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Nuclear Pulsed Space Propulsion Systems (U)

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_ UNITED STATES = =U) ATOMIC ENERGY COMMISSION >=_b ma := CONTRACT W-7405 -ENG. 36 “=% kg AEC RESEARCH AND DEVELOPMENT REPORT ~—s=== m _——0 ~~-o ~-ca 3=-m .sSm cJ—-1-—m = m ..-. . .—_. 1 ____ .—

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b This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic Energy Commission, nor any of their employees, nor any of their contrac- tors, subcontractors, or their employees, makes any warranty, expre= or im- plied, or assumes any legal liability or responsibility for the accuracy, com- pleteness or usefulness of any information, apparatus, product or process dis closed, or represents that its use would not infringe privately owned rights. . .

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This report expresses the opinions of the author or authors and does not nw- essarily reflect the opinions or viewa of the Los Alamos Scientific Lalmratory.

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‘-”● ● 00 9** .:= :~= :* -u~~~AJJ,; ● *0:000 ..: 9* ● 0:: ,0 .:00 .0 Written: October 1970 ● ..* ● . . . 900 ba LA-4541-MS C-91, NUCLEAR REACTORS Distributed: November 1970 FOR ROCKET PROPULSION M-3679 (64th Ed.)

LOS ALAMOS SCIENTIFIC LABORATORY of the University of California LOS AL.AMOS s NEW MEXICO

CtA:SIFICtiTIONCH.’dW~ ‘1’OUNCIA$SIFED BYMJ’I’W=~-f~~4wmGwii.&3

VERIFIEDBY

(SIGNA’1’JJRE

Nuclear Pulsed Space Propulsion Systems (U)

by

J. D. Balcomb L. A. Booth C. P. Robinson —— T. P. Cotter T. E. Springer J. C. Hedstrom C. W. Watson

UNITED STATES ATOMIC ENERGY COMMISSION .-- —————.—— CONTRACT W-7405 -ENG. 36

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● Abstract 1

I. Introauctial 1

II. Nucl.cmReactionsfor PulsedPropulsion 3

Fusion 3 Fission 3 LaserInittition of NuclearReactions 3

III. PreliminaryDesignConcepts &

Oeneral 4

ConceptA-1, External~stem with PusherPlate 5 ConceptA-2, ExternalSystemwith Magnetic Field and PusherPlate 6 ConceptB, InternslSystem 6

PulseUnit DesignConcepts -i’ DesignDsplicationsof PayloadShielding 8

Iv. MissicuConsiderationsand PerformanceEstimates 9

General 9 scalingIaws 10 OptimizationProcedure IL Comparisonof Externaland ~ternal ~stem Concepts 12 PerformanceLimitatIons u+ PerformanceEstimates 15

Acknowledgments 17

References l-f

Appendices

A. LaserConceptsand Consideraticms 18

B. Es* Considerationsfor pusher-propellantrnte~ction 20

c. NeutronEnvironmentalConsiderationsfor the ExternalSystem 22

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a at end of mission,relativeto the apacecrdt, a Pw3her-plateaccelerat im, n/se#

a. 2pacec~ accelention at end of mlssl~, relatiweto an Inertialf’rsms of referunce, m/sec2

b J-, a~--ss 60 Stsntlardacceleraticn of gravity,9B m/eec2

I IIIWIMSdeliveredto the spacecraftby one @se unit,!?-e.ec

Im 2pectiichqnil.ee,(1~ = Ve/go),sec in Pulse-unitmass,kg

M Totalspacecraftinitialnmas,kg

Ma M==n*-absor~r IUSSS(Ma = Me + ~), W Initifil.epacecrz&tpmpeKlaat mass,kg, n pulseunitsof % consisting of m,es Is, (~= nnl)

Me Mcmentum-conditionernuss,kg

kg Pavbad mass.k. AnY mass not desimated othexvise;Includesstructure (&her than Ma)‘andI&s laserunit, ~Mo = M - Ma - H).

pusher~latemass,kg % n Numberof @se units,dimensionless

q Pulse-unitenergym=lease,J u InitialvelncityOr the pusherplate ($ustafterthe pulse-unitinteraction) relativeto the spacecraft,m/see

Ve EffectiveVelQCitYof the propellantexhaust,m/see

Averagevelocityof propellantimpingingagainst *he puslmr-plate ‘i surface,m/see

w Materialstrength+8eightconstantdefinedby Eq. (6),m/see

Xa Ma/M,dissmaimless

xo Me/M,ditu?nsionkss

a DiJIIMMkdlSSE efficiency term definedby lul.(5) for fiteti system

at Dinb?nslonlesa efficiency term def Inedby ~. (5)for externe3system

9 collimation factor def tied by Eq. (17), dinmaionless

At T- Intenral betueen pulse-unit detonatlons$ sec

& Relative M.etauce OS travel> or stroke, of the pusher plate relative . to the spm2ecraftj m

N Mise%on-aquivalanttie-space velocityclmnge,m/see , T Efficiencyte= deftiedby Eq. (2),dimensionless

v Disk?nsbn.lessparamder for en energyscalinglaw, definedby 2q. (8)

u’ Dimensionless~ter it&&&@e”&#& ~, definedby lzq.(15) ● .: :* ● . . .- ● 9 .* ● em ● m. 9:9 :.. . . iv

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NUCIEARPUKED SPACEPROPUWION SYslms

w J. D. Balcomb L. A. Booth c. P. Robinson T. P. Cotter T. E. Springer J. C. Hedstrom c. W. Watson

AWIRACT Inltislconsiderationsare presentedfor advancedspace-propulsion conceptsthat are basedon ener~ producticn by laser-driventhermonuclear pulses. Preliminarydesignconceptsare comparedin which en individual pulseunit, locatedeitherintemaJJy or externeXlyto the system,Imparts ~ me to the SIX?.cecI’Sf’t.Max?vpulseunits- sequentiallydischarged and initiatedunt11 the desiredspacecraftvelocityis reached. various means Of shockabsorptionand shieldingagainstfast neutronsgeneratedby the therwnuclearreactionsare investigated.Indicationsare that the maximumspecificimpulsefor an internalsystemwould be . 2500 see, whereasthat for an externalsystemmightattain-7500 sec.

1. INTROI.XJCPION g- be- to studythe applicationof such energy pulsesfor spacepropulsion.VariousI.ASLgroups Studiesat Los AlamosSclentfficLaboratory (IASL)and at laboratoriesthroughoutthe world have have initiatedsupportingstudiesand experiments, indicatedthe passibilityof Lnitiating lcw-yield and are particularlyactivein investigationsof thermonuclearreactionsby the use of an intense laser-inducedthe.rnsmucleerreactions. laserpulseto heat to ignitiona very smellpellet The propulsionpotentialof nuclear-pulsedsys- of fusionablematerial. If a cufficientlyh%gh tem- tems is extraordinary:It offersthe possibilityof peratureand densitycan be achiwed in the pellet, approachingthe optimalutilizationof the ultimate thermonuclearreactionswill occurreleasingsub- kncwn sourceof energywith minimaladverseside stantialenergy. Presumably,this energyrelease effects. wouldbe much lower (O.1to 100 equivalenttons of The use of nuclearexplosionsfor propulsionwas TNT) than the minimumpracticalyieldspresently firstproposedby Uleml and has been investigatedin availablefrom fission-implosion systems. Possible detailbetween1958 and 1965 underProjectOrion.* applicationsof such small,cleanenergyreleases IiIthe finalconcept,fiss%onexplosionsof one to includeterrestrialelectricpurergenerationand 12 lcllotons(equivalentm explosiveenergyrelease) the ob~ectof this repoz’t:spacepropulsion. were to be detonatedbehinda spacec~ to accehr- Activitiesat I&L relatedto I.aeer+lriven ate propelMnt materialthatwould impe.rtmomentum thermonuclearreactionshavebeen underway for over to a massivepusherplate;this momentum,in turn, one year, includingthermonuclearburningstudies was to be transmittedmore gradusllyfrom the pusher end laserdevelopment.In J’une1970,the Advanced plateto the spacecraftthrougha pneumaticspring PropulsionConceptstask grcupwas formedwith the Wstem. @ studyWas terminatedbecauseof the in- specificob~ectiveof Investigatingadvancedpropul- herenjhuge sise of the resultingspacecraft(4000 ● 00 sion concepts;as part of this invest~O~i”&~t&’ ~ Uonr$,“becauseof the limitationsimposedby the ● * ● ● 9 9* :: **C ● ● * 9** ● ** ● O* ●:0 ● 0 1

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‘. ●*: 9:0 ● mm . . . . . ● * ● . ● * ● ●: :: ...: -o =**. nucleartest-bantreaty,and becauseof the d~i:”” “%at ~f%&k ~2&eWurt withoutths eknents deterio- cultyOf testingthe system. N was, however,gen- rat*. Nuclear [email protected] pmgmldcm systems are shi- eI’611Yacknowledgedthat the conceptwas techni- 3x@y mites, but, becanseof the extrewdy short C- soundmd mighthave achievedthe vem high interactiontb of the pulses,at much highertem- specificimpulse(1 ) of - sec. Pemtures. The interactiontimes,of the orderof Sp 25m . nKU.iseconde,are simplytoo shortto causemuch The possibilityof producingrmIsIJ.,cleannu- cleare@osions mmmves the two primaxyobstacles destnwtive damage,especiaUy if ablative materials usea to furtherprotectthe expmea surfacesfmm . in the Orionconcept--largesize end spacepollu- are high temperatures. tion. Spacecraftcan now be envisionedthatmay be even smallerthen those curnentlyproposedfor the Although,at present,it is assumedthat the nuclearspaceshuttle,yet be able to greatlyout- genemtlon of laser-driventhernxmucl.e=reactions performeventhat high-performancesystem. @ ap@kable to apacepropulsionis feasible,i.e., systemcan alsobe meenin@iUy testedon or belcw that the largeeffortsat IASL and elsewherewilJ.

-d. AMhou6h a laser-drivenpulsedpropulaim yieldthe expectedresults,It is neverthelessde- system,as any otheradvancedsystem,would requti sirableto obtainan earlyworkingkncwledgeof the a fairlylong devebpmanttires(perhapsuntil 1985- theories.9napracticesof high-energypiked lasers, lW), it shouldbe activelypursuedbecauseit of thenmnucleerburning,and of th th?nmdynamics wouldmake mannedexplorationof the solarsystem of denseplasms. This Inforfmtionis neededto as feasibleas present-daylunarexploraticm,and attackthe p-ion problem,to identifythe prod- traveltitoetih orbitinexpensiveand rout5ne. ucts of suchenergyreleasesand their interaction These,in fact,are the characteristicsof the sys- with the ~ahg6, and to investigate mmns of ternidentifiedby NASA as necessaryin a long-range tailoringthe energy=Lesses to specificpropulsion space-explorationprogmm. applications.

The high potentialperformanceof a nuclear- This reportdiscussessome aspects of using pulsedpropulsionsystemis due to (1)high specific nuclearreactions(eitkr fissionor fhsion)for energyend (2) shortinteraction times. The spa- piked propildon; cmqxms preliminarydesigncon- cificimpulseincreasesas the squareroot of the c~s$ lncludinspulseunits,ma theirperformance epeciflcenergy. Efficientconversionof chemical limitations; pnesentsmissionconsidenxtias;~a reactionenergieswcmld xweultin an I~ofcmly givesprekhmhuuy~rfonnance estimates. -460 sec. On the otherwa, if a ndierislwere Althougha detailedmeferencedesignhas not useawhich had 100 timesthe specificenergyof TNT been prepared,earlypublicationof theseprelimiruuy end this energywere convertedto kineticenergy, considemtlonsseeneddesirableto disseminateas velocit5.8scorresponatigto an I of . Sp 3090 sec quicklyas possiblethe informationgeneratedand to coold be reached. E a mixtureof D&e were totel- indicateprcmleingmea of rese~. Iy fusedand the maulting energytota13yconverted intobackwardnrnnentumof the reacticmproducts, _ts have been givento namingthe proposed [email protected] Concept,shouldfurther t~ IspW- be as h%h as 2.2 x 106 sec. Cletwlyt consideratimleadto a full.-fled@ pro~ect,and the ~fcl.e vebcities associatedwith the latterex- empk couldnot be tole~ted; thisvalue is cited nann3Sirius*wcs chosen. onlyto indicatethe upper limitof [email protected] %3.riua,the brighteststar in the heavens,is the principaletfxrin the ccmstellfbtionCenisWjor, . Sion. the GreatDog (n& Rover). Cads W@r followsin the sky closeto t~ heels of Orion,the great Nuclearpropulsicmconcepts,in general, are hunter,who by one mythologicalstay was a “fool- 13mitedby the abi13@ of the materielsused to -, heaven-daring=bel who was chainedto the . sky for his impie~.” lW nen= SIriueis franthe wm5t6na ext~me tenqwaturesratherthan by the GreekadJectiveUSIPOB,nmaningscorching,or per- specificenergyavailable.This limitis especial- haps from the Arabicsiraj,the glitteringone. The EgyptianveneratedSlrlus,regardingit as a ly c0n6tr8tiingin 601ia40r6 UUCLW rockets.q81Ps. :. toti~W tl$”@Ang of tluaXWLa and a subsequent the temperatureof the fuel elennts mwt exc~a ~ .0go~ h@res$.: ● *. 9* ● 9 ● 90 900 ●:. :.. ..

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● ● ● ● ● APPROVEDamuMu!@* FOR00 . .PUBLIC RELEASEm APPROVED FOR PUBLIC● 9 RELEASE ● O* 9* ● .*8 ● ● ** ** :

. . ● *9 9** ●:* ● ** :-c ● ✍ . ✛☛ ✜● :*O9 ✎✎ ● * .:0 9* II. NUCLEARREACTIONSFOR PULSEDPROIUS616N .:. :● ● :““-se hjgh+nergy neutronsrequireshieldingOf the spacecrrxft.Most efficiently,this shleldlng Fusion shouldbe placednear the sourceof the neutrons, The isotopesof hydrogen,helium,and lithium particularlybecauseotherconsiderationsalso sug- would fuse i.fa sufficientquantityof materialwere gest the desirabilityof dilutingthe initialenergy heatedto a sufficienttemperatureand held together releasewith a surroundingor adjacentmass. The for a sufficientlengthof time. In the laser-driven sourceparticles,movingat near relativisticveloc- fusionconcept,the necessaryconditionsare achieved ities,are simplytoo energeticto handle: their by coupl.lngthe outputfroma laserto a small.,ef- stagnationwould causeexcessiveablationof any fectivelyconfinedregionof fusicmableisotopes. surface,and theywouldpenetratea protectivemag- Only a smallquantity,of the orderof one gram,is neticfield. Also,per unit of ener~ (E = 1/2 n?), required. For example,one gramof deuteriumplus theseparticlesare carryingonly a relativelysmall tritium(D+T),totallyburned,wmld producean en- amountof momentum(P = rev).Becausethe momentum ergy equivalentto exploding80 tons of m. per unit energy(P/E)is equalto 2/v, it wouldbe In the propulsionconceptdiscuesedherein,the wastefulof energyto impartmomentumintohigh amountof energyreleasedin what seemsat the present velocities.Some nonreactingmass,much larger than the fusioningmass,must be used to dilutethe time a plausiblesize for a lasar-drivenfusionre- energyintoa moreusableform. mis UULSS,ca.uea actionis clearlyfar belowthe amountof energythat the propellant,might alsoprovidethe desired is desiredfor each pulse,i.e.,a few to many tens shieldingfunction. of tons TNT equivalent.It does not seem necessary to dwellhere on how one mightproceedfrcsnthe Fission initialsmellamountto the requiredlargeemouut; In a vexy analogousmanner,fissionableisotopes it is simplyassumed that a solutionto the problen can be compressedto the pointwhere a very smell will be availableat the time it is needed. quantitybeccsnessupercritical.Such a fissioning The productsof the basicreactionswIIJ.be systemmey producefewernet neutronsper unit of high-energychargedparticlesend neutrons. The usableenergyreleased,but would form radioactive follcwingbasicfusionreactionsare being con- fissionproducts--anundesirablebyproduct. HCW- sidered: ever,releaseof this smellamountof radioactivity may be tolenble in the alreadyhostileenvironment D+D + 3He (0.82MW) +n (2.45 WV) of spacebecausethe mwt majorityof the debriswill D+D + T (1.OIMeV) +H (3.02MeV) have velocitiessufficientto escapethe solar D+’l! +4 He (3.5 MeV) +n (14.1MeV) system. D+3He -$ ‘He (3.6 MeV) +H (14.7MeV) LaserIhitiatlonof NuclearReactions The easiestto initiateis the D+T reaction whichhas a reactioncrosssectionat lW tempera- The followingcharacteristicsmake a laser turesmore than two ordersof msgnitudegreaterthan eminentlysuitablefor achievingextremecompression that of the otherthreepairs. However,its usefiiL- end heatingof smallpellets: nessw be impairedbecause80$ of the fusionenergy ● High energydensity. Ener~ storedas excited is carriedoff by the 14.1-W? neutrons,whichhave molecularstatesof a suitableI.asingmaterial a longmean freepath. Even in burningdeuterlum,a can be sweptout of a largevolumeand focused substantialfractionof the total fusionenergyis on a smallarea. carriedoff by 14.1-MeVneutronsfrom the D+l!reac- ● ShortPUI.Se duration. Pulsewidths in the tion,whichwil.1normallyfollcmthe tritium-forming rangeof 10-10 to 10-7 sec can be achieved. branchof the D+D reaction;anotherportionof the

energyis dissipatedby 2.45-MeVneutrons. 13urning ● Controllability.Iaserpulsescan be shaped an equalatomicmtitureof D+%iewill alsoproduce and selectedat the desiredtime in a low-energy

● ● ** ● stageend then amplifiedin successivehigh- so= 14.1-Mevneutronsdue to the CO~$iW$+3:. ● 0:0 reactionend the foUloulngD+’l?reactidn: .: : ●: : ● energystages. ● 0 ● ee ● 9 ●0: ●:0 ● e* ● 00 ● m

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● ☛ ● ☛☛ ●:O ● *...... ● 0 ● ● . : ● ●:.. ::* bg- :0 :0 Presentestimate8of the laserenergyXV&”-: ● ** ● ●:. . GENERAL NOMENCLATURE to heat and compressD-W!in mllligmm quantities, Ma end to achievea net energygafn,rangefrom2 x i04 / MO Mb M, M; to 1 x 106 J (joule) deliveredin a periodvarying PROPELLANT fm 10-10to 10-7 sec. A laserof this magnitude . msy requirea hge volumeof I.asingmediumfor the finalstage (ofthe orderof 1 m3),which msy be CONDITIONINGUNIT expendedaftereach laserpulse. AlthoughgLass MOMENTUM ABSORBER CONFIGURATIONS . and gaseouslasersare presentlythe most advanced, A. EXTERNAL SYSTEMS 1.PUSHER PLATE lasersystemsutilizingchemicalreactionenergyto n achievethe requiredmolecularexcitedstatesare PULSE UNIT \l/ -o- more prumisingfor spacepropulsion.Chemical /l\ lasershave a relativelyhigh energydensi~ and offerthe possibilityof expendingthe unusedenergy + 2. BLANKET togetherwith the spentreactton products. AU at- MAGNETIC ELECTRICAL CONOUCTOR tractive possibili~ is to assemblethe finalhigh- cO’Ls-Yxl PULSE UNIT energylaserstageand the mirror-focusingsystem \l/ -o- with the individualpulseunits so that they cmld /l\ be deployedas one packagejustpr20rto pulse-unit initiatlon;the prelim.in~,lcw-ener~ laserstages would ramsinin the spacecraft.Thus,positioning B. INTERNAL-(SYSTEMS PULSE UNIT of the pulseunit aft of the spacecraftwouldbe not nearlyas crft icalas had been expected. ,1 71C NOZZLE A more completedescriptionof laserconcepts ~R~~S”R~& and considerationsrelevantto pulseprepzlaionare presentedin AppendixA. Fig. 1. Nuclearpulsedprqmlsion concepts.

configuratimsare being catalogedaccordingto III. PKELININAKYDESIGNCONCEPTS whetherthe pulseunit is explodedexternally,as General depictedIn Fig. IA, or internally,as depictedin

Threedesignsfor convertingtha energyfrom F@. 113. Tt will be sham in SectionIV that,at an explosionintopropulsivethrustare shownin leastto a first-orderapproximation,the mass of Fig. 1. Ih each case it is eupposedthat en indi- the mcns?ntumabsorberis proportionalto tb? impulse vidualpulseunit is properlylocatedand then init- deliveredto the pusherphte (inan externalsystem) or to the pulse-unitenergyrelease(inan titernsl iated by a laserpulse. m the resulting nuclear system). explosion a quantity of propellantis heatedby the releasedenergyand expendsas a high-energyplasma. Becausea aignif’icantmass fractionof the A potiionof thla expandingplasmainteractawith spacecraftwill be allocatedto the nxxn?ntumab- the spacevehicle,therebyimpartingxm?ntum to it. sorber,it wiJl be desirableto selectths proper Maw PUISeunitsare sequentiallydischargedand valuefor tk impulsedelivered(ora @se-unit Initiated,probablyat equalIntervals,resulting energyrelease) which la appropriatee for the mfnimum . in the desiredspacecraftvelocitychange. feasiblepulse-unitmesa. m too high an impulseis The spacecraftmass is consideredto consist deliveredor too mch energyreleased,the increased of a payloadplus supportingstructure,propellant maaa of the nxam?ntumabsorberwill more than offset storage,and a momentumabsorber,which smothes the gain in performance;conversely,if too low an out tk shockor inqmlseresultingfranthe explo- impuhe is deliveredor too littleenergyreleased, ● thaikxxhtn●~rfornuncewill nmre then offsetthe sionof the @Lee unit. VariousmnmMaan-abas%@ ~● ● a :: : ● 9 ● 9 ● a :: ● ● : ● * ● 0 ●:0 ● *9 ● .* :.* be 4

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● O ● 00 :00 ●:0 :*. :-c ● * ● 0:..: ● ‘*.be . ● ● * ● ☛ si reductionin nmmentum-absorbermass. fn $0%% Zases“:” “*;h; ”spacecreft, attaininga finalvelocitYeq~ to (externalend internelsystem) an optimumnsnnantum- the initialvelocitybut -site in direction.At absorbermass fraction of the spacecraftcorresponds this tius?,a pulseunit is firedconvertingthe to a mxinmm pwload del.ive=dwhich, in turn, cor- Propemt intoa plasmawhich acts at high pressure . respondsto an optimumspecificimpulseand deter- wer the surfaceof the ~sher plate for a very minesthe impulsedelivered(externalsystem)or the shorttime interval.@ net effeCt iS EUI~lse pulse-unitenergyrelease(internalsystem). deliveredto the pusherplate neversingits direc- . tion. The cych is then repeated. For the samepulse-unitmass,the externaleys- tem will greatlyoutperformthe internalsystem, Functionary,the momentumconditioneracts as S@lY becausethe nmmentum-absorbermaSS in the in- a long coil springseparatingth spacecraftfrom ternalsystemmust be largerto containthe pulse the pusherplate. If the springis greatlycom- energy,ratherthan onlyto absorbthe impulseas in pressedso that the nmtionwer a cycleis only a the externalsystem. SUELUfractionof the total compression,then the forceexertedon the spacecraftis nearlyconstant, CanceptA-1, ExternalSystemwith PusherPlate approximateing the idealconditiondescribedabwe. The pusher-plateconcept(Fig.IA-1)was devel- The totelmass of the momentum-absorbersystem oped in the Orionstudiesand, at this juncture,ap- is the sum of the messesof the pusherplate and of pearsto be the mostpremisingfor the smaLlerlmer- the energy-storageor momentum-conditioningunit. drivennuclear-pulsepropuJ.eionsystems. The pulse- 1% villbe demonstmtedin SectionIV that the total unit enemg releaseoccursat some distancefromthe mass shouldbe dividedequeUy betweenthe momentum- spacecraft,and a strong,probablyflatmtal 6truc- conditioningunit and the pusherplate if the t&al ture,calleda pusherplate,absorbsthe shockof mass is to be minimized. the explosion.A momentum-conditioningunit is =- quiredfor a gradualmonmtum transferbetweenpulses ConceptA-1 can be designedfor a high specific and for returnof the pusherplateto the proper impulsebecausethe pusher-platesurfaceis exposed locatim for the next pulse. to the propellantin very shortpulsesand the pro- pellantenergydensitycan be therefo~ quitehigh. The magnitudeof the kineticenergystoredin the pusherplate afterthe impulserequiresthat the As the propellantmaterialpilesup againstthe mowntum conditionermust a~roximatea conservative pusher-platesurface,the temperatureincreasesdue system,that is, a systemin which kineticenergyis to stagnationend much of the originalkineticen- convertedto a potentieJ.ene~ and vice-versa,with- ergy is radiatedway t~ space. Ablativematerial out appreciableener~ dissipation.(Ina dissipa- cwering the platevaporizesto form a thin film tive systemquiteunrealisticquantitiesof heat en- furtherprotectingthe subsurface.Orion studies ergywould have to be radiatedto spaceor othezwise verifiedthat ccmmonmateriels,e.g.,aluminumor removed.) Thusthe pusherplateand the momentum steel,canwithstandsurfacetemperaturesexceeding conditionerundergoa cyclicinteraction. 80,000°Kunder euch conditionswith only nominaJ. abl.ation.3 At the beginningof a typicalcyclethe pusher plate is assumedto be pmitioned at its maximum The specificiqnil.seof an externalsystemis distancefromthe spacecraftend movingtcwaxxithe the integralof the changein the axialccznponent craft. As the pusherplateapproachesthe space- of the momentumof the propellantes a resultof its craftit is deceleratedby the momentum-conditioning interactionwith the pusherplate.* !12hisspecific unit (witha nearlyconst~t force),whichbringsthe impuleeis approximatelyequalto the productof the plateto restwith respectto the spacecraft%.At propdlant impingementvelocitytimesthe fraction this pointall the original.kineticenergyof the * pusherplate is storedin potentialenergyof the The specfiicimpulse,so defined,shouldbe divided momentum-conditioningunit. Overthe ne~e~o $$ by go, the2stsndh accelerationdue to gravity “ “:%9#,m/sec ), in orderto obtatithe UZUIQY the cyclethe pusherplate is accelen$e? w.* ?@n : :a~gpted unit of seconds. ● * 9*9*** ● ** ● aa 9** ● ob ● ** ● .

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.=: ●✚ ● m, ● 00 ● *O . .

● .*.:.:: ●*

:: : : of the pulse-unitmasswhich strikesthe ~her .● ● ● ● .‘~t~r & ‘~~ed Performancewill offset*W pkte. mitid perfomuenceestimates, presented in edditionril-mass and complexityof the superconduc- SectionIv, indicatethat the desiredpropelbnt im- tive coils,etc. pingementvelocitiesarewithinthe constraintof Conce@ B, InternalSystem 150 km/seeimposedduringthe Pro$ectOrionstudy; Xn this configurationthe energyis released . furtherinvest@at ions,however,w disclosethat insidea pressurevesselthat is equippedwith a plate-surfaceenergydensitywiJJ.eventuallyUnit conventionalrocketnozzlefor the dischargeof the the perfomasnceof this cmcept. Presentspecific- heatedpropellant(Fig.I@. [email protected] impulaeestimatesfor the pusher-plateconceptfi- for controllingthe processhave emx’ged. B one dicatean upper limitof - 7P sec. conceptliquidhydrogenis fed intothe pressure Anotherconstraintwhich~ limitthe perform- vesselradia13ythrwgh the weJl and acts as a ance of the pusher-plateconceptis tbs totalpres- coolantbeforeuniformlyfillingthe vessel. The sureof the propellantactingagainstthe surfaceof energyfromthe pulseunit is releasedat the center the plate. Excessivepressureswill causeapeXla- of the tank afterthe vesselhas been xx?chargedto tion U the resultinginternal.tensilestrescesex- the fallpropellantmass. A shockwave is propa- ceedthe strengthof the platematerialwhen the gatedthroughthe hydrogenuntil it nsachesthe internalpressurewave is nsflectedfrom either wallswhere it le reflectedback towardthe center. surface. Becausethe stagnationpressureat the wall.is Esign consider&ionsfor propellant-pusher higherthan the frontalshockpmsaum by an order pbte interactions~ presentad in AppendixB. of mgnituae, this deliversa sharpinitialimpulse to the wall. @ wave is subsequentlyreflected ConceptA-2, Externalsystemwith MagneticField and PusherPlate back and forthin the vessel,continuallyincreasing the internalenergyof the l@rogen until.equilib- The limitationsimposedon ConceptA-1 by abla- rium is established.w this t5me,which is of the tion and spellation of the pusherpkte mightbe orderof milliseconds,tti hydrogenhas reacheaan were- with a magnetic-field“blanket”to protect averagepressureand an isothemal ccfxlitiondue to the plate surfacefrun the high-energypropellant the transferof shockqave kineticenergyto hydro- plasm. Figure1.B-2showsa cusp-shaped,electri- gen internalene~. The hot gas is then expandea caUf conductivepusherplate and a superconductive througha nozzlewhile the tank is being =filled coilto generatestrongmagneticfieldUnes paral.- with propellant.The expansionprocessis continued Iel to the pusher-platesurface. As the denseplaa- untilthe previousinitialconditionsin the tank ms from the e@osion expendsit pushesthe magnetic are attainea,then the cycleis repeated. fieldlinesagainstthe conductor,increasingthe fieldstrengthby inducinga circularcurrentin ths Otherpossibleinternaldesignconceptshave conductor. The increasedme8neticpzwaure (B2/8n) been considen?dbut no designwork has been ini- sbs dcwnthe plasma,turns it, and acceleratesit ttitea. One of these is a pressurevesselUnea away franthe laasherplate. The impulseis trans- with a thick~er of lithiumhydriae(Li@. The ferredto the pusherplateby magneticinteractions internalenergyreleasevaporizesS- of the Lili, which spreadout the force in apaceand time end which collectsas a hot gas in the cavity,ana then protectthe surfaceof the platefrazparticleim- flm?sout of the pressurevesselthrougha nozzle. pingement. Eecausetlw propellantparticleveloci- Thus the pressure-vesselliningis the propellant, ties can be higherthan for a plainpusherplate, whfch is graduUy vaporizedby successiveexplo- . I the specifichpulse of this conceptcan alsopre- sions(similarto coreburningIn a soliil-fuelea sumablybe higher. whethertbs apecfficimpulsede- rocketnmtor). rivedby the optimizationprocedureswill implythe Althoughthe intexnalconceptinitiallyseemxl need for zmgneticfieldsto protectthe puahe~ att.metIve, so many inhen?ntdrawbackshave emerged.-. platesurface will be determinedin futureatua~s ~ .e . as~a @..ththe extexmilsystem,that further IY so, it w5J.1&o be necess~ to deterzdnee.: { : : : . .

6

w ● m ● *99,.* ● APPROVED FOR PUBLIC RELEASE APPROVED FOR PUBLIC RELEASE

,. ● .0 ’300 ●:o :- :.. ● .:ee. . . . ● ● m ● e .:;: .0

work doesnot eeemwarranted,at leae;”~~:f’n%6 ““” ‘“• “● In an intexmal.system,sphericalgeometries= nearfuture. Eaeicpmbleme am: genemlly indicated,which offerlittlelatItudefor swat effectiveplacingof the pmpell.antmaterial.: ● For the samepulse-unitZSMS,the perfonssnce it WOUld S@~ fOllS 8 @ObS aroundthe pl’O_@biOfl of the Intenml zyatemla at laaatan orderof pellnt. Calculationsshw that 20 kg of a good . magnitudepoorerthan that of the external shieldingmaterial,e.g.,llthium-6hydride(6LiH), system. would remoreonly b5$ of the 14-MW neutronsgenerated . ● m spectfic-impulze limlt la alxmt 2500 aecj in the fusionmactlcsxs,and that hundredsof ki.lo- correspondingto excessiveradiat%veheat gramaW- ~ requiredfor effective shieldingfrom transferto the pressurevesseland nozzle aD+l!pulae. from a hydrogenpropeht. In additionto shielding,the major considera- ● l!hereis no apparentww to solvethe shielding tion in the designof tk pulseunit for an external problem. Isotropicshieldlngwill be required systemis propercollimationof the expandingpropel- to protectthe pressurevessel. lant so that a majorfmction impingeson the space-

● Positioningand initIating the pulseunitwith- craft. in the gas-filledpressurevesselmay be much With respectto shieldlng,the externalsystem more difficultthan in the vacuum&&t of the benefitsfrem an inherentadvantagebecausepulse spacecmft. unit and spacecraftaIW p~sicslly separated.This The only significantadvantageof the concept will significantlydecreasespacecraftradiation is the nrlmimel.raquinenmtfor momentwm-condition- doses,mainlyfor the follwing masons. ing;thfs functionis predominantlyperformedby the ● The qnicecraftsubtendsa amsXLsolidangle pressurevessel. Althoughthe thrustfmnsthe noz- frmsthe pulseunit. Becauseradiationswill zle mW vaxy by an orderof magnitudeover a propul- emanateisotropica13yfras the detonation,only sion cycle,the msss raqu~zsmts for snmothing a smtillfracbionmust be dealtwith. For ex- thesevariationsout in the ~cecrd’t are small ample,at a dlatanceof 8.5 m fmm a point,a comparedwith the pressure-vesselmass. 4.72-m4iam plate subtendsa 30° included-angle PulseUnit DesignConcepts ccmerepresentingonly 1.T? of the total solid anglearoundthe point. To mininrlzethe mass of the sum?ntumabsorber,

the mass of the pulseunit shouldbe as smallas ● Propellantmaterielcan be positionedto form a possible. ~ factorswhich tend to increasethe shadowshieldbetweenthe radiationsource sizeof the pulseunit are the requirementsfor (whichis nearlya point source)and the space- spacecrsd’tshielding,the desireto limitthe number craft,i.e.,shieldingmass is requiredonly in of laserpubes, and the maintenanceof a hfgh aver- the solidangle subtendedby the spacecraft. age thrustwithinthe limitationof laser-pulsera- From a shield5ngstandpoint,the logicalgeometry cycletires.As mentionedearlier,spacecraft is a cone of pzwpetit materialwith the energy shieldingis of particularconcernbecauseof the sourcelocatedat the apex of the cone and with 14-MeVneutronsproducedto a greateror lesser the coneaxis orientedalongthe spacecraft degreeby the fusionreact%onsutilized. Ih any axis. case,the pulseunitwill consistpmedaninantlyof ● Singlefast-neutronscatteringwill removemost propellantmaterialwhichwill servethe dual fsnc- of the neutronswithinthe cone anglefrom fur- tion of (1) dilutingthe energydensitythmeby re- ther conel.derati~.For sskllangles,the prob- duclngthe specificimpulseto acceptablelevelsand abiltty of a scatteringneutroncolltsionre- . increasingthe impulseobta%nedper unit of energy; eulttngin a new path stlXlremainingwithin and (2)pmvidlng priswu’yshlel.dingfor the apace- the coneangle is exceedinglysmall. The prob- craft. Pulse-unitmassesin the rangeof 1 to 100 , ,.. #blllty for msltiplescatteringback intothe ,* ● ** ● *9 kg am thoughtto be appropriate. . ● : : ● *** ● * ● ● ,originalcone angleis even smaller. Thus the ● a ● a.●: 9 :: ● ● * 9*: ●:a ●0: ●:* ● *

7

APPROVED FOR PUBLIC RELEASE APPROVEDbe FOR PUBLIC RELEASE ● ●’e

...- ● ● 0 08 .:0 ● 9S . . . ● . 9* ::0: ● a: . neutronshieldingproblemis reducedf~ ~b#,~ : ...4t$.td:*---- pm~t at-p imPIYingsame usual requirementof slcwingdcsrnand captur- directpropellantheating. lh one ccnceptthis is ing the neutronsto one of simplycausing supposedto be the dcsninantsourceof propaldant s%l.e scatteringevents. Calculationshave energy,and the designreducesto the determinathn conflrmd that,even for largeshieldingfac- of gecsmstriesleadingto good collimationof volu- tors,mat neutronsstrikingthe spacecraft metricallyheatedmaterials,somewhatsimilarto the . villbe uncollided. designof shapedchargesfor the formationof ex-

MonteCarlocal.culations,presentedin Appen- plcmivejets. One possibilityis to surroundthe neutron-heatedpropellantwith a pressurevessel- dix C, indicatethat 3.8 kg of bary~ium in the shapeof a 30” inchided-anglecone,30 cm high, end-nozzleconfigurationof thin,heavy material wiJJ.absorb~.~ of all 14-1.wfneutronsoriginally which wiX1. direct the propellant toward the push?r directedwithinthe coneangletcxmrdthe space- plate in a supersonicstream.

C-. Thus, in this example,only 0.010 x 0.017 = Another,ssn?aattractive,~ssibility is to 0.017$ of sXL neutronsemanatingfras the source desfgnfor utilizationof the energycarriedby the will traveltad the craft. chargedproductsof the fusionreactions.This en-

For an extemel system,a sophisticatedhydro- ergy CU be absorbedin a she= of highansity dynamicdesignof the pulseunitwaild favora ley- materialsurroundingthe pelletwhichwouldthen ered flatplate ratherthan a cme with the pellet confinethis ener~ to a channelaroundthe p~l- at its apex;caspromiaegeometriesw eventually lant-conesurfacewhere it waildbe absorbedin a enrge. In general,if the pulsed~cts one ele- thin layeron We surfaceof the propellantcone. ment of msss towardthe pusherplate,an equal This heatedsurfacewouldblow off at highvelocity momentummustbe carriedby snotherelementof mass ProPeUiW t~ re=fi~ coneat lcwervelocities away fromthe spacecraft.Also,the fractionof touardthe pusherplate. totalene~ carriedby tlx?firstelementis equal Othergeetries, e.g.,lqferedplatesof to the fractionof totalmass carriedIn the second varim matefiala, are alsobeingconsidered.

elementand vice-versa.One maximizesth nmmentum Designmp I.icaticnsof PayloadShieldin& carriedoff,per unit of ener~ expended,by equal- izingthe massesof the two elem?nts,which,how- Payloadshieldingis a ma~ordesignconsidera- ever,w not be a desirablegoal. ticm for all spacecrdt.ccsspcmentsparticularlybe- causemannedmissionsare aasumd. !lU spacecraft If the laser-drivenfusionconceptcan be made am thenseforeto be designedso as to prwide the to work, it can probablybe made to give any desired ~st favorableBhielding.Theywill be of elmgated energy,and the costmightbe nearlyIndependentof gecm&ries,with as much distanceand spacecraft the energylevel. This reasoningmay leadto de- materl.albetweenthe energystaarceand the peylcd st~s that are wastefulof energyin orderto save as possible. N@nteCarloneutron-shieldingcalcula- propellant. tionshavebeen perfo~d for a referencecasewith If the pusher-plateimpingementvelocityis the assumptionthat the primaryenergysmrce is the fixed,t~ mexisum-e transferredto the D+T reaction;detailedresultsare presentedin pusherplate,per unit of totalmass expended AppendixC. Conclusicasare that s- attenuating (i.e.,the specificimpulse),can be cle~ly ob- medium,in additionto tk p~H, iS required tainedby I.ettlngthe ma~orityof the mass impinge to protectthe payloadit’th? D+’l!reactionis uti- on the puskr plateat this maxhum velocity. The lized;but calculationsshouthatthe shielding .

mmainlng mass carriesthe ma$orltyof the energy maesesare acceptable:a nmdmum of 15)~ to off in the oppositedirectton at highervelocities. 40)000 kg if the mass is at the payloadlocation, . selectionof materialsfor the propellantwill.de- or 20C0 to 5000 kg if the mass is more favorably pend on the effectiveneutxmnscatteringthat can --d nearerthe energysmrce, but sti31within be achievedfrom a givenmass. Iieutrcasscat$er& lhese-ses can be includedwithin b %?.V.?%*✍✍✍✍✍ “ ● out of the propellantcone impartsam of ttiec& ~ ● a $ai#ct~ spacecraftmum of 100,000kg, etther e* some ● : ● :* S* ● 9 ● ** Bee b.. 4.. .. 8

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as structural components, utored pulse unit St or pressurevessel,of the -ntum-conditioning SYS- Me-support aupplles. Thust psvloed shielding con- tam, and of the lasersystem. l%e size of these sldemt ions, althoughto be recognisedin the design cmsponentswill dependon the stie of the pmpulsion fromthe beginning,apparentlyWU not Invalidate pulse. Althoughthe detailsof this scalingare not , th entk nuclear-pulsedprepulslonconcepteven In yet well understood,o@imlzatims for two possible the unllkelyeventthat the DW reactionis t,be sce3inglawsWe been analyzedand EUW presented pra energysource. be~. . Considera apacecmft of total initialmass, IV. MISSIONCONS~~S AND PESFOIU4ANOEESllR4WES M, ccnslatingof ccmpncnts of threekinds:Propel- lant,of mass h$; a momentumabsorber)of mss Ma; Geneml and the remainder(1.e.,payloadand otherstrut- m the normelrockatequatton, it is aaaum?d tures)of mass MOO Thus that the spacecraftconsistsof two components--a pwload, Mo, and a propellantthat fs e~nded over a M= Mo+Ma+k$. (1)

periodof time at a conBtantexhaustvelocity,Ve. ID this allocation It is appropriateto ascribeto Giventhe mission-qulvalentfree-spacevelocity Ma anY mass that scaleswith the pulse size.

c-e ~ C.V,the requiredpropellantfractlonis de- AssusEthat the propellantconsistsof n iden- terminedby the rocketequation:Me/M ‘ exp(~Vh7e)? ticalpulseunitseach of mass m. Althoughthe use whereM is the total initialmass of the spacecxwd?t. of nonuntfonspulseunitsmsy be desirablein some In enY propulsionsystemtherewill be an additional circumstances,this generallsationis deferred. mass in the .vp.acecrd’t,associatedwith the propul- Only a minorfractionof the @se unit consistsof sion system,which scalesin som mannerwith the fiel. Tbs main mass is a suitablearrangementof enginecharacteristics.This mass must be imcluded prope~t materielsdes!gnedto maximizethe total in M. For example,in an electricpropuleicmsys- me of the explosionwhile dilutingthe fuel tem, an extm ~er-supply massmust be includedin energydensi~ to a tol.erablelevelat the monEntum the spacecraftas determinedby the requiredset absorberand shieldingthe spacecraftfrom radia- ~er and the efficiencyof the system. The ticlu- tionsoriginatingin the fuel. AU ancillaryex- sionof this enginemass usueXlyresultsh sore?de- pendedmaterialis also includedin the pulse-unit signfI.exibflityin choosingthe fractionof the mass. totalspacecraftmass sillocatedto thle enginemass As a resultof the explosionof the pulseunit end in a resultingvariationin engineperformance, end its Interactionwith the spacecraft,a total Ve. Frequentlythere 1s en optimumenginemass impulse,1, will be Impartedto the spacecraftalong fractionI.eedlngto a msximumspacecraftpe@Oad Its axis. In a epectiiccasethis impulsecan be fraction.As is well lawn, this is th? casefor detensinedby integratingttw axial.componentof the the precedingexampleof electricpmpulslon--there velocitychangefor each cmsponentof the pulsemass existsan optimumfractionof the spacecraftmass as a resultof its interactionwith the spacecraft. which shouldbe allocatedto power supplyand a cor- Tt is convenientto representthe impulseas a frac- respondingoptimumspecificimpulsedependingon the titm of the total impulsewhich tail.dbe realized thrusterand power-sup@ycharacteristicsand on the if all the energy,q, were ccmvertedintokinetic mission. Tn otherWOXRIS,the inclusionof the en- energy,uniformlydistributedin the pulse-unitmass, ginemass in the q!acecraftprwides en appment end whollydirectedoppositeto the apscecraft extradegreeof freedcssIn the design,which,how- Untiono Thus ever,Is usuallyonly definedduringoptimizaticmof . the system. 1=7~ (2)

The abwe generalizationsare alsotme for where the coefficientTIis the effici.encyfor con- pulsed-propulsionsystems. In this case,the*extra vertingenem into Im@ae end O S T ~ 1. e-e ● ** ● ** ● . spacecraft masses consistof a pusherplate 8r~ ~ :0 ● :; *9 ● ● e&* e ve93 ● * ●:9 8*C G** 9*O *8

APPROVED FOR PUBLIC RELEASE APPROVED FOR PUBLIC RELEASE .* ●-*☛ ☛☛ ● b. .: * :*:,* ●

1% is convenient to cluuacterise the space- Ma = u q/k2. (5) craftperfoxmmnceh tennaof tha payloaddelivered lb? qusnti~, W, whichhas units of velcdty, char- thrcnIgha totalvelocityincrem?nt,AV, in a n?c- acterisesthe specificstrengthof the presaum- tilinearflightwith~t externalforces. The wer- vesselmaterial,and Is definedby sLU re@_retnentof an actualspacemission,which . will involveseveraldistinctpropulsiontitemala (6) in planetaryand SOI.SXgravitationslfie~, wt “f===” . maxhmm periph- then be expressedas the total rectKUnearfree- The quantity,W, is ve~ nearlythe eralvelocityof a flywkel made of the preseure- fl.ightvelocitychange,AV, to which It is dynami- vesselmateriel. The quantitya is a dimensionless callyequivalent.@ totalAV will be made up of factorwith a magnitudeapproximatelyequalto unity. n velocityincrenmtswhich increasesllghtlyae the totalmass of the spacecraftdecreases.Rather The precisevalue of a wUl dependon des%n details, on the allowanceneededfor shock-leadingof the than to expressthe totalAV as a sum it is conven- pressurevessel,cm the safetyfactordesired,and ientto approximateit by en intagrel.~ differ- on th? equationof stateof the heatedpropellant entid effectof one impulseis to changethe veloc- gas. i~ of the spacecraftBSLSSby CIVwhere: Eliminaticmof the quantities~, 1, and q MdV=I =:(44). (3) fromEqs. (1), (2),(4),and (5) leads to the expression The mass ratiofor the totalmisdon is obtainedby integratingthis equationto obtain Me/M = exp [-2/(v _ ] - Ma/14 (7)

Av . (1/xn)b [M/(M-~)] . (4) where the pmameter p is dafinedas

~is approximateprocedurewill overeatimatathe v = (m(W/AV)(Il~ . (8) correctvalueof AV by a fractionalamountof B?fone discussingtlx?implicationsof Eqs. (7)and ofiern/M. (8), equivd.enteqpationsfor impulsescalingwiI.I. be derived. The pulseunits cannotpmcticallybe made so smalland so numerousas to simultaneouslyachieve ImpulaaScaling an interestingvelocityincrementand nuintainthe l% is assumedthat Ma is proportionalto the peak vehicleacceleraticmat an acceptablelevel. total @uMe, I, delive~d to tlzsmmm?ntumabsorber Thus the mc!u?ntum-absorbersystemperfonnathe main per @se. A ccmvenientexpressionof the propor- functionof acceptingthe short4xration impulses tionalityis and deliveringthem to the spacecraftwith a nearly centtiuonsfo=e over a longerth. This is true . (9) for eitheran external(or iqinge~nt) systemor It wIU. be seen that this scaling may be appropriate an internal(or ccmtaintrsmt ) system as described in to an external concept. the previma section and sa U1.ustrnted in Fig. 1. scalingIaws The totalmass of the manentum-absorbersystem in an externalconfiguraticmis tti sum of the masses No scaling energyscalingand Inqsilae Iswaj of tha puslmrplate,1$, and the energystorageor scaling,have been Constde=d for the totalmass of nsxaentum-conditioningsystem,Me. !12x?cyclicnature the nsm?ntumabsorber. . of the pusherpl.ate-~ntum conditionersystemshas EnergyScaling been describedin SecticaIII. m a typicalcycle the changein velocityLa twicethe initialvelocity It is as-d that Ma is proportionalho the and the initialkineticenergy is energyreleaseper pulse,q. This@pa of scaling I?/8 F$. ~iS energy must be storedwkn tha pusherptite is shouldbe appruprlate,for exa@n, in en internal .s **a *** :bft#g$tP$orest. systemwherethe energymust be contain@+ A @n: *9 ●:0* venientexpressionof thispzwportional~ la’t ~ . . . e ● *O ● @* **i ●;* ●s-

10

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. . 9mi$di@:● :=. * ●✚✎ ● :.● em It can be shownthatEq. (5) to a ve~ generul Optimizatim Procedure relationbetweenthe energyand the minissammm of IXpmsaions havebeen obtainedfor two possible a 6yatem~quired to ato~ it. It aPPliesregard- WIKYCJin whichthe mmentum-aboorbermass, Ma, scales lessof the mannerin whichthe energyis stored, with the pulse size. As a fIrst conaideration, it i.e.,as compressedgas,as a rutatingMSS, or in is clearlydesirableto minimizethe pulse size in a magneticfield. Onlythe quantitya variesand orderto minimizethe correspondingMa. Several onlyover a rangeof abouta f● ctor of two. Hm - factors,-ever, will llmitthe minirmmsize of the ever,the requiremmtsof the maaentumconditloner pulseunit. exceed$ustthe raw requirementof ener~ storage. The momentumconditlonernsm.talsobQ abloto cope ● Shieldingrequirezmnts.For the Internalsys- ten this requirement~ imposea largeluier nondest~ctivelywith a failureof the pulseto fim at the correcttIms. If it is furtherrequiredthat limit,of tha orderof 100 kg. The limitfor the accelemtion of the spacecraftbe relatively an externalsystemmey be much lower--ofthe unifonzit will becomenecessaryto increasethe orderof five kilo-.

mss of the momentumcondltlonerabovethe minimum ☛ Wastageof laserreactants.A ftiedquantity by an orderof nwnitude. coding systemsto remove of gas mev be expendedaftereach pulseand the the dissipativefractionof the energy,lubrication amountwill probablybe independentof pulse systems,and othercomponentswill ultimatelyin- size. It can be shcwnthat this leadsto a creasethe requireda to well abovethe minimum desireto selecta pulse-unitmasswhichwould valueof unity. be of the saaEorderas the laserreactantmass The mass of the mcmmtum-conditioningunit can expanded. be relatedto the energyterm in a manneranalogous ● Minimumth!e Intervalbetween@ses. To avoid to Eq. (5), inefficientpropulsion,the net averagethxust shouldbe ccsmparablato the localgravitational , attraction. The averagethrustis equalto the e .a- (10) w’ me tfiesthe Pube rate. For a ftiedmaxi- wherethe primedenotesthe mcmentum-conditioning mum pulserate,thiswill tend to set a minimum unit. The totalmass of the nmnentumabsorberis impulse. then Theseconsidemtions--inparticularthe last two--mayleadto an optimumpulse-unitmass;how- Ma=~+l.ie=Mp +a’?/8~w2. (Il.) ever, In the presentdiscussionIt will be assumed

Ma wIll have a minimumvaluewhen l.$is appropri- that the minimumpulse--unitmass is fIxed. In each atelychosenas of the expressionsfor w end v‘ given in Eqs. (8) and (15), the quaintity M/m appearsas a rat10 which, (1’) Mp’”e=Ip~ In a givencase,will be determined.The quantity so that W Is determinedby the choiceof materials. The Ma= I~h. (13) missionAV will be givenand the quantities‘tl~ in Eq. (8)and ~ in Eq. (15)will.be deter- Eliminationof% end I fromEqs. (1), (4),-d (13) minedby the sophistication of the system design. leadsto the expression Thus a maximumv or p‘ will be determined. . Me/M = exp(-M/11’Ma) -Ma/M (14) where p‘ is definedae

v‘ = (M/m)(W/LS?)~ . (15)

● 9* ● ● ** ● ** ● * 99* ● ● m ● : : :e ● 9 ● *B- ● *** ● ** : ● * ● * ● *= 99* ● 00 :00 ● 0 IL

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!@m!!!m● . .:● .:● *:.*

Plotsof Eqs. (7)and (14)are presentedin Smctiea for an energy scaling law is 0.86,whereas Figs.2 and 3, respectively,for ccmatentvaluesof for an impulsescalinglaw it is 0.80. Houever,for Ilendv’. It can be observedthat there is in each valuesof p end p’ in tlm rangefran 2.7 up to 10.0 casea maximumpayloadfracticu,Me/M,and a corre- the correspondenceis within3$. Thus the perform- spondlngoptiuumsmm?ntum-abaorberfraction,Ma/M. ance of tbs two typesof systemscan be compared . Clearly,largevaluesof v end II’m desirable. directlyby C-- y end v‘. The ratio,P’111, TM locusof the maximsis indicatedon each figure, can be obtatneddirectlyfran Eqs. (8)and (15)as and the correspondingoptimm allocationsof the (16) spacecraftmass betweenpayload,pzwpellentand From this expressionit %s char that p‘ is signifi- monmtum absorberare plottedin Figs. 4 and 5 as cantlygreaterthan v in any practicalcase. An functions of v end W’$ respectively. IX can be example,hrtentionalJychosento favorthe internal seen that there is a limiting minimum value of p = p‘ = e forwhich the pwload vanisks and the system,1s: M = 100,OCOkg, m = 2CIkg, ~ = 0.9, missioncannotbe perfor!wd. a = 2.0, a’ = 47. From K. (16)we obtainV’/B = 8.1. Bscausep and v‘ appearin Eqs. (T) end (14)as Co!lq)arisonof Ilxterneland InternalSYstemConcepts dinmsionlessspecificInqxlse,it followsthat the The scalingconsiderateionsdiscussedaboveper- [email protected] of en externalsystemis mit a directperfomnancecomparisonof the extemel eightttis higherthan that of an intemsilsystem and internalsystems. ‘J@ Paremeters p (forenergy for th senEspacecraft/pulse-unitXBSSSratio. In scaling)end v? (forImpulsescallng)have titen- fact,largerpulse-unitmassesmey be requiredfor tional.lybeen def.i.nedaa rcu@ly comparable.In an internalsystemthen for en extezzvi!.system, each casethe minimumusablevalue is e. ~er prcwtdlnganotherargumentagainstan internal valuesresultin comparablemaximump@nad frac- design. tions,Me/M,as can be demonstratedby referenceto Figs. 4 end 5. The correspondence18 not exact; for exempk, at v = p‘ = 100,the Illaxiaslmp@loed

1.0 \ Lop I I I I I 1 I \ I I I I I I I I .\” ‘ LOCUS OF MAXIMA ~&LOCUS OF MAXIMA \ ~/- s! \ i

~ {-./ ! I 31 I 1 I o 0. I 0.2 0.s 0.4 ABSORBER MASS FRACTION,Me/M

-*. X,.: ● - . ●. XOC9P -. *m 9** x,. .-h - ,, Fig. 2. Pwload fractionfor m ene~ &aY$g ~.e~ P@oad fxwtion for an iqnihe scalinglaw. ● 9 ● 00 ● mm ● 00 —- .- 32 99* ● ● ● .**. ● *O ● * APPROVED FOR PUBLIC RELEASE ● APPROVED FOR PUBLIC RELEASE

PROPELLANT z (PULSE UNITS) ~ 0.6 — k u .— a E m m ().4 — x~= Ma/M 4 z PAYLOAD PLUS REMAINING )(.= MO/M STRUCTURE

0.2 — ,= a (Vv)(.m

Ma= a~/@

I I 11111 I I 20 40 60 80100 200 ‘p, minimum =e PARAMETER p Fig. 4. Optimm elJooatlonof sp.ececmftnmss for energyscalinglaw.

I .0 I I I I I I I I Ill I PUSHER PLATE ‘ — PLUS MOMENTUM 0.8 — CONDITIONING

z .— o p 0.6 — )(0 .eP’xa. xa ~ PROPELLANT I — (PULSE UNITS) E m UJ().4 — xa . Mo\M s — PAYLOAD PLUS REMAINING Xo = ‘O/M STRUCTURE 0.2 — p’ ‘(”/m) (w/AV)~

— Ma+ $ r

a I 1111 I 4 6810 20 40 60 80100 200 L p’, minimum =e PARAMETER p’ ● 990 ● ● ea ● *D ● * Fig. 5. OptimumallocationSf @@~m&.s fsrgimpulsescalinglaw. ●“ ● ;0 bbO*– ● ● : ● * ● O ●:0 ● 9* ● *O :00 9*

-<”● 0 ● ** ● ● ● ● * APPROVED FOR PUBLIC RELEASE APPROVED FOR PUBLIC RELEASE

P8X’’fOXVUUMt3Limitations The corresponding maxlmm epecificimpulseis

The basicperfonmnce limitation,that of mini- ISp “ T !/ml /80. mum pulse-unitmass,has been diecuesed.However, FOr hydrogenat 83339X (1.5,000”R),q/m = 3.5 x I-Oa otherdesignlimitat ~ons x praventthe selection J/kg, correspondingto an IW of 2430 sec. At this of tk uptinumpulse-unitIm@ne or energy. point the hydrogenis toteUy dissociatedend Is specific-e Limitation only slightlyionized. Radiativeheattransfer wouldbeccamexcessiveat highertemperatures. !l!hislimitationin smnifeatad,both sdhemati- . C~ end @YB$CWP In diffexmt wws for the en- Mamntum CcmditionerLimitations ergy scaling W and the Impulsescalingk. These In the mmmtum-conditionerconceptdescribed tvo casesare thereforetreated8eparately. above, the pusherplateexetisa constantforce,F, ImpulseScsling. In th Orionsystemstudies, on the Spacecmftthroughouta cycle of duraticnAt. the implngesmtveloci~ of the propellanton the A forceb616UICe at a time when the propellantis pusher plate was Mmlted to 150 km/see. The llmft $@ expendedcan be vrittenas: vas not baaed upon plwsical Mmita, which vere un - (20) a. (M. + Me) = Ma (aa - ao) knam, tmt ratkr on a bmdcdcun of the validityof the modelswhichwem used to predict●blationof wham : = epacec~ acceleration,at the end of the pusher-platesurface. In any case,surface 80 mission,=Iative to an inetiialframe of ablationwi21 Imposesouw 13miton the impingenmt reference. a = Puekr-plateacceleration,at the end of velocity,Vi. The net impulsecan be written a mlaslon,mslativeto the spacecraft. I=i3n vi, (17) TM distancetraveledby the pusherplate,relative which sewes as a definitionof P. In the case to the spacecraft,is the stroke,Ax. In terms of vhere the pmpallant impingeson tk pusherplate accelerationand time [email protected], it is axiallyand doesnot ncoil with appreciableveloc- (21) ity, p Is equalto the fractionof tm pulse-it & = aa At2/8. masswhich @lnges on tk pusherplate. Elimina- The tiitialvelocityof tbs pushr plate,relative tion of I betweenEqs. (4), (14),and (17)leadsto to the spacecraftis t~ follcxfingexpression: lJ=J’=L. (22) 2% P vi = tw p’ (Ma/M) . (18) Eliminationof aa end I fras%s. (10),(21),and the optimizationproceduredescribedpxwiously If (22)resultsin an expresslcmfor the mass ratio, I.eaisto a value of Av v‘ (Ma/M)gxeaterthan the b2 E Me/k$: eUouable value of S Vi, thenthe systemis limited

toa-lmm I = P@. and the p@oed plus b2 = Me/l$ = 8 a’ (AX/w At)2 . (23) manmtum-absor%’rfracticuis Bi@y Limitationssu!q’be imposedon the maximumstroke, (M. + Ma)/M . eQ(+Jv/&fi). (19) Cx? and on the minimumtire?betveenpulses,At. TO minimizethe totalmamntum-absorbermass, The hpulSe is givenby Eq. (17)sad the design Ma= ~ + Me, it is desi.xableto splitthe mass beccms!aone of minimizingMa. ‘qxsOttit ~= Me=db=l” ~isc-h Energy Sml.ins . The finalImperature of the shwn to be the case~ dll’ferentiatingEq. (u) heatedpropellantfiJlingth pnmure (prob- vessel with respectto 1$. Hw?ver, if the mmclmumper- ably hydrogen)la limitedby the acceptablelevel mittedatrolm,AX, and the mininanspermittedt~ of heat transferto the cooledpxwmure-vesseland Intemnal.,At, are suchthat b as givenby Eq. (23) nozzlewalls. This imposesa limlton the mximm Ie 38ssthaaunity,then the sdniasnn~ cannotbe value of the energydensity, q/m, and setsa limit used and Eq. (15) for v‘ mat be modifiedas follcus: on the productp ~ thrcnaghthe rebt ionahip

9 ● ** ● ● 00 ● m● “ (U/AV) ~ [2/(b + l/b) I ● ● ● k’ oh) :. . . 14 ●°0 ●

● OO** ● m. .0 APPROVED FOR PUBLIC RELEASE APPROVED FOR PUBLIC RELEASE

● ●✛✎ .OO

The fInal factor, 2/(b + l/b),VU be lessthen PeyloadPlus utherstructures(M ) = 52,542 = unity if the pensittedstmb and tim Inlxnwal XWload aoceleration = 19.6 m/se~2 (2 g) j end of rntssion combtieto limltthe designflaxibiliti.The net resultis a decreasedp~ter 1A’and, consequent- Pueher*kte stroke (a) = 14.4 m (At)= O.% eec Iy, a decreasedpayload. AS with mat mwoth mexi- lhtennd. between pulses Totalpmpulaion time = min/missiOn. ma, aevbtions from em optimumparameterresultin 20 onlyminorperfonsancereductions.For example, Thesevaluesare bsbseaon en optiammallocation ~ti~~.2 ‘.$ t~n b = ~ and V’ is reduced~ of massesfrom a mlsstinpoint of view; no corres- pondingdetailaddesignexists. cl.earti~S- Wr- tion of =ss ~ wiU. be requ~d for tls?laser,for PerfcnammeEstimates pilee-unitconveyanceand storagemechanism,shield- ExternalSystem ing,etc. (Neutronenvfronmsntelconsiderationsfor To estimate the perfonmnce of an external suchen externalsystemcue presentedin App?ndixC.) system, example pamunaters ans chosen as follows: Each of the pazzunstersaffectingp‘ has been

M = 100,000 kg. Thls initialspacecrdt mass is variedto studythe effecton systemperformance; comparableto that of the -eat spacecraft the nmitts are presentedin Table 1. The ffist contemplatedin earthorbit in tb? tires exampleis the base casepresentedabove. The ~riod 1970-85. ~ters whichIsnrebeen modifiedfrom this base caseare underlined.Examplesof the effeetsof n = 20 kg. TMS pulse-unitmass ehouldprovide variouscmstxdnts em listedin the last five rOUI@Y a factor-of-tishieldingegalnst cases. Table I clearlyillustratestk extraordi- 14-MeVneutronstf onlyhalf the muss is naryperformancewhich can be expectedfrom an ex- berylliumconfiguredin a 15” (hdf’-angle) ternalpulsedpropulsionsystem. conebetweenthe sourceend the spacecraft. m the baae casep=sented, the specificim- Ii = 3m m/see. !l!hisquantitycorrespondsto the pulsedesiredcouldbe achievedif one haE the use of steel with a lOOJOOO-PSiYieM strength. prope~t @mea on the Pusherp~te at a velac- ity of 135 km/see. This is barelywithinthe im- . 20 km/see (65,616ft/see). This free-space (w pingementvelocityconstraintof 150 lun/secimposed velocitychangewtia occurin a very advanced duringPro$ectOrion. It is not p=sently known mission. It wouldprovidefor a fast round whetherthis fmpingenmtfractioncan be achieved trip to MWS f= low earthorbit. withinthe gemnetrlcconstraintsgwerned by shield- a’ . 4’7.!l?hisefficiencyvalue is takenfrcmthe ing considerations. studyfor an actual OrIon nwmentum-condlt toner internalsystem design. TO somedegree,this highvalue can be attributedto the complextwo-stagedesign If the internalsystemis to delivera positive employedand to the largenumberof parts [email protected], tlw valueof p in Eq. (8)must exceede. which serveno enerw-storagerole. If the pelletis a major sourceof neutronsthat requireahieldlng,then the impliedus is very of P’ iS (f- Eq. The resulting value 15.57 15)) hfgh--Ofthe orderof hundmedsof kilogram. Two end correspondsto a maximumpayloadfractionof additionalparameterestinmtes,for a end for Tl,are 0.525. Cm?reepOndi”.g spacecraftparametersare: requiredto obtaina numericalestimateof v (other Optinmn epecl.ficimpulse= 6$?27 sec parameters- definedin the previousdiscussion Numberof pulses= 1275 (for ~ = 25,515 @ of externalsystems): Energyper pulse = 44 equivalenttons of m a= 2. This selectionis based on the observation (=- 5% Of propellant impin8eson pusherplate) that the mass in an internaldesignis very

Pusher-platemass (%) = 10,9(0kg ● 00 ● [email protected] O@t t~t of a s-le PXWWS Vessel. ● m Momentum-ccmditioner-s (Me) = 10)970 kg “I: ~ ~(● : ~. cd & demonstratedthat the mass of the ● ● m ● 0 ●:0 :00 ●:0 :00 ● 0

15

APPROVED FOR PUBLIC RELEASE ●.9 ●-. : ● ● .** ● ● APPROVED*m aa FOR PUBLIC RELEASE

!kkj!bsm● ●:@:.* =1

EXTEMAL NUCLEARPUU3EDPR@UIi310NSYm PAMMWCER S’NIDY U, bv, I , Mo, Ma, 1$, NP, 310, Ax, At, u, m %* A-.2- .—G .’~b —J&kkkkk 2&_ ~ .tala

100000 21-I.O300 20000 47.00 15.47 1.000 6927 52542 21941 25515 10970 10970 2.0 14.39 .930 46.0

100000 yJ 300 20000 47.f10 61.88 1.000 14998 75602 11875 12721 5937 5937 2.0 6.64 .429 51.6 loclorlo ~o 300 20000 47.00 30.94 1.000 10279 65714 16278 180n6 8139 8139 2.0 9.69 .626 48.5 lwlmlfl 50.0 300 20000 47.00 6.18 1.000 3910 28377 30966 40655 15483 15483 2.0 25.49 1.647 35.1 . 100000 lmo 300 20000 47.no 3.09 1.000 2306 4754 3652.4 58721 18262 18262 2.0 43.22 2.793 24.4

100000 20.0 150 20000 47.00 7.73 1.000 4531 35035 28705 36258 14352 14352 . 2.0 5.49 .710 18.8 100000 20.0 ~ 20000 47.00 30.94 1.000 ln279 65714 16278 18006 8139 8139 2.0 38.79 1.253 97.0

100000 20.0 300 _ 47.00 30.94 1.000 5139 65714 16278 18006 8139 8139 2.0 9.69 .626 24.2 100000 20.0 300 ~o 47.QO 7.73 1.090 9063 35035 28705 3625S 14352 14352 2.0 21.99 1.421 75.6 100000 20.0 300 100000 47.00 3.0!) 1.000 11532 4754 36524 59721 18262 18262 2.0 43.22 2.793 122.1

100000 20.0 300 2ooofl 2.00 75.00 1.000 16621 77586 10859 11554 5429 5429 2.0 140.94 1.879 253.7 100000 20.0 300 20000 ~ 47.43 1.000 12994 72042 13423 14534 6711 6711 2.0 72.11 1.520 155.0 100000 20.0 Ino 20000 20.90 23.71 1.000 8857 61121 18299 20579 9149 9149 2.0 26.45 1.115 72.0 100000 20.0 300 20000 U 10.60 1.000 5525 43592 25527 30880 12763 12763 2.0 8.47 .799 28.0 Constrdned 100000 2n.o 300 20000 47.00 15.47 1.000 ~ 59S73 16158 32967 8079 8079 2.0 .761 23.9 100000 20.0 300 20000 47.00 15.47 l.mlo 6?27 5254? 21941 25515 lo970 lo970 5.7 .323 44.0 100(IOO 2n.@ 300 20000 47.00 15.47 1.000 6927 52542 21941 25515 10970 10970 .9 * 44.0 100000 20.0 300 20000 47.00 4.87 .161 3309 2t_16~4332.?2 46022 32435 846 .6 — 10.0 100000 20.0 300 20000 47.00 14.10 .646 6561 50477 22790 26732 16074 6715 1.7 &WQ 39.s

●MinizNzukineticenergycarriedby pulse-unitmass,inequivalenttoneof ~ (4183347/tan).ibissscss tket 50$of theprope.llentmaa8lmpinszmon thepusherplatscarryins50$of theemrw.

mcmmtum-conditioningsystemis veg mmi!l--of thosefor the externalsystemexampleused previously the ofierof ten tiresthe pulse-unitmass for SUuatratesthe disadvantageof the internelsys- a consez-mtivedesign. The valueof a for an tem: In the externalsysteman energyreleaseof actunlInternaldesignwhichbaa been studied 44 equivalentton6 of TNT requireda nnmmtum- la 1.2. If SupportingStructures,parts in the abaorbern!as6 Of ody 21,941 kg-+! much s!d.br preseum vessel,pumps,a nozzh?,end otherre- mm per unit of ene~ or per unit of impulse im- quimxmsntsamountto 2/3fithe mass of the bare partedthan in the internalsystem. vessel,a will increaseto 2.0. AU Intend. systemwith a reasonableperformance Tl = 0.9. This value is takenfraa an intermitsys- can be envisionedon3yby igno~ the shieldingre- tem that has been analyzed. m no case can II quinmm?ntand choosingan acceptablysmallpu3.ae- be lessthan 0.75. unit =ss. TMS may bs practicalif suitablefusion or fissionreactionscan be used as the energy With thesevahes, II= 0.8s (fran Eq. 8) indicating that the chosenmiesionexceedsthe ca~bility of an fxxxce. internaldesignsystem. One such internaldesignhas been partially studied. A pressure-vesseldiam?terof 3.658m The mcdzzuz pulse-unitE43s6 which can be used for tbse given~tera can be obtainedby set- (12ft) was chosenarbitrarilyand hydrogenwas aammed tO be the Px’O@lant. l!hlCYCliCOperation tlng v = e. IIIthis casem = b91w. lb s~ce- of the systembaa been describedin SectionIII. crsftmass is apporticuedas ~ = 86,470 kg (pulse units or propellant)and Ma = 13$530 kg (pxws~ The byd.ax?genstagnationccmditlma in the vessel, . after the shockwave baa decayedand before expan- vessel). The cormwpondingene~ mleaae per Sh *- the nozzl.n has begun, wen? chosento pulse unit is q = MaI#Ia = 609 W or 0.19 equiva- ● ..-...... ---. . - “p .“. lent tons of TNT. cazparisonof thes~‘~ue$ w~h ~ ● ● * b ● m ● ● 00 :0 ● m ● *9 .:O ●.: ● m. ● m 16

●“:~*em...... APPROVED FOR PUBLIC RELEASE ,e APPROVED FOR PUBLIC RELEASE

●:’. Sq@diii!● :*:__a :* D. ~m ..: *:. i;m * lwperature = 8333”K(15,CK@R) use%334 de %elios system. The resultis a steel Pressure = 689 N/cm2 (1~ pSi.9) . pressuxwvesselwith a wall thicknessof 3.66 cm (1.44 in.) and a mass of 12,800kg. DetaUd hydr- At this temperatureI@rogen is almostcompletely odynamiccalculationshave been perforss?dto study disassociated,and ionizationis insignificant.At the effectof shockwaves generatedin the hydrogen highertemperatures,ionizationincreases,the . end t~ peak pressureswhich resultfrom their reflec- hydrogenbecons?sopaque,and rdiation heat trans- tion off the pressure-vesselWS3J-.The celmted fer from the opaquehydrogento the wallsprobably peak pressureof the radial.pressurewave in the wall . becomesexceseive. is 2.5 kilobars(36,!250 psi), which shouldnot cause With the aboveassumptionsthe mass of hydro- failure. gen in the vesselis 2.53 kg end the corresponding The differenceof 5430kg betweenMa and the value of v fromEq. (8) is 5.37. A mexiummpayload p=saure-vesselmass is resewed for nozzle,momentum of 23,550kg can be deliveredif the @- Ma and conditioner,and relatedstmctures. ~ edditIon, energyreleaseare selectedas 18,230kg and 0.196 scm part of the pt@oad mass must be resexvedfor equivalenttons of TWT, respectively.This is the lasersystem,tih.erstructures,and perhaps aboutequalto the energyrequiredto heat the shielding. hydrogento the postulatedconditions. * Helios: An internalco cept studiedin parallel t A pressurevesselhas been designedfor this with the Pro~ectOrion. ccmventionalfissicmex- plosionswere utSlizedwith a yield rangingfrom 1 exampleby applyingsome criteriathat had been to 40 equivalenttam of m and a repetition rate of about10 sec or longer. A plug in the nozzle was used to allcwrechargingof the vesselwith the propellant(hydrogen).These constraintsmade the Heliossystemvery large.

ACXNC%U.EIXMlmls

Specialacknowledgmentis due to KeithBoyer,IASL,DirectorLaser Pro$ects,who intreducedthe contributorsto this re~rt to the concept of laser-driventhernmuclearreactionsfor space-propulsionapplications.

1. C. J. Evezvttand S. M. Ulem, “Ona Methodof propulsionof Particles by Meansof ExternalNuclearExplosions,” TASLReportTAMS-1955, August1955 (m) . 2. 17uclearpulsedPzmpukion Project(Pr@ect Orion),~chnical summary Report (4 vol.um?s),General15mmics Corp.,GeneralAtomicDiv., RTD4?ln?-63-3o06,1%3-1964 (SR.D)●

3. J. C. Nence, !NucleerPuleePropulsion,” ReportGA-5572, Odmber 5, 1%4.

4. J. W. Hadley,T. 1?.Stubbs,M. A. Janmen{,aud L. A. Simons.‘- Helios PulsedNuclearPropulsionConcept(~), LawrenceRadiationLab., Livermore,Callf.,ReportUCRL-14238,June 2, 1965 (~).

,

● ● ●*9 ●** ● “ * ● * : ● :0:: .Cs C:t ‘ ● o 17

‘. ..,, ● ●:*. **. * ● U*U ● APPROVEDi& FOR* PUBLIC RELEASE ●* APPROVED FOR PUBLIC RELEASE

● Nd!l&ML.0 -. u ::**:*: B ;: APPENDIXA .:.* ,.&&eu~*~as reactants. LZZerpumpingis the method ● .ss . of providingthe Inputenergyrequiredto excite - CONCEPTSAND CONSIDERATIONS the materialto its upperlasinglevel.

The Nd/gl.asslasertransitionis centeradat Introduction -6 a wavelengthof 1.o6u (1.o6x 10 m). The state The essentlslch=acteristicsof en energy of the art of theselasersfor producinghigh-power, . pulse requiredfor thermonuclearignitionare high ultrashortpuleesia quiteadvanced. Thesesystems power end shwt pulsewidth. b pulsedfusion are optice3Jypumpedvia xenonflashtubesand at reactionsa sufficientquantityof materialis ion- presentcan produce200 J in a few picosecond. . ized,confined,and heatedto suchhigh levels However,the possibilityof considerablyincreasing that nuclearfusionwill occurduringthermalcol- the outputappearslimited. It is anticipatedthat lisionof the ions. Becausea laserener&ypulse the maximumoutputobtainablewill be of the order can be concentratedin both spaceand time, it is of 2000J In a few picosecondbecauseof the energy believedthat alight pulsecanbe generatedwhich densitylimitationin the glasscoupledwith the will satisfythe specificrequirementsfor initiating limitationof uniformlypumpingthe glassvolume. a fusionreaction. Also, the ener~ inputfor the requiredhigh-power pulseswouldbe excessivebecausethe overalleffi- GeneralFeaturesof LaserPulse ciencyof these systemsis orily-O.3$. lhus it is Calculationsindicatethat the minimumlaser not ent%cipatedthat a Nd/gl@%ssystemcouldproduce energyrequiredto burn D+T in milligramquantities more fusionyield energythan that requiredto power 4 6 rangesfrom 2 x 10 to 1 x 10 J. Thesevaluesare the system. Neodymium/glasslaserswidl therefore being revisedas more optimalmeansfor confining be usedprisuxrilyin experimmtsto studythe inter- and heatingthe fusionablematerielare examined. actionof high-energylaserpulseswith matter,and The D+T reactionis usue3Jyconsideredbecauseits to verifythe correctnessof the mathematicalmodels fusionthresholdIs lowerthan that of the D+D or usedto calculate laser-initiatedfusion. To date, D+3He reactions. The optimumtime widthof the neutronproductionhas been observedin experiments and 10-7 beer pulse is thoughtto be between10-~0 where laserpulsesfrom Nd/glasssystemshavebeen see,to take advantageof the resonantabsorption focusedon fusionpellets. which occursnear the plasmafrequencyof the mdium. The CO~N2 laserutilizesaresonaut energy An alternativeapproachconsistsof usinglonger trausferbetweenthe firstvibrationalstatein N2 pulsewidthsfor directcouplingof the fieldto the and the upperenergycoincidentlaserstatein C02. ions,therebypredominantlyheatingthe ions. The lasertransitionis centeredat a wavelengthof LaserSystemsUnderStu~ 10.6U (1.o6x10-5 m). Becausethe energyis dis- The laser systemmust possesssassbasicchar- tributedamonga largenumberof rotationalstates, ecteriatlcsif the requiressmtsfor the lightpulse it is necessaryto promotetransitionaamongvery are to be met. Becausethe energyneededis high, _ of thesestatesto obtdn high efficiencies a reasonableefrlciencyin convertinginputenergy with shortpulses. This canbe dcmeby operating intolaserenergyis essential. ti attemptingto at high pressures(1 to 10 atm)to collision- minimizethe energyrequiredto triggerthe fusion broadenthe lines,and then mode-lockingthe laser reaction,pulse-shapingis critical.Pulse-sha@ng outputso that all rotationallineswill lace simul- and timingare performedin en Initial,low-power taneously. CcassercielCO-22/N2 lasersusuallyoperate stage(oscillator)beforatha pulse is transmitted at pressuresof only 10 atmo . throughone or more successiveempllfierateges The pumpingschemeunder studyfor the hiCh- where Its desiredhigh power is prcduced. PressureC02/N2laser is an electricaldischargein . Threetypesof lasersare underconsideration: whichthe ionizationis producedby a high-energy (1) o@icQp_d Nd/61ass,(2) eleotricslkvpm- electronbeam,whilethe discharge-electrontempera- pedc02/N2, and (3) ch~c~px~ers US* ture and enerqpumplng are providedby an applied .* ●9- .* * **8 18 ***:. .

APPROVED FOR PUBLIC RELEASE APPROVED FOR PUBLIC RELEASE

-- .:*. ● 0 ..: ●✛✎ electricfield. !lheoverallefficiencyof this ✚ “he”kfber couldeitherbe expendedwith each lasershouldbe..-1O$ and the maximumoutputis ~losion or couldremdn on boardthe spacecraft eWeCted to be 105 J In severaltenthsof a nano- for continuedreuse. second. me outputper volumeof C02/N2operating For an expendablesystem,the problemsof at a pressurebetween1 and 10 atm is expectedto focusingthe laserbeam onto the fuelpelletwould . 5 be 10 J/m3. be essentiallyeliminatedbecausethe laser,the The chemicallaseris self-pumped,utilizing fuelpellet,and the propellantmaterielcouldbe . the ener~ output of the chemicalreactionof the constructedas one unit. Such an approachappears materialswhichform the lasermedium. The require- essentie3for internalpulse-propulsionsystems, mentsof the chemicalreactionare that It be very wherepropellantmaterialcanplet.elysurroundsthe fast (explosive)and that the resultingmediumhave energy device. Use, from a propulsionviewpoint, a largeexcitedpopulationin the upperlevelof a it wouldbe much nsYreeconomicalto imparthigh possiblelasertransition.The chemicalreaction velocityto auy not reusablelasermedium (as in a can be initiatedeitherby a llghtpulse,an elec- chemicallaser)beforeeqdling it (becausethe tricaldischarge,or an electronbeam propagating e~losive sourceis essentiallyenergyunlimited) throughthe activemedium. The energyin the ratherthan to expelthe material.with negligible upperlaser-vibrationlevelis distributedamong propulsioncontribution.lhe generationof suffi- a number of rotational.states(thoughnot as maqy cientelectricalpowerwithinthe unit and cost as in C02),and it is thereforeadvantageousto reductionto a levelwherethe disposalof laser

operateat higherpressures(1 atm) in orderto componentsbecomes economically feasibleare some extrectmaximumenergyby the laaingaction. Re- of the problemsto be encountered. searchattentionin chemicallasershas been di- For a non-e~endablelaser,the principal rectedmostlyto reactionsof hydrogenwith the problemIs the focusingof the laser energyonto ~ogens, speciflca13yF2~ C12j and (CN)2. Tu the fuelpelletand the protectionof the laserfrom increasethe ener~ densitywithinthe medium,the the ensuingexplosion.Lightweightexpendable chain-branchingreactions,e.g.,N#4 + ~, mirrorscouldsolveboth problemsfor an external seem quitepromising. The RF lasertransitionis pulse-propulsionsystem,but the problemswouldbe centeredat a wavelengthof 2.7’~. The overall more acutefor an internalsystembecausethe reac- efficiencyof chemicallasersis expectedto be at tionmust takeplace in en enclosedvolumethat is least16, and outputsof 106 to 107 J in a frec- filledwith propellantmateriel. tion of a nanosecondappearto be possible. One Of the two lasersystemspresentlyunderde- promisingconceptfor a chemicallaseramplifier velopment,the chemicallaserappearsto be more involvesthe traveling-waveignitionof the chem- suitablefor a apace-vehicleapplicationthan the icalreaction,followedimmediatelyby the laser electricallypumpedCO~N2 SYStem. It represents pulseto sweepthe cavity. The unit outputof a the most compactenergysourcein both volumeand typicelchemicalsystemoperatingat 1 atm is expectedto be 2 x 106 J/m3. weight. The chemicallaseris also Inherentlysim- pler,requixhg littleelectricalpower and storing to Propulsion LaserCharacteristicsRelating the requiredenergywithinthe atomsof the chemical Applications elements. In comparison,the electricallypumped Of prime importancefor any laser-pulsed C02/N2laserrequtresSU of its inputenergyin thermonuclearapacepropulsionsystemexe weight the form of electricalpower,which is difficultto and powerrequtiements.Becausenone of the laser provideIn a apacevehicle. Even if largeamounts systemsbeing consideredis sufficientlyadvanced of electricalenergywere storedin capacitorbanks for use in this application,detailedplanningfor suPPlledfrom a m~est Power sourceover a period incorporatingthe laserinto a spacecraftmust of time,the mass of such a storagebank for supply- awaitfuturelaserdevelopments.However,certain ing 107J of Inputenergywouldbe prohibitive.In fundamentalquestionsshouldbe con~ereiLamw...... ad..tion,the overallefficiencyof convertingthe .-.:*O A -—

19

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!:.ii!idip9:**: .* .: *.? .:. ● O* ● O energyinputto laserenergyoutputshod in~!l& becauseof the lack of radial.restraint. TO prevent the largeloss incurredby convertingthernuil. yieldingof the platematerial.from heap stresses, energyinto electricalenergyin the power supply. the platemust be thickenoughat the edge end must Thus,a theznuCUypumpedC02/N2,applyingthe ther- increasein thicknesstowardthe centerto maintain mal energy dtiectlyintolaserexcitation,might a uniformaxialplatevelocity. Theseopposingre- have a betteroverallefficiencyfor a space-vehicle quirementsdemandan unnecessaryaccumulationof . laserthan the electricallypumpedsystemand, materialat the centerof the plateto satisfythe Additionally,would obviatethe demandfor large yield-strengthcriterionalone,becausethe tangen- . capacitorstoragebanks. A singularadvantageof tial tensilestresses,i.e.,hoop stressesif the eitherthe electrically-pumpedor thermally-pumped platewere a sphere,are less than the hoop stresses CO~N2 laser systemis the factthat the laser from the edge moments. mediumis, in principle,reusableafterthe excess l!hemanentumexchangebetweenthe expanding ener~ residl.ngin the eystemhas been removed. propellantand a flat,cLrcularpusherplatedtifers fromthat of the shapedplate in that hoop stresses APPENDIXB do nOt develop. As with the shapedplate,the mass distributionof the disc shouldcorrespondto the DESIGNCONSIDERATIONSPOR radial.impulsedistributionin the axi.sldirection PUSBER-PROPELLANTIN!lERACl?ION acrossthe surfaceof the disc. Thiswouldprevent General bendingof the plate. Radialstresses~e developed OQV fia the VISCOUSshem of the radialpropelJ.ant- systemis The designof a pusher-end-pulse velocitycomponentas the wave spresdsrsdia13y constrainedby the characteristicsof the momentum- acrossthe disc. Theseradialstressesare much exchamgeinteractionof the expandingpropellant less than the tangentialtensilestressesin the wave with the pusherplate,the designcriterion shapedplate. being amaximum momxtum exchangewith no destruc- tive effectson the pusher. In sddition,the Althoughthe mcmentumexchangewith a shaped optimummass of the pusheris determinedby mission pusheris greaterthanthat with a flat disc,the considerations. extraweightof the shapedpusherto compensatefor hoop stressesq be amore severepenaltythan the PusherShape decreasedpert’ormanceof the flatdisc If both sub- The msximummomentumexchangebetweenan ax- tend the sane solidangleof the propellanteqxin- psndingpropellantand a solldpusherplate is ob- sion. tainedwhen the shapeof the platemirrorsthe sur- PusherStresses face of the propellantcloud. Assumingthat the propellant~snsion is divergent,the shapeof the Althoughthe hoop end rsdialtensilestresses pushersurfaceshouldbe spheroidal;end the use- are not a designconsiderationfor the discpusher, ful momentumexchangeat any point on the plate is both plane and raiialstrainwaveswiJl be trans- a resultof onlythe axialcomponentof the local mittedthroughthe materialfran the shockof the propelJ.antvelocity,i.e.,the directionof vehicle propa~t interaction.If the acousticthickness motion. If the mass of the plate is u!xlform,the (platethickness/speedof soundof material)is locslplatevelocltyvariesawi bendingoccurs. much less than the propellantpulsewidth (inter- The plate canbe designedto resetat uniform actiontime for propellantstagnation),the plane wave will reverbe.ratirepeate&Lyand dissipateas , velocitylf its mass distributionis made to vary the st.egnationpressurebufldsto maximum. There- radially in order to matchthe axialccanponentof the locslpropellantimpulse. The cross-sectionof fore,the strengthof the materielmust be suffi- . cientto prevantthe “blowing-off”of the pusher sucha plate is then a crescent. However,the stagnation-pressurebuildupon the plate surface back sidewhen rsxefectlonoccursfran the reflec- developshoop stressesfrom the edgemunents tion. .=e$e ~edielwavestraversethe plate in .0 ● ** ● mm .0 .0: ● ::0 .* ● ● .00. ● ●0 : . 20

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●6 ..: .:. ;:* timesexceedingthe pulsewidth,stressconcentra- g:e$+%%&n 1.5 x 107 cm/Oec,convective-flow tionswill be producedat the center. However,the Instabilitiesare calculated,thus renderingthe

radialtfaves will be weakerthan the planewaves, diffusionmodel invalid. and the stress-concentrationlevelmay thereforebe OtherEngineeringAspects less than the yieldstrenbth. This stressconcen- trationfromradialstrainwavesmustbe carefully Becauseof the Inherenthigh accelerationof the pusher,a shock-abeorbersystemmust be designed sntiLyzedfor any pushe~platedeeign. to reduceaccelerationsof the payloadmass to PusherAblation tolerablelevelsfor mannedflight. This shouldbe The energyreleasefrom the stagnatingpro- accomplishedwith a minimumdissipationof energy. pellantat the pushersurfacecausesthe pusher The attachmentof the shock-absorbersystemto the surfaceto heat with attendantablationof pusher pusherplate requiresa minimumedge thickness, materiel. To preventthis loss,the plate surface which,in turn, is a factorIn determiningthe shouldbe coatedwith an ablativematerialof high pushermass. The kinematicsof the shock-absorber heat capacity,low thermalconductivity,and large systemis stronglydependentupon the pulseperiod, Rosselandopacityat the stagnationconditions. one of the freeparametersof a pulsed-propulsion Preferably,thismaterialwouldadhereto the plate system. Becausethe pusheraccelerationis inversely end be of sufficientthicknessto last throughout proportionalto the pushermass,a minimumpusher the firingpulsesfor the mission. If sucha mate- mass wII.I.be determinedby the msximumacceleration risJ.is unavailable,an ablativefilm couldbe de- that the pushermay be subjectedto in orderto en- positedon the pushersurfacebetweeneachpulee or surethe structuralintegrityof the shock-absorber aftera groupof successivepulses. system.

The ablationphenomena,althoughcomplex,=e Althoughhoop and tensilestressestheoret- predictablefor stagnatingplasmaswith incident icaJJ.yare not developed,a mismatchof the pro- energyfluxestypicalof plasmasd-rivenby high pellantwave and the pusherfrcmpulse-system explosives.An analyticalaproximatlondeveloped positioningand nuclear-yieldtoleranceswill result in the Orionprogram2successfullypredictsabla- in bendingstresses. Thesecriteriaprobablywill tion ratesf? experimentswith propellantvelocities not affectthe pusherdesign,but must be analyzed up to 4 x 10 cm/sec. lh applyingthis expression to determinethe positioningand yieldtolerances. for energyfluxesfrom a nuclear-pulse-drivenplas- RelevantInformationfrasProjectOrion ma, the ablationratesbecomeunpredictableat pro- A wealthof Informationis availableaboutthe pellantvelocitiesgreaterthan 1.5 x 107 cm/sec. interactionof propellantand pusherfrom the design The ablationphencmenaoccurIn stages. studiesof the Orionproject. Unfortunately,much Duringthe earlyphases,the phenomenonis predic- of the informationis empiricaland relevantonly ted by a kinematicmodel,involvingpartlcle-to- to the referencevehicledesigns,and thereforemay psrticlecollisiontransferof kineticenergyand not be applicableto pulsedsystemsin general. indicatingan e~onentisl increasein the ablation However,some calculationaltechniques,e.g., the rate. As the propellantplamna,due to compression kinematicsof the shock-absorbersystemand the at the pushersurface,heatsto temperaturesin the strainpropagationIn the pusher,shouldbe appli- eV range,radiationtransportpredominates,result- cable. ing in a net decreasein the shlationrate. Next, The Informationfrom OrIonablationexperiments as the ablatingmaterialincreasesin temperature, is relevantonlyfor verifyingthe analyticaltech- the ablationrate is determinedby diffusionof nique,and is largelyempirical. the ablatingmaterialintothe propellant.For the The lack of ex- perimentaldata for propellantvelocitiesof inter- Orionconfiguration,the bulk of the mass was cal- est (> 105 m/see)resultedin a conservativecon- culatedto be ablatedby the diffusionprocess. strainton propellantvelocity x 105 m/see)for With the Orionanalyticaltechnique.at velocities (1.5 ● ✎ ✎ ✎ ✚☛☛ ✎ ✍ ... .OodeSignsof Orionvehicle~.

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● %iiidib● . :..::*. ,:0 ..@ .S*.*. APPENDIXc Totalneutronenergy= (1.275)(176)=2.24 x 105 tons 8 =9.39 x 10 Mlhec 14 ~ NEUTRONENVIRONMENMLCONSmERATIONS =9.39x 10 POR TSE EXIERM& SYSTR.f =5.87X 10w MeV. 26 l?o= neutronsource= 4.16 x 10 neutrons. This appendixpresentsmum general conclu- . slona regarding the effectof neutronenvironmental Thesemagnitudesare so largethat “attenuation”In considerationsuponthe performanceand overall the usualsenseof slowingdownand/or capturing . eyatemconfigurationof the externalpulsed-prop- the neutronsimpliesnon-survivalof the attenuating medium. ulsion concept. Resultsfrom initialoalculation- For examtple,the (uncooled)pusherplate el studiesleedlngto theseconclusionsare also in the aforementionedreferencecase can survivea given. ‘totalheat load of ~ 109 J, or some sti ordersof magnitudeless thanthe neutronenergyfrom the The calculationsgenerallyestimaterelative source. An unshieldedmannedpeylosdlocatedat quantities(e.g.,neutronreactionrates,attenua- ten metersfrom the sourcewouldreceivea total tion factors,end weight-effectivenessper source 12 dose of ~ 10 rem, which is 11 to ).2ordersof neutron). An absolutesourceintensitymust be magnitudehigherthan couldbe allowed. Further- assum?d,however,to estimateneutroneffects (e.g., more,capturey-rqf sourcesresultingfrom the temperaturerise,radiationdamage,azxldose rate). absorptionof even aminute fraction:; thesegen- A referencemissionwaa therefareselected eratedneutrons(perhapsas few as 10 to lo-’+) consistingof (identical)pulsesovera thrust 1275 wouldproduceheatingproblemsat most locations periodof 20 rein,with ~ tons (~ equivalent)of throughoutthe spacecraft. energytransferredto the propellantIn eachpulse. This energywas furtherassumedto includeall non- Two basic approachestowardalleviatingthese neutronenergyfromthe pulseand ~ neutronenergy. problemsare apparent:(1) An expendableattenuating materielcan be Interposedbetweenthe sourceand The D+T reactionwas takenas the energysource, the pusherplatefor eachpulse,and (2) the pulse which Introducesthe most severeneutronenviron- unit-spacecraftconfigurationcan be stretchedout mentalproblemsof all possiblefusionsources. so that criticalparts subtendrelativelysmall The resultsthereforeare indicativeof the upper- solidanglesfrom the pulselocation. Expendable llmit,or worst,case,end will providereference shieldingmass will reducethe sydsm specificim- valuesfor comparisonwith calculationsbaaedupon pulsedirectlyas thismass is tireased; the stretch- otherfusionreactions.Furthmmare,aD+Tburn is ing-outimpliesp@oad lossesdue to addedstructure, the easiestto achieve,axxlthis whoicecouldbe, and,m~e ~ortant~, a possiblepulse-unitdesign , therefore,realistic. problemsinceit -be more difficultto collimate The impliedtotalenergyfor each pulse in the the pulse-unitmass (to utilizeeffectivelydl non- assumedmissionis 220 tons/pulse(&$ in 14.1-MIsV neutronenergyfrcm the pulseas assumedearlier) neutronsand 2@ in non-neutronener~). l%e than If the systemwere more ccmpact. followingsourcetermsresult. l%ereare otherfundamentalconsiderationsIn Neutronenergy/pulse= (0.8)(220)= 1T6 tons such a propulsion system whichalso requireboth or (176)(4183)= 7.36x 105 Mf-sec expendablemass at the energysourceend elongation or .. = 7.36X 10~J of the spacecraft. For example,a propellantmass (7 . 36)(10U~ ;“; &x1024wv0 must be expemiedto ~ovide the impulsiveforce . ‘(1.6) (1.0-13~ “ whichdrivesthe spaceship,ad the shock-absorber - i4.60)(lo24} - Neutrons/pulse 3.26 XI.023 strokemust be long to keep the payloadacceleration (14.1) neutrons/pulse. . within acceptablelimits. Presumably,a maJor

. ● 00 ● * ● ** .:0 ● ● .em

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.0 Udi!w?● ::*::.::* fractionof thispropellantmass can alsobe”?M&I:W:0 ~ .!. : . -. .--O PAYLOAO LOCATION shielding.In addition,typicalpulse-pusherplate WWC R PLATELOCATION (oURI NO PULSE ) separatIon distancee a!xioverallspacecraftdimen- PROPELLANT CONE SOURCS SMOCK STRUCTURE, sionsimpliedby otherconsiderationsare suchas ABSORSER UNUSEO PUUE - .—-~ to provideImportantgeometricalattenuationof the SY$WM uNiTs, ●to J > neutronfluxesthat impingeuponthe pusherplate

end uponthe payload. % ‘“.’+-+=+=

Soresbasicoverallconsiderationsthusbegin Fig. C-1. Basic spacecraftschematic. A to emerge. We are led to considera petit source have undergone one, or at umst a few, scattering of neutronson the uis of a circularpusherplate, eventsescapeinto spaceand can no longerIntercept at a distance,a, fromthe plate as shownIn Fig. the targetma. Ideally,every scatteringevent c-l. l%e plate subtendsa relativelysmellsolid would,with high probability,removethat neutron anglefromthe source,as definedby the angle 9. from considerationand the neutronfieldsin the For maximumshieldingeffectivenessper unitmass eystemwouldthenbe prcducedpredominantlyby un- * (thiswil.lbeelaborateduponlater),the pulse- coXllQedlk-MeVneutrons. As we shellsee later, unitmass or someportionof it is placedin a cone this idealcan be approachedcloselywith a rather of height,1, with its apex at the source.* A p8Y- straightforward,albeitelongated,systemconfigura- loadregionis locatedat a distance,b, from the tion. positionof the pusherplate at the time the fusion The neutronenvironmentalproblemareasas pulse occurs,or at a distancea + b from the neu- currentlyenvisionedfor sucha spacecraftcan be tron source. The interveningspaceis occupiedby S.s classifiedas follows: a shock-absorbersystem,the spacecraftstructure, propellantstorage,etc. 1. Neutronheatingof the pusherplate. 2. Neutronheatingami radiationdamagein the The principelneutron attenuatorsfor each remainderof the spacecraft(the stored main componentcan be identifiedse follows: pulse unitsmay pose a particularproblem). 3. payloaddoses (e.g.,crew shielding). Comporient Attenuators 4. Neutroncapturey-rw effects. (lllismay Pusherplate The “propeU.ant” be en importautfactorIn Items2 and 3.) The separationdistance,a 5. Neutronactivationlevels,particularly Shockabsorber The above frau the standpointof long-termcrew doses. (Thisis closelyrelatedto Item 4). The pusherplate Main spacecraft The above Althoughtheseareasare clearlynot independentof (incl;dingun- one enother,they are being emphasizedin the order usedpulse units)The shock-absorbersystem The distancec given. Most calculationsto date have been directed Payload The above towardItem1; the remainderof this appendixwill Main spacecraftstructure deal mainlywith theseresultsand with somere- Unusedpulse units sultsfor Item 3. The distanced In additionto the .sIngle-scatteringneutronic If the pertinentsolidauglesare kept small criterionmentionedearlier,smallnessof the solid enough,materielattenuationbetweenthe source angledefinedby the angle9 might alsobe deter- end any positionin the system(asmeasuredalong mined on the basisof geometryrequiredto minimize

the x-axis)will be primarilyvia scattering-out * of the solidanglesubtendedfrom the sourceby the ~deed, this is the principalneutronicscriter- componentin question;i.e.,all neutronawhich ion by vhichthe smallnessof the solidanglecan be judged. * *In the remainderof this appendix,“prOpell~t” This listingis in orderof decreasingexpected will mean onlythatportionof the pulseunit seriousness(on a ratherintuitivebasis)from which is In the indicatedshieldingcone. the standpointof overallmissionperformanceand ● O* . conceptualfeasibility. 9** . . ●:O ● *. .. ● : :0- 23

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● .b ● !i!iMi@f● ● * ● ::.*:.:: .:O :.* ● 0 .* ● ““”& C-I

Cone bd.f+agls (9),dog 15 15 u 30301515 15 15 15 6 Prqd.bntmterlal so so ~o ~o so Cl$ ~ %2 LIE ‘~ ~ -at d-Kv (P),LTfCX3 1.0 1.0 1.0 1.0 1.0 O.sl o.91 0.65 0.65 0.65 0.65 1.87 .

P=JPuantCOna length(t),m 10 20303.0202030 20 30 !WW30 R’cpel.lantattemxtlca factor (A) 2.5 6.%? 15.9 2.2 5.0 7.4 19.8 8.0 z?.8 I@ 1339 la? . Frsctimxl solidawl.exubtxudedlW ~nher plxti, $ 1.? 1.7 1.7 6.7 6.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7

&emll ●ttenuatlm fxctor 144 365 935 33 74 433 11.65 459 3339 9e60 ?8,803 6W0 Frxcti.morneut.musatpuxberplxta tintXM emeatlal.lyuncollldml o.g9 O.* 0.80 0.89 O.* 0.94 O.SO 0.95 0.89 0.74 0.66 O.* -1 E - (inA)/l,cx 0.0!3% o.og12 0.0922 o.cq79 0.0803 0.1001 0.0935 0.1040 0.1042 o.mm 0.1029 o.15~2

0.0962 0.0962o.qx2 0.0962 O.* O.uyd0.1051o..Ll360.L156 o.Iv6 0.u56 0.u363 Elq 0.93 0.93 0.96 0.81 0.84 0.95 0.95 0.90 0.9 0.89 O.@ 0.83

Propalxnt-s (m),g 219 5433 WE 391 1320 61.10 @770 37W x/(;.A)3 6P 68 69 43 43 46 45 38

the propellantmass neededto achievea givenneu- As we shall see laterfor severalcasesof tron attenuation.A coro31aryquestion-lees as Interest,Z is (almost)independentof 1 and 16 to the sensitivityof the propellantmass to pro- approximatelyequalto the macroscopictotal pellantshape,for a givenoverallattenuation. (lk-MeV)neutron cross-sectionof the propellant,Zt. To addressthis question,equatIonswere derived Further,if the propel.Umtis a simplecme, its whichrelateboth totalpropellantmesg and the mass is givenby radialvariationof pugher-plateneutroncurrent (c-2) to prope3knt shape(e.g., convexor concavecone face) and to 9. Thesegeometricalrelationships becomecomplex,and overeJloptimizationis not straightforward.However,someprincipalcon- wherepis propellantdensityand A is the propel- clusionsare apparent:(1) the propellantmess is lant attenuation,definedhere as (pusher-plate insensitiveto variationsin the shapeof the conets neutron-ener~currentwith no propellantccae

face (thus,the simplecone is very near3yoptimum), present)+ (pusher-plateneutron-energycurrent end (2) the radialdistributionof pusher-plate with the propellantcone in place). Note that,for heatingIs relativelyflat and also Insensitiveto a givenpropellantattenuationfactor,the propel- the cone shape,es well as to 9. This is true for lantmass dependsvery simp3yupon 8, i.e.,m= W valuesof 13considered,up to 13= 30”. (tan8)2, becausep, Z, endAare independentofe.

We can arrivevia theseoverallconsiderations Sone calculat&l quantitativeresultsmay help at saneusefulshplifiedrelationships.If the to clarlfythe abovediscussions.*mble C-I shows neutronsarrivingat the pusherplate are essen- pertinentresultsfor s- typicalconfigurations tiallyuncollided,the propellantattenuationfec- (seeFig. C-l). tor la very near3yJugt * n Because of the diversity and multidimensional A=e (c-1) character of the geometries of interest,end be- causeof the rathersubstantialattenuationfac- whereZ is a constantappropriateto the propellant tors lilcelytobe encountered,Monte Carlo has been materiel,and 1 is shownin Fig. C-1. selected as the c.elculational method of choice.

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✚✚☛ ✚☛✚ ☛✎☛ ● 0 ●.: .:O ● Fran TableC-I we see the fo3J.owing: ● .{. :o~ TABLEC-II 1. Z is generallywithin1O$ of Et (at least fore _15°). FIGURE-OF-MERITV- FOR VARIOUS SIIUi!LD~GMATERIALS 2. Between70 and lC@ of the neutronsIncident CromSOoticm upon the pusherplateare uncollided,de- .Jt(14.1rev), pendinguponthe propellantattenuation Msterlnl 0/(0 n@3 factor. 2 612 > ~2 0.69 1.4 3. The relative shieldingperformanceper unit 0.65 2.ca mass of the variousmaterialsis.evident, 6Li2 ‘r 90 e.g., 7LlS 8 0.75 2.16 @o . H20: m S 100 (finA)3, g = 15” M 9.0 1.86 1.49 64 ~ 700 (ln A)3, t3 = 30° B lo.a 2.54 1.3k 80 CH2: m: 70 (ln A)3, e =15” c K 2.2 1.30 163 LiR: m> 45 (4nA)3, s = 15” ‘% 14 0.91 2.7 168 Be: m: 40 (enA)3, e =15° qo U 1.0 2.89 2k2 U 2.7 l.p 5& The approximatevelfdityof Eqs. (C-1)and 27 T1 47.9 4.5 2.26 470 (C-2)for the configurationsof TableC-I sIJ.ows Cr 52 7.1 2.k2 197 us to derivea convenientfigureof meritwith Fe 3.14 90 whichto screenpotentitipropellantshielding 55.9 7.9 nl !3.? 8.9 2.a 133 materiels. For thispurposewe assume Cu 63.5 8.9 2.?5 125 Z ~Zt (14.1MeV) Sr 91.2 6.4 4.cA 280 .Q@Q=t m 95.9 8.k 2.1 w M no 96 10.2 4.03 130 whereM is the propellantmolecularweightand Ot cd 1..12 8.6 4,4 223 is the microscopictotalcross-sectionfor 14.1-MeV T8 101 16.6 5.07 165 neutrons. w 184 19.3 5.3 1.12 E% 207 Ilk 5.32 453 Thus, u 23a M.’r 6.23 159 or SinceEq. C-3 Is approximateand,more im- portantly,becausethe relativeimportanceof (n,2n) Also, reactionsvariesconsiderablyfrom speciesto spe- cies,thesevalues are not an exactmeasureof re- lativepropellanteffectiveness.However,they are consistentwith the calculatedrelativemassesin Thus , TableC-I, with Be beingthe predictedbest propel- 0.0752 0 (In A)3 m = (0.6023)3(Pmt/M)3 lant shieldingmaterialon a mass basis. However, . detailedcalculationsindicatethat Be is only about =0.344 (4nA)J ~ . 5 to 10% betterthanLiH. (P et/M)3 For two propdlants,1 end 2, and a givenattenua- AS an approachto examininga more realistic tion,A, v: have pulse unit,attenuationcurves*havebeen calculated for the configurationof Fig. C-2. The variation , ~ [P/( PCJt/M)3]1 . *Here,thepropellantattenuationfactoris defined ~= [o/(oc@312 as (pusher-plateheatingwith no propellantcone present)+ (pusher-plateheatingwith the propel- . The quantity~ is a measureof the propel- (P u /M)3 lant cone). Note the differencefrom the previous lent shieldlng~s requiredto achievethe given definitionbaaed upon neutroncurrent attenuationfactor,A. TableC-II givesvaluesof thisparameterfor severalX%?pHW?d&$V@~$ritQ~e :.O ..

25

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C= be cotiigmed as In Fig. c-2, the pusher-plate PUSHER PLATE Y PROP heatingfromFig. c-k is stilltoo largeby a fec- tor ofN25. (LO: The implicationsare fairlyobvious. Either the pulse-unitshieldlngmass must be substantially , CASE increased(introducinga severeperformancepenalty), or the pusherplatemust have a coolingmechanism that acts duxingthe courseof the propul.sionper- . I +#--- Fig. C-2.Pulse-unitconfiguration. icd (whichmight seriouslycompromiseits struc- of X, ~rw , and m witht iS shownin Fig. tural inte~ity),or the pulse energymustbe ob-

C-3 for a LIH prope~~~. Usingthesecurvesand tainedfrom a fusionreactionotherthanD+T (e.g., the neutronsourceintensi~ presentedearlier D&e). Roughly40 kg of pulse-unitshieldingmess, or- ~olo 11. (appropriateto a D-T energysourceaud the given to 10 J of pusher-platecooling,or referencecase),the pusher-plateheatingcurveof someappropriatecombinationof theseapproachesis Fig. C-k can beplottid. (FigureC-4aJ.eoshows requiredif D+T fusionis to be the energysource. one calculationusingBe as the propellant.) Certainly,if the prisxxryenergycomesfrom the D+3Hereaction,or evenfrom D+D, the pueher-plate The result@ pusher-lateheat load is too heatingproblemis greatlyrelieved;however,no high to be tolerable. It has been estimatedthat quantitativeresultsare as yet availablefor such the pusherplate of Fig. C-2 (uncooled)can survive systems. a neutronheatingload of- 109 J duringthe course of the referencemission. Thus,even if we assume In concludingthis discussion,a few comments perfecthydrodynamiccollimation,so that the en- willbe made regardingthe payloed(crew)neutron tire 20 kg of pulse-unitmass in the referencecase dose to be expectedin the D+T drivensystem. Some Initialcalculatlonalresultsare also available.

I I I I I I I 24 — 2, I I I I I I I 22 —

20 — ~G” ~ASS

18—

16—

, I I I 0.090 ,ocLJJ_LLd 00 lo 20 so 40 50 60 70 00 60 o 4 8 20 24 2S 2,cm iiASS,k; Fig. C-3.Variationwlthk of massesand~ for Fig. C-4U Neutronheatinginpueher platevs configurationof Fig. C-2. . •~~&~ Of canpinents,in c–tiiguration .0 be* .:- ●

26

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●:.:. ● ::. : ●: : .’ ● o ●0: : TABIAC-II; ●:. “.o

PAYLOADBIOLOGICALDOSE CA14ULATIONS Shield Unshielded “Shield”Masses,W PropelJ.antLength (i), P&ell.ant E@fload Shield Thickness Material cm Mass,h Dose,rem Material _&k?& !!z?241Wl?!41

, LIH 20 0.39 7.8 x 108 LiH 205 21,8&3 2790

LIH 20 0.39 7.8 x 108 B 145 44,300 5540

. LIH 50 6.11 4.09 x 107 LiH 175 18,600 2350

LIH 50 6 .U 4.09 x 107 @ 125 38,200 4736

Be 30 3.80 7.02 x 107 LiH 181 19, 2C0 24L0

B 30 3.80 7.02X 107 Be 129 39,4~ II$X20

Be 60 30.40 1.58 x 106 LIH 143 15,200 190

Be 60 30.40 1.58 x 106 Be 102 31,100 3830

Once againit is evidentthat elongationof the Pusher-platethickness= ‘7.62cm spacecraftis advantageous.Indeed,thisproblem Source= D+T neutronsource(describedearlier) is quiteanalogousto the pusher-plateproblemin Flux-to-doseconversionfactor(~ 14-MeVneutrons) thatthe requiredattenuationcan best be achieved = 4.2 X10-8 rem/(neutron/cm2) througha smallsolidangle (subtendedby the pay- t = variable. load)and throughmaterialattenuationvia scatter- The fractionalsolidanglesubt~ded by the ing-outof this solidangle. payloadfrom the sourceis 2.77 x 10 , and for a However,thereare fundamentaldifferencesbe- totaldose of-. 1 rem, the additionalmaterial tweenthesetwo problems. Much largerattenuation attenuationrequiredis factorsmust be attainedfor payloadprotection, A = (k.16)(lo26)(2.~)(lo-4)(4.2)(lo-8) o and the resultingimplicationsare quitedifferent (1.63)(105) from thosefor the pusherplate. Any shielding = 2.97 x 101O. mass thatmust be addedfor payloadprotectiondoes This must be providedby the combinedpropel.lant- not affectperformanceby diminishingthe factorP. pusherplate-spacecraftmateriel,plus any necessary Rather,it simplyreplaces(roughlypound-for-pound) additionalshielding. “useful”payloadthatcan be carried. Furthermore, The actualmaterialattenuation thatmight be Interveningmass (e.g.,storedpulse units)that availablecannotbe calculatedwithoutassumingmany must be carriedanywaywill substantiallyreduce of the spacecraftdesigndetails. However,the (or eliminate)thepayloadshieldingmass that must attenuationavailablein a propellantand pusher be carried. platecan be estimated,whichgivesan indicationof To help put theseconsiderationsin perspec- how much shieldingmust be acccmrplishedby the re- tive,we considerthe “generalized”spacecraftcon- mainderof the system. TO do this,a vacuumbetw~n figurationof Fig. C-1, with the pulse-unitand the pusherplate and the payloadis assumed,end the pusher-plateconfigurationof Fig. C-2. We assume, resulting(upperlimit)payloaddoses are calculated. * consistentwith the aforementionedreferencecase, TableC-III showssuch resultsusingthe configura- 9 = 15° tion shownin Fig. C-2. Also given in TableC-III Pusherplatediameter= 4.5T2m are two estimatedshieldingmasses. Shieldmass . Payloaddiameter= 4.572m (area= 1.63x105cm2) Type (1) Is the estimatedmass of the given shielding a=8.5m materialthatwouldbe requiredat the payload(diam- b=60m eter,4.572m) to reducethe dose to 1 rem. Shield

27

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that these shieldingmassesassumethat no other mass Is locatedbetweenthe pusherplateand the ‘~~~~y$~~~: ‘:~~~~~~”a~~=le

8oSm~ ~x-=1-= x l-T’pE(’) for the shieldingrequire=nts. Indeed,it seems likelythat a much more serloueneutronenviron- . l===< mentalproblemwill be that of protect- the structureand equipment(includingthe lnitisJ- Fig. C-5. Psyloedshieldconfigurations. 25,000kg of storedpulse units)betweenthe pusher mass Type (2) is the correspondingrequirementin plateand the pwlosd location,Wd tmt littleor a shedowshieldplaced15 m from the pusher-plate no additionalshieldingmass wiJl be requiredto positionat the time of the pulse (seeFig. C-5). protectthe payload. Note that l!ype(1) is indicativeof the maximum shieldmass that couldbe implied,whereasType (2)

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