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NASA Contractor Report 187154AIAA-91-3451

An Historical Perspective of theNERVA Nuclear Rocket EngineTechnology Program

W.H. RobbinsAnalytical Engineering CorporationNorth Olmsted, Ohio

and

H.B. FingerConsultantWashington, D. C.forSverdrup Technology, Inc.NASA Lewis Research Center GroupBrook Park, Ohio

July 1991

Prepared forLewis Research CenterUnder Contract NAS3-25266

NASANational Aeronautics andSpace Administration

https://ntrs.nasa.gov/search.jsp?R=19910017902 2018-02-12T15:07:48+00:00Z

AN HISTORICAL PERSPECTIVE OF THE NERVA NUCLEAR ROCKET ENGINE TECHNOLOGYPROGRAM

W. H. Robbins*Analytical Engineering Corporation

H. B. FingerConsultant

Abstract

Nuclear rocket research and development wasinitiated in the United States in 1955 and is still beingpursued to a limited extent. The major technologyemphasis occurred in the decade of the 1960's and wasprimarily associated with the Rover/NERVA Programwhere the technology for a nuclear rocket enginesystem for space application was developed anddemonstrated. The NERVA (Nuclear Engine for RocketVehicle Application) technology developed twentyyears ago provides a comprehensive and viablepropulsion technology base that can be applied andwill prove to be valuable for application to NASA'sSpace Exploration Initiative. Also the uniqueorganization that evolved to successfully manage thiseffort requiring major contributions and involvementof two government agencies in partnership withindustry is a useful model when a similar developmentis initiated for SEI. This paper, which is historical inscope, provides an overview of the conduct of theNERVA Engine Program, its organization andmanagement, development philosophy, the engineconfiguration and significant accomplishments.

Introduction

Manned exploration missions to the near planetsplanned in NASA's Space Exploration Initiative (SEI)are particularly difficult. Long term exposure to thehostile space environment requires the developmentand improvement of mission critical technologiesincluding life support systems, electric power andpropulsion. The space vehicle propulsion system isparticularly important since it has a major influenceon the vehicle configuration including its size, weight,and perhaps the most important consideration, costand trip time required to perform the mission.Therefore, it is necessary that planetary propulsionsystems be developed with a step increase inperformance and specific impulse over conventionalchemical rockets. The nuclear rocket, with a specificimpulse potential two to three times that of chemicalsystems, is an attractive option and perhaps the onlyoption in the foreseeable future. The reduction in triptime with increasing specific impulse is illustrated inFigure 1. Trip time could be cut at least 100 days fromthat of advanced chemical systems.

Figure 1 - Propulsion System PerformancePotential for a Manned Mars Mission

10000

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a 1000

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loo100 200 300 400 500

Mission Round Trip Time - Days

Nuclear rocket research and technologydevelopment was initiated in the United States in 1955and is still being pursued to a limited extent. The realtechnology emphasis occurred in the decade of the1960's and was primarily associated with theRover/NERVA Program. The Rover program consistedof both research and technology development directedto nuclear rockets for space application. Theprincipal research thrust, conducted by the LosAlamos National Laboratory, LANL, was directed to thedevelopment of reactor fuel and reactor systems thatwould operate with hydrogen at temperatures above2200°K. The technology development, called NERVA,the focus of this paper, was directed to nuclear enginesystem components and the conduct of ground testengine system demonstrations. The NERVAtechnology, developed twenty years ago andsummarized herein provides a viable technology basethat will prove valuable for application to NASA'scurrent Space Exploration Initiative. Also the uniqueorganization that evolved to successfully manage thiseffort requiring major contributions and involvementof two government agencies in partnership withindustry is a useful model when a similar developmentis Initiated for SEI. This paper, which is historical inscope, provides an overview of the conduct of theNERVA Program, its organization and management,the development philosophy and the engineconfiguration. In addition, the verification approach,test philosophy and the significant accomplishmentsof this highly successful technology program are

'Member AIAA 1

presented. The emphasis of this paper has beendirected toward the engine system development. Twotopics are emphasized - the rationale for establishingthe engine configuration and the "proof of concept"approach that demonstrated engine operationalfeasibility. LANL reactor research, although a majorand enabling activity in the program, is not discussedin detail.

The Information presented herein is a synopsis ofthe authors' experience as overall Program Directorand Project Manager of the Engine SystemDevelopment Program through the decade of the1960's. Fortunately, the NERVA Program was welldocumented. Approximately 100,000 reports andmemoranda were generated and many of thesedocuments were utilized as a "memory check".References 1 and 2 were particularly helpful and someof the information from these references is containedherein.

the nozzle and the engine control system. Facilities forengine assembly/ disassembly and engine systemtests were also recognized as being essential and asignificant challenge. Therefore, the design andconstruction of these facilities was initiated early inthe program.

In addition to the major program thrust, which wasthe determination of engine system characteristicsusing graphite reactor technology with hydrogencoolant, a small but significant activity was directed tothe pursuit of higher risk, high- benefit technologies.Two primary alternative reactor technologies werefunded; refractory metal reactors, and liquid and gascore reactors.

There was also a continuing systems engineeringactivity associated with mission and flight enginedefinition which guided technology development.

Organization and ManagementTechnology Development Philosophy

After several years of nuclear reactor researchconducted by the Los Alamos National Laboratory.rocket engine development began in 1961 with theselection of the industrial contractors, Aerojet andWestinghouse, to develop the NERVA engine. Initially,a nuclear rocket engine flight test was planned.However, in 1963 the nuclear rocket program was re-viewed and redirected toward an engine technologyprogram focused on ground tests only. Nevertheless,space mission plans associated with mannedplanetary exploration, unmanned solar systemexploration, and extended lunar exploration continuedto strongly influence definition of propulsion systemrequirements and guided ground test engine definition.

The objective of the NERVA Technology Programwas to establish a technology base for nuclear rocketengine systems to be utilized In the design anddevelopment of propulsion systems for space missionapplication. The principal task was associated withthe assessment of real system performance andoperating characteristics in advance of firm missionrequirements. This approach ensured that systemperformance would be well understood when missionobjectives and propulsion system requirements wereclearly specified.

Technology priorities were determined at theoutset — safety and reliability requirements tookprecedence over weight and performanceconsiderations. Emphasis was placed on thedevelopment of critical engine components whichsignificantly affected system interactions and systemcharacteristics, namely the propellant feed system,

The initial NERVA management issue wasassociated with the fact that the nature of the programrequired the technical capabilities and functions oftwo government agencies, NASA and the AtomicEnergy Commission (AEC). Perhaps the mostsignificant management issue of all was associatedwith the fact that it was impractical to separatereactor development from the development of the non-nuclear engine components because of theinteractions and critical interfaces that existed amongcomponents of a rocket engine. The technicalmanagement solution to this issue became obvious. Asingle program/project organization was mandatoryfor efficient and effective program implementation.There were, of course, many institutional and politicalproblems that required joint agency resolutions suchas the flow of funds, agency roles and responsibilities,and staffing. The organizational approach thatevolved, however, gave priority to technicalconsiderations and was structured to maximize thepossibility of technical success.

The NERVA program was organized as a jointNASA/Atomic Energy Commission (AEC) program andboth agencies provided resources. AEC funded reactorresearch and technology development as well as thedevelopment of reactor and fuel test facilities. NASAfunding was directed to non-nuclear componenttechnology, engine system development, and thedesign and construction of engine test facilities.

The NERVA program was managed by the SpaceNuclear Propulsion Office (SNPO). This "ProgramOffice" was located in Washington D.C. and was staffedby both NASA and AEC employees. The director of the

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Figure 2 - Nuclear Rocket Program OrganizationAEC NASA

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Space Nuclear Propulsion Office ,dameacWiston of Rtor .

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Figure 3 - Space Nuclear Propulsion Ofnce Cleveland Extension (SNPO - C)

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office was a NASA employee and his deputy was on theAEC staff. The balance of the Program Office staff,approadmately twenty people, were a combination ofAEC and NASA employees. Specific responsibilities ofthe Program Office included program and resourceplanning and evaluation, the justification anddistribution of program resources, the definition andcontrol of overall program requirements, monitoringand reporting of progress and problems to NASA andAEC management, and the preparation of testimony tothe Congress.

The Nuclear Rocket Program Organization as itexisted In 1961 is shown in Figure 2. As shown in thechart, the Director of SNPO reported to both agencies.The NASA reporting chain was through the AssociateAdministrator of the Office of Advanced Research andTechnology to the NASA Administrator. In the case of

AEC, the Director reported through the Division ofReactor Development and the AEC General Manager tothe Atomic Energy Commission.

Three SNPO Field Offices reported directly to theSNPO Director. The Cleveland extension (SNPO-C)was a - Project Office" responsible for the managementof NERVA Engine Technology Development. The officewas located at the NASA-Lewis Research Center(LeRC). The NASA-Le RC staff provided a major portionof the engineering support throughout the technologyprogram. The SNPO-C Project Office managed theactivities of the industrial contractors. Aerojet andWestinghouse. In addition, SNPO-C responsibilitiesincluded the management of the design, constructionand activation of engine assembly/disassembly andengine test facilities at the Nevada Test Site.

The 1966 SNPO-C Organization is shown in Figure3. The Project Office contained all the technical,administrative and support functions necessary forthe management of the technology developmentactivity that was conducted by government, industryand university teams.

This Program/Project organization was unique intwo respects. 'Me Project Office (SNPO-C) was a linemanagement organization, and it was comprised of aco-located AEC/NASA staff, that reportedinstitutionally as well as programmatically to theProgram Office, SNPO. The Program Office had directline management authority over all Project functions.The Project Office, in turn, was responsible for the dayto day project management activities includingtechnical, procurement, budget, schedule and safety.With this organizational structure, roles andresponsibilities were clear and the response to allProject Manager actions, particularly procurementactions, was greatly accelerated. The projectorganization also benefited from its location andutilized NASA-Lewis engineering support in theconduct of day to day project management activities.

The Nevada Extension, located at the NuclearRocket Development Station, managed the test sitebase support activities and provided on site supportduring facility construction, activation and operation.

The Albuquerque Extension provided liaison withthe Los Alamos Laboratory who had the responsibilityfor the KIWI and Phoebus research reactor programs.KIWI and Phoebus reactor research was directed toadvanced concepts to demonstrate reactor operationat progressively higher temperature, higher powerdensity and higher power with reduced fuel corrosion.

In addition to the line management organizationdescribed, SNPO retained technical direction authorityover the Los Alamos Laboratory for the KIWI andPhoebus reactor development activities.

This organization was unique in NASA history inthat it was a single joint Program Office that wasresponsible for implementation of policies andpractices of two government agencies. In addition,field offices, responsible for Project Management andliaison, were line organizations reporting to theProgram Office institutionally and programmatically.

This organizational approach proved to beefficient and effective in managing institutional,programmatic and technical issues. The keys to itssuccess were associated with the ability to effectivelyresolve interagency issues, provide clear leadershipand direction to the Project organization, and maintain

the support of the agencies involved, as well as theAdministration, the Congress and the general public.

NERVA Engine Technology Development

The long lead times required for the acquisition ofsystem and reactor test facilities forced earlyestablishment of engine size and propulsion systemperformance goals. An early NERVA priority task wasflight system definition derived from NASA missionplanning and the analysis of missions which werethen thought to be 10 to 20 years in the future.Because it was recognized that firm missions wouldtake years to evolve, the nuclear rocket engine systemand the associated technology thrusts were definedsuch that the resulting technology developmentactivities would, in so far as possible, be applicable toa variety of space missions.

System Definition

The missions of interest in the early 1960's werenot substantially different than those beingconsidered today in NASA's Space ExplorationInitiative. Extended lunar exploration and principallymanned Mars expeditions received the most attention.The propulsion system requirements for the Marsmission were the most severe. Chemical propulsionsystems that existed or were under development werenot considered practical for manned planetaryapplication because of the large vehicle weight and thetrip time required to perform the mission. For nuclearrockets, spacecraft weights between 1.5 and 3 millionpounds were considered necessary and propulsionstages with specific impulse values in excess of 800seconds and thrust levels of 200,000 pounds or morewere desired. In contrast, engine thrust requirementsfor the lunar missions, because of reduced vehicleweights, were much lower, below 100.000 pounds.

Because it was necessary to make an early enginesize selection for multiple missions, one option waspropulsion modules with multiple engines with thenumber of engines per propulsion modulecommensurate with the required mission thrustrequirements. Using this approach, a single flightbaseline engine was defined with a thrust level of75,000 pounds and 825 seconds of specific impulse. Itis important to note that the reactor powerrequirements, somewhat in excess of 1000 MW, weresimilar to the design requirements of the KIWI-Breactor under development by the Los AlamosNational Laboratory.

The flight engine configuration is shown in Figure4. The engine was 22 feet high from the upper thruststructure which mates to the hydrogen propellant tank

4

PUMP

TURBINECONTROLS

REFLECTOR PROPELLANTPRESSURE VESSEL TANK

LINES

there. The flow then cools the shield and enters thereactor inlet plenum.

Propellant is distributed from the inlet plenuminto several parallel paths. The bulk of the flow entersthe reactor fuel-element cooling passages and isheated to a high temperature. The remainingpropellant is distributed to flow passages which

Figure 4 - NERVA Flight Engine Configuration

to the nozzle exhaust exit. The components shownfrom top to bottom consist of the conical upper thruststructure, spherical bottles which contain actuationgas, and the gimbal assembly between the upper andlower thrust structure. The turbomachinery ismounted in the lower thrust structure. The reactor andinternal shield is contained within the pressure vessel.The rocket nozzle mounted to the pressure vessel iscooled by the main propellant flow.

Prooellant Flow Path

The propellant flow path is shown in Figure 5.During steady-state operation, main propellant flowbegins with liquid hydrogen, under tankpressurization, passing through the tank shutoff valveinto the pump suction line. This propellent inlet linecontains a gimbal bearing for thrust vectoradjustment. A centrifugal flow pump pressurizes thepropellant. ^ The pressurized propellant enters thepump discharge line and flows to the nozzle coolingpassages, removing heat transferred to the nozzle fromthe main exhaust stream as well as heat generated inthe nozzle and pressure vessel by deposition ofnuclear radiation energy. The coolant leaves thenozzle as a low temperature low-density fluid and issplit into parallel flows to cool the pressure vessel,reflector and control drums. Propellant exits from thereflector region and is directed along the pressurevessel dome to remove radiation energy deposited

TANKVALVE

4 TURBINE EXHAUST

^ REACTOR

BLEED PORT

NOZZLEC-90-09790

Figure 5 - Nuclear Rocket Engine Schematic

provide coolant to various reactor structural elementsand to the peripheral region between the hot reactorcore and the regeneratively-cooled reflector. Thesevarious cooling flows merge at the reactor exit and flowthrough the nozzle reaching high exhaust velocitiesand high specific impulse.

Cycle Selection

The choice of the turbine-drive cycle was one ofthe principal system design selections that wasrequired because the turbine-drive cycle can have asignificant impact on propulsion system performance(specific impulse) and engine start-up characteristics.In addition, major component designs andinteractions between components are also affected.Included among the components are the turbopump,reactor, and nozzle.

The approach and rationale for the turbine-drivecycle selection that is discussed herein was typical ofthe process that was utilized for decision making. TheNERVA engine utilized a hot bleed cycle wherein hot

5

Figure 6 - Turbine Drive Cycle Options

CYCLE DESCRIPTION REMARKS

Cold Bleed • Hydrogen bled from pressure vessel dome for I,w temperature turbineturbine drive gas large hydrogen now rates required

• Large impulse penalty• Minimal acceleration margins during startup• Large area ratlon nozzle requtred for heat input

Gas Pressurization Cx o(hlgh pressure liquid steStmph, reliable sy mhydrogen and flow control system Results In high H 2 storage volumes and high tank weights• Does not require a turbopump Impractical for a flight system

Chemical Gas Generator H2- O 2Combustion Products drive a Independent power source avoids system integration

nurbopump systemIssues

Liquid oxygen must be carried - complicates stage design• increased weight - decreased reliability precluded selection

• Uses reactor heat energy to drive the • Provides highest specific impulseTopping Cycle turbopump Heat energy derived from reflector

e operates at low temperature• Total propellant flow utilized to drive theProPe • Complicates reactor design

Compl

turbtne • Reactor design effected' strength requirrments• sealing requirements' reflector design

• Discarded for first generation system

Hot Bleed Hydrogen from nozzle chamber mixed Bleed gas flow rate flied by turbine inlet temperaturewith cold hydrogen to a turbine Inlet Small 1 penalty• n

• Nozzle and bleed port development can proceed withouttemperature compatible with material system interactionscapability • Bleed port Is a significant development

The Selected System

gas is tapped from the nozzle chamber, diluted withcold hydrogen gas to reduce the temperature to thelevel acceptable to the turbine materials, and passedthrough the turbine to drive the turbopump. Theturbine exhaust gas was dumped overboard with somethrust recovery (Figure 5). This cycle was selectedafter a thorough evaluation of engineering alternatives.Alternatives included pressurized gas cycle, achemical gas generator system cycle, topping cycle,and hot and cold bleed cycles. These turbine drivecycle options are summarized in Figure 6 where a cycledescription and remarks on each cycle, which includeadvantages and disadvantages of each alternative, arelisted. The hot bleed cycle was selected primarilybecause the bleed cycle components could bedeveloped and the performance could be verified withcomponent testing. Complete system testing was notrequired to successfully demonstrate the technology.

Test/Verification Approach

Following the completion of the NERVA enginedesign a series of design reviews were conductedwhich addressed both component and system designs.The design review process included reviews of theconfiguration and supporting analyses and test datathat established material properties, structuralcapabilities and operating environment. Aftercompletion of component fabrication, componenttests were initiated to verify that the functional,environmental and structural capabilities of eachcomponent met or exceeded the design requirements.

Components whose performance were key to thesuccess of the engine system, namely the fuelelements, reactor support system turbopump andnozzle received the most emphasis. Test verificationcriteria were established at each assembly level andhigher level tests of sub-systems and systems wereinitiated only after meeting well-defined testverification criteria at the prior assembly level. Theapproach is shown in Figure 7.

Figure 7 - Design/ Test Verification

Design/ Verification

Design Component/ System Analysis Simulation

Design Revicwlsl Structural/ EnvironmentalAnalysts/ Tests

Test/ Verification ^

Non-Nuclear Component/Component Subsystem I I Furl ElcmentTests

Functional Tests Structural Tests

Cold Flow Test I I Reactor SubsystemVehicle Test

Engine SystemTest

Testing of major sub-assemblies followedsuccessful component testing. Many reactor sub-system development tests were performed by both theLos Alamos National Laboratory (KIWI and PhoebusTest Series) and the NERVA contractors. In addition, acold flow test of an engine with an unfueled reactor

6

Figure 8 - NRX Reactor Assembly andFlow Path

Inlet Plenum

\ShieldReflector Support PlateOutletPlenum

Pressure LateralVessel upportCore SystemReflector---b

Noale

r Inlet

Charm Plenum>44, ;^ Nozzle

was made to determine the engine systemperformance during the initial start transient. Thiswas a critical test which determined that a "bootstrap"start, using the stored heat energy of the nozzle andreflector, was a feasible startup procedure.

Finally, only after successful performance of allcritical sub-assemblies, engine system tests wereperformed. This technique proved to be sound for theNERVA program and is still utilized to a large extentfor space system development and qualification today.

This test verification approach forced a signL'icantamount of discipline into NERVA technologydevelopment. Each test had criteria for successfulcompletion that were met prior to proceeding to thenext assembly level.

KIWI/NERVA Reactor

The NERVA reactor, called NRX (Nuclear ReactorExperimental), selection was based upon the LosAlamos National Laboratory KIWI-134 researchreactor. The KIWI-I3 reactor concept had evolved as aresult of several years of intensive research and

development at LANL prior to the start of NERVA and itwas logical therefore that the NERVA (NR)Q reactorshould take maximum advantage of the prior KIWIresearch and development effort.

The NRX reactor assembly included the reactorcore, reflector, control drums and internal shield andis shown in Figure 8. The reactor core was composedof graphite fuel elements impregnated with pyrolyicgraphite uranium carbide particles and supportedboth axially and laterally. The reflector was made ofberyllium. Twelve rotary control drums with a boralsheet located in the reflector were utilized for powercontrol. The reactor was configured so that, whenincorporated into a flight engine system configuration,the engine would produce approximately 75,000pounds of thrust and a specific impulse of 825seconds. The reactor operating temperature wasapproximately 2300°K and the initial reactoroperating Lifetime goal was one hour. A Mars missioncould be successfully accomplished with a propulsionsystem with this performance. The KIWI/NERVAreactor research and technology program was clearlya major key to the success of NERVA and has been welldocumented.

Nuclear Reactor Experimental/Engine System Test -NRX/EST

KIWI/NERVA reactor research and technologymade great strides in terms of structural design andthe reduction of fuel element corrosion in the early1960's. On the basis of significant advancements inreactor and fuel technology, an early test to determinethe feasibility of an engine system was both desirableand necessary. In addition, many of the experimentalengine test objectives could be achieved years earlierthan planned. It became obvious that the fastest wayto accomplish this major program milestone would beto combine an engine system demonstration with aplanned reactor test. NRX/EST (Nuclear ReactorExperimental/ Engine System Test) therefore emergedas a key element of the NERVA program. The NRX/ESTtest article is shown in Figure 9. A turbopump wasadded and installed within the enclosure on the testcart as shown in Figure 10. The arrangement of theengine components, although physically different froma conventional engine, was functionally similar to aflight system. The critical engine components, namelythe reactor, the turbopump, and the nozzle, were alsofunctionally "flight-like" components.

7

yn

C-91-06729`7

J TO REACTO^LE INLET

-^_ _^-fJ^_ PUMP DISCHARGE VALVE

'URBINE, POWER CONTROL VALVE

Figure 9- NRX/ EST Test Configuration

The NRX/EST Test Proiiram

The primary objectives of the NRX/EST test wereas follows:

1. Demonstrate the feasibility of starting andrestarting the engine without an external powersource.

2 Evaluate the control system characteristics(stability and control mode) during startup,shutdown, cooldown, and restart for a variety ofinitial conditions.

3 Investigate the system stability over a broadoperating range.

4 Investigate the endurance capability of the enginecomponents, especially the reactor, duringtransient and steady state operation with multiplerestarts.

Real issues were associated with engineperformance. The engine start transient was a majorconsideration, that is, could the turbopump beaccelerated (bootstrapped) utilizing the latent heat ofthe nozzle and reflector? Another concern was thedesign performance of the nozzle bleed port whichprovided hot drive gas to the turbomachinery.Operation of the engine control system over a widerange of conditions also had to be demonstrated, sincethe only information, prior to system testing, wasobtained from control system analysis and simulation.Results from prior reactor tests, however, providedsome confidence that the engine controls wouldperform satisfactorily.

The NRX/EST test program was conducted inFebruary 1966. All test objectives were successfully

Figure 10 - Engine components within NP-K/ ESTEnclosure

accomplished. The engine system was started and re-started several times; the engine was demonstrated tobe stable over a wide range of operating conditions;required temperature ramp rates were shown to beachievable; and the engine control systemperformance proved to be predictable, safe andreliable during both transient and steady stateoperation. In addition, the endurance capability ofengine components was demonstrated both at ratedpower, thrust and temperature as well as duringtransient operating conditions. NRX/EST operatednearly two hours of which 28 minutes were at fullpower. This exceeded the operating time of priorreactors by more than a factor of two.

'Die NRX/EST test program was important in thatit was the culmination of a long line of Rover researchand development tasks. The test demonstrated thefeasibility of a nuclear rocket engine for spaceapplication. The NERVA activities following NRX/ESTresulted in significant incremental technologyimprovements beyond that embodied in NRX/EST thatwere necessary to accomplish prior to proceeding withflight system development.

Ground Experimental Engine XE

The second nuclear rocket engine test wasconducted following the completion of constructionand activation of engine test stand # 1, ETS-1, in 1969.In contrast to NRX/EST run three years earlier, the XEengine configuration (Figure 11) closely simulated aflight system both physically and functionally. Flightcomponent designs were utilized selectively, i.e., onlywhen component characteristics had an importantinfluence on overall system performance. An example

g

was the use of a flight-design turbopump becausemass and inertia effects had an influence on chilldowntime and acceleration characteristics. In so far aspossible, facility type components and subsystemswere used to save cost and time. Examples were manyvalves and the pneumatic system that were notdeemed to affect engine system performance. Inaddition, an external radiation shield was added to theconfiguration to protect engine components. Thiseliminated the need to radiation harden manycomponents utilized in system testing.

s-

i ^ ---- — PROPELLANT SHUT OFF VALVE

TEST STAND ADAPTER

UPPER THRUST STRUCTURE—

Ai

TURBOPUMP ASSEMBLY

TURBINE BLOCK VALVE

COOLDOWN LINE-- —Jr! i-- TURBINE POWER CONTROL VALVE

EXTERNAL SHIELD._S ?(''^ CONTROL DRUM ACTUATOR

LOWER THRUST STRUCTURE

PRESSURE VESSEL

TURBINE INLET LINE r-r str-T DILUENT LINE

TURBINE EXHAUST LINE I It i TURBINE EXHAUST LINE

PUMP DISCHARGE LINE - - NOZZLE

C-90-09789

Figure 11 - XE Engine

The test facility, ETS-1, was designed so that theengine could be operated in a down firing attitude in anenclosed compartment with reduced atmosphericpressure (1-PS1A) so that the space environment waspartially simulated. A picture of the XE engineinstalled in the ETS-1 facility is shown in Figure 12.The installation and removal vehicle (EM can be seenbehind the engine. The EfV was capable of remotelyinstalling and removing the engine from the facility.XE Test Program

The objectives of the XE Test Program were similarto those of the NRX/EST. However, many incrementalimprovements in the reactor, other enginecomponents, and the control system wereincorporated into XE as a result of the informationderived from NRX/EST and other prior reactor tests.These improvements were to be evaluated in terms oftheir effect on startup, power operation, off nominaloperation, shutdown, and cooldown. Because this testwas also the first test in a new down-firing test facility,a major objective of the test series was to demonstratethe practicality of the facility for flight enginequalification and acceptance. Clearly, however, theparamount objective of this test was to demonstratethat engine system operational feasibility was

Figure 12 - XE Engine Installed in ETS - 1

successfully demonstrated and that no enablingtechnology issues remained as a barrier to flightengine development.

The engine was successfully started withoutexternal power and operated over a range of transientand steady state conditions including full power. Infact, in this test series, completely automatic startupwas successfully demonstrated. The engine wasoperated safely and as predicted over the wide range ofoperating conditions in 'Several control modesincluding the range encompassed by the solid lines ofFigure 13. One that was particularly significant wasoperation under closed loop pressure and temperaturecontrol. This control mode was planned in flight forcontrol of thrust and specific impulse.

soot

q^3 ^l' Fuel Tem^crature IJ nIt

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"rIM Impulse Design Thr .

o. 3000 c

E 2000F ^

_^ coo1000

100 200 300 400 Soo 600 700

Nozzle chamber Prr—um, PSI(Related to Thruetl

Figure 13 - Engine Operating Map

9

The total run time on the XE engine was 115minutes and included twenty-eight starts to poweroperation. The XE test series was significant in that itconfirmed that a nuclear rocket engine was suitablefor space flight application and was able to operate ata specific impulse twice that of chemical rocketsystem. All engine feasibility issues were successfullyaddressed, no component or systems issues wereobserved, and the development of a flight nuclearrocket system could proceed with confidence.

In addition, the engine test facility was proved tobe practical for qualification and acceptance testing ofnuclear rockets. In today's environment, however, anexhaust gas scrubber would be required to controlfission product release.

Summary of Rover/ NERVA Technology Base

The Rover/NERVA program conducted through thedecade of the 1960's was a highly successfultechnology program. It's goals and objectives were todemonstrate the feasibility of a nuclear rocket enginesystem for space application. This "Proof of Concept"program was mission oriented and culminated in thesuccessful demonstration of a ground test enginesystem.

Reactor/Engine Technology

The Rover reactor program started when a seriesof research reactors, called the KIWI Series, weredesigned, built and tested to determine feasibility of areactor concept which would operate at hightemperature with liquid hydrogen. The first series oftests, called the KIWI-A Series, were conducted in1959 and 1960. The KIWI test series met its objectivesby demonstrating operational feasibility withhydrogen and at the same time meeting performancegoals of high temperature operation required for spaceapplication.

The final KIWI reactor, 134E, the seventh and lastreactor in the series, operated successfully at atemperature of approximately 2000°K for elevenminutes and was utilized as the basis for the design ofsubsequent reactors.

The NRX series of developmental reactor test werestarted concurrently with the KIWI-B tests. The NRXseries of reactors continued the advancement ofreactor technology. The NRX Series demonstratedimproved structural integrity, reduced fuel corrosionrates (longer life) and higher temperature operation, inexcess of 2200°K. NRX-A6, the last in a series,operated one hour continually. The fuel corrosion ratewas low. The extrapolated reactor lifetime, 2-3 hours,would be commensurate with today's manned Marsmission requirement. In parallel with the NRX TestSeries, research reactors (Phoebus, Pewee and NuclearFurnace) demonstrated high power density, highpower (4000 MW), and the potential of long life fueloperating at high temperature.

In addition to the reactor test performance, enginesystems tests NRX/EST and XE, described previously,were successful. No engine system feasibility barriersto flight development were manifested.

A summary of the major tests conducted in theRover/ NERVA program is discussed in Reference 2and shown in Figure 14. A total of twenty reactor testsand two engine tests were performed. The operatinghistory is shown in Figures 15 and 16. These twofigures dramatically illustrate the major investmentthat has been made to produce the significantadvancement in nuclear rocket technology. Seventeenhours of operating time were accumulated with sixhours in excess of 2000°K.

Fuel

A significant technology thrust was directed to fueldevelopment. Graphite was chosen as the fuel element

1959 1960 1961 1962 1 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972

NRX NRX-Al • NRX -A3 NRX -A6Ta REACTOR I •

d TEST NR) -A2 • • NRX -A5

•ENGINETESTS •

NRX/EST O-XE

FCF

&A• WI B16 KIWI B4DKI

KIWI A• K1T1 A3 • •KIWI TNI'

KIWA KIWIKIWI B4A • KIWI B4E

B'APHOEBUS PHOEBUS IA PHOEBUS IB

U • • PHOEBUS 2A

•V)

zPEWEE 40 PEWEE

NUCLEARFURNACE NT'-1 •

Figure 14 - ChronolopV of Maior Nuclear Reactor Tests

10

1

0.9

0.8

0.7

0.6

a5

Q4

0.3

0.2

0.1

0

CC

OLOU

Rf

ti

s'wc-n2 -aaFigure 15 - Operating Time VersusCoolant Exit Temperature

for the Full Power Reactor Tests

60 TFE> 2500 K

40 TFE 2200 to 2500 K

n' 20TFE' 2200 K

GC

c2X0

Calendar Ttme (years)

material because of its favorable neutronicscharacteristics and potential to operate at hightemperature. However, it was recognized thathydrogen reacted with carbon which resulted in fuelcorrosion. Therefore, all fuel elements were coated toprevent high corrosion rates. Niobium carbide and

zirconium carbide proved to be the most effective inminimizing corrosion. Progress in reducing corrosionis shown in Figure 17. Improvements in coatingtechnology reduced the corrosion rate by an order ofmagnitude.

Test Facilities

A major key to the success of Rover/NERVA wasthe development of test facilities at the Nuclear Rocket

Figure 16 - ROVER/NERVA Reactor Operating Time

1000

900

cumulattvc T1me at loafers^ Above Onc Megawatt

200

\\\\\\\\\\\\

100 Cumulative nme ai

01964' 1965 ' 1966 1 1967 ' 1968' 1969' 1970' 1971 ' 1972

1965 1966 1967 1968 1969

Figure 17 - ROVER/.NERVA Corrosion Rate History

Development Station at Jackass Flats, Nevada. Tworeactor test stands were built and utilized for reactorsub-system tests. The reactor was oriented In anupward firing position. The engine test facility was adownward firing facility with an exhaust system thatprovided altitude simulation so that space conditionscould be partially duplicated.

Maintenance, assembly, and disassemblybuildings were also utilized for assembling and disas-sembling engine and reactor systems. The buildingscontained all the necessary equipment for remoteassembly and disassembly of reactors and enginesand were, in themselves, significant technologicaladvancements over current "state of the art" facilities.

Perhaps the most significant facility whenreviewed from todays environmental test requirementsassociated with no fission product release was thenuclear furnace test facility. The nuclear furnace wasa test reactor which was capable of evaluating fuelelements and fuel clusters in a nuclear environment.The nuclear furnace test facility was operated with anexhaust gas scrubber which provided completecontainment of all radioactive material. Although thescrubber had a much smaller capacity than would berequired for testing reactors planned in the future, thefeasibility of scrubbers has been demonstrated insmall scale.Safety

The safety record at the Nevada Test Site duringthe period 1959 to 1972 was excellent. Personnelinjuries were limited, the most serious was a result of ahydrogen explosion in which two workers sustainedfoot and ear drum injuries. In addition, three reactorssustained damage during reactor testing. The damageto two of the reactors was the result of structuraldesign deficiencies that led to failure caused by flow-

11

induced vibrations. In both cases fuel elements wereejected from the core. The damage sustained by thethird reactor was caused by a procedural error whichresulted in the exhaustion of all propellant which ofcourse is the reactor coolant. The core overheated andextensive damage to the core including the release offuel elements resulted. Fortunately, no personnelinjuries were sustained in these reactor mishaps andthe failure mechanisms were understood andcorrected.

Ei)iloEiue

Currently, nearly twenty years after thetermination of the Rover/NERVA Program, there isconsiderable new interest in space nuclearpropulsion. As part of the on-going activity associatedwith NASA's Space Exploration Initiative, nuclearthermal propulsion has been identified as an enablingtechnology for manned missions to Mars. In this light,as future propulsion system requirements evolve forplanetary exploration, technology effort similar toNERVA is likely to be reborn in this decade; perhapswith the name Rip Van Winkle. Fortunately, NERVAtechnology is still applicable to our understanding ofcurrent propulsion needs. The demonstratedRover/NERVA technology base evolved as a result of along and comprehensive program, including a series ofanalyses and tests progressing from parts tocomponents, subsystems and finally engine systems.The legacy of twenty reactor tests and two full-scaleengine system tests is impressive evidence of theproven state of available nuclear rocket technology.

Reactor and engine system tests wereconducted at temperature, pressure, power levels, anddurations commensurate with today's propulsionsystem requirements. Feasibility of the nuclear rockethas been clearly established so that future nuclearpropulsion development associated with new spaceexploration initiatives can be directed to incrementalperformance, reliability and lifetime improvements. Inaddition, a model for an effective managementapproach involving two government agencies in amajor technology development has beendemonstrated. The real future development challengewill be associated with engine and reactor systemground testing in an environmentally acceptablefashion.

The management of multi-agency programs is adifficult task. It remains to be seen whether a SpaceExploration Initiative management structure evolveswhich maximizes the probability of successfullyaddressing the significant technical challengesassociated with nuclear rocket development and at the

same time attenuates the institutional biases of thegovernment agencies involved.

References

L Nuclear. Thermal, and Electric Rocket Propulsion:Fundamentals, Systems and ApplicationsR. W. BuscandGordan and Breach Science Publishers1964

2 Experience Gained form the Space Nuclear RocketProiiram (Rover)Daniel R. KoeningLos Alamos National LaboratoryLos Alamos, New Mexico 87545, LA10062H

12

NASANational Aeronautics and Report Documentation PageSpace Administration

1. Report No- NASA CR-187154 2. Government Accession No. 3. Recipient's Catalog No.

AIAA -91-3451

4. Title and Subtitle 5. Report Date

An Historical Perspective of the NERVA Nuclear Rocket July 1991Engine Technology Program

6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No.

W.H. Robbins and H.B. Finger

10. Work Unit No.

505 -43-119. Performing Organization Name and Address

11. Contract or Grant No.Sverdrup Technology, Inc.Lewis Research Center Group NAS3-252662001 Aerospace Parkway

13. Type of Report and Period CoveredBrook Park, Ohio 44142Contractor ReportFinal

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration14. Sponsoring Agency CodeLewis Research Center

Cleveland, Ohio 44135 - 3191

15. Supplementary Notes

Project Manager, Thomas J. Miller, Nuclear Propulsion Office, NASA Lewis Research Center, (216) 433 -7102.W.H. Robbins, Analytical Engineering Corporation, 25111 Country Club Blvd., North Olmsted, Ohio 44070, subcon-tractor to Sverdrup Technology, Inc. (Subcontract No. 6401-85). H.B. Finger, Consultant. Prepared for the Confer-ence on Advanced Space Exploration Initiative Technologies cosponsored by the AIAA, NASA, and OAI, Cleveland,Ohio, September 4 - 6, 1991.

16. Abstract

Nuclear rocket research and development was initiated in the United States in 1955 and is still being pursued to alimited extent. The major technology emphasis occurred in the decade of the 1960's and was primarily associated withthe Rover/NERVA Program where the technology for a nuclear rocket engine system for space application wasdeveloped and demonstrated. The NERVA (Nuclear Engine for Rocket Vehicle Application) technology developedtwenty years ago provides a comprehensive and viable propulsion technology base that can be applied and will prove tobe valuable for application to NASA's Space Exploration Initiative. Also the unique organization that evolved tosuccessfully manage this effort requiring major contributions and involvement of two government agencies in partner-ship with industry is a useful model when a similar development is initiated for SEI. This paper, which is historical inscope, provides an overview of the conduct of the NERVA Engine Program, its organization and management, develop-ment philosophy, the engine configuration and significant accomplishments.

17. Key Words (Suggested by Author(s)) 18. Distribution Statement

Nuclear engine for rocket vehicles Unclassified - UnlimitedNuclear rocket engines Subject Categories 20 and 91

19. Security Classif. (of the report) 20. Security Classif. (of this page) 21. No. of pages 22. Price'

Unclassified Unclassified 14 A03­ . I- " I M *For sale by the National Technical Information Service, Springfield, Virginia 22161