NASA Technical Memorandum 106698

AIAA-94--3175

/ ) ;s-,-_..

Cryogenic Gellant and Fuel Formulationfor Metallized Gelled Propellants:

Hydrocarbons and HydrogenWith Aluminum

Wing Wong, John Starkovich,

and Scott Adams

TRW Space and Technology Division

Redondo Beach, California

Bryan Palaszewski

Lewis Research Center

Cleveland, Ohio

Prepared for the

30th Joint Propulsion Conference

cosponsored by AIAA, ASME, SAE, and ASEE

Indianapolis, Indiana, June 27-29, 1994

(NASA-TM-lO6698) CRYOGENIC GELLANT

AND FUEL FORMULATION FOR METALLIZED

GELLEO PROPELLANTS: HYDROCARBONS

AND HYDROGEN WITH ALUMINUM ReporttNov. 1989- Sep. 1993 (NASA. Lewis

National Aeronautics and Research Center) 2I p

Space Administration G3120

N95-I082I

Unclas

0019873

https://ntrs.nasa.gov/search.jsp?R=19950004409 2020-03-13T21:01:12+00:00Z

Cryogenic Gellant and Fuel Formulation for Metallized Gelled Propellants:Hydrocarbons and Hydrogen with Aluminum

Drs. Wing Wong, John Starkovich, and Scott AdamsTRW Space and Technology Division

Redondo Beach, CA

Bryan PalaszewskiNASA Lewis Research Center

Cleveland, OH

Abstract

An experimental program to determine the BETviability of nanoparticulate gellant materials forgelled hydrocarbons and gelled liquid hydrogen BTMSE

was conducted. The geUants includedalkoxides (BTMSE and BTMSH) and silica- BTMSHbased materials. Hexane, ethane, propane andhydrogen were gelled with the newly- CO2formulated materials and their rheological H21Ai

properties were determined: shear stress versus Ispshear rate and their attendant viscosities. KMetallized hexane with aluminum particles was MMH/Ala/so rheologically characterized. The propellantand gellant formulations were selected for the NASAvery high surface area and relatively-high

energy content of the gellants. These new NBPgellants can therefore improve rocket engine NFPspecific impulse over that obtained with NTOtraditional cryogenic-fuel gellant materials: rt**

silicon dioxide, frozen methane, or frozen O2/H2ethane particles. Significant reductions in the ptotal mass of the geUant were enabled in the FZT

fuels. In gelled liquid hydrogen, the total mass Qof geUant was reduced from 10-40 wt % of RP-1/Aifrozen hydrocarbon particles to less than 8 wt SAD% with the alkoxide. TEOS

TEM

Ngmenclature

Brunauer-Emmett-Teller surface

area measurement

Alkoxides with EthylHydrocarbons

Alkoxides with Hexyl

HydrocarbonsCarbon Dioxide

Hydrogen/Aluminum

Specific Impulse 0bf-s/Ibm)

Temperature in KelvinMonomethyl Hydrazine/Aluminum

National Aeronautics and SpaceAdministration

Normal Boiling PointNormal Freezing PointNitrogen TetroxideLimiting Viscosity if'a-s)

Oxygen/Hydrogen

Expulsion PressureHezoclccu'icMass Flow Rate

Rocket PropeUant- I/AluminumSelected Area Diffraction

TetraethoxysilaneTransmission Electron

Microscopy

TRW Thompson, Ramo, andWoolridge

ty Yield Stress (Pa)wt % Weight Percent

Introduction

Liquid rocket propellant engines, includingO_Hz engines, have significantly contributed

to many magnificent achievements in human

and robotic space flight.1 However, advanced

propulsion systems that offer improvedperformance are required to make the nextgeneration of launch vehicles and spacetransportation and exploration missionspossible. Studies by the NASA-LewisResearch Center_ have identified advanced

gelled propellants such as H2/AI, RP-1/A1, andother gelled propellants as showing significant

performance gains. Metallized propellantsmay have modestly higher specific impulses

(Isp increases of 5 to 6 lbt'-s/lbm for O_-I2/A1system, 60 wt % AI in the H2/A1 fuel)

compared to nonmetallized hydrogen fuels.For proposed NASA Mars evolution andexpedition missions, it has been estimated thatmetallized propellants can result in a 20 to 33%improvement in surface payload delivery

capability.2 More importantly for O2/RP-1/A1and NTO/MMI-FA1 propellants, adding metalcan deliver considerably higher propellant

density, depending on the application. Hence,both the tankage mass as well as the overallpropulsion system dry mass can besubstantially reduced. The propellant density

increaases and their attendant Isp changes withthe aluminum additives allow a payloadincrease of 14 to 35 percent by replacing theSpace Shuttle Solid Rocket Booster with aLiquid Rocket Booster using O2/RP-I/AI and

NTO/MMH/A1, respectively.3

In metallized fuels, it is imperative that thedense metal particles are dispersedhomogeneously throughout the fluid, andremain arrested in the liquid propellant duringstorage and transport. The distribution ofparticles is achieved by proper mixing, whilethe particle suspension requirement can be met

by gelling the metallized fueL Gelling alsoprovides additional benefits including thereduction of sloshing, potential reduction of

boil off or vaporization losses, betteroperational safety, and fuel storability. Thegelation of H2 fuel was successfully

demonstrated in earlier NASA studies.4,5. 6

More recently, gelation of metallized liquidpropellants, such as Inhibited Red FumingNitric Acid/Monomethyl Hydrazine (IRb'NA/MMH), has also been demonstrated at ambient

condition.7 However, the studies of cryogenic

fuels employed high loadings of silica particles(10 to 40 wt % of the fuel)8 to prevent the

metal particles from settling. However, silicadoes not make the combustion process more

energetic, making these type of gelledpropellants impractical for propulsionapplications. More efficient gellants thatprovide higher combustion energy aredesirable.

The silica gellant materials employed in earlierstudies were produced pyrogenically and havenominal surface areas and particle diameters of

approximately 100 m2 and 15 nm, respectively.The higher surface area translates into highergelling efficiency, and therefore less gellant isused. These smaller dimension gellant particlesmay be prepared using sol-gel synthesisprocedures and advanced processing methods.In particular, tailored organic polymers areattractive candidate gelling agents since thesematerials are efficient gelling agents for non-polar liquids and they contribute energy to thecombustion of the cryogenic gel propellants.

Technical Description

Objectives

The principal objective of this work was toexplore the feasibility of developing a newclass of gellant materials useful for formulatingadvanced cryogenic gel propellants. The goalwas to demonstrate the synthesis and

preparation of nanometer-dimension polymergellant materials which are capable ofefficiently gelling cryogenic liquid fuels anduseful for preparing attractive metallized gel

propellant formulations. A second objectivewas to provide a preliminary evaluation of the

rheological properties of candidate gelformulations to determine whether they haveattractive flow properties for propulsionapplications

Approach

The technical approach included thedevelopment of advanced gellant materialsthrough sol-gel syntheses and zero-surfacetension processing techniques, includingsupercritical drying and freeze drying, therebyobtaining the desirable features, such asnanoparticulate size and positive combustionenergy.

A second part of the program was thecharacterization of the rheology of gelled liquidpropellants (neat and metallized) at room andcryogenic temperatures, thereby demonstratingthe pertinent flow characteristics, and thestability of the gels and the particle dispersion.Early in the program, gelled liquid propellantsincluded hexane, propane, and ethane served tosimulate some of properties of gelled metallizedhydrogen at reduced cost. One of the mostimportant features was the metal loading in thegelled fuel. The measurement of the viscosityof gelled, metallized, liquid hydrogen wassought to demonstrate the gelling capability ofthe nanoparticulate gellants at extremely lowtemperatures.

The work performed under this project wasexploratory in nature and principally directed atproducing a new class of gellant materialsuseful for extremely low temperature service.The project was highly successful in meetingthis aim and has confu'med the attractive gelformation properties of nanoparticulatematerials for cryogenic fluids.

Technical Discussion

Gellant Sol-Gel Synthesis

Sol-gel material synthesis methods wereselected for the preparation of candidate

advanced gellant materials. Sol-gel processingtechnology has been extensively studied andused to prepare a wide variety of fine-microstructure film, foam and powder

materials.9 The technology utilizes solution-based polymer synthesis reactions and wellestablished product drying/recovery methodsfor preparing multi-component materials withhomogeneous molecules. The technology hasseveral attractive features; it is readily scalablefor producing large product volumes if

eventually required, it utilizes commerciallyavailable precursors for starting materials, andit is uniquely capable of preparingcombinational type materials like organo-ceramic polymers. This latter feature isparticularly needed for preparing gellantmaterials with selectable chemical propertiesand affinities for fluids with widely differingpolarities.

The candidate gellant materials were preparedfrom organic compound polymerizationreactions carried out in alcohol solution. The

starting compounds used in the synthesisreactions were boron and silicon esters of ethyland methyl alcohol. These compounds areknown as alkoxides and are commonly usedreagents in sol-gel synthesis of various glassand ceramic composition materials. Alkoxidescontaining ethyl (BTMSE) or hexyl (BTMSh0hydrocarbons were also used for synthesispurposes where an organic group was desiredin the final polymer gellant material. Thepolymerization reactions were initiated throughacid-catalyzed hydrolysis of the alkoxidecompound. Illustrative examples of the overallreactions involved in the synthesis of singleand copolymer composition products are givenin Equations 1, 2, and 3.

The hydrolysis and polymerization reactionsproceed to form weakly-coherent, translucentalcogels in the reaction medium. Gel formationtypically required anywhere from a fewminutes to 24 hours depending on the catalyst,monomer concentration and temperatureconditions employed.

m B(OCH3)3 + 3 m H20 -> m H3BO3 + 3 m CH3OHBoric Acid Methanol

n Si(OC2H5)4 + 4 n H20 -> n Si(OH)4 + 4 n C2H5OHSilanol Ethanol

HO[-B (OH)O]m[-Si(OH)20]nn

Linear borosilicate polymer

m H3BO3 + n Si(OH)4 -> or

[B203]mn[SiOz]n

Crosslinked borosilicate polymer

(1)

(2)

(3)

Product Drvin_ and Recovery. The

precursors react to-forin, coherent polymeralcogels from homogeneous solutionscontaining as low as 0.05 molar monomer.These polymer alcogels consist of delicate three-dimensional structural networks that cannot be

dried using conventional methods, such asvacuum bake and oven heating since thesemethods can lead to high surface tension forcesat the solid-fluid interface and cause the

networks to collapse. Two-zero surface tensiondrying techniques,freeze-drying andsupercdtical fluid p_ing, were used torecover products for this study. The freeze-dryprocess is depicted in Figure 1. It involvessolvent freezing followed by evaporation of thefrozen solvent at reduced pressure. Acommercial unit having a processing capabilityof 5 liters was used in this study. Critical fluid

processing involves the use of a fluid such asCO2 in a supercdtical state. By raising the

pressure and temperature above the criticalpoint of the exchange fluid (304.2 K and 73alto for CO2), it is possible to extract the

solvent from the alcogel without encounteringsignificant surface tension forces at the solid-fluid interface. The apparatus used forsupercritical processing is shown in Figure 2.

Nanoparticulate gellant materials with largespecific surface areas were successfully

prepared using both methods.

Physical Characterization. The particlesize and specific surface area of the polymergellant materials prepared by freeze-drying andsupercritical fluid processing were compared toselect the drying technique for preparingnanogellant materials. Film samples of gellantpowder were examined by transmissionelectron microscopy (TEM) and selected areadiffraction (SAD) todetermine the morphology

and the crystalline nature. Both drying methodsproduced particles which have similarfilamentousmorphology and are highlyamorphous as denoted in Figure 3.The

filamentous particles appear to have largelength-to-diameter aspect ratios (at least greaterthan 5 to 1). The particles produced bysupercritical fluid processing appear to besomewhat smaller in size. This fmding issupported by Brunauer-Eramett-Teller (BET)surface area measurements and supercritiealfluid processing was consequently selected for

production of nanogellant materials for furtherstudy.

Experimentally determined surface area (by

BET), the elemental composition (bycombustion analysis), and the heat of

combustion(by calorimetry) of supercriticalfluid processed gellant materials aresummarized in Table I. The diameters for the

individual, particles were calculated based onthe assumptions that the individual particles areuniform spheres, with a true density of 2.2

g/cm3. A true density is one where the totalmass of gellant in the particles was fullycompacted. The diameters of the uniformspheres were found to be 13, 4 and 2 nm, for

BTMSH, silica, and BTMSE respectively. TheBTMSE and BTMSH gellant polymers havelow H/C and C/Si ratios, which suggest thatthe polymers are highly crosslinked. TheBTMSH gellant exhibits slightly higher heat ofcombustion than the BTMSE gellant, which isattributed to BTMSH's longer aliphatic chain.The overall merits make the BTMSE gellant thepreferred gellant material for cryogenic fuels.

Rheological Characterization of Neatand Metallized Hexane Gels

Hexane was chosen as the liquid hydrogensimulant for initial gel rheological propertyinvestigation since it is a non-polar liquid witha moderately low surface tension and easy towork with at ambient temperature.

Cone/Plate Viscometer. Figure 4depicts the rheological properties ofnanoparticulate hexane gels that were measuredusing a Brookfield cone-plate viscometer. Coneand plate inslruments are preferred becausethey measure an absolute fluid viscosity.

Pseudoplastic Behavior. The viscosityof neat hexane gels containing differentconcentrations of nanoparticulate gellants wasmeasured at different shear rates to determine

their shear thinning properties. Gellant

concentrations were varied from 5 to 20 kg/m3(0.75 to 3.05 wt %), which is equivalent to ageUant concentration range of 0.86 to 3.45 wt% in liquid propane and ethane gels. Based onthe relative fuel densities, this concentration

corresponds to 7.0 to 28 wt % in liquidhydrogen gels. These nanoparticulate gelledpropane and ethane exhibited considerable

shear thinning, and the gel viscosity decreasesby over two orders of magnitude upon being

sheared at rates higher than 200 s-i, as seen inFigure 4. The shear-thinning properties ofthese gels are nearly identical for the BTMSE-derived gel and silica gel (derived fromtetraethoXysilane, TEOS) for fuels with thesame gellant loading. These results suggestthat the specific surface area and particle size ofthe gellant materials critically affect gellingefficiency.

Aluminum particle suspensions in hexane

gelled with 5 kg/m 3 (0.76 wt %) of BTMSEwere successfully prepared and observed to hestable for at least 4 months when kept in aspecially fabricated plexiglas viewingcontainer. Coarse (325 mesh, 44 microns)aluminum particles were selected for these testsand the loading level selected was 3.7 volume% (13.8 wt %) which corresponds (on arelative density basis) to a 60 wt % aluminumloaded hydrogen fuel gel. This resultdemonstrates that, even at a low gellantconcentration, sufficient gel yield strength isattained and large grain additives may be stablydispersed.

The metal-loaded dispersions exhibitpseudoplastic behavior similar to that observedfor the neat gels. Analysis of the effect of shearrate on viscosity and the effect of gellantconcentration on gel yield stress demonstratesthat both neat and metallized nanoparticulate

gelled fluids behave as _power-law" fluids.With this fluid model, the viscosities and the

corresponding shear rates can be correlated bya power curve. This behavior is illustrated inthe viscosity-shear rate data and it is shown inFigure 5. These gelled propellant systems

were prepared using only 7.5 kg/m3 (or 1.1 wt% gellant) and are loaded with 13.8 wt %aluminum. Viscosities for these gels also fellwithin the target viscosity profiles set byNASA-Lewis for gelled propellants. Thepseudoplastie index decreases from 0.40 to0.02 upon the addition of aluminum particles.

It is apparent from Figure 5 that both neat andmetallized nanoparticulate gels behave like aviscous fluid at very low shear rates. The yield

stress, ty, a relative measure of gel strength,can be determined by extrapolating to zeroshear rate. The yield stresses for a series ofgels are plotted as a function of the gellantconcentration in Figure 6. The inset log-log

plot shows that yield stress may be correlatedwith a simple equation:

ty= to(C -C_)_

where to is a prefactor, C is the gellant

concentration, a is an empirical constant, andCm is the minimum gellant concentration

necessary to form a coherent gel structure. Thefitted values for a, Cm, and to are 3.06, 0.7

kg/m3, and 2.6, respectively. Such equations

are indicative of percolating fractal stmctures.n

Rheolo_cal Characterization of Neat andMetallized Cryogenic Gels

The gelation properties of the nanoparticulategellant at cryogenic temperatures weredemonstrated with propane, ethane andmethane. The normal boiling points of these

cryogenic propellants are 230.1 K, 184.7 K,and 111.6 K, respectively, and are shown inTable IL

Liquefaction Apparatus and Cry_o-_. The liquefaction apparatusused ingelation demonstration and rheological testingemployed a copper condenser chilledwith

liquidnitrogen. This equipment was

successfully used to condense liquid ethane andpropane. However, attempts to condense theliquid methane were beset with recurrentplugging of the condenser/delivery tube sincemethane has a much narrower liquid

temperature range (21 K) than otherhydrocarbon fuels (81 K for ethane and 144 Kfor propane). Efforts were thereforeconcentrated on the demonstration of propane

and ethane gels with BTMSE.

Cryogenic gels were successfully prepared inan apparatus in which the condensed liquid fuelis mixed with gellant materials in an open-cup,vacuum insulated, glass dewar and agitatedwith an ultrasonic probe. A Searle viscometer

was modified to determine the cryo-rheological

properties of the cryogenic fuel gels. Themeasuring element consists of two coaxialcylinders. Cryogenic fuel was placed in thegap between the outer (stationary) and the inner(rotating) cylinders. The viscometer is vacuuminsulated to maintain thermal isolation. The

measuring element is connected to the torquesensing element of a Haake Rotoviscometerthrough a ferrofluidics feedthrough.

Cry_ogertic Gel Demonstration:Pseudoulastic Behavior. Neat propane gel was

preparecl using the gel preparation system and

7.5 kg/m3 (or 1.3 wt %) BTMSE gellant. Therheological properties of the propane and

ethane gels, containing 7.5 kg/m3 BTMSEgellant, were measured using the cryo-viscometer. The shear stress versus shear rate

profiles are given in Figure 7. The flowbehavior of the gels may he described by aCasson model12 :

tl/2- t yl/2 -- (n.. [gamma dot]) I/2

gamma dot = r (dr2/dr)

t=Tl(2_r2L)

where T = torque on the cylinder, L is thelength of the cylinder immersed in the gelledliquid, r is the radiusin the cylindrical gap ofthe viscometer, t2 is the rotation rateofthe

cylinder, ty is the yield stress of the gel and n**is the limiting viscosity at very high shear rates.

The values of ty and n_ are 6.8 Pa and 6.4 mPa-s for ethane gel and 1.7 Pa and 6.4 mPa-s forpropane gel. These results indicatethat thesetwo gels have similar limiting viscosity at highshear rates, but the ethane gel has a slightlyhigher yield stress.

The viscosity properties of the neat gels inFigure 8 can be correlated using the power-lawexpression. A regression analysis showed thatthe pseudoplastic indexes are 0.22 for bothethane and propane gels. The ethane gel has amuch thicker consistency compared to the

propane gel as evident by the higher k value

(0.47 Pa-sO.22 versus 0.22 Pa-sO.22). The high

pseudoplastic index for these gel systems isvery attractive for the design of low energy lossfluid transport systems and efficientatomization components.

Cry_o-Rheometer for Metallized

The existing literature on hydrogen viscositymeasurements describe techniques that involveoscillating disks, torsional disks, and a varietyof fine capillary mbes.12,13,14,1s,16 It wasanticipated that these approaches would not beapplicable to the metallized hydrogen gels dueto their non-Newtonian behavior. A direct

measurement approach, using a cone-and-platecryo-rheometer design, was therefore initiallysought. Additional comiderations for thedesign are the capability to condense liquidhydrogen, mix the metal additives and gellantmaterials with the liquid hydrogen, maintain

temperature within a narrow liquid window ofbetween 15 and 20 K during the rheologicalmeasurements, maintain an oxygen-depletedenvironment, and measure at low shear rate

region for rheologieal properties, such as yieldpoint and pseudoplastic behavior.

The initial design was developed on the basisof the successful experience with cone/plateviscometers, and with preparing propane andethane gels using an ultrasonic probe. Thedesign involved the reconfiguration of theviscometer to submerse a heat exchangechamber containing the cone-plate measuringelement into a liquid helium dewar. Thecondensed hydrogen would then be mixed withthe metallic additives and nanoparficulategellant inside the housing with the ultrasonicagitation of the in-situ piezoelectric (PZT)crystals. However, preliminaryexperimentation revealed that adequateultrasonic mixing could not be reliably attained,and that mechanical stirring would be required.The design was hence modified to allow the gelingredients be stirred in an external mixingchamber prior to transferring the hydrogen gelto the heat exchange chamber through aninsulated delivery tube. The external mixerdesign was still considered to be toocomplicated to operate and its design could not

ensure effective transfer of the gel due to theexcessive thermal losses from the cryogenic(liquid helium) valves and long delivery tube.Due to these difficulties, the cone/plateviscometer design concepts were eventuallyabandoned.

Capillary_ Flow Viscometer Design.The final design selected was a capillary flowviscometer which is typically used to measureflow behavior of fluids in the high shear rateregion. The key elements consist of twointernal mixing cups of known volumeconnected with a transfer tube of predeterminedinner diameter (0.001 m) and length (1 m).Each mixing cup is equipped with silicondiodes and heaters for temperature sensing andcontrol. They are housed in a heat exchangechamber which is submerged in liquid heliuminside a magnetic dewar. The interior of theheat exchange chamber is either filled withhelium gas for thermal exchange betweenmixing cups and dewar, or in a vacuum toprovide thermal isolation of the mixing cups.Hydrogen gas is condensed in the temperature-controlled buffers which function as liquidhydrogen reservoirs. The liquid hydrogen ismixed with metallic additives and

nanoparticulate geIlant in the mixing cupsequipped with long-shaft stirrers and poweredby twin external motors. An in-situ liquid levelprobe, which consists of a string of series-connected carbon resistors, is used to

determine the quantity of hydrogen gel insideeach mixing cup.

The key measurement is the time required toexpel a known quantity of gel from one mixingcup to another. The gel expulsion is achievedthrough releasing helium gas into one of thetwo the mixing cups to establish a constantpressure gradient between the two mixingcups. These measurements provide the flowrate versus expulsion pressure (Q vs. P) profilefor a given gel. The viscosity for Newtonianfluids can be determined using the Hagen-Poiseuille equation:

n = JtAPRc4/(8 QL)

andfor power-lawfluids:

t = AP Rc / (2 L) = K (gamma-dot) a

n = K (gamma-do0 n-1

where Re and L refer to the inner radius and the

length of the delivery tube, respectively.

Demonstration Test. A check-out run

of the cryo-rheometer was conducted withliquid neon (normal boiling point [NBP] of27.1 K) and demonstrated that the system cansafely be operated at liquid hydrogen

temperatures.

This equipment was subsequently used todetermine the viscosity of gelled hydrogen. Inthis experiment, nanoparticlulates derived fromBTMSE (7 wt %) were charged into themixers, followed by liquid hydrogen. Afterthermal and chemical equilibria were attained(as evidenced by the temperatures becomingconstant and by and zero pressure/gradientacross the two mixers), helium gas wasintroduced into the first mixer, thereby creatinga positive pressure of 1,000 Pa which expelledthe hydrogen gel into the second mixer. Bymonitoring the liquid level probe inside the firstmixer, the mounts of hydrogen gel transferredwas determined, where each incremental

voltage increase corresponds to a rise inresistance of the level probe which reflects adrop in liquid level between the two sequentialcarbon resistors.

h was found that approximately 48 mL weretransferred in 120 seconds. AssumingNewtonian behavior,thistransfer roteamounts

to a viscosity of 0.064 mPa-s at a shear rate of

4,000 s-l. This is about 5 times higher than theviscosity of liquid hydrogen (0.013 Pa-s)reported in the literature and demonstrates thethickening effect. These viscosities are shownin Table IL These shear rates and higherviscosities are not representative of those in afull-scale propulsion application, but they showa trend that implies that the gelled hydrogenwill likely have a minimal viscosity difference

from the ungelled hydrogen fuel under highvelocity flow conditions.

Concluding Remarks

Advanced gellant materials were synthesized bymeans of the sol-gel methodology usingorganometallic compounds. The primary goalof this work was to explore the feasibility of anew class of nano-size (10-9 m) materials for

the effective gelation of metallized cryogenicfuels including suspended aluminum particlesin liquid hydrogen.Two gellant materials wereformulated specifically to provide a positiveenergy output upon combustion. The newly-formulated alkoxide gellants can provide 3.1 to5.5 kcaFg. Zero-surface tension dryingtechniques,such asfreeze-dryingandsupercritical fluid drying, were employed toprocess the polymer products into large specificsurfacearea,nano-size,particulateswhichserve as effective gellants. Polymer gellantsproduced by supercritical fluid process exhibitspecific surface areas in the range of 600 to

1,000 m2/g and possess average particlediameters of 2 to 4 nm. On the basis of its

overall energy and high surface area merits,BTMSE and BTMSH polymer gellant materialswas selected for rheological studies.

The gelling efficiency of polymer-derivednanoparticulates was successfully demonstratedusing various neat and metallized propellantsimulants includinghexane, propane and

ethane. Stable metallized hexane gels were

prepared with as little as 5 kg/m3 (0.75 wt %)nanoparticulate concentration, which wouldcorrespond to a 7.0 wt % gellant loading forhydrogen. This equates to a six:fold reductionin gellant concenwation over thebest gellingagents demonstrated in the 1960's for gellingliquid hydrogen. The rheology of hexane gels

in the low (less than 250 s-l) shear rate rangewas characterizedwith a Brooldieldcone-plateviseometer. Both neat and metallized gels act as"power-law" fluids and exhibit pseudoplasticbehavior, a behavior that is desirable for the

storage and transportation of gelled propellants.

Upon the addition of 13.8 wt % aluminumparticles, these gels thickened and exhibitedincreased viscosity and the predictedpseudoplastic behavior. The yield strength ofthe 0.75 wt % polymer gellant in hexane wassufficient to prepare stable aluminumsuspensions of interest to this project.

Cryogenic gels with 7.5 kg/m3 (1.3 wt %) ofpolymer-derived particulates in propane (230-Kelvin, NBP) and ethane (185 K, NBP) were

successfullyprepared using a specially

designed liquefaction and ultrasonic-dispersionsystem. The rheological properties in the lowshearraterange were characterizedusing a

modified Searle eryo-viscometer. These gelledcryogens also exhibit "power-law" fluid andpseudoplastic behavior. Their flow behaviorcan be described by a Casson fluid model. Theethane gels have a thicker consistency (0.47 Pa-

so.22 vs. 0.22 Pa-s0.22) and a higher relativeyield strength (6.8 vs. 1.7) than the propanegels.

In order to evaluate the gelling efficiency ofpolymer-derived nanoparticulates for extremelylow temperature service, a 15-K capillary-flowviscometer was designed and assembled with acapability to mix pre-loaded metal particleadditives and gellant with condensed hydrogenin-situ. A demonstration run was conducted

with liquid hydrogen gel whose polymergellant mass was 7 wt %. This hydrogen gelexhibited a viscosity of 0.064 Pa-s at amoderate shear rate of 4,000 s-1. This is

approximately only about 5 times higher thanthat for liquid hydrogen alone. These shearrates and higher viscosities are notrepresentative of those in a full-scalepropulsion application, but they show a trendthat implies that the gelled hydrogen will likelyhave a minimal viscosity difference from theungelled hydrogen fuel under high velocityflow conditions.

These results successfully demonstrated theeffectiveness of polymer-derived gellants forcryogenic propellants, and verified the generalapproach of using nanoparticulates to gelcryogenic fluids. Additional studies tocharacterize the rheology of neat and metallized

hydrogen gels have also been conducted. The

objective of any future gellant optimizationwould be toproduce a gellant material that isefficient for gelling liquid hydrogen atconcentrations less than 2 to 3 wt %. At these

concentration levels gellant material will have amuch-reduced thermodynamic and enginesystem performance penalty over the currently-required 7 to 8 wt % gellant level.

Aclqn0wledgements

This paper describes work conducted by TRWSpace and Technology Division under ContractNAS3-25793 during the period of November1989 to September 1993. The project wasmanaged by Drs. John A. Starkovich and WingC. Wong of TRW. Major contributors includedthe following:

Dr. William Davison for gellant synthesis,Hareesh Thridandam for the rheologicalmeasurements and data analysis, William Burrfor the cryo-rheometer design, Hsiao-Hu Pengfor the cryo-rheometer fabrication anddemonstration, Dr. Scott Adams for hydrogengel rheology. Administrative review wasconducted by Myrrl J. Santy, Manager ofTRW's Advanced Materials and Products

Department and NASA project managementwas provided by Bryan A. Palaszewski.

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A. Van Itterbeek, H. Zink, and O. VanPaemel, "Viscosity Measurement inLiquified Gases", Cryogenics, 2, 210,1962

N. S. Rudenko and V. G. Konareva,Zh. Fiz. Khim., 37, 2761 (1963)

M. Fatehi, M. Kaviany, R. Sonntag,B. Squires, and C. Kim, "Gelation ofLiquid Nitrogen with Butane", FinalReport, NASP Contractor Report,University of Michigan, Grant NAG3-850, May 1990.

R. Barton, "Cryogenic Systems",McGraw-Hill, 1966.

10

_ _ O_ iI_ m_l _

O0 _q_

- 7

_ VAPOR

a. Process Design

b. Commercial Unit

Figure 1. Nanogellant Preparation by Freeze Drying

12

t

EMIrmmlATUR

a. Process Design

b. Research Unit

Figure 2. Nanogellant Preparation by Supercritical Fluid Processing

13

Figure 3. TEM Photomicrographs of Supercritical Fluid Processed BTMSE Gellant Particles(SAD Pattern shown in inserted picture)

14

lo,o0oCONEAIND_PLATEVISCOMETER

--5

250 IO0 1SO 2OO 2SO

SHEAR RATE.S"1

Figure 4. Pseudoplastic Behavior of BTMSE-Hexane Gelled Propellants

15

lOO,000

OgOo'J5;

1,000

4O

SJO2,AJ

lo

40.1 1 10 100 1,0o0

SllEAJB RATE, $-1

Figure 5. Power-Law Behavior of Nanoparficulate Gelled Hexane

16

4.0004.OO0

" 1,000

GELLANT CONCENTRATION ko/m 3

Figure 6. Yield Stress Dependence on Gellant Concentration(BTMSE-Hexane Gel at 298 K)

17

21

24

20

|

4

SEARIi:V1SCOMETER

BTMSE GELLANT

7.5 kg/m 3

! I I I I [ I I60 120 180 240 300 360 " 420 480

SHEARRATE,1/SEC

Figure 7. Rheological Properties of Nanoparticulate Gelled Cryogens(with Fitted Casson Fluid Models)

18

100

80 -

60-

40-

m

2 -

4_

a. 20E

o

10=>

Zu,lIlc< 6o.

4

I0

BTMSE GELLANT

7.5 kg/m3

I I I ! I I !20 40 60 80 100 290 400

SHEARRATE,1/SEG

I8OO

Figure 8. Pseudoplastic Behavior of Nanoparticulate Gelled Cryogens(with Fitted Power-Law Models)

19

Form ApprovedREPORT DOCUMENTATION PAGE OMBNo 0704-0m8

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1. AGENCY USE ONLY (Leave b/ank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

August 1994 Technical Memorandum4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Cryogenic Gellant and Fuel Formulation for Metallized Gelled Propellants:Hydrocarbons and Hydrogen With Aluminum

6. AUTHOR(S)

Wing Wong, John Starkovich, Scott Adams, and Bryan Palaszewski

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, D.C. 20546- 0001

WU-506-42-72

8. PERFORMING ORGANIZATIONREPORT NUMBER

E-9059

10. SPONSORING/MONITORINGAGENCY REPORT NUMBER

NASA TM- 106698AIAA-94-3175

11. SUPPLEMENTARY NOTESPrepared for the 30th Joint Propulsion Conference cosponsoredby AIAA, ASME, SAE, and ASEE, Indianapolis, Indiana. June 27-29, 1994. W'mgWong, John Starkovich, and Scott Adams, TRW Space and Technology Division, One Space Park, Redondo Beach, California 90278 (work funded byNASA Contract NAS3-25793); Bryan Palaszewski, NASA Lewis Research Center. Responsible person, Bryan Palaszewski, organization code5310,

(216) 977-7493.12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Categories 15, 16 and 20

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

An experimental program to determine the viability of nanoparticulate gellant materials for gelled hydrocarbons andgelled liquid hydrogen was conducted. The gellants included alkoxides (BTMSE and BTMSH) and silica-based materi-als. Hexane, ethane, propane and hydrogen were gelled with the newly-formulated materials and their rheologicalproperties were determined: shear stress versus shear rate and their attendant viscosities. Metallized hexane withaluminum particles was also rheologically characterized. The propellant and gellant formulations were selected for thevery high surface area and relatively-high energy content of the gellants. These new gellants can therefore improverocket engine specific impulse over that obtained with traditional cryogenic-fuel gellant materials: silicon dioxide,frozen methane, or frozen ethane particles. Significant reductions in the total mass of the gellant were enabled in thefuels. In gelled liquid hydrogen, the total mass of gellant was reduced from 10-40 wt % of frozen hydrocarbon particlesto less that 8 wt % with the alkoxide.

14. SUBJECT TERMS

Gelled propellants; Gellants; Metal propellants

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATIONOF REPORT OF THIS PAGE

Unclassified Unclassified

NSN 7540-01-280-5500

19. SECURITYCLASSIFICATIONOF ABSTRACT

Unclassified

15. NUMBER OF PAGES

2116. PRICE CODE

A0320. LIMITATION OF ABSTRACT

Standard Form 298 (Rov. 2-89)Prescribed by ANSI Std. Z39-18298-102