LASER ABLATIVE PROPULSION.pdf

7/22/2019 LASER ABLATIVE PROPULSION.pdf

1/51

PROPULSIONSUBSYSTEMSIZINGTOOL:

LASERABLATIVEPROPULSION

AProject

Presentedto

TheFacultyoftheDepartmentofAerospaceEngineering

SanJosStateUniversity

InPartialFulfillment

oftheRequirementsfortheDegree

MasterofEngineering

by

JohnA.DonovanV

May2012


2/51

ii

2012

JohnA.DonovanV

ALLRIGHTSRESERVED


3/51


4/51

iv

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.


5/51

v

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.


6/51

vi

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


7/51

vii

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


8/51

viii

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)


9/51

1

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.


10/51

2

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.


11/51

3

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.


12/51

4

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:


13/51

5

Figure1: GeometryofLaserIrradiationonaSolidSurface

S(t)=0priortothelaserbeamheatingthesolidmaterial(t=0). Oncethelaserenergy

isdeliveredtothesurface,thesurfaceofthesolidismeltedandchangesphasesto

liquid.

Liquidmaterialisvaporizedtothegaseousstate

Themaximumtemperatureachievableforthisstatewillbetheboilingpointofthe

material,atwhichpointthematerialablatesintheformofgas. Theliquidregionseen

inFigure1willvaporizetoagaseousstateintheoppositedirectionofthelaser.


14/51

6

Inaccordancewiththephasechangefigurebelow,theenergyrequiredtovaporizethe

solidmaterialcanbedeterminedbyintegratingtheenergyequationsforeachregionof

thermalabsorption: Heatingofsolid,heatoffusion,heatingofliquid,andheatof

vaporization.

Figure2: PhaseChangeExampleforWater(H2O)

GasmovesawaywithVelocity(VE)

Theadditionofenergy(intheformofheat)excitesthemoleculeswithinthematerial,

causingthemtoincreasetheirvelocityastheyexittherearofthespacecraft. Thisgasis

expelledtoprovidethrustforthespacecraft.


15/51

7

Figure3: LAPThrustConceptVaporizationofLiquefiedSolidMaterial

Inthisreport,thefocusiswhatoccurswhenhighlypressurized,heatedmaterialexpands

intovacuumandtransformsintoagaseousstate.

1.4 PotentialBenefitsandApplicationsThebenefitsofLAPincludeadramaticweightsavingsintermsofpropellantneededfor

themission. Asaresult,morepayloadcanbedeliveredforasimilarsizedcraft. Another

approachistominimizethesizeofacrafttoachieveasimilarmission. Thiscanalsoincrease

thefrequencyofmissions,assystemsbecomelesscomplexandcanbemanufacturedona

largerscale.

Initialmissionscouldbepowergenerationsatellitesand/orplatforms,whichcouldabsorb

theraysofthesunwhileonorbit. Thispowercanbeconvertedintolaserenergyand


16/51

8

transmittedtospacecraftthatarefartherfromthesun,wheresolarpowerislessfeasible,or

notfeasibleatall.

BenefitsofLasersforEnergyTransmission

Thedevelopmentofprogressivelymorepowerfullasersiscloselyrelatedtothe

advancementofthistechnology,butwillnotsolelybenefitLaserAblativePropulsion.

Astheselaserpowersystemsgrowmorepowerful,theyprovideawidevarietyofpossibleuses:

Beamedenergytoearthfromspace

Beamedenergyfromspacetospace

Shortrangesatellitedefensefromincomingspacedebris

Beamedlaserlightreflectedoffasatellitecouldprovidegroundtogroundenergy

transmission

Beamedlaserlightreflectedoffasatellitecouldprovidedaylighttoanotherareaofthe

earth

Atmosphericapplicationsinclude:

Boostphasemissiledefense(AirborneLaserTestBedALTB)

Laserpoweredaircraft

Highpowereddata/signaltransmissions


17/51

9

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.


18/51

10

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


19/51

11

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]


20/51

12

2.2 ConservationofEnergyTransmittedlaserenergyabsorbedbythepropellantisbasedontheabsorptivityofthe

surface. Somesurfacecoatingswillreflectmoreenergythantheyabsorb. Othersaredesigned

tomaximizeabsorptivity. Inthisstudy,thesurfaceisassumedtoabsorballdirectedenergy.

Possibletypesofenergylosses

Therearenumeroustypesofenergylosswhichoccurfromthetimetheenergyisgenerated

untilitisdeliveredtothetarget. Theseinclude,butarenotlimitedto:

Energyconversiontolightlosses

Imperfectfocusingofthebeam

Heatenergylostintransmissionthroughspace

Oncetheenergyhasreachedthecraft,therearemoreopportunitiesforenergylosses

topresentthemselves:

Imperfectreflectionofthecollectoronthespacecraft

Focusedenergyreflectedbythepropellant(Absorptivity


21/51

13

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.


22/51

14

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


23/51

15

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


24/51

16

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


25/51

17

outsideofthescopeofthispaper. Intheconclusionsectionofreference7,itisstatedthatthe

ablatedmassperareaappearedtoremainapproximatelyconstant. Thisisfurtherproofthata

nozzleisnotnecessaryinanLAPsystem.

Oneconcept[8]tookthenozzlelessapproachevenfurther. Amicrothrusterwas

developedandtestedthatutilizesapropellanttapethatispassedoveranonboardlaserfor

thrustgeneration.

Figure7:LAPConceptSequenceofEvents


26/51

18

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.


27/51

19

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



Specific Heat 384.56 J/kg-K [11]




Melting Temperature 1357.75 K [11]


TableIII:Silver(Ag)Properties



Specific Heat 235 J/kg-K [12]




Melting Temperature 1234 K [12]



28/51

20

TableIV:Silicon(Si)Properties



Specific Heat 710 J/kg-K [13]Density 2330 kg/m^3 [13]





TableV:Uranium(U)Properties








TableVI:DelrinProperties


Molar Mass - kg/mol -


Enthalpy of Fusion - J/kg -



Boiling Temperature - K -

ThereisnotenoughinformationabouttheDelrinmaterialtoincludeitinthisstudy,butit

islistedasadditionalinformationwhichmaybeusefulinafuturestudy.


29/51

21

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.


30/51

22

4 BENCHMARKING4.1 CorrelationtoExperimentalResults

Thetoolcannotbecheckedforcorrelationwithanyfullsizeexperimentorstudy,

becausethisconcepthasnotbeentestedinspace. Thesmallscaleexperimentsavailablewillbe

utilizedtodiscusstheunderstandingoftheprocess,despitetheresultsnotcorrelatingtothe

conditionspresentinthistool. Sincethereisnodirectcorrelationforthisstudy,thisreportwill

discussthedifferencesofthoseexperiments,aswellaswhattheyimplyaboutthefeasibilityof

theconditionspresentedinthistool.

Severalorganizationshaveconductedexperimentsusingsmallscalelaserpowerlevels

ablatingtinyamountsofmaterialwithapulsedlaserapproach. Themostcommonpower

sourcewasaCO2laser.


31/51

23

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.


32/51

24

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%


33/51

25

AnexperimentoutlinedinReference[15]documentsaccelerationduetoalaser

inducedshockimpulse. However,itwasdiscoveredthroughexperimentationthataprojectile

constructedofpolyacetal(POM)withmassof1.36gwaspropelledatavelocityof32.2m/sin

atmosphere. Thiswasaccomplishedbyablatingamassof10.7mgofmaterialperpulse.

Withoutthespecificmassoftheprojectileandmassablatedateachpointintime,the

momentumoftheablatedmaterialcannotbedetermined.

Figure9:Momentumofprojectilevs.massreductionbyablation[15]


34/51

26

Figure10:ElectronandLatticetemperatureforCoppertarget[16]

Figure10showsthetimedependenceofelectronandlatticetemperatureofsurfacefor

coppertargetirradiatedby60fs,800nmpulseatdifferentlaserenergyirradiancelevels.

Higherirradiancesleadtohigherequilibriumtemperaturesduetohigherenergylevelsbeing

addedtothesystem.

Thisshowsthatitispossibletoaddalargeamountofenergyandraisethetemperature

ofanobjectveryquickly. Forexample,witha200nanosecondpulse(200,000picoseconds),the

temperatureofthematerialwillcertainlyequalizeatahighenoughvaluetovaporizethe

propellantmaterial.


35/51

27

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.


36/51

28

Figure12:Surfacestructuralfeaturesinducedbytrainsoflaserpulses[17]

Figure13:Surfaceripplesinducedbytrainof17laserpulsesforAg(a)andCu(b)[17]


37/51

29

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.


38/51

30

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.


39/51

31

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.


40/51

32

Forreference,asamplediagramofbeamareafromanotherstudy[2]wasincluded.

Figure14: Beamprofilerecordedontothermalpaper. (a)Beampatternofcrosssection

(150mmx150mmsquare),(b)Focusedbreampatternonablator(30mmx30mmsquare)[18]

5.2 DeterminingpropulsivecharacteristicsTheoutputsofthetoolinthisconfigurationarethekeypropulsivecharacteristics,including:

MassAblatedperpulse

ExitVelocity(ve)ofablatedmaterial

Force

SpecificImpulse(Isp)


41/51

33

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.


42/51

34

Figure15: ToolscreenshotwithLaserParameterInputsandPropulsiveCharacteristicOutputs

forAluminumpropellant.


43/51

35

Figure16: ScreenshotofGeneratedDatawithequationsofeachcelllistedforAluminum

propellant.


44/51

36

5.4 FutureAnalysesThistoolcanbeutilizedinmultipleways. Ifthedesiredoutputsareknown,acceptable

inputscanbedeterminedfromthoseanswers. Theequationsgoverningthisconditioncanbe

rearrangedtosolvearelateddesignproblem.

Determininglaserinputrequirements

Theoutputsofthetoolinthisconfigurationwouldbealistofkeylaserproperties

requiredtoprovidesufficientpowerforthemission. Thekeylaserpropertiesinclude:

LaserPower

LaserPulseWidth

PulseFrequency

Density

Thiscanbeusefulifthereareseveralmaterialsbeingconsideredduetovolumetricor

weightconstraintsofthemission. Itcanalsoservetoalleviatepossibleinteractionsbetween

materialsofthespacecraftandthepropellantsbeingablated.


45/51

37

Determiningpropellantmaterialproperties

Thisconditionwasnotstudiedinthisproject,butcouldeasilybeconstructed.

Theoutputsofthetoolinthisconfigurationwouldbealistofkeymaterialproperties

thatwouldfitthemissionconstraints. Thekeymaterialpropertiesdeterminedinthismission

designsituationinclude:

Molarmass

HeatofVaporization

SpecificHeat

Density

Thiscanbeusefulifthereareseveralmaterialsbeingconsideredduetovolumetricor

weightconstraintsofthemission. Itcanalsoservetoalleviatepossibleinteractionsbetween

materialsofthespacecraftandthepropellantsbeingablated.


46/51

38

6 DISCUSSIONANDRESULTS6.1 DiscussionofContinuousWavevs.PulsedLaserPower

Intheliteraturesearchtherewereseveralexperimentsusingsmallscalelaserpower

levelsablatingtinyamountsofmaterialwithapulsedlaserpowersource.

TheresultsoftheseexperimentswouldworkdifferentlyifaContinuousWavelaser

wereusedasthepowersource. TheContinuousWavelaserwoulddelivermorepower,but

systemgeneratingthepowerwouldneedtobemuchlargertodeliverthesamelevelofenergy

tothepropellant. TheremaybeanissuewiththeCWlaserprovidingenergycontinually,where

theremaybeexcessenergyputintothematerialthatisntusedtogeneratepropulsion.

Withoutthetimeinbetweenpulses,thephysicswouldbeentirelydifferent. Therewould

almostimmediatelybegenerationofplasma,whichwasavoidedinthisreportbykeepingthe

pulsesfarenoughaparttoensureonereactionwascompletedbeforethenextbegan.


47/51

39

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.


48/51

40

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.


49/51

41

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.


50/51

42

REFERENCES[1] CHARLESC.PlasmasforspacecraftpropulsionJournalofPhysicsD:AppliedPhysics

163001.

[2] SINKOJ,PAKHOMOVA.DelrinforPropulsionwithCO2Laser:CarbonDopingEffects

AmericanInstituteofPhysics9780735405165/08.

[3] PAKHOMOVA,THOMPSONMS,GREGORYD.AblativeLaserPropulsion: AStudyof

SpecificImpulse,ThrustandEfficiencyAmericanInstituteofPhysics0735401268/03.

[4] COHERENTIncorporated.UnderstandingLaserBeamParametersLeadstoBetter

SystemPerformanceandCanSaveMoneyAccessedonOctober25,2011from

http://www.coherent.com/download/6807&cmpid=EMC450

[5] NEWPORTCorporation.AverageandPeakPowerATutorialAccessedonOctober23,

2011fromhttp://www.newport.com/images/webdocumentsen/images/20063.pdf

[6] PHIPPSCR,etal.ImpulsecouplingtotargetsinvacuumbyKrF,HF,andCO2single

pulselasersJournalofAppliedPhysicsvolume64(3),1August1988

[7] SINKOJE,etal.ConicalnozzlesforpulsedlaserpropulsionSocietyofPhotoOptical

InstrumentationEngineersSPIE0277786X/08

[8] PHIPPSCR,etal.LaserAblationPoweredMiniThrusterSocietyofPhotoOptical

InstrumentationEngineersSPIE0277786X/02

[9] TRANDV,LAMYC,ZHENGHY,WONGBS,HARDTDE.Directobservationofthe

temperaturefieldduringablationofmaterialsbymultiplefemtosecondlaserpulses

AppliedSurfaceScienceVolume253(2007)p72907294

[10]PeriodicTableofElements:AluminumAlAccessedonDecember8,2011from

http://environmentalchemistry.com/yogi/periodic/Al.html

[11]PeriodicTableofElements:CopperCuAccessedonDecember8,2011from

http://environmentalchemistry.com/yogi/periodic/Cu.html


51/51

[12]PeriodicTableofElements:SilverAgAccessedonDecember8,2011from

http://environmentalchemistry.com/yogi/periodic/Ag.html

[13]PeriodicTableofElements:SiliconSiAccessedonDecember8,2011from

http://environmentalchemistry.com/yogi/periodic/Si.html

[14]PeriodicTableofElements:UraniumUAccessedonDecember8,2011from

http://environmentalchemistry.com/yogi/periodic/U.html

[15]MEYERM,JOHNSONL,PALASZEWSKIB,GOEBELD,WHITEH,COOTED.NASADRAFT

InSpacePropulsionSystemsRoadmapTechnologyArea02November2010

[16]WATANABEK,TAKAHASHIT,SASOHA.UsefulInspaceImpulseGenerationPoweredby

LaserEnergyAmericanInstituteofPhysics0735401756/04.

[17]TANX,ZHANGD,MAOF,LIZ,YIDI,ZHANGX.Theoreticalandexperimentalstudyof

energytransportationandaccumulationsinfemtosecondlaserablationonmetals

TransactionsofNonferrousMetalsSocietyofChinaVolume19(2009)p16451650

[18]NASACEAThermoBuildOnlineToollocatedat:

http://www.grc.nasa.gov/WWW/CEAWeb/ceaThermoBuild.htm

[19]RETHFELDB,TEMNOVVV,SOKOLOWSKITINTENK,ANISIMOVSI,VONDERLINDE,D.

Dynamicsofultrashortpulselaserablation:equationofstateconsiderationsSociety

ofPhotoOpticalInstrumentationEngineersSPIE0277786X/02

[20]MicrosoftExcelToolforPropulsionSubsystemSizing

Documents

LASER ABLATIVE PROPULSION.pdf