laser ablative propulsion.pdf
TRANSCRIPT
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PROPULSIONSUBSYSTEMSIZINGTOOL:
LASERABLATIVEPROPULSION
AProject
Presentedto
TheFacultyoftheDepartmentofAerospaceEngineering
SanJosStateUniversity
InPartialFulfillment
oftheRequirementsfortheDegree
MasterofEngineering
by
JohnA.DonovanV
May2012
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2012
JohnA.DonovanV
ALLRIGHTSRESERVED
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Abstract
Conventionalpropulsionsystemswillnotfeasiblyallowmankindtoexploreoutsideof
thissolarsystem,oreventhegalaxy. Costsaretoolargetoraisematerialsintospacetobuild
largerobjectsonorbit.Moreefficient,advancedpropulsionsystemsareneededtoenablethe
dreamsoflargeronorbitstructures,possiblecolonizationofotherobjectswithinoursolar
system,andexplorationbeyond. LaserAblativePropulsion(LAP)isoneoftheseadvanced
propulsionsystems,andthetopicofthisreport.
Oneofthemajorissuesconventionalpropulsiontechnologiesisthemassratio. These
technologies(solid andliquidfueledchemicalsystems)relyonalargeamountofpropellant
carriedonboardtopropelarelativelysmallamountofmassintospace. LAPandother
advancedtechnologieslooktoimproveefficiencybyremovingpropellantweightfromthecraft,
whilestilldeliveringtherequiredenergytoachievethedesiredincreaseinvelocity(v).
Thisisaninnovativetechnology,withverylittleresearchandfieldtestingoflarge
systems. Someexperimentsandanalyseshavebeendocumentedonasmallscalewithspecific
solidmaterials. Thegoalofthisreportistofurtherunderstandingofthistopic,anddevelopa
tooltobeginsizing/tradingthistechnologyforthepropulsionsubsystem. UsingConservationof
Energyequations,thesimplerelationshipsbetweeneventsintheenergytransferchainwillbe
describedanddiscussed.
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AcknowledgementsIwouldliketothankmyfatherforalwaysenablingandencouragingmetostudyand
learnmore. HefosteredmyloveofouterspaceandtalentforengineeringsinceIwasachild.
IwouldliketothankDr.AlecGallimore,professorattheUniversityofMichigan
AerospaceEngineeringdepartment. Hetaughtandinspiredmetothinkofmoreadvanced
propulsionsystemsandthedoorstheywillopenforhumankind.
JerryOckermanandBlakeHaufofLockheedMartinwereinstrumentalinadministering
theSJSUoncampusprograms,enablingmetoearnthisdegreewhileworkingintheindustry.
Dr.NikosMourtos,Dr.PeriklisPapadopoulos,andMs.CandySimelassistedmewith
manyhurdlesandmuchpaperworkthatwererequiredtoenablemyworkonthisproject,and
finallycommencemydegree.
IwouldliketothankNikDjordjevicandLeeLundsfordfortheirguidanceontheresearch
andpresentationofthematerialscontainedwithinthisreport.
Also,thankstomygirlfriendwholetmetaketimetoworkonthisproject,whilestill
ensuringIhadenoughtimetopayattentiontoher.
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TableofContents1 INTRODUCTION........................................................................................................................ 1
1.1 Motivation........................................................................................................................ 1
1.2 HistoryandBackground................................................................................................... 2
1.3 LaserAblativePropulsionConcept.................................................................................. 3
1.4 PotentialBenefitsandApplications................................................................................. 7
2 THEORY.................................................................................................................................... 9
2.1 LaserProperties............................................................................................................... 9
2.2 ConservationofEnergy.................................................................................................. 12
2.3 RocketPropulsion.......................................................................................................... 13
2.4 LAPSubsystemDesign................................................................................................... 16
3 PROPELLANTEVALUATIONANDSELECTION......................................................................... 18
3.1 PropellantsConsidered.................................................................................................. 18
3.2 PropellantProperties..................................................................................................... 19
3.3 FuturePropellantConsiderations.................................................................................. 21
4 BENCHMARKING.................................................................................................................... 22
4.1 CorrelationtoExperimentalResults.............................................................................. 22
4.2 DiscussionofDifferences............................................................................................... 29
5 PARAMETRICAPPROACH....................................................................................................... 31
5.1 ToolParameters............................................................................................................. 31
5.2 Determiningpropulsivecharacteristics......................................................................... 32
5.3 ToolLayoutandCalculations......................................................................................... 33
5.4 FutureAnalyses.............................................................................................................. 36
6 DISCUSSIONANDRESULTS.................................................................................................... 38
6.1 DiscussionofContinuousWavevs.PulsedLaserPower............................................... 38
6.2 ToolAnalysisResults...................................................................................................... 396.3 FutureStudy................................................................................................................... 40
7 CONCLUSIONANDRECOMMENDATIONS.............................................................................. 41
REFERENCES................................................................................................................................... 42
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ListofFiguresFigure1 GeometryofLaserIrradiationonaSolidSurface 5
Figure2 PhaseChangeExampleforWater(H2O) 6
Figure3 LAPThrustConceptVaporizationofLiquefiedSolidMaterial 7
Figure4 PulsedLaserOperation 10
Figure5 Asurveyofreportedmaximumcouplingintensities 11
Figure6 ConservationofMomentumVisualExample 14
Figure7 LAPConceptSequenceofEvents 17
Figure8 Ablatedmasspershotvs.irradianceonPOM 23
Figure9 Momentumofprojectilevs.massreductionbyablation 25
Figure10 ElectronandLatticetemperatureforCoppertarget 26
Figure11 Surfacestructuralfeaturesinducedbytrainsoflaserpulses 27
Figure12 Surfacestructuralfeaturesinducedbytrainsoflaserpulses 28
Figure13 Surfaceripplesinducedbytrainof17laserpulsesforAg(a)andCu(b) 28Figure14 Beamprofilerecordedontothermalpaper 32
Figure15 ToolscreenshotwithLaserParameterInputsforAluminumpropellant 34
Figure16 ScreenshotofGeneratedDataforAluminumpropellant 35
ListofTablesTableI Aluminum(Al)Properties 19
TableII Copper(Cu)Properties 19
TableIII Silver(Ag)Properties 19
TableIV Silicon(Si)Properties 20
TableV Uranium(U)Properties 20
TableVI DelrinProperties 20
TableVII PercentageofLaserPowerforHeatFlow 24
TableVIII MaterialAnalysisResultsforOne200ns,1x109W/cm2Pulse 25
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NomenclatureA beamspotsize(m2)
CP specificheatatconstantpressure(J/kgK)
dm depth(thickness)ofmaterialablated(m)E changeinenergy(J)
E totalenergy(J)
Ep energyperpulse(J)
F thrust(N)
f pulsefrequency(1/s)
g0 gravitationalacceleration
Isp specificimpulse(s)
ma massablated(kg)
mi initial,orwetmass(kg)
mf final,ordrymass(kg)
n numberofmolesP laserpower(W)
PPeak peaklaserpower(W)
PAvg averagelaserpower(W)
p pressure(N/m2)
R specificgasconstant(J/kgK)
Ru universalgasconstant(8.314J/molK)
T0 initialtemperature(K)
Tc chambertemperature(K)
Te exit(ambient)temperature(K)
TM meltingtemperature(K)
T incrementaltemperature
t time(s)
t timeincrementorlaserpulsewidth(s)
V volume(m3)
Vv volumeofmaterialvaporized(m3)
ve exitvelocity(m/s)
vrms electronmeanvelocity(m/s)
Greek
incrementorchange
density(kg/m3)
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1 INTRODUCTION1.1
Motivation
Conventionalpropulsionsystemswillnotfeasiblyallowmankindtoexploreoutsideofthis
solarsystem,oreventhegalaxy. Costsaretoolargetoraisematerialsintospacetobuildlarger
objectsonorbit.Withoutanachievableandaffordablespacetransportationsystem,
explorationofspacewillnotreachitspotential.Moreefficient,advancedpropulsionsystems
areneededtoenablethedreamsoflargeronorbitstructures,possiblecolonizationofother
objectswithinoursolarsystem,andexplorationbeyond.
Oneofthemajorissuesfacingconventionalpropulsiontechnologiesisthemassratio.
Thesetechnologies(solid andliquidfueledchemicalsystems)relyonalargeamountof
propellantcarriedonboardtopropelarelativelysmallamountofmassintospace. Laser
propulsionandotheradvancedtechnologieslooktoimproveefficiencybyremovingpropellant
weightfromthecraft,whilestilldeliveringtherequiredenergytoachievethedesiredincrease
invelocity(v).
Thisisaninnovativepropulsiontechnology,withverylittleresearchandfieldtestingof
largesystems. Someexperimentsandanalyseshavebeendocumentedonasmallscalewith
specificsolidmaterials. Thegoalofthisreportistofurtherunderstandingofthistopic,and
developatooltoanalyzetheconceptformultiplematerials.
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1.2 History and BackgroundConventionalrocketpropulsiondependsoncarryingtheentireamountofenergy
requiredforthemission. Thisisamajorcontributortothelargepropellantmassratio. Typical
missionscontainupwardsof85%ofthevehiclemassaspropellant. Theseenergyrequirements
varydependingonthetypeofmaneuverstobeexecuted:orbitraisingorinsertion,
orbit/attitudecontrolanddeorbitingattheendofasatelliteslifetime.
Forfuturemissionssuchasdeepspaceexploration,thepowerrequirementforafixed
payloadmasssimplyscaleswiththedistance:typically200kWforareturnmissiontotheouter
solarsystem,200600kWforacargotugtoMars,orover1MWforamannedMarsmission[1].
Carryingthisamountofpoweronboardcannotbeachievedwithtodaysconventional
propulsionsystems. Thesetypesofmissionswouldrequireestablishmentofanorbiting
platformfromwhichtobuildlargerstructuresforlongrangemissions. Theenergyrequiredto
beginbuildingsuchaplatform,keepitonorbit,andmanthisplatformwouldbeimmense!
Then,transportationofmaterialstothisplatformcanbeginforconstructionofalargescale
spacecraft.
Clearly,futuremissionswillneedtodevelopmuchmoreefficientenergyusageintheir
propulsionsystems,ortheywillneedtoharnessexternalenergysources. Theuseoflasersto
beampowertothespacecraftprovidesafavorablealternative. Sincethereisanexternal
energysource,thespacecraftcanshedthelargeofthepropellantweightrequiredby
conventionalpropulsionsystems.
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Oneofthemajoradvantagesoflaserpropulsioncomparedtotraditionalchemical
propulsionisthatpracticallyanyobjectofcondensedmattercanbeusedasafuel. Aslongas
thematterwillablatewhenalaserisfocusedonit,theenergeticexhaustwillactasthrust. This
ablationexhaustwillbeintheformofvaporizedmatteruntiltheplasmagenerationthreshold,
whereplasmaswillbegenerated.[2]
Conventionalpropellantsarematerialsthatareexplosive,corrosive,and/orpoisonous,
makingthemgenerallydangerousanddifficulttohandle. Sincenearlyanymaterialcanbe
selectedasthepropellantforthistypeofsystem,safetyisnotalargeconcern.Thisfactoralone
willdrivedownthecost,complexity,andsocialpressuresagainstconventionalpropulsion
systems,aswellasadvancedsystemsusingsomeformofnuclearpower.
Theperfectpropellantforlaserpropulsionisnotknownatthistime,duetolimited
researchinthearea. Researchconductedsofarhasnotidentifiedwhichmaterialproperties
leadtothebestthrustorspecificimpulseforagivenmission.
1.3 LaserAblativePropulsionConceptLaserAblativePropulsion(LAP)canresultinvaryingphysicalmechanisms,includingthermal
(vaporization,explosiveboiling)andnonthermal(plasmagenerationandacceleration)[3]. In
thisreport,thethermalaspectsofthisprocesswillbeassessedduetotimeconstraintsforthe
explorationoftheplasmaphysicsatthistime. Theplasmaphysicsareanareaoffurtherstudy
forfollowonprojects.
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ThebasicsofLAPcanbedescribedinthefollowingenergytransferchain:
Laserenergyistransmittedthroughspace
Thisenergybeingtransmittedbylaserdoesnotpassthroughanyatmosphere,orvery
littleatmosphere,sothatahigherpercentageofthepowertransmittedisusableto
generatepropulsionforthespacecraft. Furtherdiscussionsoftheselossesareincluded
inSection2.3titledConservationofEnergy.
Futurestudy: Theinstallationofvariouslaserpowerstationsonorbitoronthesurface
ofothercelestialbodieswillenabletravelthroughthespacewithoutrequiringlarge
amountsofpropellant.
Laserisfocusedontothesolidmaterialthatactsaspropellant
ThiscanbeANYmaterial! Thisincreaseseaseofmanufacturingandavailability,and
decreasessafetyconcerns,costs,handling,permits,storagerequirements,etc.
Laserheatssolidmaterialtomeltingpointandchangesphasetotheliquidstate
Thepropellantisexposedtothevacuumofspace,sotheinitialtemperatureofthe
propellantisassumedtospaceambient,ornearabsolutezero(Approximatedat3.15
Kelvin). Themaximumtemperatureachievableforthisstateofmatterwillbethe
meltingpointofthematerial,atwhichpointthesolidtransformstoaliquid(with
thicknessS(t)asseeninFigure1below:
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Figure1: GeometryofLaserIrradiationonaSolidSurface
S(t)=0priortothelaserbeamheatingthesolidmaterial(t=0). Oncethelaserenergy
isdeliveredtothesurface,thesurfaceofthesolidismeltedandchangesphasesto
liquid.
Liquidmaterialisvaporizedtothegaseousstate
Themaximumtemperatureachievableforthisstatewillbetheboilingpointofthe
material,atwhichpointthematerialablatesintheformofgas. Theliquidregionseen
inFigure1willvaporizetoagaseousstateintheoppositedirectionofthelaser.
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Inaccordancewiththephasechangefigurebelow,theenergyrequiredtovaporizethe
solidmaterialcanbedeterminedbyintegratingtheenergyequationsforeachregionof
thermalabsorption: Heatingofsolid,heatoffusion,heatingofliquid,andheatof
vaporization.
Figure2: PhaseChangeExampleforWater(H2O)
GasmovesawaywithVelocity(VE)
Theadditionofenergy(intheformofheat)excitesthemoleculeswithinthematerial,
causingthemtoincreasetheirvelocityastheyexittherearofthespacecraft. Thisgasis
expelledtoprovidethrustforthespacecraft.
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Figure3: LAPThrustConceptVaporizationofLiquefiedSolidMaterial
Inthisreport,thefocusiswhatoccurswhenhighlypressurized,heatedmaterialexpands
intovacuumandtransformsintoagaseousstate.
1.4 PotentialBenefitsandApplicationsThebenefitsofLAPincludeadramaticweightsavingsintermsofpropellantneededfor
themission. Asaresult,morepayloadcanbedeliveredforasimilarsizedcraft. Another
approachistominimizethesizeofacrafttoachieveasimilarmission. Thiscanalsoincrease
thefrequencyofmissions,assystemsbecomelesscomplexandcanbemanufacturedona
largerscale.
Initialmissionscouldbepowergenerationsatellitesand/orplatforms,whichcouldabsorb
theraysofthesunwhileonorbit. Thispowercanbeconvertedintolaserenergyand
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transmittedtospacecraftthatarefartherfromthesun,wheresolarpowerislessfeasible,or
notfeasibleatall.
BenefitsofLasersforEnergyTransmission
Thedevelopmentofprogressivelymorepowerfullasersiscloselyrelatedtothe
advancementofthistechnology,butwillnotsolelybenefitLaserAblativePropulsion.
Astheselaserpowersystemsgrowmorepowerful,theyprovideawidevarietyofpossibleuses:
Beamedenergytoearthfromspace
Beamedenergyfromspacetospace
Shortrangesatellitedefensefromincomingspacedebris
Beamedlaserlightreflectedoffasatellitecouldprovidegroundtogroundenergy
transmission
Beamedlaserlightreflectedoffasatellitecouldprovidedaylighttoanotherareaofthe
earth
Atmosphericapplicationsinclude:
Boostphasemissiledefense(AirborneLaserTestBedALTB)
Laserpoweredaircraft
Highpowereddata/signaltransmissions
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2 THEORY2.1 LaserProperties
ThewordLASERstandsforLightAmplifiedbyStimulatedEmissionofRadiation. Energy
istransferredintheformoflightwhenenergyisremovedfromtheelectronatacertain
potential. Absorptionoflaserenergy(light)willcauseanelectrontobebumpeduptothenext
highestlevel. Sincelaserstransmitenergyintheformoflight,thetargetrequiresadirectlineof
sight,andanydisturbancesinthemediumitistravellingthroughwillreducetheenergy
transmitted.
Typicallaserproperties
TheenergytransmittedbyalaserismeasuredinJoules(J). Lasersystemsaretypically
ratedintermsofthepower(P,inWatts)thattheycandeliver. Powerisameasureofthelasers
rateofenergytransfer,inJoulespersecond(J/s). Acommonlyusedpropertyisthefocused
intensity(I,inW/cm2). Thisparameterdescribesthepowerofthelaserdeliveredtoaspecific
area,whichisusefulindeterminingthepowerdeliveredforaparticularbeamsize. Thisbeam
spotsizeisdenotedasanarea(A,inm2).
Thereareseveralmethodstodescribethepoweroutputofalasersystem. Peakpower
andaveragepowerdescribeusefulcharacteristicsforanenergyfocusedviewoflasers. This
projectisspecificallyexaminingthelaserpropellantinteraction,soanassumedamountof
energyisdeliveredtothesurfaceofthepropellant. Thedesigndetailsregardingenergylosses,
pointingrequirementsandotherconsiderationsaretopicsforfuturestudiesonthistechnology.
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Therearemanyotherpropertiestoconsider,dependingontheparticularapplicationof
thelasersystem. Examplesare:beamdiameter,spatialintensitydistribution(beamprofile),
divergence,andbeamqualityfactor[4].
Whyuseapulsedlaser?
Oneadvantageofpulsedlasersforthistechnologyisthatenergyisdeliveredin
consistentlysizedpulses,withenoughtiminginbetweensothatthepropellantreactionis
completebeforethenextpulseisdelivered. Thisallowsthereactiontoremainina
thermodynamicstate(solidtogasreaction)versushighertemperaturesandionizationthat
wouldleadtoplasmageneration.
Pulsedlasersalsoenablehigherenergyleveldelivery. Powersourcescangeneratea
fixedamountofpoweroveronewholesecond,butahigherlevelofpowercanbeachievedfora
fractionofthatsecond. Inthefigurebelow,aconstantenergylevelisdeliveredperpulse(Ep).
Thefrequencyofrepetitionisdefinedbyf=1/t.
Figure4: PulsedLaserOperation[5]
PeakPowerisdefinedastherateofenergyflowineachpulse,ofdurationt.
(1) PPeak=Ep/t
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Foralaserenergyof0.01Jandpulsedurationof8nanoseconds,thePeakPowerisasfollows
PPeak=0.01J/.000000008s=1,250,000Wor1.25MW
AveragePowerisdefinedastherateofenergyflowaveragedovereachperiod,ofdurationt.
(2) PAvg=Ep/t
Forafrequency(f)of10,000Hz,thedurationbetweenpulsesis0.0001seconds,whichimplies
PAvg=0.01J/0.0001s=100W
Whyshorterpulses?
Practicallyspeaking,shorterpulsesallowthepowersourceofthelasertocoolbetween
pulsessothatitdoesnotoverheat,andcanoperateforalongerduration. Asseeninequation
(1)above,ashorterpulseyieldshigherPeakPowerdeliveredtothesystem. Thefigurebelow
showshowthemaximumintensityofalaserdecreasesasthelaserpulsedurationincreases.
Therefore,shorterpulseswillenablehigherintensitiestobeachieved.
Figure5: Asurveyofreportedmaximumcouplingintensities[6]
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2.2 ConservationofEnergyTransmittedlaserenergyabsorbedbythepropellantisbasedontheabsorptivityofthe
surface. Somesurfacecoatingswillreflectmoreenergythantheyabsorb. Othersaredesigned
tomaximizeabsorptivity. Inthisstudy,thesurfaceisassumedtoabsorballdirectedenergy.
Possibletypesofenergylosses
Therearenumeroustypesofenergylosswhichoccurfromthetimetheenergyisgenerated
untilitisdeliveredtothetarget. Theseinclude,butarenotlimitedto:
Energyconversiontolightlosses
Imperfectfocusingofthebeam
Heatenergylostintransmissionthroughspace
Oncetheenergyhasreachedthecraft,therearemoreopportunitiesforenergylosses
topresentthemselves:
Imperfectreflectionofthecollectoronthespacecraft
Focusedenergyreflectedbythepropellant(Absorptivity
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Energyeffectonsolidmaterial
Typicalphasechangesprogressfromsolidtoliquidtogas(thenplasma,butthiswillnot
beexaminedinthisreport). Intermsofenergy,thematerialwillheatasasolidaccordingtothe
specificheatcapacityofthematerialinsolidform. Oncethemeltingpointisreached,aphase
changeoccurs. Thisphasechangefromsolidtoliquidrequiresanenergyinputcalledthe
enthalpyoffusion. Duringthisphasechange,thereisnochangeintemperature,eventhough
thereisheatbeingaddedtothematerial. Oncemelted,thematerialwillheatasaliquid
accordingtothespecificheatcapacityofthematerialinliquidform. Oncetheboilingpointis
reached,asecondphasechangeoccurs. Thisphasechangefromliquidtogasrequiresan
energyinputcalledtheenthalpyofvaporization. Thetemperatureremainsconstantduringthis
phasechange,butthenheatingcontinuesaccordingtothespecificheatcapacityofthematerial
ingaseousform. EnthalpyisdefinedastheEnergyInput(laserenergyinthiscase)dividedby
themassofthematerialandthechangeintemperature.
2.3 RocketPropulsionConservationofMomentumisthefoundationforthebasicthrustequationsutilizedinthis
paper. Theconservationofmomentumsaysthatinthebelowfigure,thefollowingequation
holdstrue:
(1) (mm)*(v+v)=m*ve
Theunitsofmomentum(orimpulse)arekgm/s.
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Figure6:ConservationofMomentumvisualexample
Thrust,aforcemeasuredinNewtons(N,orkgm/s2),isthederivativeofmomentum(impulse).
Byapplyingthetimederivativetotheexitvelocity(ve),theequationbecomesthestandard
forceequationofmasstimesthesecondderivativeofposition,whichisacceleration.
(2) F=m*
(ve)=F=m*acceleration
Sincetheexitvelocityismoreeasilydeterminedthantheaccelerationofparticlesleavingthe
spacecraft,thetimederivativeisappliedtothemassofthepropellantinsteadoftheexit
velocityasshownbelow
(3)
(m)*ve=*ve
Withaconsistentpulsedlaserenergysource,themassflowratewillbeveryconstantand
predictable. Sincemassflowrateisdefinedasmasspersecond,thentheaveragematerial
ablatedpersecondwouldbeusedinthisequation. Theexitvelocityisalsorelatedtothe
specificimpulse(Isp)bythegravitationalaccelerationoftheobject(g0)
4 ve Isp* g0
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ThrustEquations
Chemicalpropulsionsystemshaveathermodynamiccharacterization,whichrelates
temperature in the chamber to the temperature at the exit, shownbelow
5 Egas Cp* mp* Tc Te
Startingfromthekineticenergyequation
6 E * m * v2
Thenrearrangingtosolveforvelocity
7 v 2 * E / m0.5
Combiningequations(7)and(9)provideanewrelationforvelocity
8 v 2 * Cp* Tc Te 0.5
Equation 10 states the variables affecting velocity in a constant pressure condition, where
there is only heating of the material.
IdealRocketEquationTheIdealRocketEquationapplieswhenacraftcanapplyaccelerationtoitselfby
ejectingsomeofitsmassatahighspeedintheoppositedirection,asgovernedbythe
conservationofmomentum. Theequation,derivedbyTsiolkovsky,statesthatthechangein
velocityofacraftisproportionaltothemassratio(initialmass,includingpropellant,dividedby
thefinalmass).
9 v veln mi/ mf
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Thisequationisusefultoshowusthelimitationsofconventionalpropulsionsystems.
Thebasisofthisequationisthatthecraftistheonlysourceofenergy. Therefore,thecraft
needsalargeamountofpropellanttogeneratetheenergynecessarytoachieveacertainvalue
ofv.Withalaserpoweredpropulsionsystem,theenergyisnotstoredonthecraft,and
thereforesignificantlyreducestheinitialmassofthecraft.
Ifthesamevcanbeachievedwithamuchsmallerinitialmass,thenthecraftcanbe
sizeddowntoperformthemission. Anotheroptionistoincreasethepayloadofthemission,
whichwouldrequireasimilaroverallsizeofthecraft,butahigherpercentageofthemass
wouldbepayload.
2.4 LAPSubsystemDesignThedesignofthespacecrafttakesonsomeuniquecharacteristicsduetotheadvancesin
technologythatcomewithusingtheLAPsystem. Thefirstexampleisthesignificantreduction
ofonboardpropellantstorage. Thelaserenergycapturewillrequirereceivingopticsmounted
onthespacecraftfacingthedirectionofthelasersource. Alimitationtousinglaserenergy
transmissionisthatdirectlineofsightisnecessary. Oncethelaserenergyiscaptured,focusing
mirrorswillshinethelightontothepropellantsurface.
Duetotheequalheatingwithinthelaserspotarea,theangleofincidencebetweenthe
laserandthesurfaceofthepropellantisnotafactorinthedirectionofthrust. Theablationof
propellantmaterialwillejectmaterialnormaltothesurfaceofthepropellant,inagaseous
form. AstudyontheeffectsofdifferentconicalnozzlesontheperformanceofanLAPsystem
[7]describestheperformanceimprovementsthatcanbeachievedbytheuseofanozzleonthe
gasplume. However,theuseofanozzle,whichnozzleworksbest,andgasperformanceare
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outsideofthescopeofthispaper. Intheconclusionsectionofreference7,itisstatedthatthe
ablatedmassperareaappearedtoremainapproximatelyconstant. Thisisfurtherproofthata
nozzleisnotnecessaryinanLAPsystem.
Oneconcept[8]tookthenozzlelessapproachevenfurther. Amicrothrusterwas
developedandtestedthatutilizesapropellanttapethatispassedoveranonboardlaserfor
thrustgeneration.
Figure7:LAPConceptSequenceofEvents
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3 PROPELLANTEVALUATIONANDSELECTION3.1 PropellantsConsidered
Severalexperiments[3,9,18,19]wereconductedusingAluminum(Al),Copper(Cu),
Silver(Ag),and/orSilicon(Si)aspropellants. Theseexperimentsusedshort(femto tomilli
second)laserpulsestodeliverenergytothematerials. Inadditiontothosepropellantsfoundin
experiments,Uranium(U)wasaddedtounderstandthebehaviorofamuchheavierelement.
Propellantdesign
Nonozzleisusedinthedesign,sothecrosssectionalareaofthepropellantisequaland
theareaoftheexitareequal. Theambientpressureisthevacuumofspace,whichhasan
approximatevalueof0Pa Thepropellantforthiscraftisshapedsuchthattheareaheatedby
thelaserenergyisexactlythespotareaofthelaser. Thiswillensureevenheating,a
consistentlysizedheatingsurface,andacontinuousthrustlevel. Thelaserfocusing/pointing
systemcanaccountforthedistancefromsourceandanglefromsourcetoalwaysfocusthe
energyonthepropellantface.
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3.2 PropellantPropertiesTablesinthissectionoutlinerelevantmaterialpropertiesforeachpropellantselected.
TableI:Aluminum(Al)Properties
Property Value Units Source
Molar Mass 0.02698 kg/mol [10]
Specific Heat 900 J/kg-K [10]
Density 2702 kg/m^3 [10]
Enthalpy of Fusion 395478 J/kg [10]
Enthalpy of Vaporization 10885841 J/kg [10]
Melting Temperature 933.4 K [10]
Boiling Temperature 2740 K [10]
TableII:Copper(Cu)Properties
Property Value Units Source
Molar Mass 0.06355 kg/mol [11]
Specific Heat 384.56 J/kg-K [11]
Density 8960 kg/m^3 [11]
Enthalpy of Fusion 204721 J/kg [11]
Enthalpy of Vaporization 4793076 J/kg [11]
Melting Temperature 1357.75 K [11]
Boiling Temperature 2840 K [11]
TableIII:Silver(Ag)Properties
Property Value Units Source
Molar Mass 0.10787 kg/mol [12]
Specific Heat 235 J/kg-K [12]
Density 10500 kg/m^3 [12]
Enthalpy of Fusion 104756 J/kg [12]
Enthalpy of Vaporization 2364884 J/kg [12]
Melting Temperature 1234 K [12]
Boiling Temperature 2436 K [12]
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TableIV:Silicon(Si)Properties
Property Value Units Source
Molar Mass 0.02809 kg/mol [13]
Specific Heat 710 J/kg-K [13]Density 2330 kg/m^3 [13]
Enthalpy of Fusion 1653257 J/kg [13]
Enthalpy of Vaporization 15628337 J/kg [13]
Melting Temperature 1683 K [13]
Boiling Temperature 2628 K [13]
TableV:Uranium(U)Properties
Property Value Units Source
Molar Mass 0.23803 kg/mol [14]
Specific Heat 120 J/kg-K [14]Density 18950 kg/m^3 [14]
Enthalpy of Fusion 65034 J/kg [14]
Enthalpy of Vaporization 2003949 J/kg [14]
Melting Temperature 1405 K [14]
Boiling Temperature 4407 K [14]
TableVI:DelrinProperties
Property Value Units Source
Molar Mass - kg/mol -
Specific Heat 1450 J/kg-K [2]Density 1410 kg/m^3 [2]
Enthalpy of Fusion - J/kg -
Enthalpy of Vaporization 172000 J/kg [2]
Melting Temperature 448 K [2]
Boiling Temperature - K -
ThereisnotenoughinformationabouttheDelrinmaterialtoincludeitinthisstudy,butit
islistedasadditionalinformationwhichmaybeusefulinafuturestudy.
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3.3 FuturePropellantConsiderationsOnlysolidmaterialswereconsideredatthistime,butliquidscouldalsobeutilizedina
systemsuchasthis. Theliquidpropellantswouldhavetobeanalyzedtodetermineiftheir
materialpropertiesweremoresuitabletothisapplication. Thedesignofthesystemwouldbe
morecomplex,includingpressurizationtanks,spraynozzles,etc. Oneimmediatelyobvious
advantageofusingliquidpropellantisthataconsistentsprayofpropellantcanbe
accomplished. Also,theseliquidswouldrequirelessexternalenergytovaporizeintothe
gaseousstate.
Therearemorepossibilitiesforcreatingandtailoringmaterialstofittheneedsofthe
mission. Ifahigherdensitymaterialisdesired,whilestillworkingwithliquids,thenhighdensity
solidscanbedissolvedinacidtoyieldahighdensityliquid.
Advancedsolidmaterialswithtailoredproperties
Liquidswithhighdensity,molecularweight,etc.
AdvancedmaterialsperNASAInSpacePropulsionSystemsRoadmap[15]
Evensolidwater(H2O)wasdiscussedinafewstudies.Waterhasthehighestspecificheat
ofANYknownmaterial. Asmentionedabove,itisunknownwhichpropertiesmaketheperfect
propellant,butsolidwaterwouldbeagoodwaytoexaminespecificheatasaperformance
contributor.
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4 BENCHMARKING4.1 CorrelationtoExperimentalResults
Thetoolcannotbecheckedforcorrelationwithanyfullsizeexperimentorstudy,
becausethisconcepthasnotbeentestedinspace. Thesmallscaleexperimentsavailablewillbe
utilizedtodiscusstheunderstandingoftheprocess,despitetheresultsnotcorrelatingtothe
conditionspresentinthistool. Sincethereisnodirectcorrelationforthisstudy,thisreportwill
discussthedifferencesofthoseexperiments,aswellaswhattheyimplyaboutthefeasibilityof
theconditionspresentedinthistool.
Severalorganizationshaveconductedexperimentsusingsmallscalelaserpowerlevels
ablatingtinyamountsofmaterialwithapulsedlaserapproach. Themostcommonpower
sourcewasaCO2laser.
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Onestudy[2]examinedthepropulsivecharacteristicsofPolyoxymethylene(POM),a.k.a.Delrin
astheyarerelatedtolaserirradiance. Thisstudyassessedveryrelevanttopicssuchas
momentumcoupling,specificimpulse,efficiency,andablatedmasspershot,whichisseenin
thefigurebelow:
Figure8:Ablatedmasspershotvs.irradianceonPOM[2]
ThevaluesinTableVIIbelowareagoodmeasureoftheactualpowerlossesdueto
conversionoflaserenergyintoheating. Thisinformationwasntincorporatedintothetool,
becausetheywereperformedwithinatmosphere,andothertestconditionsmayhavebeen
presenttofurtheralterthesevalues. Futurestudiescanlookatreplicatingthesetestsina
vacuumenvironmentattemperaturesthataremorerepresentativeofthespaceenvironment.
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TableVII: Percentageoflaserpowerforheatflow[9]
Specimens
Spotsize
(mm)
Averageincident
laserpower(mW)
Peakfluence
(mJ/cm2)
Percentageoflaser
powerforheatflow(%)
Steel 0.3 26 74 76.9
50 141 74
90 254 67.7
150 424 70
210 594 67.6
Silicon(a) 0.3 26 74 69.2
50 141 82
66 186 87.8
150 424 68210 594 66.7
0.6 30 21 73.3
75 53 73.3
120 84 72.5
170 120 70.6
300 212 68.3
Copper(b) 0.3 90 254 77.8
150 424 86.7260 736 50
300 849 50
467 1321 50.3
Fromtheabovetable,theexperimentalaveragepercentageofpowerforheatflowisasfollows:
Steel(with0.3mmdiameterbeam)is71.24%
Silicon(with0.3mmdiameterbeam)is74.74%
Silicon(with0.6mmdiameterbeam)is71.60%
Copper(with0.3mmdiameterbeam)is62.96%
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AnexperimentoutlinedinReference[15]documentsaccelerationduetoalaser
inducedshockimpulse. However,itwasdiscoveredthroughexperimentationthataprojectile
constructedofpolyacetal(POM)withmassof1.36gwaspropelledatavelocityof32.2m/sin
atmosphere. Thiswasaccomplishedbyablatingamassof10.7mgofmaterialperpulse.
Withoutthespecificmassoftheprojectileandmassablatedateachpointintime,the
momentumoftheablatedmaterialcannotbedetermined.
Figure9:Momentumofprojectilevs.massreductionbyablation[15]
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Figure10:ElectronandLatticetemperatureforCoppertarget[16]
Figure10showsthetimedependenceofelectronandlatticetemperatureofsurfacefor
coppertargetirradiatedby60fs,800nmpulseatdifferentlaserenergyirradiancelevels.
Higherirradiancesleadtohigherequilibriumtemperaturesduetohigherenergylevelsbeing
addedtothesystem.
Thisshowsthatitispossibletoaddalargeamountofenergyandraisethetemperature
ofanobjectveryquickly. Forexample,witha200nanosecondpulse(200,000picoseconds),the
temperatureofthematerialwillcertainlyequalizeatahighenoughvaluetovaporizethe
propellantmaterial.
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Figure11:Surfacestructuralfeaturesinducedbytrainsoflaserpulses[17]
Theabovefigureshowsthesurfacestructuralfeaturesinducedbydifferenttrainof
pulsesat0.8mJpulseforCopper. Thenumberofpulsesusedforeachpictureare(Nisnumber
ofpulses):(a),(a)N=4;(b),(b)N=17. Thebelowfigureshowsthesameforalargernumberof
pulses:(c),(c)N=67;(d),(d)N=500.
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Figure12:Surfacestructuralfeaturesinducedbytrainsoflaserpulses[17]
Figure13:Surfaceripplesinducedbytrainof17laserpulsesforAg(a)andCu(b)[17]
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Figure13showsthesurfaceripplesinducedbytrainof17femtosecondlaserpulsesat0.8
mJpulseforSilver(a)andCopper(b). Thisstudywasexaminingthesurfaceripplesthat
occurredonthemetalsurfaces,butillustratesthebehaviorofLAPsurfaceinteractionsona
smalltimescale.
4.2 DiscussionofDifferencesTheexperimentsconductedbytheseorganizationswerewitnessingtherealworld
powertransmissionlossesthatwerediscussedinsection1.3,aswellasatmosphericconditions
whicharenotpresentinthespaceenvironmentforwhichthistoolisbeingdesigned.
Forthisstudyinreference[2],amaximumof20Jperpulsewasdelivered,which
resultedfromamaximumirradianceof500W/cm2. Itisdifficulttocorrelatetothisdata,
becausethecorrelationbetweenIrradianceandenergydeliveredisdependentonthebeam
size,andareabeingirradiated. FromFigure10,itappearsthattheirradiatedareawasa150mm
x150mmsquare,whichwouldbe15cmx15cmsquare,oranareaof225cm2.Withthis
information,since1W=1J/s,itcanbeestimatedthatthetimeofthepulsewasequalto500*
225/20=5625s1,soeachpulsewas0.00018s,or180s. Thisseemslikealongerpulsethan
mostotherstudies,sothemissinginformationaboutthebeamprofileisnecessarybeforeitcan
bedeterminedwhetherthisdatacorrelatesornot.
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ThelaserinducedshockimpulseexperimentoutlinedinReference[15]isnot
representativeofLAP,sinceitreliesonashockwavepropagatingthroughatmosphereto
providethrust. Thisexperimentlookedverypromisingwiththelevelofdetailthatwasprovided
documentingtheexperiment. However,withoutthespecificmassoftheprojectileandmass
ablatedateachpointintime,themomentumoftheablatedmaterialcannotbedetermined.
Figure10[16]showstheelectrontemperaturesunderdifferentlaserirradiancelevels,
whicharenearthelevelsutilizedinthisreport. However,temperatureconditionsthishighwill
produceplasmasthatareoutofthescopeofthisreport. NASAsonlineThermoBuildtool[18]
isusefulinexaminingthebehaviorofelementalmaterialsatthetemperaturesseenhere.
However,agreaterunderstandingofthenomenclatureusedinNASAsprogramisneeded.
Thereisalotofroominthisareaforfuturestudies,andespeciallyexperimentstogaina
betterunderstandingofthisprocess,andthegoverningphysics.
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5 PARAMETRICAPPROACH5.1 ToolParameters
Thetooldevelopedforthisprojectfocusedonseveralkeyvariablesbasedonthedesired
outputs. Theoutputscouldbeusedasinputsiftheproperinformationisnotavailable.
UsingtheConservationofEnergyequationsabove,andtheMaterialPropertiesofSection
3.2,theamountofenergycominginfromthelaserisusedtovaporizethesolidpropellant
material. Thismaterialthenexitsthespacecraftwithvelocity(ve),whichprovidesthethrustfor
thecraft.
ThetoolwasdevelopedusingMicrosoftExcel,andcanbeappliedforanymaterialfor
whichthecharacteristicsinsection3.2areknown. Screenshotsofthecode,alongwiththe
equationsusedinthecode,willbeprovidedinthissection.
BeamPropertiesThelaserenergydeliveredtothepropellantisassumedtobetheamountofenergythat
istransmittedtothepropellant. Thisenergyisalsoassumedtobe100%usedforheating. The
valuesshownareforonelaserpulse,sothattheresultscanbescaledupforlargersystems,or
correlatedtofutureexperimentsthatwillproveordisprovethisanalysis.
Inthisstudy,abeamsizeof0.3mmwasused. This0.3mmrepresentsthediameterofa
beamsize,inacircularbeamshape. Thebeamspotsizeistheareaofthecircle,or7.069x108
m2. Laserirradianceisassumedtobe1x109W/cm
2,withapulsewidthof200nanoseconds.
Thisyieldsavalueof14.138Jperpulse.
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Forreference,asamplediagramofbeamareafromanotherstudy[2]wasincluded.
Figure14: Beamprofilerecordedontothermalpaper. (a)Beampatternofcrosssection
(150mmx150mmsquare),(b)Focusedbreampatternonablator(30mmx30mmsquare)[18]
5.2 DeterminingpropulsivecharacteristicsTheoutputsofthetoolinthisconfigurationarethekeypropulsivecharacteristics,including:
MassAblatedperpulse
ExitVelocity(ve)ofablatedmaterial
Force
SpecificImpulse(Isp)
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Thiscanbeusefulifthereareseveraltradestobemadeeitherinthepropellant
materialorthelaserbeingutilizedforthemission.
BasisofEquations
Oncethe14.138Jofenergyisreceived,thisenergyheatsthesurfaceaccordingtothe
specificheatofthematerial,passesthroughenthalpyoffusiontoliquidform,heatsagain
accordingtospecificheat,andpassesthroughenthalpyofvaporizationbeforeablatingto
providethrust.
Fromthemassablatedvalue,densitycanbeusedtocalculatethedepthofvaporization
inconjunctionwiththespotarea.
ExitvelocityisdeterminedusingtheZerothLawofThermodynamicsattheboiling
temperatureofthematerial.
Ispisdeterminedbydividingtheexitvelocitybythegravitationalforce.(Assumedtobe
9.8m/s2fornearearthoperations)
Forceisdeterminedbytheexitvelocitytimestheproductofnumberofpulsesper
secondandthemassablatedineachpulse(massflowrate).
5.3 ToolLayoutandCalculationsThistoolcanbeutilizedinmultipleways. Thissectionshowsanexampleofthetoolused
forthepurposesstatedinsection5.2above. ItisshownforthecaseofanAluminum
Propellant.
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Figure15: ToolscreenshotwithLaserParameterInputsandPropulsiveCharacteristicOutputs
forAluminumpropellant.
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Figure16: ScreenshotofGeneratedDatawithequationsofeachcelllistedforAluminum
propellant.
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5.4 FutureAnalysesThistoolcanbeutilizedinmultipleways. Ifthedesiredoutputsareknown,acceptable
inputscanbedeterminedfromthoseanswers. Theequationsgoverningthisconditioncanbe
rearrangedtosolvearelateddesignproblem.
Determininglaserinputrequirements
Theoutputsofthetoolinthisconfigurationwouldbealistofkeylaserproperties
requiredtoprovidesufficientpowerforthemission. Thekeylaserpropertiesinclude:
LaserPower
LaserPulseWidth
PulseFrequency
Density
Thiscanbeusefulifthereareseveralmaterialsbeingconsideredduetovolumetricor
weightconstraintsofthemission. Itcanalsoservetoalleviatepossibleinteractionsbetween
materialsofthespacecraftandthepropellantsbeingablated.
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Determiningpropellantmaterialproperties
Thisconditionwasnotstudiedinthisproject,butcouldeasilybeconstructed.
Theoutputsofthetoolinthisconfigurationwouldbealistofkeymaterialproperties
thatwouldfitthemissionconstraints. Thekeymaterialpropertiesdeterminedinthismission
designsituationinclude:
Molarmass
HeatofVaporization
SpecificHeat
Density
Thiscanbeusefulifthereareseveralmaterialsbeingconsideredduetovolumetricor
weightconstraintsofthemission. Itcanalsoservetoalleviatepossibleinteractionsbetween
materialsofthespacecraftandthepropellantsbeingablated.
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6 DISCUSSIONANDRESULTS6.1 DiscussionofContinuousWavevs.PulsedLaserPower
Intheliteraturesearchtherewereseveralexperimentsusingsmallscalelaserpower
levelsablatingtinyamountsofmaterialwithapulsedlaserpowersource.
TheresultsoftheseexperimentswouldworkdifferentlyifaContinuousWavelaser
wereusedasthepowersource. TheContinuousWavelaserwoulddelivermorepower,but
systemgeneratingthepowerwouldneedtobemuchlargertodeliverthesamelevelofenergy
tothepropellant. TheremaybeanissuewiththeCWlaserprovidingenergycontinually,where
theremaybeexcessenergyputintothematerialthatisntusedtogeneratepropulsion.
Withoutthetimeinbetweenpulses,thephysicswouldbeentirelydifferent. Therewould
almostimmediatelybegenerationofplasma,whichwasavoidedinthisreportbykeepingthe
pulsesfarenoughaparttoensureonereactionwascompletedbeforethenextbegan.
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6.2 ToolAnalysisResultsTheresultsofthetoolprovidedthekeypropulsiveparametersdeliveredforonelaser
pulseofenergy. Asexplainedearlier,thepulsedlaserapproachallowstimeforthereactionto
completebeforeantherreactionisstarted. Thiswillalsomaketheresultseasiertocorrelateto
futurestudiessincetherehavenotbeenanystudiesofthistypetodate.
TableVIII:MaterialAnalysisResultsforOne200ns,1x109W/cm2Pulse
MaterialMass Ablated
(grams)Exit Velocity
(m/s)Force
(N)Isp
(Seconds)
Aluminum 0.00001029 2219.5 0.000023 226.5
Copper 0.15329947 1477.1 0.2264 150.7Silver 0.4462870 1069.3 0.4774 109.1
Silicon 0.06445467 1930.6 0.1244 197.0
Uranium 0.64794445 1028.1 0.6661 104.9
Basedontheseparametersalone,thereislittlethatcanbeknownaboutthe
applicabilityofeachmaterialtoaspecificmission. AluminumandSiliconseemtoablatethe
leastamountofmaterialperpulse,whichmeanstheycanbeusedforlongerdurationmissions,
wherehigherIspismoreimportantthanalargeamountofthrust. SilverandUraniumseemto
ablatesignificantlylargeramountsofmaterial,whichresultsinalargerforceperpulse. Copper
seemstobeawellbalancedmaterialamongstthefive. Itprovidesamoderateamountofboth
ThrustandIsp. Therearemanymorematerialstoselectfrom,sincealmostanymaterialcanbe
thepropellantforLaserAblativePropulsion.
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6.3 FutureStudyVaryingthetimescalewillyielddifferentresultsinthephysicsoftheproblem. Inthis
report,thelengthsofpulsesdiscussedareconsideredlongintermsofpulsedlaserpower. If
muchshorterpulsesaredelivered,thesolidwouldnotfullytransitiontogaseousstatewithin
thetimeframeofonepulse. Inthisscenario,heatingwouldoccuroveranumberofpulses,and
oncetheenergyinputwassufficienttoablatethematerial,thereactionwouldbecompleted
andthrustgenerated. Reference[19]coverstheseinteractionsinmoredetail.
FutureMissionDesign: Theinstallationofvariouslaserpowerstationonorbitoronthe
surfaceofothercelestialbodieswillenabletravelthroughspacewithoutrequiringlarge
amountsofpropellant.
Thereisalotofroominthisareaforfuturestudies,andespeciallyexperimentstogaina
betterunderstandingofthisprocessandthegoverningphysics.
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7 CONCLUSIONANDRECOMMENDATIONSAttheonsetofthisresearch,itwasexpectedthatathoroughunderstandingofthekey
propertiesofmaterialswouldbeobtainedandanalyzed. Basedontheresultsofthisstudy,
thereismuchmoretolearnaboutthesematerialsandtheirinteractionswithlaserexcitation.
Thelistofacceptablematerialsforthistechnologyissovastthatitwouldtakeaverylongtime
tofindtheperfectpropellant.
Significantresearchinthehardwareaspectofthistechnologyisreallyneededtoenable
widespreadacceptanceanduseofthistechnology. Thedifficultyofpointingand
collecting/focusingthelaserenergywillprovetobeverydifficult.
Thecalculationsperformedinthisreportwereverydifficultwithoutexperimental
resultstocorrelatewith. Experimentationundertheconditionsofthespaceenvironmentis
difficult,butthetechnologywillbestoperatewherethereareasfewenergylossesaspossible
duringtheenergytransmission.
Thisreportrepresentsamajorleapfromthedataandstudiesthathavepreviouslybeen
conducted. Itrepresentsmoreofatheoreticalapproachtothetechnology,sincethereisno
datatoprovethattheconceptworksaspresentedhere.
Onceconcretedatacanbeobtainedfromexperiments,thematerialpropertiesthat
contributemosttothrustefficiencycanbeidentified,andthesearchfortheperfect
propellantcanbegin.
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[12]PeriodicTableofElements:SilverAgAccessedonDecember8,2011from
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