
• Number ofmaintenanceactIOnsunplannedbeforeorbetweenmissions
• Number ofmaintenanceactIOnsplannedbeforeorbetweenmiSSIOns
Step6. ExtensivelyTest/Adjustthe Design to Qualifythe System and Achieve the Objectives
A. Discussion/Specifics
Extensively qualify the components, verify & validate the systems, and flight test the vehicle to prove
accomplishment ofall needs and objectives prior to commitment to full operational capability and/or production;
thencontinuouslyimproveto maintainpre-eminenceandcompetitiveness. Specifically,
1. Prove-out design assumptions for simplicity and build technical and managerial confidence in a Simple.
robustsystem priorto committingto production.
2. Allow the flight test program to schedule improvements in the system design prIOr to committing to
production. This is done to avoid production cost impacts to design changes intended to verify for the
owner-operatorthatitcanmeetoperabilityandsupportabilityobjectives.
3. Maintain a separate. developmental component. subsystem. system. and flight test capability that is offline
from operatIOnaltransportationservice; do notputthese intheserial-criticalpath ofthe owner-operatorof
thesystem. Theowner-operatorshouldnotbeexpectedto interrupttheirtransportationserviceto makerisky
changes and conduct flight experiments. Nor shouldtechnical changes to the systems design be required to
impactproduction costs which should be surfaced in the offlinetest mode. A separate effortto demonstrate
upgradedcapabilitieswillmitigateanyaddedoperationalprogramriskfromdesignimprovementchanges.
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B. SpecificEngineeringCriteriaAddressed for Improvement
Step 6 enables or directly addresses a large number of the nineteen (19) engineering criteria for achieving
affordableandsustainablespacetransportationsystems(referenceTable 1for specificcorrelationsto affordability);
• Percentageofelementsandsystemsthatarehighlyreusable
• Number ofmamtenanceactionsunplannedbeforeorbetweenmissions
• Number ofmamtenanceactIOnsplannedbeforeorbetweenmissions
• NumberofCriticality 1system andfailureanalysis modes
FourEssentialTechnologiesEnablingtheHighPayoffDesign Approach
A. Concentric-NestedPropellantTanksforLaunch VehicleStagesand Spacecraft
To simply the architectural design, in-depth consideration should be given to using concentric-nested main
propellanttanks, wherethe innerwalloftheexternalofthetwotanks isintegraltothe structureofthe spacevehicle.
Suchconfigurationswoulduse at leastonetoroidaltankconfigurationlocatedontheexteriortotakethe vehicle and
engine structural loads (depending on how many stages employ this technique). This enables the overall vehicle
architecture to achieve a far more compact vehicle structure whose cryogenic propulsion system is far simpler to
operate, in-flightandonthe ground, thantoday's morecomplexdesignapproaches.
The"stackheight," ortotalintegratedspacevehicle height, canbeareal costdriverto launchsystemdesignsfor
a number ofground system processing stations and elements. For example, the total stack height drives assembly
facilities, launch towers (both mobile and fixed), and launch pad structures (see Figure 3). Advanced development
investments for innovative parallel tanks for cryogenic systems will give the designer more tank arrangement
optionsto considerduringconceptualdesign.
Tandemtank arrangements can also drive the concepttowards slender dimensions with large overall length-to
diameter(LID)ratios. This leads to vehicle structural dynamicstabilityproblemsduringflight. Additionally,tandem
tankarrangement lengthens propellantfeed line, protrudesthe vehicleoutermoldline(OML), anddrives the engine
startboxthermalconditioningrequirements; "pogo" affects, andadded geyserprotection systems. This intum leads
to added system design, production, and ground operations cost for stabilizers, dampeners, and so forth. Finally,
tandemvehiclearrangements presentissuesofliftoffdriftand launchtowerre-contact.
Toroidaltankarrangements for spacevehiclesandstagescangreatlyimprovethe length-to-diameter(LID) ratio,
with far stronger inherent structural stiffness of the stage (as opposed to traditional tandem tank and tandem
common bulkhead arrangements). This not only reduces the complexities of flexible-body stability and control
dynamics ofthe vehicle in flight, it also improves the handling and access characteristics ofthe vehicle during
groundoperations. As withthe common-bulkheadarrangement, itinherentlyavoidsroutine groundpersonnelaccess
that can arise from the creation ofan interstitial area between traditional, tandem tank arrangements (i.e., avoids
creatingan inter-tankcompartment).
To date, the use ofconcentric-nestedtanks for mainpropellantcontainmentsuchas toroidaltank configurations
tends to be avoided in the conceptual design processes. However, storable propellant applications are in production
and operationusingtoroidaltanks in anumberofRussian launch system designs, e.g.,the Briz-M upper stage. The
Sea Launch Zenit 3SL second and third stages also use toroidal kerosene fuel tanks with a L02 propellant
combination.
One drawback of a longitudinally-arranged inter-tank compartment is the temptation of locating functional
hardware and ground interfaces within such a compartment. This immediately creates an opportunity for locating
flighthardware functions requiringroutine groundservicingatthis location. Routine access bypersonnelmaycome
in two forms: external access for mold-line connections/disconnections (which may have some potential for
automation), but also internal access. Internal access requires added mass and complexity for entry hatches.
However, if full personnel entry is required and not just a partial reach inside an opening, it will require the
following:
• Timeandeffortto installandremoveworkplatforms
• Establish lighting; establish oxygen/conditioned air flow for personnel to avoid confined space oxygen
deficiencyhazards
• Time and effort to install and remove various protective caps, covers and other remove-before-flight
itemsstrictlytheretoprotectcriticalanddelicateflighthardware from inadvertentdamage
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• Ground-supplied purge systems to convert inert inter-tank atmosphere (gaseous nitrogen) to air for
personnel entry and vice versa. These purge systems are active systems and are costly and time
consumingto designandoperate.
Another benefit ofthe toroidal tank arrangement is greater thermodynamic efficiency. Liquid hydrogen (LH2)
has a lower boiling point value than liquid oxygen. Therefore, if the L02 tank is the outer tank in a toroidal
configuration, and a traditional cylindrical LH2 tank is nested inside, it can dramatically reduce hydrogen boil-off,
as the outerL02 tank acts as a more effective thermal radiation insulatorthan atraditional tank arrangement. This
reducesthe weightofvehiclethermalprotectionsystem(TPS)onthe LH2 tankandcouldbeespeciallybeneficialin
spaceoronanextraterrestrialsurface.
For surface landing applications, the use ofLH2 as a fuel for the propulsion function has been avoidedbecause
ofboil-offduring a medium stay time (say, a month). This boil offrate can be reduced by using concentric tank
arrangements where the LH2 commodity is located onthe intenor, where its radiation heat barrier is the outerL02
tank.
Anothercryogenic use forthis tank arrangement involves L02/CH propulsion; perhapstakingadvantage ofthe
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availabilityofCH4 onMars, for example. Inthis case,the boilingtemperatures are verysimilar, offeringadifferent
setofdesigncircumstancesandconsiderations. Since liquidoxygenhasagreaterdensitythanmethane, itshouldbe
the outertankandan integralpartofthestructure, andcarrythe loadofthestageorvehicle.
Acommonly-raisedissuetobeaddressedisthatthe innercylindricalwallofanelongatedtoroidaltankwillbein
radialcompressionwhenpressurized. Typically,the cylindricalwallofapressure vessel is mostly intension. Being
primarily in compression, the inner cylindrical wall must be designed for high strength. However, the mass and
strengthrequired ofsuchawall, aspartofthepressurevessel, canalsodouble asthe meansofcarryingthemajority
ofvehicle's weight and thrust loads. The architectural implications are quite profound ifthe life cycle costs and
ownership benefits are fully considered. The payoffmay be well worth the investment to explore innovations that
overcometheperceiveddifficulties.
A promising configurationto be exploreduses atraditional cylindrical cryogenictank inside anotherconcentric
toroidal cryogenic tank. The two parallel "nested" tanks are separated by compliant filler material as the two
cryogenic tanks expand and contract between ambient and cryogenic temperatures. A common barrel section
(similar to common bulkhead arrangements) to reduce weight is not what is envisioned. This is primarily due to
operational safety considerations, along with the production and propellant loadingloffloading complexities of a
common structural face encountering two different cryogenic fluids, each having unique thermal transport
properties. Also, in this configuration, the outer propellant tank as an integral structure to the vehicle while the
interior tank (either cylindrical or toroidal) is a non-load bearing structure. Whether the interior hydrogen tank is
toroidal or a standard cylindrical design depends on the application. The L02 tank interior wall should carry the
longitudinal load ofthe vehicle. Ifso, the interior hydrogen tank can be a very lightweight, traditional cylindrical
tank with standardenddomes. If, however, the interiortank is also toroidal, acrewaccess airlockcanbe insertedin
the interiorspacecreatedinaspacecraftapplication.
The concentric-nested tank arrangement, unlike the common-bulkhead, with compliant insulation in the
expanding and contracting interstitial volume between the tanks allows free flow of the upper inter-tank
compartment's purge gas to flow into the aft compartment. This arrangement deletes one ofthe traditional tank
design's ground-supplied compartment purges. In addition, the procedural constraints required of the common
bulkheaddome (sometimes calledthe "crotch" area) during cryogenic loadingbecause ofmajor stresses inthe face
sheetsareeliminated. These issues areavoided inthe concentrictankapproach. Productioncomplexitieswithuseof
a common bulkhead are likewise avoided, but not addressed here. The insulation betweenthe interior and exterior
tanksneedstobecompliantandallowforexpansionandcontractionwithchangesinthetankdiameters.
B. L02-Rich/L02-CooledFirstStageEarth-to-Orbit(ETO)MainEngine
This proposedtechnology uses large-scalethrust cryogenic engines (say 1-to 2 million-poundthrust class) with
an oxidizer-rich (L02-rich), rather than the traditional fuel-rich (LH2-rich) main combustion mixture ratio. A
promising approach would have the mixture ratio set at ~12:1at liftoffand reduced during ascent. This technology
would then allow a much higher propellant bulk density for the fust stage propellant tanks or a greatly increased
massfraction overtraditionalconfigurations.
As with concentric-nestedtanks, this technology allows a far more compact vehicle structure. It does so bythe
reductionofthe volumerequirementofthehydrogentanksubstantiallymorethanthe increasedoxygentankvolume
required to supply the oxygen-rich engine (due to the relative densities ofthe propellants). It also results in a far
simpler cryogenic propulsion system to operate in-flight and on the ground than today's more complex design
approaches. This approach will achieve the required thrust at liftoffwith fewer main engines and will decrease the
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