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AN EVALUATION OF GREEN PROPELLANTS FOR AN ICBM POST-BOOST PROPULSION SYSTEMBrian J. German*, E. Caleb Branscome*, Andrew P. Frits*, Nicholas C. Yiakas*, and Dr. Dimitri N. Mavris†

Aerospace Systems Design LaboratorySchool of Aerospace EngineeringGeorgia Institute of Technology

Atlanta, GA 30332-0150

ABSTRACT *†

Propellant toxicity is a major concern in storing,maintaining, and transporting strategic missiles. Manylow toxicity “green” propellants have been developedwhich hold the potential of increasing the safety andlowering the operation and support costs of liquid-fuelled strategic missile propulsion systems. This studyevaluates several green propellants for use in a notionalnext-generation post-boost propulsion system (PBPS).The mission and physical dimensions for this PBPSwere defined by the requirements of the currentMinuteman III propulsion system rocket engine(PSRE). Possible propellants were initially screened interms of toxicity, performance, and technical feasibilityfor the PBPS application with a multi-attribute rankingmethod based on an overall evaluation criterion (OEC).Promising propellants were identified, and candidatePBPS concepts were developed and sized for each ofthese propellants. These concepts were evaluated interms of weight, cost, and technical risk to determinewhich concepts, and hence propellants, show the mostpromise for the application. Probabilistic techniqueswere employed to explore the effects of uncertainty inthe propellant performance and structural weightestimates. The results indicate that high-test peroxide(HTP) combined with either an ethanol-based nontoxichypergolic miscible fuel (NHMF) or competitiveimpulse non-carcinogenic hypergol (CINCH) is a veryviable propellant solution.

NOMENCLATUREACS Attitude Control SystemCDF Cumulative Distribution FunctionCINCH Competitive Impulse Non-Carcinogenic HypergolDACS Divert Attitude Control SystemHTP High Test Peroxide (High Concentration H2O2)Isp Specific ImpulseHTPB Hydroxy-Terminated PolybutadieneMM MinutemanMM III Minuteman IIIMMH Monomethyl HydrazineNIOSH National Institute for Occupational Safety and

Health

*

Graduate Research Assistant† Director, Aerospace Systems Design Laboratory, Boeing Prof.Copyright © 2000 by Dimitri Mavris. Published by the DefenseTechnical Information Center, with permission. Presented at 2000Missile Sciences Conference, Monterey, CA.DISTRUBUTION STATEMENT: Approved for public release;distribution is unlimited

NHMF Ethanol Nontoxic Hypergolic Miscible FuelNTO Nitrogen TetroxideOEC Overall Evaluation CriterionPBPS Post Boost Propulsion SystemPBV Post Boost VehiclePE PolyethylenePSRE Propulsion System Rocket EngineREL Recommended Exposure Limits

INTRODUCTIONAs evidenced by the Minuteman III and Peacekeepersystems, storable liquid rocket propellants have beenhistorically preferred for land-based ICBM post-boostpropulsion. Typically consisting of a hydrazine-basedfuel and a nitrogen-based oxidizer, they have beenselected for many ICBM upper stage and spacecraftapplications because they offer very high performancefor storable (non-cryogenic and non-degrading)propellants. The specific impulse for many of theseformulations approaches 300 seconds. As hypergolicliquids (propellants that ignite automatically uponmixing) these formulations also allow for the precisethrust metering/impulse cycling necessary for thestringent angular positioning and range maneuveringrequirements of the post-boost application.

These propellants have one major detriment, however:toxicity. Any significant exposure to either the liquidsor vapors can be extremely harmful or fatal. A leak orspill could be devastating both in loss of life and inenvironmental damage. The threat of a spill incidentmandates the need for costly safety systems andprocedures and causes concern in transporting thepropellants. Additionally, planned future reductions inexposure limits from the Occupational Safety andHealth Administration (OSHA) and the NationalInstitute for Occupational Safety and Health (NIOSH)will further constrain the handling of the propellants.

Based on these concerns, many new lower toxicitypropellants have been undergoing development. Thesepropellants include fuels such as dimethyl-2-azidoethylamine, known by the 3M trade nameCompetitive Impulse, Non-Carcinogenic Hypergol(CINCH)1, and a doped ethanol Nontoxic HypergolicMiscible Fuel (NHMF)2,3 that may offer comparableperformance to monomethyl hydrazine (MMH).Renewed interest in high concentration hydrogenperoxide, also called high-test peroxide (HTP), as a lesshazardous oxidizer has also spurred significant

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research4. These, in addition to inherently lessdangerous solid, hybrid, and gel formulations, are oftenclassified as “green” propellants because of the lowerenvironmental and personnel dangers that they posecompared to other high-performance propellants.

Although less toxic, these propellants do not have aproven record for ICBM application. Research andtesting must prove them to be capable of hypergolic ornear instantaneous ignition, to provide high levels ofspecific impulse, and to be storable without significantdegradation for a service life of as much as 30 years.Hypergolicity is a challenge because many of theproposed formulations require soluble catalysts toprovide the capability. Storage life is a concern forHTP because it degrades significantly over time atnormal storage temperatures.

This study evaluates the potential of the various greenpropellants for use in a notional next-generation Post-Boost Propulsion System (PBPS). The authors assertthat the most effective method of implementing thisevaluation is through discerning the system level effectsof the propellant selection. This evaluation isaccomplished by the following process:

• Identify propellant formulations that have the mostpotential for the application

• Formulate and size a PBPS concept for eachpropellant, incorporating proposed propulsiontechnologies that may be required to realize systemperformance

• Evaluate the concepts in terms of key metrics suchas weight, cost, and technical risk

To identify green propellants with the most potential forthe application, an Overall Evaluation Criterion (OEC)method was used. This method allowed the ranking ofpropellant candidates based on qualitative andquantitative attributes such as performance, systemcomplexity, and toxicity. After comparing propellantcombinations through the OEC method, the mostplausible candidates from each propellant “family” (e.g.liquid, solid, etc.) were selected for further analysis. APBPS concept was sized for each candidate propellantbased on extrapolations from current systems. Theseconcepts were then compared based on considerationssuch as their total weight, cost, and possible technologydevelopment issues. These results were used to drawconclusions on the applicability of the various greenpropellants for a future PBPS system. This process issummarized in Figure 1.

The current Minuteman III Propulsion System RocketEngine (PSRE) uses monomethyl-hydrazine andnitrogen tetroxide, two highly toxic hypergolicpropellants that provide good specific impulse as well

as the rapid restart capabilities required for the precisepositioning of multiple reentry vehicles. For thepurposes of this study, the MM III PSRE has beentaken as the archetype for the mission definition andphysical dimensions of the candidate concepts.

PropellantCombinations

OverallEvaluationCriterion

Candidate GreenPropellants

ConceptDefinitions

SizedConcepts

Most PlausibleGreen Propellants

LargestOEC Value

LowestSystemWeight,

Cost,and Risk

Figure 1: Overview of Propellant Evaluation Method

PROPELLANT IDENTIFICATIONAs a first step in identifying candidate greenpropellants, extensive data available in the literaturewere collected for a wide variety of propellantformulations. The data consisted of metrics that arevaluable for evaluating propellant applicability in theareas of environmental and/or personnel hazard (i.e.“greenness”), performance, and PBPS designintegration. The following quantitative metrics werecompiled for each propellant formulation:

• Mixture ratio• Densities of oxidizer and fuel• Viscosities of oxidizer and fuel• Vapor pressures of oxidizer and fuel• Frozen equilibrium specific impulse for an

expansion from 300 psia to 14.7 psia• National Institute for Occupational Safety and

Health (NIOSH) recommended exposure limits(REL) for oxidizer and fuel

The NIOSH recommended exposure limits were used toquantify propellant toxicity because these are the moststringent government guidelines5.

In addition to these quantitative metrics, qualitativerankings were established for certain characteristics thatare either not easily represented as continuous numericvalues or not widely available other than asgeneralizations. These rankings were specified asintegers ranging from one to five with threecorresponding to the metric ranking of the current MMIII PSRE, one corresponding to significantly worse thanthe existing system, and five indicating significantlybetter than the baseline. These metrics include thefollowing:

• Storability of oxidizer and fuel• Ease of inerting oxidizer and fuel

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• Ignition delay• Complexity of implementing the propellant in a

propulsion system

Data was obtained for 68 distinct propellantformulations (i.e. combinations of oxidizer and fuel at acertain mixture ratio) that have been used in productionrocket engines or have been studied extensively inlaboratory experiments. These formulations includeliquid (including mono- and bi-propellants), solid,hybrid, and gel propellants. In addition to greenpropellants, traditional propellant formulations wereincluded for comparative purposes. Cryogenicpropellants were eliminated from consideration becauseof their incompatibility with the Minutemaninfrastructure and their very low storage lives, andgaseous propellants were omitted because their verylow density make packaging impractical. Thepropellant data was culled from several sourcesincluding technical reports and papers, the proceedingsof the Second International Hydrogen PeroxidePropulsion Conference, the NIOSH internet web site,and the Weight Engineers Handbook. These sourcesare listed as references 6-13.6,7,8,9,10,11,12,13

In order to identify the propellants that represent themost suitable blend of performance and “greenness”, anoverall evaluation criterion method was used. TheOEC is a single numerical metric that can be used tocompare alternatives. The OEC is formulated such thathigher values represent more highly preferred solutions.The core of the OEC method is its defining equation.The equation consists of a sum of several terms, each ofwhich represents a particular characteristic upon whichthe alternative is evaluated14. Each term is formed as aquotient comprised of the value of the respectivecharacteristic for a particular alternative and the valuefor a chosen baseline alternative. Because highervalues of the OEC are intended to represent morefavorable options, the numerator and denominator ofthis quotient are interchanged as necessary to obtain thedesired direction of optimality. For instance, ifincreasing a certain metric is desirable, the value for thealternative is placed in the numerator with the baselinevalue in the denominator. This formulation will yield avalue of the quotient greater than 1.0 for an alternativewith a higher value of the metric than the baseline. Theratio is inverted for metrics that are more desirablewhen minimized. The general form of the OECequation is shown in Figure 2.

OEC = α MetricBaseline Metric

+ β Baseline Metric

Metric

+ γ Metric

Baseline Metric

Weights indicate importance of metric in decision

Maximize Metric Minimize Metric Maximize Metric

Figure 2: General Form of the OEC Equation

As shown in the figure, the terms of the OEC equationare preceded by scaling factors. These factors are usedto weight the importance of each term relative to theOEC metric. The weights are chosen as fractionswhose sum is 1.0. By choosing such a format, the valueof the OEC for the baseline alternative is 1.0.Alternatives with an OEC value greater than that of thebaseline are more desirable, while alternatives withvalues less than the baseline are less favorable.

For the purposes of propellant selection, an OECequation was developed that is a function of thepreviously identified propellant metrics. The OEC isshown in Equation 1 below.

Equation 1: Propellant Selection OEC

+

+

+

=

BLBLBL Greenness

Greenness

Isp

Isp

WeightSpecific

WeightSpecificOEC γβα

+

DelayIgnition

DelayIgnition

Complexity Sys.

Complexity Sys. BL

BL

εδ

The specific weight and greenness terms shown in theequation are defined as functions of multiple metrics.The specific weight is the inverse of the weightedaverage of the oxidizer and fuel densities. As such, it isa function of mixture ratio that indicates the volumetricefficiency of the propellant combination. Equation 2shows the expression for the greenness metric.Formulated as a general indicator of the propellantenvironmental and personnel hazard, it is intended tocapture not only the direct toxicity of the propellant butalso the ease of containing and inerting any propellantspills. For this reason, it was specified as a function ofthe REL to indicate toxicity directly and metrics such asvapor pressure (VP) and ease of inerting to indicate thehazard associated with a propellant leak. Thelogarithms of the REL and vapor pressure metrics thatappear within the equation are used to compress theorders of magnitude variations that can occur in theparameters for different propellants into a linear formsuitable for application in an OEC.

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Equation 2: Toxicity OEC Term

+

+

=

)RELFuel*100log(

)RELFuel*100log()REL.Ox*100log(

)REL.Ox*100log(6Greenness

Greenness

BLBLBL

γγ

+

+

+

BLBL

BLBL

FuelInert

FuelInert

Ox.Inert

Ox.Inert

)VPFuellog(

)VP(Fuellog

)VPOx.log(

)VPOx.log(

This OEC method for propellant selection is not uniqueand does not necessarily assure the best selection. It isonly one of several decision methods for rankingalternatives based on subjective and objective data. Italso suffers from some shortcomings including thepresumption that all attributes are independent. TheOEC method was chosen, however, because it wastractable and provided good visibility and traceablityfor the propellant pre-screening process.

After developing the propellant evaluation OECequation, a series of weighting scenarios was developedagainst which the propellants should be judged. Eachscenario is comprised of a set of weighting scalars,whose sum is 1.0, that indicate the relative importanceof each metric to the OEC. Multiple scenarios wereused to prevent a biasing of the results caused by asingle particular choice of weightings. This methodensures that no propellants are eliminated fromconsideration because of poor performance on only onescenario. Propellants were judged on their suitabilityacross the entire spectrum of scenarios studied. Table Ishows the scenarios developed for propellant screening.As shown in the table, the six scenarios rangesuccessively in emphasis from performance to toxicity.

Table I: Propellant Evaluation Scenarios

Scenario 1 Scenario 3Scenario 2 Scenario 4 Scenario 5 Scenario 6Performance

ScenarioToxicity

Scenario

Scenario

Specific Weight, Isp, Toxicity,

System Complexity,

Ignition Delay,

Performance (1) 0.3 0.5 0 0.1 0.12 0.3 0.4 0.1 0.1 0.13 0.3 0.3 0.2 0.1 0.14 0.25 0.25 0.3 0.1 0.15 0.25 0.25 0.4 0.1 0

Toxicity (6) 0.2 0.2 0.5 0.1 0

α β δγ ε

Using the weightings within each scenario, the OECwas calculated for each propellant within the databaseusing the current MM III PSRE propellants, NTO andMMH, as the baseline. From the results, the propellantwithin each family with the highest average OEC valueacross the scenarios was noted. An example of this“average” OEC performance for some of thepropellants examined is shown in Figure 3. In thisexcerpt from the OEC results, it is clear theHTP/ethanol NHMF is, on average, superior to theother propellants listed for the spectrum of weighting

scenarios considered. The HTP/CINCH formulationclosely follows. It is interesting to note that theHTP/NHMF is superior to the NTO/MMH that is usedin the current PSRE in every case except for the purelyperformance weighted scenario in which it trails onlyslightly. The CINCH formulation outperforms theNTO/MMH even for the performance scenario.

0

0.5

1

1.5

2

2.5

3

NitrogenTetroxide/MMH

98%HTP/Ethanol

90% HTP/Hydrazine

Whole Fuming Nitric Acid/JP-4

HTP/CINCHO

EC

Performance Scenario

Scenario 2

Scenario 3

Scenario 4

Scenario 5

Toxicity Scenario

Figure 3: Example of Average OEC Across the Spectrumof Scenarios

The propellants within each family of similar concepts(e.g., liquid, solid, etc.) with the highest average OECperformance were selected as candidates for which asized PBPS concept would be developed. Thesepropellants are the following:

• Ethanol Nontoxic Hypergolic Miscible Fuel(NHMF) / High Test Peroxide (HTP) liquid

• Competitive Impulse Non-Carcinogenic Hypergol(CINCH) / HTP liquid

• Monomethyl hydrazine (MMH) / nitrogen tetroxide(NTO) loaded gel

• Polyethylene (PE) / HTP hybrid• Aluminum / hydroxy-terminated polybutadiene

(HTPB) / ammonium perchlorate solid

Figure 4 shows the weighted values of each OEC termfor the selected liquid, hybrid, and gel propellantsevaluated for the performance and toxicity scenarios.Because desirability relative to the baseline iscomprised of OEC values greater than 1.0, it is clearthat all of the selected propellants exhibit significantlylower toxicity and, at worst, only marginally decreasedperformance relative to the MMH/NTO liquid used inthe existing MM III PSRE. These results indicate thatthe selected green propellants should prove goodcandidates for the PBPS application.

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0

0.5

1

1.5

2

2.5

OE

C

Ignition Delay

System Complexity

Greenness

Specific Impulse

Specific Weight

98% HTP / Ethanol NHMFLiquid

Gelled NTO/MMH98% HTP / Polyethylene

Hybrid

Per

form

ance

Sce

nario

Tox

icity

Sce

nario

Per

form

ance

Sce

nario

Tox

icity

Sce

nario

Per

form

ance

Sce

nario

Tox

icity

Sce

nario

Per

form

ance

Sce

nario

Tox

icity

Sce

nario

98% HTP / CINCH

Figure 4: OEC Component Breakdown for Selected Propellants

Current PSRE 1 2 3 4 5

Propellant MMH / NTOliquid

HTP / alcohol liquid

HTP / CINCH liquid

HTP / polyethylene

NTO / MMH gel

HTPB / ammonium perchlorate solid

Thrust metering system

valving valving valving valving valvingpintle/hot gas bypass

system

Thrust vectoring

gimballed engine

gimballed engine

gimballed engine

gimballed engine

gimballed engine

gimballed engine

System typeducted primary

propellant thrusters

ducted primary propellant thrusters

ducted primary propellant thrusters

ducted primary propellant thrusters

ducted primary propellant thrusters

ducted hot gas thrusters/DACS

Thrust metering system

valving valving valving valving valvinghot gas bypass

system

Feed systempressurized

cold gaspressurized

cold gaspressurized

cold gaspressurized

cold gaspressurized

cold gasno feed system

Pressure tank shape

cylindrical spherical spherical spherical spherical no tank

Fuel tank shape cylindrical spherical spherical cylindrical sphericalcylindrical thrust

chamberOxidizer tank

shapecylindrical spherical spherical spherical spherical

integral to fuel tank/thrust chamber

Post Boost Propulsion System Concepts

Propellant Storage

Assemblies

Primary Propulsion

System

Attitude Control System

Propellant Expulsion and

Distribution Systems

Figure 5: Matrix of Candidate PBPS Concepts

CANDIDATE CONCEPT DEFINITIONIn order to develop a PBPS concept for each propellant,several alternative subsystem design and technologyconcepts were explored. The subsystem categoriesinclude the primary propulsion system, the attitudecontrol system, the propellant expulsion anddistribution systems, and the propellant storageassemblies. Structural support alternatives were notinitially explored during the development of PBPSconcepts. This decision to isolate structuraltechnologies was made in order to allow comparison ofthe concepts only through the implications of thepropellant selection on the subsystem and components.

For the purposes of developing conceptualconfigurations, modified versions of thealuminum/magnesium monocoque structure from theexisting MM III PSRE were assumed.

The principal design decisions in the development of aPBPS configuration are those defining the primarypropulsion system. The primary propulsion systemdesign decisions were classified as selection of thepropellant formulation, the impulse cycling method,and the thrust vectoring technique. Because conceptswere developed for each propellant formulation, the

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decisions on impulse cycling and vectoring mustcorrespond to the particular propellant.

Based on these subsystem alternatives, a notional PBPSsubsystem configuration was developed for each of thefive selected propellants. The five configurations areoutlined in Figure 5. These configurations were formedby considering the subsystem options that wereexpected to yield the minimum weight and minimumtechnological risk solutions. For instance, all of theconfigurations employing liquid propellants containspherical propellant storage tanks. Trade studiesindicated that these tanks provided the minimum weightsolutions and could be packaged within the dimensionalenvelope of the current MM III PSRE. Also, the liquidsystems were all designed with cold gas expulsionsystems. Cold gas systems were believed to providelower weight (relative to a pump) and more accurateexpulsion control (relative to a hot gas system). Forthrust vectoring, all designs incorporate mechanicallygimbaled engines. These systems were viewed to bepotentially lighter and lower risk than fluidic control,jet tabs, or jet vanes.

The systems incorporating solid propellant componentshave some significant differences from the liquidpropellant concepts. For instance, both the hybridHTP/PE and the solid ammonium perchlorate/HTPBsystem use cylindrical fuel storage assemblies thatserve as the combustion chambers. The cylindricaldesign allows for integration of a solid grain conducivefor even burning. In the pure solid system, the oxidizerconsists of grains commingled with the fuel, while inthe hybrid system, the oxidizer is stored as a liquid inseparate spherical tanks. These concepts also havesignificantly different attitude control systems (ACS)configurations than their liquid counterparts. Thehybrid system uses the oxidizer as a monopropellant forthe attitude control engines. The pure solid systememploys ducted hot gas from the solid propellant gasgenerators and a divert attitude control system (DACS)to provide a multi-tiered approach to attain precisevernier positioning.

HTP/NHMF, HTP/CINCH LIQUID CONCEPTSThe schematic for the HTP/NHMF concept is shown inFigure 6. The HTP/CINCH concept is identical exceptfor the change in fuel (and the corresponding tanksizing). One of the major design challenges that mustbe overcome in developing a feasible HTP system is themitigation of the oxidizer’s tendency to degrade. Highpurity HTP is required to maintain acceptable levels ofperformance; however, HTP slowly decomposes towater and oxygen. This decomposition is a strongfunction of temperature. At a typical silo temperatureof 70°F, the HTP decomposition rate is between 1%

and 2% per year15. With a 1%/year rate, HTP at aninitial concentration of 98% degrades to 72.5%concentration within the 30 year storage life expectedfor the next generation PBPS. If it is maintained at atemperature near 25°F, however, the yearlydecomposition rate is reduced to approximately 0.01%.This rate of decomposition means that an initialconcentration of 98% HTP would decompose to 94.5%during the 30 year storage life. This level ofdegradation is manageable from a propellant sizingstandpoint.

One possible solution to hold the hydrogen peroxidetemperature near 25°F during nominal silo storage is toattach solid-state cooling devices, called thermo-electriccoolers (TECs) to the oxidizer tanks. These devices arecommercially available, inexpensive, lightweight, andhave demonstrated 30 years of continuous operation inspace-based applications15. Due to their low weightand cost, the TECs could be made multiply redundant.

ACS engines

Mechanically gimballed axial nozzle

OxidizerManifold

InsulatedH2O2

Tanks

GasStorage

Assembly

Fuel Manifold

NHMF(Ethanol) Tank

Thermoelectricdevices andheat sinks

Vent valve(1 of 2)

Figure 6: HTP/NHMF Candidate Schematic

NTO/MMH LOADED GEL CONCEPTThe NTO/MMH gelled propellant concept consists of asingle spherical oxidizer storage assembly, a sphericalfuel storage assembly, and a spherical gas storageassembly in addition to the axial and attitude engines.

One of the key drivers of the gelled propellant design isthe high system pressure that is required. Because thegels are thixotropic (they become liquid underpressure), high pressures are needed before the gels willflow through the propellant ducting. In order to handlethe system pressures, both the gas and propellantstorage assemblies and the propellant distributionsystem must be strengthened, adding significant weightto the structure.

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Another unique challenge of the gelled propellantconcept is design of the engine propellant valves.Although loading the gelled fuel with aluminumparticulates increases the propellant performance, theparticulates have a tendency to clog valves andinjection orifices. Because of the technological riskassociated with a large loading fraction, the concept forthis study employs only the minimal loading levelrequired to negate the performance degradation due tothe presence of the gellant material. This low level ofloading should preclude the need for complex valveassemblies, but some changes over a liquid system mayprove necessary. If future studies indicate that valveclogging will be an issue, a valve design employing atranslating centerbody with a wiping capability couldbe incorporated.

HTP/POLYETHYLENE HYBRIDThe primary assemblies comprising the configuration ofthe HTP/polyethylene hybrid concept are three HTPoxidizer storage assemblies, a gas storage assembly,and a fuel storage/combustion chamber. The necessityfor three oxidizer storage assemblies is driven by thehigh value of the optimum mixture ratio.Correspondingly, the fuel storage/combustion chamberis comparatively small.

The primary advantage of a hybrid propulsion conceptis that it incorporates many of the favorablecharacteristics of both liquid and solid propellantsystems. As with liquid systems, hybrid engines allowimpulse cycling. This cycling is achieved byincorporating a throttleable valve that controls theoxidizer injection into the combustion chamber. Thehybrid system gains two critical advantages over liquidrocket systems. These are the increased density anddecreased environmental hazard of the fuel. Because ofthe increased density, more fuel mass can be packagedin a smaller volume. This characteristic is especiallyadvantageous for a volume-constrained upper stageapplication such as a PBPS. The solid fuel has a verylow environmental and personnel hazard both becauseit cannot leak and flow and because it has negligibletoxicity.

In order to facilitate thrust chamber packaging, acircumferential swirl injection combustion chamberdesign was adopted. This design concept, similar to theSurrey Vortex Flow “Pancake” Engine16, allows a lowaspect ratio construction because oxidizer is injected atports located around the circumference of thecombustion chamber rather than in one. The injectorsare oriented such that the liquid stream has a significantcircumferential component. This orientation causes theoxidizer to “swirl” around the combustion chambertoward the center in a similar fashion as water drains

from a sink. As combustion occurs, the hot gas isexpelled through the centrally located nozzle throat.The general configuration of this concept is shown inFigure 7.

Chamber

Nozzle

FuelGrain

Injection VelocityVectors

Oxidizer Injectors

Figure 7: Circumferential Swirl Injection ThrustChamber

This “swirl” effect has additional advantages. Thecircumferential component of the oxidizer injectionvelocity serves to improve propellant mixing and toreduce the chamber size necessary to achieve aresidence time greater than the ignition delay. Theswirling also results in a layer of cool oxidizer on theperiphery of the combustion chamber. This layershields the chamber walls from high heat loads.

Another unique feature of the HTP/PE concept is thedesign of the attitude control engines. Hybrid systemsare often characterized by poor ignition delay times, sothe development of an effective ACS engine mayinvolve significant technical risk. An ACS thrusterconcept that uses HTP as a monopropellant may provemore risk averse. The hydrogen peroxide fuel can beused as a monopropellant by catalyzing the HTP suchthat it exothermically decomposes into water vapor andoxygen gas17. The resulting hot gaseous products canbe expelled through a nozzle to produce the requiredthrust. Though the reaction time of this catalytic HTPdecomposition would not be low enough to ensure anadequate ignition delay and minimum impulse for theACS engines, an integral accumulator reservoir withinthe ACS engine has been incorporated for storingcatalyzed decomposition products. The presence of thistank within the thrusters allows the HTP to be catalyzedin a quasi-steady manner such that hot gas is alwaysavailable for rapid engine firings. This thrusterconfiguration is depicted in Figure 8.

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Accumulator Reservoir

Hot Gas Valve

Propellant Valve

Catalyst Bed

Nozzle

Figure 8: HTP Monopropellant ACS Engine

Although the HTP monopropellant thruster conceptshould overcome the issue of ignition delay for theACS engines, ignition delay could remain a problem forthe axial engine. Many current hybrid rocket engineshave ignition delays of several seconds. Such delaysare unacceptable for the PBPS application. Thisconcern is amplified by the fact that hybrid motors havenot been demonstrated for upper stage application. Anadditional concern for the hybrid concept is inpackaging the combustion chamber. The low aspectratio of the chamber allows it to fit within the outerPBPS geometric envelope, but sizing studies haveindicated that a purely cylindrical chamber may intrudeinto the volume occupied by the guidance set gyroplatform of the current MM III. This intrusion wouldnecessitate a more complicated combustion chambergeometry. All of these concerns indicate that a hybridconcept would entail significant developmental risk.

SOLID PROPELLANT CONCEPTFigure 9 depicts a schematic of the aluminum/HTPB/ammonium perchlorate solid propellant concept.The concept consists of three primary solid propellantgas generators, three vernier positioning gas generators,and a final positioning divert attitude control system.This 3-tiered propulsion scheme is the key attribute ofthe solid propellant concept.

Valves ACS Engine

Primary Gas Generators(with thrust termination

capability)

Vernier Positioning GasGenerators

(with thrust terminationcapability)

DACS

Primary Propulsion / ACS

Radial Vent Nozzles

Mechanically GimballedAxial Nozzle

Valve

Hot Gas Manifold

Figure 9: Solid Propellant PBPS Concept

To attain acceptable levels of pointing accuracy andprecise downrange and crossrange velocity increments,a PBPS design must allow high fidelity thrust controlfor both the axial and the attitude control engines. Thisthrust control is characterized by a small minimumimpulse capability, low ignition delay, and impulsecycling capability. A high level of thrust control isdifficult to achieve in solid propellant systems becausethe engines fire at a designed burn rate until all of thepropellant is expended. Although some control may beafforded by designing the propellant grain structure tovary thrust over a certain profile during the burn, theexact burn profile requirement for a given PBPSmission cannot be known a priori and certainly cannotbe generalized to a single grain design. This difficultyarises from the flexibility that the PBPS must afford: itmust be capable of correcting any positioning errorsresulting from the boost phase of the mission, and itmust allow independent targeting of three RVs over awide range of flight paths. To allow this flexibilitywithin a solid propellant system, a multi-tiered“tunneling” approach was devised to attain accuratepositioning. The system envisioned for this studyconsists of three discrete levels of propulsive capabilityin which each subsequent tier provides more precisethrust control that can correct for the positioning errorsimposed by the previous propulsive level.

The first propulsive tier is provided by three primarygas generators. These solid propellant thrust chambersare fired to produce major velocity increments for bulkdownrange and crossrange maneuvering. One primarychamber is provided for each RV. The chambers areconnected to the attitude control and axial nozzles viahigh-temperature hot gas ducting. During a burn, theeffluent products from the chamber provide both axialand attitude control thrust. Should the thrustrequirements at a given point during the burn be lessthan the capability of the chamber, the excess hot gas isexpelled through a series of radially-oriented ventnozzles. These nozzles are placed symmetricallyaround the PBPS circumference and opened in unisonsuch that the net radial thrust is negligible.

Three vernier positioning gas generators comprise thesecond tier of the propulsion system. These chambers,similar to those employed by the Trident SLBM18, arefired to allow positioning during “coast” phasesbetween major downrange or crossrange maneuvers.Although connected to the same hot gas manifold as theprimary gas generators so that both axial and ACSimpulse can be provided, the vernier positioningchambers are sized primarily for the ACS duty cycle.

The final level of propulsive control is afforded by thedivert attitude control system. This system consists of

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multiple banks of small solid propellant impulsecharges that are oriented in a pattern similar to that ofthe 10-engine primary ACS of the current MM PSRE.These charges are used to provide precise positioning ofthe post-boost vehicle (PBV) immediately prior to RVrelease. Because the impulse of these charges is smalland can be accurately predicted, multiple charges canbe fired immediately before each release to damp outangular positioning errors and to allow PBV back-awayfrom the RV without applying any significant jolt to theRV being released.

Although this solid propellant concept offers significantadvantages in propellant “greenness”, it has severaldisadvantages. One issue is weight. The hot gasducting and valves needed for this concept must befabricated from materials capable of withstanding hightemperatures and pressures. For this reason, the hot gasdistribution system will be heavy. The use of multiplecombustion chambers also amplifies structural weightsignificantly. In addition to high weight, the concept isalso very aggressive from a technology standpoint.Although some elements of the concept have beendemonstrated in the Trident SLBM system, thetechnological risk associated with the hot gasdistribution system and the DACS is significant. Also,a solid propellant system has not been demonstrated forthe stringent land-based ICBM accuracy requirements.Another disadvantage of the solid propellant system isthat the propellant is a dangerous ordnance item.

CANDIDATE CONCEPT SIZINGCandidate concepts were sized by using a rocketequation approach. Each sized candidate concept wasprovided with enough propellant mass to maintain thesame velocity budget as the current Minuteman IIIsystem (including boost stages and the PSRE). PBPSempty weight was estimated by scaling the currentPSRE sub-system weights based on propellant massand pressure. Figure 10 presents the methodologyformulated used for sizing the candidate concepts.

Assumptions for the concept sizing are listed in TableII. Additional details on the sizing process arepresented in reference 15.

Select a propellant for theconcept

Determine the theoreticalIsp

Calculate the requiredpropellant mass from the Rocket

Equation

Determine the requiredcomponents and draw

a schematic

Estimate the component andtotal system masses

Has the structural massestimate changed?

YesDevelop a packaging

concept

NoDraw an accurate concept layout

Can the system be packaged into the

required volume?

No

Yes

Converged Design

Figure 10: Concept Development Process Flowchart

Table II: Table of Sizing Assumptions and Characteristics

Sizing Characteristic Sizing AssumptionsChamber pressure for liquidconcepts

Fixed for all concepts at 125 psia

Chamber pressure for solid,hybrid, and gelled concepts

Fixed for all concepts at 600 psia

Expansion ratio/exit pressure Fixed to estimated baseline expansionratio of 26 for all except hybrid, whichwas sized for fixed length nozzle withcorrespondingly higher expansion ratio

Structural weight Fixed for all conceptsPropellant storage assemblies Scaled according to required chamber

pressures, fuel volumes, mixture ratiosand densities

Gas storage assemblies Scaled according to required chamberpressure

Fittings (pressure transducers,filters, squibs, etc.)

Weight fixed, but quantity adjustedaccording to concept

Examples of concept layouts are given in Figure 11.Converged weight breakdowns for the systems aregiven in Table III.

Main Engine Nozzle

Oxidizer TankFuel Tank

GSA Tank

Oxidizer Tank Guidance Intrusion

Vernier Gas Generator(1 of 3)

Guidance Intrusion

Primary Gas Generator(1 of 3)

Main Engine Nozzle

Figure 11: Layout of HTP/NHMF Concept (left) and Solid Propellant Concept (right)

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Table III: Converged Mass Breakdowns for the Candidate Systems

CurrentMMIII PSRE(Estimated) ‡

HTP/NHMF

HTP/CINCH

HTP/PE NTO/MMH Gel

Solid

System Average Isp (sec) 280.2 269.8 273.9 274.1 284.7 264.7Subsystem Mass

(lbm)Mass(lbm)

Mass(lbm)

Mass(lbm)

Mass(lbm)

Mass(lbm)

Gas Storage Assembly 55.0 55.0 55.0 26.0 60.0 --Internal Structure 27.0 27.0 27.0 27.0 27.0 27.0Rocket Engine Assemblies / Thrust Chamber Assemblies 69.0 69.0 69.0 82.0 69.0 72.0Pyrotechnic Command Cable 7.0 7.0 7.0 7.0 7.0 7.0Thruster Actuation and Gimble Command Cable 14.0 14.0 14.0 14.0 14.0 14.0Thermal Insulation Blanket Assembly 15.0 15.0 15.0 15.0 15.0 15.0External Shell/Interstage Assembly 73.0 73.0 73.0 73.0 73.0 73.0Propellant Distribution System 19.5 19.5 19.5 10.8 19.5 --(Propellant Gas Combustion and Distribution System) --- -- -- -- -- 216.6Oxidizer Propellant Storage Assembly 26.6 13.2 13.2 19.1 19.8 --Fuel Propellant Storage Assembly / Solid Fuel Grain Casing 26.6 6.9 6.9 26.0 19.8 --Fasteners / Miscellaneous 9.0 9.0 9.0 9.0 9.0 9.0Oxidizer Coolant System --- 10 10 -- -- --Total Structural Mass 341.7 318.6 318.6 295.9 333.1 433.6Oxidizer Mass (Primary Gas Generator Propellant) 160.0 171.4 151.2 269.8 147.7 400.0Fuel Mass (Secondary Gas Generator Propellant) 100.0 45.2 58.2 22.3 92.3 115.0(DACS Propellant) --- -- -- -- -- 40.0Total Propellant Mass 260.0 216.6 209.4 292.1 240.0 555.0Total Mass 601.7 535.2 528.0 588.0 573.0 988.6

( ) indicates a subsystem that is only present in the solid propellant concept ‡ references 15,19, and 20

UNCERTAINTY ANALYSISA refined version of the HTP/NHMF concept withadditional weight savings achieved through modernstructural technologies was chosen as an example toevaluate the effects of uncertainty in the sizing processon total PBPS weight. A probabilistic study wasconducted to capture uncertainty in both the structuralweight and the propellant specific impulse, as theseparameters are the primary drivers on system sizing tomeet the fixed performance requirements. The specificgoals of the design study were as follows:

• Define reasonable bounds for uncertainty in thespecific impulse and structural weight

• Bound the uncertainty in the total PBPS weightthat might result from variation in the specificimpulse and structural weight from their presumedvalues

• Ensure that the specific impulse and structuralweight uncertainty does not result in an inability tocreate a sized design to meet the missionrequirements

The first step in the study involved establishing boundsfor the maximum expected uncertainty for the specificimpulse and structural weight estimates. Because of ahigh confidence in the predicted propellantperformance, a small uncertainty bound was placed onthe nominal Isp predictions. This uncertainty bound wasset at ±2.5% of the calculated propellant Isp.

The structural weight estimates, however, are moreuncertain. This uncertainty is partially a function of thelower fidelity and predictive capability of the structuralweight estimation method used in preliminary sizing.Another factor in the uncertianty is in the estimates ofthe fixed weights (e.g. valves, ducting, etc.). Becausethis uncertainty is signficantly greater, bounds of ±20%of the nominal structural weight estimate was chosen.Since the actual structural weight and Isp values aremost likely close to the nominal value, and because thelikelihoods of errors being either high or low are likelyequal, the parameters were defined as normallydistributed random variables for the purposes of thisprobailisitic design study. In order to generate themean and standard deviation necessary to define thesevariables, the nomial predictions were set as the means,and the standard deviations were chosen such that thepredicted bounds in the uncertainties were set toenclose ± 3 standard deviations. An illustration of therandom variable definition for Isp is shown in Figure 12.

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Isp (sec)

263.1-2.5%-3σ

269.8Nominal

276.6+2.5% +3σ

Lik

elih

ood

Figure 12: Specific Impulse Random Variable Definition

After the specific impulse and structural mass weredefined as probability distributions, a Monte Carloanalysis was coupled with the PBPS sizing algorithm.In this analysis, the Monte Carlo simulation selected10,000 discrete values of structural weight and specificimpulse according to their respective probabilitydistributions. The simulation then generated a sizedPBPS concept for each of the 10,000 cases. Theresulting PBPS total mass outputs from the sizingalgorithm were collected to produce a cumulativedistribution function (CDF). This CDF indicates thelikelihood that the total mass is less than a specifiedvalue. The CDF for PBPS total mass is shown inFigure 13.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

400 420 440 460 480 500 520 540 560

PBPS Total Mass (lbm)

Pro

babi

lity

75% probability < 510 lbm

99.79% probability < 550 lbm

Figure 13: PBPS Total Mass Cumulative DistributionFunction

The probabilistic analysis shows that likelihood of thePBPS weight being less than 550 lbm for the presumeduncertainty in structural mass and specific impulse isnearly 100%. This result indicates that the HTP/NHMFPBPS concept is certain to weigh less than the currentMM III post boost system given the assumptions forstructural weight and Isp made in the study. The resultsalso indicate that a HTP/NHMF PBPS can successfullybe sized in the entire presumed range of uncertainties,i.e., no combination of structural weight and Isp result in

an unreasonable fuel mass requirement. Similar resultswere obtained for the HTP/CINCH concept (i.e. theweight was less than that of the current PSRE). Asshould be expected from the deterministic resultsshown in Table III, the other concepts showedsignificant probability of weights greater than that ofthe PSRE.

CANDIDATE CONCEPT EVALUATIONFrom a weight standpoint, the HTP/NHMF,HTP/CINCH, and NTO/MMH gel concepts were allpredicted to be comparable or superior to the baselinePSRE. The CINCH and NHMF were especiallypromising because of the relatively high propellantspecific impulse values and the lower-weight sphericalpropellant tanks. As weight is often directly related tocost, these systems may also have lower acquisitioncosts.

Weight and cost are not the only discriminating factors,however; technical risk is also a key consideration. Allof the concepts presented have some degree ofdevelopmental risk. The HTP concepts would needtechnology development and validation of the oxidizercooling system and the fuel catalysts, and the gelconcept would need to demonstrate a feasiblepropellant distribution and valving system. The mostrisky concepts, however, are the hybrid and solidpropellant concepts. The hybrid has two very difficulttechnology challenges: proving that ignition delay canbe reduced to acceptable levels and demonstrating anadequate HTP monopropellant thruster. The solidpropellant concept must be shown capable of producingthe precise positioning control with a multi-tieredattitude control propulsion system and must also useaggressive materials technologies to keep the weight ofthe system reasonable.

Based on these considerations, the HTP/NHMF andHTP/CINCH concepts appear to offer the most promisefor the PBPS application. This study has indicated thatthe systems can be produced at a weight (and possiblycost) lower than that of the current MM III PSRE, evenconsidering uncertainty in the sizing presumptions. Theweight may be further reduced if other structuraltechnologies such as an isogrid shell and compositeinterior structure are implemented. In addition to theirweight advantage, the systems also appear to befeasible in terms of technical risk, especially ascontrasted to the solid and hybrid propellant conceptspresented.

CONCLUSIONSFrom this study, it is apparent that several “green”propellants are adequate for the PBPS application. Themost promising green propellant formulations appear to

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be HTP/CINCH and HTP/NHMF. The propellants’performance, which approaches that of the NTO/MMH,results in sized PBPS concepts that have equivalentperformance and weigh less than current Minuteman IIIPSRE. These propellant formulations also have areasonable level of technical risk, mostly residing in thedevelopment of soluble fuel catalysts that are required

for hypergolic ignition with HTP and in thedemonstration of a low-weight oxidizer cooling system.Based on these results, it is clear that in addition to theirinherent advantages in storage, personnel safety, andhandling, green propellants offer the necessaryperformance required for land-based strategic ICBMpost-boost propulsion systems.

REFERENCES 1 “Dimethyl-2-azidoethylamine, Available Data and Recommended

Additional Data”, Johnson Space Center White Sands TestFacility, 28 April 2000.

2 Funk, J.E. and Rusek, J., “Assessment of United States NavyBlock 0 NHMF/RGHP Propellants”, 2nd International HydrogenPeroxide Propulsion Conference, 7-10 November 1999.

3 Meade, C.J., Lormand, B., and Purcell, N., “ Status of Non-ToxicHypergolic Miscible Fuel Development at NAWCWD ChinaLake”, 2nd International Hydrogen Peroxide PropulsionConference, 7-10 November 1999.

4 “1st Hydrogen Peroxide Propulsion Workshop”,http://www.ee.surrey.ac.uk/SSC/H2O2CONF/index.htm, SurreySpace Centre website, 20 April 2000.

5 “Niosh Pocket Guide To Chemical Hazards”,http://www.cdc.gov/niosh/npg/pgintrod.html, Center for DiseaseControl Website, 28 May 2000.

6 Anderson, W., “Low Cost Propulsion Using a High-Density,

Storable, and Clean Propellant Combination”, NASA MarshallSpace Flight Center.

7 Palaszewski, B., “Gelled Liquid Hydrogen: A White Paper”, NASA

Lewis Research Center.

8 Whitehead, J.C., “Hydrogen Peroxide Propulsion for Smaller

Satellites”, Lawrence Livermore National Laboratory.

9 Sarner, S.F., Propellant Chemistry, Reinhold, New York, 1966.

10 Siegel, B. and Schieler, L., Energetics of Propellant Chemistry, J.Wiley and Sons, New York, 1964.

11 Sutton, Rocket Propulsion Elements, An Introduction to theEngineering of Rockets, J. Wiley and Sons, New York, SixthEdition, 1992.

12 Keith, E., “The Peroxide Counter Revolution: Replacing ToxicPropellants in Space”, International Space PropulsionIncorporated.

13 Palaszewski, B. “Propellant Technologies: A Persuasive Wave ofFuture Propulsion Benefits”, NASA Glenn Research Center.

14 Mavris, D.N., DeLaurentis, D.A., "An Integrated Approach toMilitary Aircraft Selection and Concept Evaluation," 1st AIAAAircraft Engineering, Technology, and Operations Congress, LosAngeles, CA, September 19-21, 1995. AIAA-95-3921

15 German, Branscome, Frits, Yiakas, et.al., “Next Generation ICBMPost Boost Propulsion System”, Proposal to the 2000 AIAAMSTC Graduate Missile Design Competition, 5 June 2000.

16 Haag, G.S., Paul, M., Brown, R., Gibbon, D., Richardson, G., “AnExperimental Investigation of ‘Rocket Grade’ HydrogenPeroxide,” 2nd International Hydrogen Peroxide PropulsionConference, 7-10 November 1999.

17 “The Decomposition of H2O2”,http://www.uiowa.edu/~c004016b/exp10/lec10/sld007.htm,University of Iowa Website, 27 May 2000.

18 “Trident II D-5 Fleet Ballistic Missile”,http://www.fas.org/nuke/guide/usa/slbm/d-5.htm, Federation ofAmerican Scientists Website, 28 May 2000.

19 “Minuteman Propulsion System Handbook”, ICBM SE/TAProgram, Strategic Systems Division, TRW, inc., September1997.

20 Photographs of PSRE on Display at SAC Museum, Ashland, NE,from Brian York, curator, January 2000.