_A3A - t_,t..¢uu _o NASA Technical Memorandum 100951 • NASA-TM-100951 [ AIAA-88-2821 t _Ol_l_O_l_ _j ' Characterization of Aluminum/RP-1 " Gel Propellant Properties Douglas C. Rapp Sverdrup Technology, Inc. NASA Lewis Research Center Group Cleveland, • Ohio and Robert L. Zurawski National Aeronautics and Space Administration Lewis Research Center Cleveland, Ohio OCT! 8 19gg Prepared for the 24th Joint Propulsion Conference L*NGLEYL,_R.:.,_._ "REEEARC_t,!',._;, CENTER HAMPION,ViF:GINIA ,. cosponsored by the AIAA, ASME, SAE, and ASEE ! Boston, Massachusetts, July 11-13, 1988 IU/LSA https://ntrs.nasa.gov/search.jsp?R=19880015424 2018-05-23T18:03:35+00:00Z
Transcript
_A3A - t_,t..¢uu _o
NASA Technical Memorandum 100951• NASA-TM-100951 [
AIAA-88-2821 t _Ol_l_O_l_ _ j
' Characterization of Aluminum/RP-1" Gel Propellant Properties
Douglas C. RappSverdrup Technology, Inc.NASA Lewis Research Center GroupCleveland,•Ohio
and
Robert L. ZurawskiNational Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio
OCT! 8 19ggPrepared for the24th Joint Propulsion Conference L*NGLEYL,_R.:.,_._"REEEARC_t,!',._;,CENTERHAMPION, ViF:GINIA
Researcheffortsare being conductedby the NASA Lewis ResearchCenter toformulateand characterizethe propertiesof AI/RP-I and RP-I gelled propel-]ants for rocket propulsionsystems. Twenty four differentcompositionsofgelledfuels have been formulatedwlth 5 and 16 I_m,atomizedaluminumpowderin RP-I. The total solids concentrationIn the propellantvaried from 5 to
_L
I 60 wt %. Tests were conductedto evaluate the stabilityand rheologicalchar--_i acterlstlcsof the fuels. Physicalseparationof the solidsoccurred in fuels_ with less than 50 wt % sollds concentration. The rheologicalcharacteristics-.. of the AI/RP-Ifuels varied with solids concentration. Both thlxotropicand
rheopectlcgel behaviorwere observed. The unmetallizedRP-I gels, which wereformulatedby a differenttechniquethan the AI/RP-I gels, were highly vlsco-elastic. A historyof researcheffortswhich have been conductedto formulateand characterizethe propertiesof metallizedpropellantsfor variousapplicationsis also given.
INTRODUCTION
The addition of metals to liquid bipropellant systems has been investi-gated for a number of years because of the potential improvements in rocketpropulsion system and vehicle performance. The specific impulse advantagesderived from metallized propellants, such as LI/F2/H2, have been documented(ref. I). However, recent evaluations of the concept for rocket propulsionapplicationshave focusedon the density-lmpulserelationshipand deliveredpayloadadvantages. These studieshave shown that metallizedpropellantsoffer substantialadvantagescomparedto conventionalliquid bipropellantsinvolume-llmltedpropulsionsystem applications(ref. 2) and are promisingpro-pellantsfor rocket-poweredslngle-stage-to-orbltvehicles and advancedboosterpropulsionsystems. In addition,supplementingliquidpropellantswith a solidmetallicphase often requires the gelationof the neat liquid carrier. Theresultantsemisolidstate created by the gel additionprovides a safety advan-tage with respectto tank ruptureand spillage. Thls report summarizesrecenteffortsby the NASA Lewis ResearchCenter to formulate,stabilize,andcharacterizethe propertiesof aluminum/RP-I(AI/RP-I)fuels.
Metallizedpropellantsconsistof a solid, low molecularweight partlcu-late metal or metallic compound in a liquidfuel or oxidizer. High density,
lowmolecularweightand highcombustionenergyare desiredpropertiesof eachpropellantcomponent.The metalor metalliccompoundis typicallysuspendedin theneat liquidcarrierfor fluidictransportthroughthe systemtank,lines,manifold,and Injectorports. Thesetwo-phasefluidsmay be slurries(smallparticulates,_I _m), gels(largerparticulatesand a polymericgellantadditive),blmodalstabilizedgels(largeand smallparticulatesand a poly-mericgellantadditive)or emulsions(two immiscibleliquidsand an emulsifier)(ref.3). Variouspreparationtechnlquesand additivesare usedto formulatethesepropellants.Detailsof the preparationtechniqueand stabillzlngagentsusedfor theAI/RP-Igelsreportedhereinare confidentialdue to similarworkeffortsby the U.S.Air Force. Therefore,somepropellantcompositionsarereportedwiththe additivesgenericallycoded.
A literaturereviewof previousmetallizedpropellantwork revealstherelatlvelylonghistoryand rangeof appllcatlonsof the metalllzedpropellantconcept. Earlystudiesof metallizedfuelsconductedby the NationalAdvisoryCommitteeforAeronautics(NACA)LewisResearchCenterfrom 1947to 1957focusedon aluminum,boron,and magnesiumin hydrocarbon(JP)fuelsto increasethe thrustand flightrangeof ramjets. Aluminumwas eliminatedearlyin thatprogramInfavorof magnesiumand boron. Depositionof sticky,moltenaluminumoxideinsidethe combustorwas a seriousproblemdue to the relativelylow com-bustiontemperaturein the ramjet. It was also learnedthat boron-slurryfuelswerenot satisfactoryfor use in afterburnersor ramjetenginesbecauseof thepoorcombustionefficiencyof fuelscontainingboron. Magnesium-slurryfuelsperformedsuccessfullyand couldprovidethrustgreaterthan conventionalliquidhydrocarbonfuelsbut couldnot increasethe flightrangeof the ramjet(ref.4).
Researchon metalllzedfuelsfor propulsionsystemssubsequentlyfocusedon hypergollcpropellantcombinations,suchas hydrazineand monomethylhydra-zlnewithnitrogentetroxlde,IRFNA(inhibitedred fumingnitricacid)andhydrogenperoxide.The U.S.Air Forceformulatedan aluminum/hydrazlnegelledfuelcalledalumlzlnefor liquidrocketenglnesystems(ref.5) and aberyllium/hydrazinegelledfuelcalledberylllzlnefor use In advancedpropul-sionsystemsfrom 1959to 1966(ref.6). Similarly,the U.S. Navyconductedsafetyrelatedgelledpropellantprogramsfrom 1959to 1965with hydrazinetypefuelsand nitricacid. In the periodfrom 1960to 1970,the Navy.success-fullyformulatedNOTSGEL-A(60wt % AI In hydrazine)and NOTSGEL-Z(80wt % Zrin hydrazine)in theirHighEnergyBi-PropellantFuelsProgram(refs.7 and8). More recentlytheU.S.Army has fundedprogramsto formulate,character-Ize,andevaluatehigh-energyliquidpropellants,and developedthe rheologl-callymatchedpropellantcombinationof MICOM(MissileCommand)gel and IRFNAgel (ref.9).
In the early1970'sInterestwas revlvedIn metalllzedllquldhydrocarbon ',fuelsdue to the advantageof highervolumetrlcheatof combustlonwlth respectto the neatllquldhydrocarbonfuelsthatwere usedfor alrbreathlngsystems.From 1969to 1973,theU.S. Navydevelopedhydrocarbon-gelfuelswlth 60 wt %carbonblack,75 wt _ alumlnum,and 65 wt _ boroncarbldeduringa HighEnergyRamjetFuelsprogram(refs.7 and 10), Carbonslurryfuelformulatlonworkwas alsofundedby theU.S.Alr Force,NASA LewlsResearchCenterand theDepartmentof Energy(DOE)In the late1970sand early19BOs. The patentedSF2carbonslurryfuel(carbonIn JP-lO)was formulatedand Itspropertleswerecharacterizedas a partof thlswork. The energycrisisalso supportedtheneedfor fuelalternatlvesfor hlghwaytransportatlon,and othercarbonslurry
fuels were developedfor this appllcatlon(refs. II and 12). Investigationsof meta111zedhydrocarbonpropellantscontinuetoday, but much of the work issensitiveIn nature due to its milltaryapplicatlonand is not reported here.
The NASA Lewis ResearchCenter is presentlyinvestigatingthe aluminum/RP-11oxygen(AIIRP-IIO2) propellantcombinationfor rocket engine appllca-tlons. Previouspropellantdevelopmentwork in the area of aluminizedfuelshas focusedon alrbreathlngengine combustionwhere only limited successhasbeen achieved. The additlonalstudy of aluminum-hydrocarbonpropellantis
• requiredfor the developmentof a rocketfuel, not an alrbreathingfuel, andthe successfuldemonstrationof injection,ignition,and combustionwithin ahigh temperature-pressurerocket engine. The objectiveof the research inves-tigationsis to formulateand characterizethe propertiesof stable AI/RP-Igels with acceptablerheologlcalproperties. The AI/RP-I fuel is of specificinterestto NASA for severalreasons. Aluminum additioncan potentiallyimproverocketperformancebecauseof its density and high combustionenergywith oxygen (ref. 13). Aluminum is also nontoxicand safer to handle thanother promisingmetal fuels such as berylliumand lithium. RP-I (Kerosine,MIL-P-25576C)is a high density storableliquid hydrocarbonthat has beenprevlouslyused as a rocket propellantand is currentlybeing evaluatedforadvancedspace transportationboosterengines. This report summarizesrecenteffortsby NASA Lewis to formulate,stabilize,and characterizethe propertiesof AIIRP-Ifuels.
DISCUSSION
The suspensionof a so1|d materlalin a neat liquid involvesnumerousconsiderationsand trade-offsin order to maximizethe potentialbenefitsofthe fuel. Table I summarizesfuel propertieswhich shouldbe consideredinformulatlnga metallizedpropellant. The formulation,stability,and rheologl-cal propertiesare dependentupon the physicaland chemicalpropertiesof themetallicphase, liquid hydrocarbon,and the stabilizingadditivesand theirinteractionsin solutlon. Two importantpropertiesIn formulatingmetallizedpropellantsfor rocket propulsionapplicationsare the bulk density andspecificimpulseof the propellants. The combinationof these two parametersmust be optimizedto maximize deliveredpayload. Bulk density increaseswiththe additionof the dense solid metallicphase and specificimpulsemay improveupon metal addition. It Is desirableto minimize solid particle size (21 pmmean particlesize) for efficientcombustion;however, the smallerthe parti-cle size, the more particulatesthat are requiredfor a given heat of combus-tion. In general,less gellant is requiredwith the smallerparticle size andviscosityincreaseswith increasingnumber of particulates. The other formula-tion propertiesare characteristicsprimarilyderivedfrom the liquid carrier.
The physical,chemlcal,and thermalstabilityof a metallizedpropellantmust also be maximized. Physlcal stabilityrefers to the disablinginter-actions (agglomeration,flocculatlon,or aggregation)of the solid phase whilein suspension. Definitionsof terms are locatedin the appendix. While theseprocessesweaken the physical stabllltyof the AI/RP-Ifuels, the gelationofllquidhydrogendependsupon the surfaceinteractionsof a large number offinely dividedparticles(ref. 14). As the small particulatesgrow in sizedue to these partlcle-particleInteractions(which are dependentupon particlemorphology),they will settle under normal gravimetricconditions. Hence, sta-billzlngagents (surfactantsand gellants)and/or bimodal (two size) particle
dlstrlbutionsareoftenrequiredto accountfor theseforces(seeappendix).Surfactantscan preventflocculatlonvia stearlchinderancewith the adsorp-tionof a monolayeronto the particle.The adsorbedmonolayerovercomesthevan derWaalsforcesthatcreatethe attractivemigrationforceamongparti-clesby assumingan inducedchargewhichrepelsparticlessimilarlycoatedbythesurfactant.Gellantsand blmodalparticledistributionscan preventagglomerationand aggregationby forminga matrixor networkaboutdefloccu-fatedparticles.Gellantsconstructthe three-dimenslonalmatrixby the inter-mixingof theirlongpolymericchains. Blmodalparticledistributiondevelopsthenetworkby theflocculatingforcesof smallerparticulatesaroundlargerparticles.However,theadditionof stabllizlngcomponentsto the metallizedfluidmustbe minimizedbecausetheirintroductioncomplicatesthe preparationprocedureand tendsto increaseviscosityand chemicalinstability.Ingeneral,stabilizingcomponentsalsorepresentthe lowestenergyportionofthepropellantcombinatlon.
Chemlcal Instabilltyresultsfrom chemlcalreactionsbetween the(sometlmesnumerous)formulatloncomponents,impurities(sulfur,water, etc.),and additlves(dryingagents,vapor pressurereducers,etc.) in the metalllzedpropellant. Additivesand impurltlesin metallizedpropellantsshould be mini-mized as they add to the potentialchemical imbalanceof the system. Tempera-ture variatlonsmay promotechemicalreactionrates and vaporizatlonlosseswhich in turn, could increasesyneresls,pressuredevelopment,bulk growthrate, and physicaland chemical instability. Thermallystable metallizedfuels are generallyproducedby trial and error formulationwhere the concen-tratlonsof the stabillzingagents and additivesare varied parametrically.
The viscosityof metallizedpropellantsis the most difficultproperty tocontrolin formulationbut is criticalsince it determinesthe final propel-lant'sflow characteristics. The additionof stabilizingagents to the solid-11quldmlxturetypicallycreatesan internalstructurethat requires a finiteyield stress to initiateflow and a nonlinearshear stress-shearrate relation-shlp that is often time-dependent. Figure I shows shear stress-shearraterelatlonshipsfor varioustypes of these tlme-independentnon-Newtonianfluids.Viscosityis the ratio of shear stress to shear rate. Figure2 illustratestime-dependentnon-Newtonianviscoelasticfluid behavior. Time-dependentbehaviorIs illustratedby uniformlyincreasingshear rate to a maximum limitand then uniformlydecreasingshear rate to a minimum limit. Thlxotroplcfluids demonstratea decrease in shear stresswlth time at a constant shearrate. Rheopecticfluids demonstratean increasein shear stresswith time ata constantshear rate. A detailedreview of non-Newtonlanflow and heat trans-fer is containedin references15 and 16. The ideal metalllzedpropellantfrom a rheologlcalstandpointis a reproducibleyield pseudoplastlcthixotrope(shearthinning)that approachesNewtonlanbehaviorat hlgh shear rates. Theyield point, finite stressrequired to inltiateflow, providesa measure ofspill and tank rupturesafety since a minimum shear force is required forflow. Shear thinningbehavioris desiredso that the energy required to trans-port the material diminisheswith shear rate and time. Newtonianbehavior Isdesiredat high shear rates (i.e. propulsionsystemoperationalshear rates)so the rheologlcalcharacteristlcsof the fuel and oxldizer match as closelyas possibleand to assist in maintaininga constantmass flow rate. The yieldpolnt and viscosity,especiallyat low temperatures,shouldbe minimized byminimizlngthe concentrationof the stabilizingagents,mlnimizingthe distor-tion of particleshape during formulation,and minimizingthe solids concentra-tlon and particlesurfacearea per unit volume.
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To achievesuccessfulfuelformulatlonswithuniformpartlcledispersion,theaggregates,flocculates,and agglomeratesof the metallicpowdermust bebrokendownto the elementalparticlesize. The millingprocessused to accom-plishthlsmustbe conductedwlthminimalflatteningof the particulatessincelongerand flatterplateletswillproducea moreviscousproduct. Figure3showssometypesof laboratoryequipmenttypicallyusedduringthe formulation
, of metalllzedpropellants.The hlghshearmixerIs usedto uniformlydispersethe solidsand add|tivesintothe liquidpropellant.The rollingballmill ischargedwithcylindricalgrindingmediato slowlybreakup the flocculatedpar-ticulates.Finally,the centrifugeIs usedfor fuelstabilitytestlng.
Physical,chemlcal,and thermalstabI11tyare requirementsof an accept-ablemeta111zedpropellant.The propellantmustbe stableduringall phasesof the fuelformu1atlonprocessandfor storageof the propellant.Staticshelfllfetestsand acceleratedstoragestabilitytestscan be usedto evalu-ate the stabI11tyof a propellantformulation.Staticshelflifetestscan beusedto evaluatethe longtermphysical,chemical,and thermalstoragesta-bilityof a propellantformulationunderactualstorageconditions.In astaticshelflifetestthemetalllzedpropellantIs sealedIntoa storageves-selwhichIs exposedto a thermallyvaryingenvironment.The vesselshouldnot containany u11agefor the liquidcarrierto evaporateand condenseand,hence,yieldfalsestabilityinformation.InstablIityof the suspensionwlthtime Is witnessedby supernatantfluidat the topof the containerand soliddensitygradientsalongthecontainerheight. Althoughthesestaticshelflifetestsareeasy to conduct,usefulresultsmay takel to 5 yearsto attain.
Acceleratedstoragestabilitytestscan be usedto rapidlyevaluatethephysicalstabllityof the propellant.Physlcalstabilityis achievedby limit-ing the amountof aggregation,flocculation,and agglomeratlonthatcreateslargerparticleswhichsettlewithgravity. Solidparticulatestendto settleundertheirown massbecausetheyare typicallymoredensethanthe IIquldcar-rler. Acceleratedstoragestabilitytestsare basedon StokesLaw. Stokeslaw,whichgovernsthe rateof settlingof a singleparticlein a gravitatlonalfield,statesthatthe rateof fall is proportionalto the gravitationalfleldstrength(ref.17). Modified StokeslawextendsthisproportlonaIItyto con-centratedsuspensionsundergoinghinderedsettling.A centrifuge,suchas theone shownin figure3, can be usedto producean artificialgravitatioDalfleldwhosecentrifugalfieldstrengthmay be derivedfrom:
G = l.ll8x10-5 (rpm)2(r)
where rpm is theoperatingrevolutionsper minuteof the centrifugeand ris theradius(in centimeters)of the centrifugeas measuredfrom the centerof thecentrifugeto the tipof the testtube. G Is expressedas multiplesof thegravltatlonalconstant,g (9.8m/s2). Stabilityvaluesare reportedIntermsof a dlmensionlessratioof concentrations,C/Co, where C° Is the orig-Inalsolidsconcentrationand C Is the solldsconcentrationof the top I mlaftercentrifuging.C/Co valuesdeterminedat a particularcentrifugalfieldstrengthand centrifugetlmedurationcan be relatedto a longterm storagestabilltyperiod.
ChemicalInstabllitiesare witnessedduringfuelprocessingby gas forma-tionfromchemicalreactionspromotedby the IncompatiblIItyof the propellantcomponents.A shortterm testfor chemlcalstabilityIs to containthe
.
propellantsampleIn a sealedcontainerand measurethe pressureincreasedur-ing shorttermstorage.An adverseenvironmentmay be imposedon the fuel sam-ple by ovensor refrigerationunitsto testfor thermalinstabilities.
The flowcurvesfor themetallizedpropellantswere acquiredwlth rota-tlonalviscometers.A Brookf|eldvlscometerwas usedfor quickevaluationorscreeningof gellantsand surfactantsto verifyan adequateyleldpointforthe suspensionof solidsand acceptableviscosityfor flow. The Brookfleldviscometermeasuresthe torque(proportionalto shearstress)of a rotatingspindle(rpmproportionalto shearrate)in a largevolumeto determlneviscos-ityvalues. Highershearrateviscositymeasurementsweredeterminedwith aHaakerotationalviscometer.The Haakeis lesssensitiveto low shearratesin comparisonto the Brookfleld,howeverIt canbe muchmoreaccurateandprecise. The Haakevlscometerwhichwas usedmeasuresthe torque(proportionalto shearstress)of a rotatingbob (rpmproportionalto shearrate)in a con-centriccyllndricalarrangement.Differentrangesof viscositycan be measuredby ut111zingthe differentavailablegeometrles.
RESULTS
Fuel Formulation
Propellantformulationeffortswere conductedto prepareAI/RP-Ifuelswith the aforementionedpropertiesdesiredfor metalllzedpropellants. Twelvedifferentcomposltlonsof AI/RP-Ifuel were formulatedwith 5 and 16 _m, atom-Ized aluminumpowder in RP-I for a total of 24 fuel samples. All formulationswere producedby the same preparationtechniquewlth the same proportionofadditives. The total solids concentrationvaried from 5 to 60 wt % at 5 wt %increments. The metallizedpropellantcompositionand componentconcentrationsin weight percentfor both the 5 and 16 pm AI/RP-I gels are given in table II.Densityvalues for the 5 and 16 _m AI/RP-I gels are tabulatedwith the stabil-ity and rheologlcalInformatlonin tables III and IV, respectlvely. Manufac-turer speciflcationssuggestthat the 400 and 200 series aluminumpowdersshould have averageparticlesizes of 5 and 16 _m, respectlvely. However,theresultsreportedin table V of a particlesize image analysis,a standardopti-cal sizingtechnique,suggestthat the mean particlesize Is smallerthan themanufacturerspecifications. The 200 series aluminumpowder containsa largenumber of very fine particles,loweringthe mean particlesize. This can beseen from a scanningelectronmicrographof the 200 seriesaluminum powder asshown in figure 4. The irregular,nonsphericalshape of the aluminum powderpartlcIesis also evident.
Fuel formulationeffortswere also conductedto preparean RP-I gel(unmetallized)using a water solubleresin describedIn reference18. The com-positlonof the RP-I gel is listed In table VI. The resin is an acrylic acidpolymerthat is insolublein kerosene,but when added togetherwith the laury-lamlne,the resin becomeshydrophobicand mildly solublein kerosene. Themethyl alcoholis added to adjust the solubllityof the kerosene so that thegellant/aminecombinationcan go into solutlon. The RP-I gel was prepared byfirst addlng the laurylamineto the RP-I. The resin was then carefullydis-persed Into this mixture,and the mixturewas agitatedto a thin homogeneoussolution. Finally,the methyl alcoholwas added with vigorousagitationusing
a high shear mixer to blend the mixtureto uniformlty. The resultingRP-I gelwas highly viscoelastic. Furtherwork focusedon decreasingthe RP-I gel vls-cosity by adjustingthe concentrationsof the resin, laurylamlne,and methylalcohol.
: StabilityTests
The acceleratedstoragestabilitytestson theAIIRP-Imetalllzedfuelswere conductedat a centrifugalfieldstrengthof 96g for 6 hr. Solidsconcen-trationof a samplewas determinedby a mass analysisbeforeand aftertheevaporationof the liquidphaseby an oven. Gelledphysicalstateswereachievedinfuelswith solidsloadlngsgreaterthan50 wt %, basedon overnightvisualobservationsof densitygradients,for boththe 5 and 16 Nm aluminumpowder. However,C/Co stabllltyvalueswerewell belowthemaximumvalueofone aftercentrifuging.Stabilityvaluesfor theAI/RP-Imetalllzedpropellantformulationsare listedin tablesIII and IV. Physicalseparationoccurredfor all fuelsbelow50 wt % basedon overnightvisualobservationsof densitygradients.Betterstabilitywas achievedwith the 16-_mgradealuminumsamplesbecausethe surfactantadditivesareworkingmore efficientlyon the smallersurfaceareaper unitvolumeparticles.Thisfindingdoesnot necessarilyimplylargerparticlesarerequiredfor stabilitybecausethe physicalInsta'bllltiesof the formulationsoriginatedfrom a misproportionlngof stablllz|ngagents. Chemicalinstabilitiesduringfuelformulation,althoughnot fullyconclusiveevidenceof chemicalstabillty,were not detectedwith theAI/RP-Igels. Thermalstabilitytestswere not conducted.
RheologyTests
Brookfieldapparentvlscosityvaluesfor the twenty-foursamplesarereported in tables III and IV. The 0.3 rpm reading correspondsto a yieldpoint value, and the 60 rpm (less than 16 s-l) reading correspondsto the high-est shear rate of the instrument. The viscosityvalues were measuredafterag_tatlonat 60 rpm until a steady readingwas obtained and then decreasedto0.3 rpm for the yield point reading. The viscositiesdecreasedwith solidsloadingfor the gelled samplesand had erratic behaviorfor the unstablesepa-rated samples of both test series. Experimentalviscosityresultsfor propel-lants with 45 wt % and below solids concentrationare questionablebecauseofthe instabilityof the samples. The fuels with 16-Nm aluminumpowder hadhigher viscositiesthan those with 5-_m aluminumpowder at the same solidsloading. The stabilizingagents worked betterwith the lower surfacearea perunit volume particles(16-pm)because less of the liquid carrierwas immobi-lized by the particulates.
The Haake yleld point and viscosityvalues for the AI/RP-Ifuel formula-tions are 11sted In tables III and IV. The yield point values are peak shearstress values (gelledsamplesonly with overnightstanding)achievedupon aramping increaseof shear rate to If7 s-I (lO percentmax shear rate). Shearstress peaks for AI/RP-Ifuels wlth 60 wt % solldsconcentrationof 5 and16 _m powder are shown in figures5(a) and 6(a), respectively. The tabulatedviscosityvalues are reportedwith a correspondingshear rate from the downramping line (wherepossible)becausethe viscosityIs not constantwith shear(non-Newtonian). Haake flow curves to the maximumshear rate (1170 s-l) forthe AI/RP-Ifuels with 60 wt % solidsconcentrationof 5 and 16 pm powder are
shown in figures 5(b) and 6(b). Figures5(a) and 6(a) correspondto the 0 tolit s-I shear rate range of figures 5(b) and 6(b) since these flow curvesweretaken at the same time shortlyafter formulationof the fuels. A second setof measurementswas made approximately5 months after the originalmeasure-ments with a similarHaake vlscometer,but differentsensingheads,to investi-gate changes in the rheologicalbehaviorof the fuels with time. This secondset of Haake flow curves for fuel sampleswith 60 wt _ solids loadingof 5 and16 _m aluminumpowder is provided infigures 5(c) and 6(c), respectively. Acomparlsonof vlscosltyvalues with time for 45 wt % solids concentrationandabove is tabulatedin tablesVll and Vlll.
The resultsof the rheologlcalinvestigationsof the AI/RP-I fuels arequalitativebecauseof the observed instabilities,the use of differentmeas-urementproceduresand the apparentevaporationloss of the liquidphase dur-ing storageof the fuels. Stable gelled AI/RP-Ifuels may behave differentlydependingupon the method of stabilizationand additiveconcentrations. How-ever, severalgeneral conclusionswere drawn from these Investlgations. Theyield points of the AI/RP-I fuels increasewith solids loadingdue to theIncreasedImmobllizationof the liquid carrierby the additionalsolids. Theyield points also increasewith particle slze at equal solids concentrationsbecause the stabilizingagents become more effectiveas the surfacearea perunit volume of the particledecreasesand, hence, less liquid phase is immobl-lized. The yield points diminishwith time due to the predictedphysicalinstabilltiesand potentiallyfrom the long term chemicaland thermal Instab11-Ities. In general viscosityincreasedwith time at equal shear rates andsolids loadingdue to the apparent evaporationOf the liquid phase. Viscositydecreasedwith increasingshear rate (the fuels were shear thinning)due tothe breakdownof the gel structure,and thixotroplcand rheopectlcbehaviorofthe fuel was observed. Finally,at equal shear rates, viscositydecreasedwith decreasingsolids loadingbecausethe liquidcarrieris less conf|nedbythe particulates.
CONCLUDINGREMARKS
Research efforts were conducted at the NASALewis Research Center toformulate and characterize the properties of AI/RP-I and RP-I gels for rocketpropulsion applications. The AI/RP-I fuel has been previously Identifled aspotentially improving propulsion system and vehicle performance because of theincreased energy and density of aluminum relative to the neat liquid fuel.Safety advantages are also possible with these propellants in certain appIIca-tions because of the gelled physical state of the propellant. While a finalAI/RP-I fuel formulation has not been produced to date, research efforts con-tlnue to formulate gelled and metallIzed fuels and characterize their stabilityand rheologlca] properties.
Numerous considerations with respect to rocket propulsion applicationswere presentedthat influencethe final metalllzedpropellantformulationprop- ,ertles. The fuels must be physically,thermally,and chemicallystable duringall phasesof the fuel formulatlonprocessand for storageof the propellant.Propellantformulationeffortswere conductedto prepareAI/RP-I and RP-Igelled fuels. Twenty-fourdifferentcompositionsof AI/RP-I fuels were formu-lated with 5 and 16 Nm aluminum powder In RP-I. All AI/RP-Iformulationswereproducedby the same preparationtechniquew|th the same proportionof addl-tives. Acceleratedstoragestabilitytests were conductedto evaluate the
8
stabilityof the propellant.Physicalinstabilityof the AI/RP-Isampleswaspredictedfrom96g acceleratedstoragestabilitytests. Physicalseparationof the solidsoccurredin fuelswith lessthan50 wt % solidsconcentrationbecauseof a misproportloningof additivesat the lowersolidsconcentrations.The unmetalllzedRP-Igels,whichwereformulatedby a differenttechniquethantheAI/RP-Igels,were stable.
I. Arb|t, H.A., Clapp, S.D , Dickerson,R.A., and Nagai, C.K.,"Llthlum-Flourine-HydrogenPropellantStudy," R-7257,RockwellInternationalCorp., Canoga Park, CA, 1968, NASA CR-72325.
2. Zurawski,R.L., and Green, J.M., "An Evaluationof MetallizedPropellantsBased on VehiclePerformance,"AIAA Paper 87-1773,June 19B7. (NASATM-IOOI04.)
3. Burdette,G.W., and Couch, D.H., "Non-NewtonianRocket Fuels,"U.S. NavalOrdnanceTest Station,China Lake, CA. [Declassifiedby G._. BurdetteJune 14, 1977 in accordancewith E.O. I1652.]
7. Burdette,_., "A Reviewof Navy Gelled Fuel Programs,"17th JANNAFCombustionMeeting, Workshopon the Combustionof Slurry Fuels inAirbreathingSystems,Hampton,Virginia,Sept. 1980.
8. Burdette,G.W., Hoffman,H.J., and Nyberg,D.G., "GelledLiquidPropellantsWith ImprovedPerformance,"U.S. Naval Ordnance Test Station,China Lake, CA. [Declassifiedby G.W. BurdetteJune 14, 1977 inaccordancewith E.O. I1652.]
9. Allan, B.D., "A Gelled Oxidizerfor TacticalMissiles,"U.S. Army MissileCommand,RedstoneArsenal, Huntsville,AL, 1982. (Avail.NTIS,AD-CO28699L.)
lO. Bryant,J.T., and Burdette,G._., "RamjetFuel Studies,Part l - Carbon,"NHC-TP-4810,Parts I-3, U.S. Naval WeaponsCenter, China Lake, CA, July1971. (Avail.NTIS, AD-B86596L,AD-913289L,AD-BOI6607L.)
If. Ryan, T._., III, and Dodge, L.G., "Developmentof Carbon Slurry Fuels forTransportation(HybridFuels - Phase 2)," SHRI-694B,SouthwestResearchInstitute,San Antonio, TX, May 1984, DOEINASA/0263-1,NASA CR-174659.
12. Ryan, T._., III, Callahan,T.J., Likos,_.E., Moses, C.A., and Dodge,L.G., "Alternatlve(Hybrid)Fuels for HighwayTransportation,Vol. l,"SouthwestResearch Institute,San Antonio,TX, 1980.
13. Zurawsk|,R.L., "CurrentEvaluationof the TripropellantConcept," NASATP-2602, 19B6.
aObservation made after standing overnight.bMeasurement made, after standing overnight, at 700 rpm for 6 hr, CO is original solids concentration and C iscentrifugedsample solids concentration.Measured after agitation startingat 60 rpm.Densityvalues were calculatedexcept values in parenthesiswhich were measured.eFor gelled samplesHaake measurementswere made after overnightstanding. For the remainder,measurementsweremade as rapidlyas possible.fall results for 45 wt % and below are questionabledue to physical separation.
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TABLE IV. - 16 IJm(200 SERIES)ALUMINUM/RP-IGEL PROPELLANTPROPERTIES
Observationmade after standingovernight.Measurementmade, after standing overnight,at 700 rpm for 6 hr. CO is original solidsconcentrationand C iscentrifugedsamplesolids concentration.Measured.afteragitation startingat 60 rpm.Densityvalueswere calculatedexceptvalues in parenthesiswhich were measured.
eFor gelled.samplesHaake measurementswere made after overnightstanding. For the remainder,measurementswerefmade as rapidlyas possible.All resultsfor 45 wt % and below are questionabledue to physical separation.
TABLEV. -PARTICLE SIZE IMAGEANALYSISRESULTS...... FOR ATOMIZEDALUMINUMPOWDER
DouglasC. Rapp and Robert L. Zurawski10. Work Unit No. o_
506-42-llg. PerformingOrganizationName and Address
, 11. Contractor Grant No.
National Aeronautics and Space AdministrationLewis ResearchCenterCleveland,Ohio 44135-3191 13.TypeofReportandPeriodCovered
12. SponsoringAgencyName and Address Technical Memorandum '
National Aeronauti cs and Space Admini stration 14.SponsoringAgencyCodeWashington, D.C. 20546-0001
15. Supplementary Notes
Prepared for the 24th Joint Propulsion Conference Cosponsored by the AIAA,ASME, SAE, and ASEE, Boston, Massachusetts, July ll-13, 1988. Douglas C. Rapp,Sverdrup Technology, Inc., NASALewis Research Center Group, Cleveland, Ohlo44135; Robert L. Zurawski; NASALewis Research Center.
'16. Abstract
Research efforts are being Conducted by the NASALewis Research Center to formu-late and characterize the properties of AI/RP-1 and RP-1 gelled propellants forrocket propulsion systems. Twenty four different compositions of gelled fuelshave been formulated with 5 and 16 pm, atomized aluminum powder in RP-1. Thetotal solids _:oncentration in the propellant varied from 5 to 60 wt %. Testswere conducted to evalua_te the stability and rheologlcal characteristics of thefuels. Physical separation of the solids occurred in fuels with less than50 wt _osolids concentration. The rheological characteristics of the AI/RP-1fuels varied with solids concentration. Both thixotropic and rheopectic gel ,.behaviorwere observed. The unmetallizedRP-I gels, which were formulatedby a ,_differenttechniquethan the AI/RP-Igels, were highly viscoelastic. A history Iof researchefforts which have been conductedto formulateand characterizethepropertiesof metallizedpropellantsfor variousapplicationsis also given.
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