Experimental Investigation of High-Burning-RateComposite Solid Propellants

Timothy D. Manship,∗ Stephen D. Heister,† and Patrick T. O’Neil‡

Purdue University, West Lafayette, Indiana 47906

DOI: 10.2514/1.B34559

High-burn-rate propellants help maintain high levels of thrust without requiring complex, high-surface-area

grain geometries, thereby increasing volumetric loading and overall motor performance for many applications. The

objective of this study was to identify propellant candidates that burn at 3:80 cm=s at 6.9 MPa (1:50 in:=s at

1000 psig) while maintaining a hazard classification of class 1.3 and good processing characteristics. Selected

propellant formulations included hydroxyl-terminated polybutadiene-based propellants with burn-rate modifiers

and dicyclopentadiene-based propellants. Propellants with varying levels of nano-aluminum, nano-iron oxide, iron

complex of the energetic ligand bistetrazolamine, and overall solids loading were evaluated experimentally. Results

from the study show that nano-additives have a substantial effect on propellant burning rate with nano-iron oxide

having the largest influence. Of the formulations investigated, the highest burning rate belonged to a high-burn-rate

dicyclopentadiene-based propellant that featured anano- andmicron-aluminumblend,micron-sized iron oxide, and

ammonium perchlorate in a 3:1 (20:200 �m) fine-to-coarse ratio, which achieved a burning rate of 4:62 cm=s at6.9 MPa (1:82 in:=s at 1000 psig). Furthermore, dicyclopentadiene-based propellants were shown to burn at

approximately twice the rate of hydroxyl-terminated polybutadiene-based propellants, most likely as a result of

dicyclopentadiene’s lower decomposition temperature.

Nomenclature

a = preexponential factor in St. Robert’s lawn = pressure sensitivity in St. Robert’s lawr = burning rate, cm=s (in=s)P = pressure, MPa (psi)

Introduction

B ECAUSE motor thrust depends on the product of the exposedburning surface area and the propellant burning rate, high

burning rates provide the ability to generate high thrust levelswithoutthe need for elaborate grain designs that provide high-burningsurface areas. Complex, high-surface-area grain designs suffer fromthe fact that they reduce volumetric loading fraction (the ratio ofpropellant volume to chamber volume), which is one of the leadingreasons for missiles’ size and weight. In addition, complex graindesigns are more subject to mechanical failure under the harshoperational conditions of high-thrust/high-acceleration missiles. Forthese reasons, high-burning-rate propellants provide an enablingtechnology for a number of important missions. Ideally, alternativescan be identified that lie within the hazards class 1.3 propellantclassification such that handling is less involved and the devices areinherently safer to personnel working with the system.

It is well known that burning rates increase as particle sizes aredecreased because the flame zone lies closer to the propellant surfacewhere it provides more subsurface heating which drives the overallprocess. Because ammonium perchlorate (AP) is the most commonoxidizer used in today’s composite solid propellants, there are

numerous studies [1–5] that have shown burning rates increaseinversely with AP particle size. The well-known Beckstead et al.multiple-flamemodel predicted a substantial increase in burning rateas AP particle size was decreased [6]. For unimodal distributions,their results showed an asymptotic ‘premixed flame’ limit that wasapproached at AP particle sizes somewhat below 10 �m. From apractical standpoint, propellant processing limits preclude the use ofthis variable to a significant extent. In addition, very fine AP powderis more hazardous; particle sizes less than 15 �m are classified asexplosive by the U.S. Department of Transportation. Despite thesedrawbacks, one of the most effective strategies to increase burningrate is to minimize AP particle size such that mix viscosity and thedesired hazards classification are met.

Aluminum powder is the most common fuel in composite solidpropellants, and reduction of its particle size has similar effects inincreasing burning rate as with AP. The effect of aluminum contentand powder size on burning rate has been studied in the classicalliterature [7–9] and has been the subject of more recent literature[10,11] with the development of nano-aluminum (nAl). Nano-aluminum has been proven to considerably enhance propellantburning rates over coarse aluminum. Shalom et al. showed in [12]that replacement of half of a propellant formulation’s 18% (byweight) coarse aluminum with nano-aluminum or ultrafinealuminum (UFAl) increased the burning rate by 84%. Using aunimodal aluminum distribution to determine the effect of particlesize, Galfetti et al. [13] showed that replacing all of the micron-sizealuminum in an AP/Al/hydroxyl-terminated polybutadiene (HTPB)(68/15/17) propellant increased the burning rate by 100%. Dokhanet al. in [11] showed that the addition of UFAl increased burning rateand that a higher fine-to-coarse AP ratio, in addition to the UFAl,further enhanced burning rate. With only 20% of the aluminumloading being UFAl, Dokhan et al. [11] showed that a burning-rateincrease of 160% was delivered by increasing the fine AP loadingfrom 20 to 40% and that a further 38% increase in burning rate couldbe realized with a fine AP loading of 60%. In addition to the effect ofthe fine AP loading, Dokhan et al. [11,14] obtained results thatshowed, for bimodal cases, substantial burning-rate increases occurat UFAl loadings as small as 20% of the total aluminum loading.

Burn-rate catalysts or ‘modifiers’ are another commonmethod forenhancing burning rates of composite solid propellants. Extensiveresearch regarding burn-rate modifiers has been conducted since thesecond half of the 1960s [1,3,15–25]. It has been shown that

Received 18 January 2012; revision received 23 April 2012; accepted forpublication 28 April 2012. Copyright © 2012 by Timothy D. Manship,Stephen D. Heister, and Patrick T. O’Neil. Published by the AmericanInstitute of Aeronautics and Astronautics, Inc., with permission. Copies ofthis paper may be made for personal or internal use, on condition that thecopier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc.,222 Rosewood Drive, Danvers, MA 01923; include the code 0748-4658/12and $10.00 in correspondence with the CCC.

∗Graduate Student, School of Aeronautics and Astronautics, 500 AllisonRoad. Member AIAA.

†Professor, School of Aeronautics and Astronautics, 500 Allison Road.Associate Fellow AIAA.

‡Graduate Student, School of Mechanical Engineering, 500 Allison Road.Member AIAA (Corresponding Author).

JOURNAL OF PROPULSION AND POWER

Vol. 28, No. 6, November–December 2012

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transition metal oxides with Ag, Cu, Fe, Cd, Mg, Zn, and Li havebeen some of the most-effective catalysts for AP decomposition.Although some researchers believe this is due to a proton transfer, thecommonly accepted mechanism is that the metal ions promote anelectron transfer of ClO�14 to the NH�34 . Although there is somediscussion as to whether this reaction is a solid-phase or a gas-phasereaction and, thus, where the catalyst should be located to provide themost affect, it has been generally agreed that the surface area of thecatalyst plays a major role.

Solid catalysts are typically added in powder form. Most of theprior work has been with nonenergetic catalysts such as iron oxideand copper chromite using micron-sized powders. The recentavailability of nano-sized catalysts has led to much interest inindustry, as the smaller sized particles amplify the catalytic effect fora given catalyst loading [20,21]. Mach I, Inc., provides a superfineiron oxide, named NANOCAT, which is currently some of the finestiron oxide on themarket with a diameter of 3 nm (1:18�10�7 in:) anda specific surface area of�250 m2=g (�1:76�108 in:2=lbm) [20]. Inlow concentrations (�0:1%) NANOCAT has a 25% higher burningrate than Catocene at 6.9 MPa (1000 psig) and, at 1.3% (by weight)loading, has the same burning rate. Exploiting the synergistic effect,Wei and Hua in [21] achieved burning-rate enhancements of�42–102%, depending on the ratio of Cu/Cr in the Cu-Cr-Onanocomposites. They were also able to achieve a burning rate of7:11 cm=s at 7 MPa (2:80 in:=s at 1015 psig); it would be desirableto get other groups to verify this astounding rate for similar mixtures.

There is some recent work by Gilbert et al. aimed at developing anenergetic catalyst based on an iron complex of the energetic ligandbistetrazolamine (BTA) dubbed Fe-BTA [15]. Modest gains inburning rate, similar to those obtained with micron-sized iron oxideparticles, were obtained in Gilbert’s work that provided the collateralbenefit of a slight increase in specific impulse (0.4–0.8% on atheoretical basis) due to the energetic BTA material. There are alsosome recent efforts in suspending nano-sized catalyst powders inlarger-sized AP crystals [16]. This unique approach affords theadvantage of large burning-rate increases associated with nano-sizedcatalysts while keeping the processability of micron-sized materials.As this work is in its formative stages; no data is yet available onpotential augmentation that might be realized by this technique.

Liquid catalysts, typically from the ferrocene family of materials,have also been employed for burning-rate augmentation. Liquidshave the advantage of reducing mix viscosities, thereby allowinghigher solids loading and, hence, higher energy content. Addition-ally, although the affectivity of solid catalysts tend to peak at aloading of around 2–5%, liquid catalysts could be loaded to 9% [22].Ferrocene derivatives come in many different forms, but some of themore common ones are n-butyl ferrocene [1,23], di-n-butylferrocene [1,23], acetyl iron [22], ferrocene polyglycol oligomer[23], and 2,2-Bis(Ethylferrocenyl) propane; or Catocene. Burning-rate increases in the range of 30–70% have been reported in[23,24,26,27] for ferrocene-based materials. One of the most-explored liquid catalysts has been Catocene, which has been shownnot only to catalyze more efficiently than copper chromite and ironoxide but has been employed in propellants that were able to burn at7:00 cm=s at 7 MPa (�2:80 in:=s at 1015 psig) [25].

Unfortunately, liquid ferrocenes, especially Catocene, have allbeen recognized as posing serious safety issues [22–28]. The mainissue for these concerns is that the ferrocene migrates through thepropellant. It is noted that migration of the ferrocene is furtheraccelerated under elevated temperatures. The ferrocenes tend tomigrate toward the propellant boundaries, themissile’swalls, and thepropellant’s surface, for example. In the best-case scenario, thepropellant will burn uniformly and without incident. In most cases,however, what results is accidental ignition due to the propellant’ssurface being sensitized to friction, impact, and electrostatic dis-charge from the higher concentration of ferrocene. Althoughresearchers in [22] have tried to model this phenomenon, Gerardset al. [26] have tried to justify the use of ferrocenes by limiting theircontent and thus ‘increase safety,’ and others [23,27] have triedcreating molecules that would entangle themselves with the binderpolymer. The researches that have had the most success in limiting

ferrocene migration have been those that have chemically grafted theferrocenes to the polymer molecule.

The Société Nationale des Poudres et des Explosives (SNPE)began synthesizing Butacene in 1981 to address the issues thatferrocene derivatives posed. By grafting a silico-ferrocene derivativeonto an HTPB backbone, SNPE was able to significantly reducemigration. Butacene provides numerous advantages in addition tomigration mitigation. These advantages include increased safety,similar mechanical and rheological properties toHTPB, the ability tobe substituted for large amounts of HTPB binder, and an excellentburning-rate enhancement, which suggests that Butacene is thepremier burn-rate modifier [25,29,30].§ Additionally, the Butaceneiron content can be tailored (e.g., 3, 7, or 8% iron content) fordifferent applications [29].§ To cap off the ability of Butacene as aburning-rate enhancer, it was shown that a propellant using Butaceneas 100%of the binderwas able to achieve a burning rate of 7:00 cm=sat 7 MPa (�2:80 in:=s at 1015 psig), which is comparable to theferrocene derivatives. Some criticisms that have arisen aboutButacene are its tendency to retain some amount of drift, similar toCatocene, leading to reduced safety. Also, use of Butacene inpropellant has been observed by [28] to degrade the propellant’smechanical and aging properties in addition to greatly increasingpropellant viscosity during mixing.

Alternatives to the AP oxidizer could potentially be employed toenhance burning rate. Ammonium perchlorate has been themainstayof solid rocket oxidizers since its inception, but there have beennumerous attempts to substitute other oxidizers or energetics thatprovide either environmentally friendly exhaust, smokeless exhaust,different (and occasionally better) mechanics of combustion, higherenergy content, or higher burning rates. Nitronium perchlorate morethan doubles AP’s oxygen concentration, making it an extremelygood oxidizer. Unfortunately, nitronium perchlorate is extremelyhygroscopic, and the products of its reaction with air are nitric acidand perchloric acid [1].

Most alternative oxidizers come in the formof nitramines. Thefirstof the nitramines is ammonium dinitramide (ADN). Ammoniumdinitramide offers several advantages over AP, the first of these is ahigh heat of formation (144 kJ=mol), resulting in better propellantperformance [26,31]. In addition, ADN, like ammonium nitrate, hasno halogen or metal atoms and, thus, is considered for smokelesspropellants [1]. Finally, although the chemical mechanisms are notfully understood, ADN undergoes highly exothermic combustionreactions near the propellant’s surface, and its burning rate iscontrolled by reactions in the condensed phase. A polycaprolactonepropellant containing 89% ADN has shown a burning rate of3:50 cm=s at 7 MPa (1:38 in:=s at 1015 psig), which was furtherenhanced with 2% CuO [32]. Not only did the CuO increase theburning rate slightly, but it considerably reduced the pressureexponent from a dangerously high 0.70 to an acceptable 0.44. Theprinciple disadvantage of ADN is an increased sensitivity andhazards classification.

Energetic additives can also enhance burning rates of compositepropellants. The two main nitramines used in solid propellantsare cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX) and 1,3,5,7-tetramethylene-2,4,6,8-tetranitramine (HMX). Both compoundshave melting temperatures nearly 200 K (360 R) lower than AP andgenerate considerable energy when burning with flame temperaturesof 3300 K (5940 R) and 3290 K (5922 R) for RDX and HMX,respectively. RDX and HMX both contain nearly all of their ownoxidizer and fuel and can basically burn by themselves. As such,RDX and HMX do not create considerable diffusion flames with thebindermatrix as does burningAP. In addition to this, RDX andHMXare explosives and their inclusion will raise the sensitivity andpotentially the hazards classification of a propellant. Because thesenitramines burn much faster than the binder, the main mechanismthat determines the burning rate of the entire propellant is howquickly one fast-burning nitramine inclusion can burn through the

§Gauthier, C., “Properties of Butacene and of Butacene-Based CompositePropellants,” Presentation of Butacene 800 to Mach I, Inc., by SNPE,July 2007.

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binder to another fast-burning nitramine inclusion. Propellants withnitramine inclusions, especially in high concentrations, are verydependent upon the binder type and the interaction of the binder andnitramine. It is noted that this binder–nitramine interaction can have adetrimental effect on the propellant’s bulk burning rate.

Another nitramine that could be used to augment propellantburning rate is hexanitrohexaazisowurtzitane (CL-20). Considerablework has been done with this caged nitramine by the U.S. Naval AirWarfare Center, Alliant Techsystems Inc., SNPE in France, and India[33]. CL-20 possesses a higher propellant burning rate than AP andHMX and has one of the highest monopropellant flame temperaturesof any oxidizer at approximately 3750 K (6750 R) [2]. If a catalystcould be included that would increase the reaction rates and bring theCL-20flame closer to the surface, large potential increases in burningrate may be possible. Unfortunately, development of CL-20 is stillongoing, with the largest batches only at pilot scale [2,31]. It is notedin [31] that production issues may limit CL-20 production to pilotscale for the foreseeable future.

Although binders and plasticizers only make up a fraction of thepropellant (8–25%), their influence on the burning rate, especiallywith nitramines, can be considerable. The binder in most use today ishydroxyl-terminated polybutadiene (HTPB), which provides notonly a good regression rate but also good processing characteristics,goodmechanical properties, and good response to solids loading. Aswas mentioned previously, Butacene provides a mechanism toenhance the burning rate of HTPB formulations. Looking to againexploit the energy delivered by breaking nitrogen bonds, researchersbegan looking into azide polymers such as glycidyl azide polymer(GAP), bis-azide methyl oxetane, 3-azido-methyl-e-methyl oxetane,polynitratomethyl methyloxetane, and polyglycidylnitrate. Becauseof its high density, large heat of formation (490:7 kJ=mol), andrelative insensitivity, GAP has received the most attention, andinformation on it is abundant [1,34–45]. An aspect of GAP thatmakes it such an ideal binder for high-burning-rate propellants is thatit has a decomposition temperature of only 515 K (928 R) [39] and,despite its low flame temperature of 1465 K (2637 R) [36], this peaktemperature occurs nearly at the surface. The result of this is thatGAP burns by itself at a rate slightly faster than 1:10 cm=s at 7 MPa(0:44 in:=s at 1015 psig). The burning-rate characteristics of GAPchange further with the addition of solid oxidizers and nitramines.

Between nitramines and AP, AP-GAP propellants show higherpromise for use in high-burn-rate propellants. In fact, the use of anAP-GAP-TMETN (Trimethylolethane trinitrate) propellant resultedin burning rates greater than 4:57 cm=s at 6.9 MPa (1:80 in:=s at1000 psig) [34]. Although this propellant had a good burning rate,both its pressure exponent and hazard sensitivity were considerablyhigh. Although GAP was not selected for the present study, dueprimarily to hazards classification concerns, there may be a pathforward for use of this binder in less-sensitive high-burning-rateformulations.

High-regression-rate binders take a different approach to increasea propellant’s burning rate. As opposed to delivering energy to thesystem, these binders tend to melt, decompose, and/or sublimate atlower temperatures and thus gasify as fast as (if not faster than) thesolid energetics embedded within their matrix. An example of thesetypes of propellants can be found in hybrid rocket systems, wherehigh regression rates help achieve stoichiometric conditions withoutsacrificing the oxidizer flow rate or increasing the combustionchamber’s size. Unfortunately, most of these fuels, such as paraffins,have low mechanical strength, high glass transition temperatures,and relatively low performance, making them unsuitable for solidrocket applications.

Like paraffins, dicyclopentadiene (DCPD) has its origins as arocket propellant based in hybrid rocketry. Used extensively inautomotive and marine parts, DCPD is a very common material,making it an affordable binder. The material behaves more as aplastic with very high strength and fracture toughness, but it can beplasticized to create a range of elongation behaviors. The initialapplication of the material in rocket combustion was as a hybridrocket-fuel grain [46,47]. More recently, the material has beeninvestigated as a binder for use in solid propellants, where it

displayed burning rates roughly double that of HTPB in aformulation that used the same AP and solids loading [46,48].Dicyclopentadiene has the advantage of maintaining a lower hazardsclassification similar to an HTPB formulation with similar solidsloading. Material properties of DCPD and HTPB are compared inTable 1.

Experimental Setup

Because the hazards classification was a prime consideration inour study, many of the alternatives discussed previously wereimmediately eliminated from consideration. Availability of thevarious materials was also a consideration in the selection ofcandidate formulations for the study. A Kepner–Tregoe evaluationwas conducted to determine the most desirable parameters toinvestigate, and the following design variables flowed from thisevaluation.

AP loading (APL): Increasing theAP loading drives the propellanttoward stoichiometric conditions and higher flame temperatures,which increases the heat feedback and, thus, burning rate. Increasingthe AP loading also provides an increase in Isp due to the increasedflame temperature. Minimizing AP particle sizes subject toprocessing limits also provides a mechanism to enhance burningrates.

Nano-aluminum loading (nAlL): Nano-aluminum loading effectsare well known, but processability was the main reason for includingthis parameter in the study.

NANOCAT loading (NCAT): NANOCAT is a commerciallyavailable nano-iron oxide catalyst that has been shown to con-siderably improve burning rates in loadings as small 2.5%.

Fe-BTA loading (BTA): Availability of this chemical provided forits assessment in our study.

Overall Solids loading (OSL): Increasing the solids loadingincreases the amount of energetics and/or catalysts in the propellant,all of which contribute to the burning rate.

Binder Type (BT): HTPB and DCPD binders were investigated.The initial test matrix was designed to evaluate the low, medium,

and high loadings of the previous parameters by doing small, 50 ghand mixes to assess rheology and processability of the propellants.Candidate formulationswith poor rheologywere eliminated from thetest matrix, and the most attractive formulations were selected forscale-up. A total of nine desired lab-scale formulations were createdas summarized in Table 2.Mixes of these formulationswere obtainedto independently assess each of the six design variables from theprevious list. Because DCPD is a new binder system, there is muchless known about its combustion characteristics compared to thewealth of data on HTPB-based formulations. In preliminaryexperiments, Bluestone [46] had shown that DCPD burned at twicethe rate of a similar HTPB formulation. For this reason, head-to-headcomparisonswere conducted using both nonaluminized (mixes 1 and2 in Table 2) and aluminized (mixes 3 and 4 in Table 2) formulations.

Two different mixers were employed in the study. A 1 qt dual-planetary Ross mixer model DPM-1Qt (DPM) was employed formost of the HTPB mixes (mixes 3, 5, 6, 7, and 8). The DPMis integrated into a vacuum-capable facility that also includesprovisions for heated-water-jacket flow during mixing [54]. For theDPM mixes, approximately 500 g of propellant was manufactured

Table 1 Material properties of DCPD and HTPB

[46,49–53]

Parameter DCPD HTPB

Molecular formula �C10H12�n �C4H6�n�OH�2Molecular weight, g=mol 132.21 �2800Density, g=cm3 0.97 0.90Decomposition temperature, K 373.15 523.15Dynamic viscosity, cP 0.706 5,000–20,000Boiling point, K 443.25 N/AMelting point, K 306.80 N/AMaximum stress, MPa 39.354 0.425–0.800Strain at maximum stress, % 5.436 155–340

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and ambient cast into rectangular propellant blocks (typically calledice cream cartons in the industry). A two-day 60�C (140�F) curecycle was used for these mixes, and more details on the overallprocedures can be found in Manship [55]. There was some evidenceof gas bubbles on the edges of the propellant block due primarily tothe ambient pressure casting, but trimming of the propellant providedfor high density 1.27 by 1.27 by 1.27—3.18 cm (0.50 by 0.50 by 0.50—1.25 in.) strands for burning-rate tests. Figure 1 shows typicalstrand samples after inhibiting sides with gloss spray paint.

The low viscosity of the DCPD monomer, which is similar to thatof water, presented challenges in using established HTPB mixingprocedures in the DPM because s the binder would tend to settle tothe bottomof themixer immediately asmixingwas halted. Bluestone[46] obtained much greater mixing uniformity using a Resodynmodel LabRAM resonant acoustic mixer (RAM). The DCPD mixes(mixes 2 and 4) used theRAMas did theHTPBmix 1. Roughly 100 gof propellant was manufactured in these mixes, using 40g ofacceleration for 5 min in the RAM. A detailed discussion of mixingprocedures employed for DCPD propellants is contained inBluestone’s thesis [46]. As was mentioned in the literature review,DCPD is very tough when cured; thus, instead of casting thepropellant into a block and cutting strands like the HTPB-basedpropellants, the DCPD-based propellant was cast directly into aplastic tube, which produced strands with an approximate diameterand length of 0.73 cm (0.29 in.) and 7.00 cm (2.76 in.), respectively.

Awindowed Crawford bomb rated to 41.4 MPa (6000 psig) wasused to obtain strand burning rates for all samples [56]. A VisionResearch V7.1 high-speed black-and-white Phantom camera wasused, which provided the high frame rate and exposure settingsrequired to visually record the strand’s surface regression used todetermine burning rates. To explore a potential mechanism for the

enhanced DCPD burning, microscopic imaging of the propellant’ssurface during burning was employed. The Phantom camera wasonce again used, but instead of a normal lens arrangement an Infinitymodel K2 long-distance microscope was used to evaluate samplesunder atmospheric combustion. Multiple tests were performedon the nonaluminized DCPD and HTPB propellants, as well asthe aluminized, high-burn-rate DCPD and HTPB propellants. Toprovide enough light for videos of the nonaluminized surfaces, axenon lamp was used instead of a work lamp.

Results and Discussion

Nonaluminized Propellant Results

As discussed in previous sections, the DCPD polymer has beenshown to exhibit physical properties, chiefly mechanical propertiesand burning rate, which may make it an advantageous binder for usein solid rocket motors. DCPD has been shown through research byIN Space and Bluestone [46] and Bluestone et al. [48] to havesome mechanical properties, such as high strength, that make it apotentially more desirable binder than HTPB in rockets undergoinghigh-acceleration maneuvers. A difference in the polymer matricescan be seen in Fig. 2 for the mix 1 and 2 propellants, respectively.

From Fig. 2, it can be seen that the polymers react differently toshearing forces, as indicated by the surface and edge roughnessproperties. The difference in surface properties is best seen in Figs. 2aand 2c. With DCPD, the polymer is cut cleanly, suggesting that thecross-linked molecules of the short-chain monomer resist forces upto a certain point and then give away freely. It is possible that, withHTPB, the long-chainmonomer hasmultiple degrees of freedom andcan be elongated farther but, due to its larger size, would tend to haveless cross-linking than DCPD. As a result of this possible differencein cross-linking, if a force is applied, the HTPB matrix tears atmultiple points, creating a rough, coral-like surface after the cut, asshown in Fig. 2d. This theory is supported by the stress/strain curves

Table 2 Test matrix of lab-scale formulations

Catalyst, %

Mix number Binder, % Solids, % APa, % 80 nm Al, % NANOCAT Fe-BTA Design variable

1 HTPB, 20 80 80.0 0.0 0.0 0.0 Baseline2 DCPD, 20 80 80.0 0.0 0.0 0.0 BT3 HTPB, 20 80 73.7 6.0 0.3 0.0 nAlL4 DCPD, 20 80 73.7 6.0 0.3 0.0 nAlL, BT4ab DCPD, 20 80 73.7 0.25 0.0 0.0 nAlL, BT5 HTPB, 20 80 73.7 4.8 1.5 0.0 nAlL, NCAT6 HTPB, 20 80 73.7 4.8 0.0 1.5 nAlL, BTA7 HTPB, 20 80 70.7 9.0 0.3 0.0 nAlL, APL8 HTPB, 16 84 77.7 6.0 0.3 0.0 OSL

aAll formulations in Table 2 featured a 20:200 �m fine-to-coarse AP ratio of 3:1.bIn addition to the materials shown in Table 2, mix 4a also featured 5.75% 12 �m Al and 0.3% micron iron oxide.

Fig. 1 Inhibited propellant strand.

Smooth surface with no visible AP crystals

Smooth surface with no visible AP crystals

AP

Rough coral-like polymer surface

Rough coral-like polymer surface

Void from AP crystal torn

during cutting

Direct Isometric

Viewing Angle

Prop

ella

nt T

ype

AP/

HT

PBA

P/D

CPD

a) b)

c) d)

200 µm

200 µm

200 µm

200 µm

Fig. 2 Surface images of nonaluminized DCPD (mix 2) and HTPB

(mix 1) propellants.

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of HTPB and DCPD, as shown by Bluestone [46]. Bluestone’s dataindicated that, although DCPD had a higher strength then HTPB, itwas capable of undergoing less elongation while under tension [46].The polymer matrix composition may similarly affect the binders’decomposition mechanisms as will be discussed later.

In addition to DCPD’s unique mechanical properties, it alsodecomposes at a significantly lower temperature thanHTPB,makingit an ideal high-regression-rate binder. As shown in Table 1, DCPDand HTPB begin to decompose at temperatures of 373.15 K(671.67 R) and 523.15 K (941.67 R), respectively. In his thesis,Bluestone had shown that, for similar formulations with 80% solidsloading and coarse aluminum, with just the binder switched betweenformulations, DCPD strands had burned at twice the rate of HTPBstrands [46]. To determine why this burning-rate enhancementoccurred, high-speed video was taken of the HTPB and DCPDsurfaces while burning at atmospheric pressure. Images of the HTPBsurface indicated that the nano-aluminum powder tended to remainon the burning surface of the propellant where it agglomerated morethanwhatwas seen on theDCPD strands. Because smaller aluminumagglomerates are ejected from the DCPD propellant surface, theoverall aluminum agglomerate surface area is greater; thus, fasterconsumption of the aluminum can take place. This is particularlyadvantageous in motors with small combustion chambers where theresidence time of combustion products are very small. Conversely,the lower exposed surface area due to larger agglomerates (like thoseobserved on theHTPB strands) leads to specific impulse losses due toreduced aluminum combustion efficiency and two-phase floweffects.

Surface images from the atmospheric burning of aluminizedDCPD propellant from mix 4 are shown in Fig. 3. From the pictures,it is clear to see that the AP crystals are almost entirely visible on thepropellant’s surface, which implies that the binder is decomposingand regressing faster than theAPparticles.Another phenomenon thatcan be noticed in the sequence of pictures is the location of thecombustion sites of the nano-aluminum, shown as the bright spots inthe pictures. The proximity of the aluminum ignition to the surface issuch that the aluminum is actually igniting near the base of the APcrystals (Fig. 3b). This supports the theory that the nano-aluminumignition occurs mainly in the primary diffusion flames present nearthe base of the oxidizer crystal. Were the nano-aluminum ignition

dependent upon themonopropellant flame or the secondary diffusionflame, ignition sources would be seen above the oxidizer crystal.

Surface images from the atmospheric burning of aluminizedHTPB propellant featuring Fe-BTA from mix 6 are shown in Fig. 4.One of the first things that can be noticed is the apparent lack ofvisible crystals on the propellant’s surface. This suggests that the APcrystals are regressing as fast as (if not faster than) the HTPB binder.Second, the ignition sites of the nano-aluminum are not as clear asthose in the DCPD images (Fig. 3). This is not due to a differentproximity of the ignition sites to the surface as much as the ignitionsites being obscured by unburned coral-like structures. Thisphenomenon can be seen in detail in the second image, Fig. 4b. Theobscuration occurs to the right of the propellant’s surface and can becharacterized as a light coral-like structure in front of bright ignitionspots. These coral-like structures are also capable of luminescence,as can be seen toward the left of Fig. 4b,where a bright coral branch issticking out of the propellant’s surface. These coral-like structuresmay be composed of unburned binder, unburned nano-aluminum, ora combination of the two.Although a direct confirmation of the coral-like structure’s composition was beyond the scope of the experiment,observations from the surface burning video data indicates the latteras 1) the coral-like structures showed resistance to burning and werepresent for the burn’s duration (indicative of a reinforced/metalizedbinder), 2) the luminescence occurred occasionally and inter-mittently (occurring perhaps as the nano-aluminum was heated toluminescence), and 3) the coral-like structures do not melt togetherbefore leaving the propellant’s surface as aluminum agglomerateswould.

Based on these images, a theory was posed that the interaction ofthe DCPD and aluminum may be what is causing the increasedburning rate instead of just the binder itself, as was suggested byBluestone [46]. Thus, a total of 41 strands were fired at pressuresranging from 1.4–13.8 MPa (200–2000 psig) in the Crawford bombto assess burning rates of the nonaluminized propellant formulations.Of the 41 strands tested, 23 featured DCPD as the binder, while theremainder featured HTPB as the binder. Measured burning-rate datafor the two binder systems is depicted in Fig. 5, where two sigmaoutliers, based on the population data, have been removed. It is notedthat, for the inset reduced-burning-rate equations, burning rate r hasunits of cm=s and pressure P has units of MPa. Tabulated empirical

nAl

200 µm AP

AP

a) b) c)

200 µm 200 µm 200 µm

Fig. 3 Surface images from atmospheric burning of aluminized DCPD propellant.

nAl

Luminous Coral

Binder Obstruction

Coral-Like Structures

a) b) c)

200 µm 200 µm 200 µm

Fig. 4 Surface images from atmospheric burning of aluminized HTPB propellant.

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values of the St. Robert’s Law for both propellants and their 6.9MPa(1000 psig) burning rates are shown in Table 3.

Similar to what Bluestone [46] described in his thesis, the burningrate of the nonaluminized DCPD is roughly double the burning rateof the nonaluminized HTPB.

Aluminized Propellant Results

Employing nano-additives to modify the propellant’s burning ratecreated some particularly interesting processing issues. Mainly, thehigh surface area of the nano-particles effectively consumes more ofthe binder volume than coarse particles. Thus, the propellant’sviscosity is higher for the same solids loadings as a propellant withcoarse particles. This phenomenon was most prevalent in thealuminized DCPD mix (mix 4), which lead to severe processingissues that necessitated several reformulations until adequate resultscould be obtained.

Despite slight challenges in processing, mixes for all of thepropellants listed in Table 2 were successful. For each mix, 15–20 strands were burned at different pressures between 0.5–20.7 MPa(70–3000 psig) to determine the propellant’s pressure–burning-raterelationship. Particular attention was given to the 6.9 MPa(1000 psig) pressure level because that is the pressure at which thedesired 3:80 cm=s (1:5 in:=s) burning rate must be achieved. Moredetails on the test procedure are available in Manship [55]. Asummary of the power-law-fit burning-rate correlations are providedin Table 4. Figures 6–11 provide the resulting burning-ratemeasurements for the six mixes successfully produced in this effort.

The addition of nano-aluminum and NANOCAT to an HTPBpropellant leads to an 89% increase in burning rate relative to themix 1 baseline. The addition of the nano-materials lead to anoticeable increase in viscosity; however, this increase did notgreatly affect mix or casting operations. Although baseline mix 1flowed like paste, mix 3 had malleability more akin to putty and didnot flow easily. Additionally, the pressure exponent is comparable tothe AP-HTPB formulation. The burning-rate measurements andreduced data for mix 3 are shown in Fig. 6.

The nano-aluminum/NANOCAT/DCPD mix (mix 4) presentedthe greatest challenges due to the large surface area of the nano-materials, and initial attempts at preparing this formulation wereunsuccessful. The DCPD was readily absorbed into the nano-materials like water soaking into a sponge, leaving a wet-sand-typetexture. Limited success was obtained by vibrating this mix on ashaker table and then introducing additional solids. It is noted that allstrands cast from this mixture exploded in the Crawford bomb atpressures less than 1.7 MPa (250 psig).

It was suspected that failure of themix 4 propellant was due to thatformulation’s high surface area. The two failure modes consideredwere 1) the high-surface-area particles adsorbed all the DCPD, and2) the high surface area of the particles allowed for a significantportion of the DCPD binder to evaporate during the cure cycle. Ineither case, a complete, binding matrix of DCPD, where all particleswere wetted, could never be formed due to a lack of DCPD. Thus,subsequent mixes were conducted where the total surface area of theformulation was reduced. The reduction in surface areawas achievedby reducing the amount of nano-particles present in the formulationrather than adjusting the fine-to-coarse AP ratio. After severaliterations, a successful version of themix 4, termedmix 4a in Table 2,was achieved.

Mix 4a featured 0.25% nano-aluminum and 5.75% micronaluminum (versus 6% nano-aluminum, as was attempted for mix 4)and 0.3% micron-sized iron oxide (versus 0.3% NANOCAT). Theaddition of only 0.25% nano-aluminum and micron-sized iron oxideto a DCPD propellant leads to large increases in burning rate.Burning-rate measurements for mix 4a are presented in Fig. 7. The6.9 MPa (1000 psig) burning rates of mix 4a are 4:62 cm=s(1:82 in:=s), which represents a 243% increase over the nonalumi-nized baseline HTPB mix (mix 1). This mixture had the highestburning rate of the nine cases evaluated in this study. Additionally,the mix 4a pressure exponent is the highest of all formulationsinvestigated, with a value of 0.529.

Mix 5 (Fig. 8) was designed to assess the loading effect ofNANOCAT, an iron-oxide catalyst. The first differencewe find in theNANOCAT mix is a slight increase in burning rate and pressureexponent. There is a diminished return in terms of catalystaugmentation, and the five-fold increase in loading from mix 3 to

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Mix 1 Strand Data

Mix 2 Strand Data

Mix 1 Formulation: HTPB/AP 20/80 Reduced Burning Rate: r = 0.7732(P) 0.291 R2 = 0.656Mix 2 Formulation: DCPD/AP 20/80 Reduced Burning Rate: r = 1.3260(P) 0.414 R2 = 0.880

Fig. 5 Burning-rate comparison of nonaluminized DCPD and HTPB

propellants.

Table 3 Comparison of burning rates for nonaluminized DCPDand HTPB propellants

Mix number a n r at 6.9 MPa (1000 psig), cm=s (in:=s)

1 0.7732 0.291 1.35 (0.53)2 1.3260 0.414 2.94 (1.16)

Table 4 Burning-rate coefficients and 6.9 MPa (1000 psig)

burning rates

Mix number a n r at 6.9 MPa(1000 psig),cm=s (in:=s)

Burning-rate increaseto mix 1, %

3 1.3358 0.334 2.54 (1.00) 894a 1.6623 0.529 4.62 (1.82) 2435 1.1122 0.477 2.80 (1.10) 1086 1.0600 0.347 2.07 (0.81) 537 1.0722 0.483 2.72 (1.07) 1028 1.1172 0.524 3.08 (1.21) 128

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Mix 3Formulation: HTPB/AP/80-nm Al/NANOCAT 20/73.7/6.0/0.3 Reduced Burning Rate: r = 1.3358(P)0.334 R2 = 0.883

Fig. 6 Mix 3 burning-rate pressure profile.

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mix 5 showed only modest improvements. The replacement of thenano-aluminum in return for a higher loading of NANOCAT led to aslightly higher viscosity ofmix 5 overmix 3. This could be attributedto the considerable higher surface area of NANOCAT. The viscosityincrease, however, was not so significant as to restrict mixing orcasting; it had just produced a propellant that was slightly tougher,with the malleability shifting from putty to more clay-like. Table 4shows the 6.9 MPa (1000 psig) burning rate increasing from 2.54 to2:80 cm=s (1.00 to 1:10 in:=s) under the higherNANOCAT loading.It is noted that greater enhancement in burning rate is present athigher pressures. The observed behavior is most likely due to theNANOCAT causing the AP to decompose at a lower temperature viaan augmentation of the electron transfer process. As a result of thelow-temperature AP decomposition, more of the combustionreactions are shifted toward gas-phase reactions, which are moresensitive to pressure.

Mix 6 was designed to assess an alternate burning-rate catalyst,Fe-BTA. The Fe-BTA burning rate shown in Fig. 9 is substantiallylower than that for the mix 3 formulation. It is noted that the burningrate for mix 6 was the lowest of the aluminized formulations studied.Processing of the propellant was also easier with the Fe-BTA than theNANOCATbecause the particle sizewas significantly higher and thesurface area that needed to be wetted was drastically less. Althoughthe Fe-BTA mixture may have delivered more energy to the system,the large size of the Fe-BTA particles provides a greatly diminishedcatalytic effect. Because the catalytic ability of a material is

dependent upon the surface area, the Fe-BTA particle, due to its size,essentially delivers similar catalytic performance as coarse iron oxide[15], which leads to a propellant burning rate lower than that ofmix 3 (which usesNANOCAT).Additionally, because 1.5%Fe-BTAloading was used, 1.2% nano-aluminumwas sacrificed from the mixto accommodate the additional Fe-BTA.

Mix 7 was designed to assess the impact of a higher loading ofnano-aluminum as compared with mix 3. Figure 10 provides thepower-law fit of the strand data from this mix. With its higher nano-aluminum content, mix 7 has a burning rate about 7% higher thanmix 3. It is noted that the 9% nano-aluminum content did lead todifficulties in mixing and casting. The large amount of surface areathat was required to be wetted led to a propellant that had themalleability of hard clay. During casting, it was felt that an additionalfew percent of nano-aluminum would have led to dry spots and acorresponding loss of mechanical strength. Mix 7 also has a higherexponent of 0.483 versus 0.334 as compared tomix 3. The increase inburning rate is likely a result of the additional energy provided by theincrease in nano-aluminum, which increases the flame temperatureand heat feedback. The increased pressure sensitivity due to thehigher nano-aluminum content matches the trend seen in thework ofShalom et al.’s [12]. A possible reason for this phenomenon is that,because the nano-aluminum particles’ ignition is highly dependentupon the location of hot zones (mainly primary diffusion flameszones), the increase in pressure lowers the flame standoff distance,causing earlier ignition of the nano-aluminum. It is noted that anincrease in pressure leads to a two-part increase in heat feedbackbecause not only are AP andAP/Binder flames in closer proximity tothe surface but the aluminum combustion is as well.

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Mix 4a Strand Data

Mix 4aFormulation: DCPD/AP/80-nm Al/12-µm Al/Fe2O3 20/73.3/0.25/5.75/0.3 Reduced Burning Rate: r = 1.6623(P)0.529 R2 = 0.825

Fig. 7 Mix 4a burning-rate pressure profile.

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Mix 5 Strand Data

Mix 5 Formulation: HTPB/AP/80-nm Al/NANOCAT 20/73.7/4.8/1.5 Reduced Burning Rate: r = 1.1122(P)0.477 R2 = 0.839

Fig. 8 Mix 5 burning-rate pressure profile.

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Mix 6 Strand Data

Mix 6 Formulation: HTPB/AP/80-nm Al/Fe-BTA 20/73.7/4.8/1.5 Reduced Burning Rate: r = 1.0600(P)0.347 R2 = 0.912

Fig. 9 Mix 6 burning-rate pressure profile.

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Mix 7 Strand Data

Mix 7 Formulation: HTPB/AP/80-nm Al/NANOCAT 20/70.7/9.0/0.3 Reduced Burning Rate: r = 1.0722(P)0.483 R2 = 0.913

Fig. 10 Mix 7 burning-rate pressure profile.

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Mix8 (Fig. 11) shows the effect of additional solids loading,whichincreases the burning rate due to a higher concentration of energeticmaterial. This mix produced a burning rate of 3:08 cm=s at 6.9 MPa(1:21 in:=s at 1000 psig). The additional energeticmaterial also leadsto a higher pressure exponent than mix 3: 0.524 versus 0.334. Thehigher solids loading of 84% had still shown some malleability afterthe mix. The end-of-mix propellant had a texture of rough putty andwas easily cast into molds, suggesting that a slightly higher solidsloading is perhaps feasible. Although this set of propellantwasmixedin PurdueUniversity’sDPMas described previously, the use ofRAMcould potentially allow an even higher solids loading. Increasing thesolids loading may cause additional increase in pressure exponent,but the tradeoff for burning-rate increase (21% burning-rate increasefor 4% increased solids loading) could outweigh the increasedpressure sensitivity. The degradation in propellant physicalproperties with increased solids loading is another obviouslimitation. In applications for small divert engines, lower propertiesmay be acceptable due to the small web thicknesses employed insuch an application.

There are numerous reasons for the increase in burning rate aswellas the increase in pressure exponent. The first reason for the increasein burning rate comes from the fact that the propellant is closer tostoichiometric conditions, leading to a higher overall flametemperature and greater heat feedback. Second, increasing thefraction of AP to binder leads to a higher concentration of primarydiffusion flames near the propellant surfaces, which are a leadingcontributor to burning rate. Interestingly, as the AP loading getshigher in a propellant, increasing theNANOCATcontent, as opposedto the nano-aluminum content, may have a greater contribution toincreasing the burning rate.Aswasmentioned previouslywithmix 5,adding NANOCAT led to an increase in burning rate. In fact,NANOCAT had a better ‘burn rate increase per percentage loading’

than nano-aluminum. Unfortunately, iron oxide catalyst affectivitytends to plateau around 3% solids loading, but fortunately this ishighly dependent upon the amount of AP present. With more APpresent, the NANOCAT addition may be greater than 3%.

Conclusions

An experimental study was conducted to assess mechanisms toincrease burning rates in composite solid propellants under theconstraint of keeping a hazards class 1.3 formulation. A review ofapproaches for increasing composite propellant burning ratesrevealed six techniques that have been used in the past:

1) Decreasing the AP particle size increasing mass diffusion andthe number of primary diffusion flames at the burning surface.

2) Using nano-aluminum to enhance energy release (andsubsurface heat conduction) near the burning surface.

3) Using burn-rate catalysts to reduce the decompositiontemperature of ammonium perchlorate (AP).

Liquid catalyst: Reduces solids loading while releasing transitionmetal atoms in a cloud above the propellant surface. Unfortunately,liquid catalysts suffer from migration and safety issues and wereeliminated due to hazards classification concerns.

Solid catalysts : Safer but less efficient than liquid catalysts. A newarea of research is in nano-catalysts that maximize the surface areaavailable.

4) Using alternative oxidizers and energetic materials. This optionsuffers from hazards classification concerns and somewhat pooreffectiveness for some options relative to the common AP oxidizer.

5) Using energetic binders and plasticizers and high-regression-rate inert binders. The GAP binder represents a viable alternative butwas not studied here due to hazards class and availability issues. Thedicyclopentadiene (DCPD) binder, which has not been usedsubstantially in solid propellants but offers some promise, wasselected as an item to study.

Nine different lab-scale mixes (see Table 5) were prepared forstrand burning-rate evaluation in a Crawford bomb. Photomicro-graphs were also generated for two nonaluminized mixes to assessdifferences between the baseline hydroxyl-terminated polybutadiene(HTPB) and the DCPD binders. These images showed more rapiddecomposition of the DCPDmaterial near the surface that supporteda higher burning rate. Additionally, the more-rapid decomposition ofDCPD resulted in what appeared to be faster ejection of aluminumfrom the burning surface with a concurrent reduction in aluminumagglomeration. Nano-ingredients such as nano-catalysts and nano-aluminum were evaluated as well as a new iron complex Fe-BTAmaterial. The 6.9 MPa (1000 psig) burning rates for the mixesinvestigated varied from 1:4–4:8 cm=s (0:5–1:8 in:=s), with theDCPD formulation incorporating nano-sized and micron-sizedaluminum and catalyst additives, showing the highest burning rates.The nano-iron oxide material (NANOCAT) and nano-aluminumboth provided strong enhancements to burning rates at the expense ofincreased challenges in processability. Although physical propertyevaluations and aging studies are required, the DCPD binder showspromise for high-burning-rate composite solid propellants.

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Mix 8 Strand Data

Mix 8 Formulation: HTPB/AP/80-nm Al/NANOCAT 16/77.7/6.0/0.3 Reduced Burning Rate: r = 1.1172(P)0.524 R2 = 0.972

Fig. 11 Mix 8 burning-rate pressure profile.

Table 5 Compilation of 6.9 MPa (1000 psig) burning-rate data with propellant formulation data

Al, % Catalyst, % r at 6.9 MPa (1000 psig)

Mix number Binder, % 80 nm 12 �m APa, % NANOCAT Fe-BTA n cm=s in:=s

1 HTPB, 20 0.0 0.0 80.0 0.0 0.0 0.291 1.35 0.532 DCPD, 20 0.0 0.0 80.0 0.0 0.0 0.414 2.94 1.163 HTPB, 20 6.0 0.0 73.7 0.3 0.0 0.334 2.54 1.004 DCPD, 20 6.0 0.0 73.7 0.3 0.0 —— —— ——

4ab DCPD, 20 0.25 5.75 73.7 0.0 0.0 0.529 4.62 1.825 HTPB, 20 4.8 0.0 73.7 1.5 0.0 0.477 2.80 1.106 HTPB, 20 4.8 0.0 73.7 0.0 1.5 0.347 2.07 0.817 HTPB, 20 9.0 0.0 70.7 0.3 0.0 0.483 2.72 1.078 HTPB, 16 6.0 0.0 77.7 0.3 0.0 0.524 3.08 1.21

aAll formulations in Table 5 featured a 20:200 �m fine-to-coarse AP ratio of 3:1.bIn addition to the materials shown in Table 5, mix 4a also featured micron iron oxide.

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Acknowledgment

The authors would like to thank graduate student Joseph Moorefor his assistance in capturing the surface-burning videos.

References

[1] Kubota, N., Propellants and Explosives: Thermodynamical Aspects ofCombustion, Wiley-VCH, Weignheim, Germany, 2002, pp. 66, 69,110–113, 158–162, 164–168.

[2] Beckstead,M.W., “AnOverview of CombustionMechanics and FlameStructures for Advanced Solid Propellants,” Solid Propellant

Chemistry, Combustion, and Motor Interior Ballistics, edited by V.Yang, T. Brill, and W. Ren, Vol. 185, Progress in Astronautics andAeronautics, AIAA, New York, 2000, pp. 267–286.

[3] Kilshore, K., and Gayathri, V., “Chemistry of Ignition and Combustionof Ammonium-Perchlorate-Based Propellants,” Fundamentals of

Solid-Propellant Combustion, Vol. 90, Progress in Astronautics andAeronautics, AIAA, New York, 1984, pp. 53–119.

[4] Ramohalli, K., “Steady State Burning of Composite Propellants UnderZero Cross-Flow Situation,” Fundamentals of Solid-Propellant

Combustion, Vol. 90, Progress in Astronautics and Aeronautics,AIAA, New York, 1984, pp. 409–477.

[5] Summerfield, M., Sutherland, G. S., Webb, M. J., Taback, H. J., andHall, K. P., “The Burning Mechanism of Ammonium PerchloratePropellants,” ARS Progress in Astronautics and Rocketry Vol 1: Solid

Propellant Rocket Research, Academic Press, New York, 1960,pp. 141–182.

[6] Beckstead, M.W., Derr, R. L., and Price, C. F., “AModel of CompositeSolid Propellant Combustion Based on Multiple Flames,” AIAA

Journal, Vol. 8, No. 12, 1970, pp. 2200–2207.doi:10.2514/3.6087

[7] Price, E. W., “Combustion of Metalized Propellants,” Fundamentals ofSolid-Propellant Combustion, Vol. 90, Progress in Astronautics andAeronautics, AIAA, New York, 1984, pp. 479–513.

[8] Sutton, G. P., and Biblarz, O., Rocket Propulsion Elements, 7th ed.,Wiley, New York, 2001, pp. 417–540.

[9] Beckstead, M. W., “A Summary of Aluminum Combustion,” RTO/VKISpecial Course on Internal Aerodynamics in Solid Rocket Propulsion,Von Karman Inst., Rhode-Saint-Genèse, Belgium, May 2002,pp. 1–47;alsoRTO Paper EN-023.

[10] Kuo, K. K., Risha, G. A., Evans, B. J., and Boyer, E., “Potential Usageof Energetic Nano-Sized Powders for Combustion and RocketPropulsion,” Proceedings of Materials Research Society Symposium,Vol. 800, 2004, pp. 3–14.

[11] Dokhan, A., Price, E. W., Seitzman, J. M., and Sigman, R. K., “TheIgnition of Ultra-Fine Aluminum in Ammonium Perchlorate SolidPropellant Flames,” 39th AIAA/ASME/SAE/ASEE Joint Propulsion

Conference and Exhibit, Huntsville, AL, July 2003;alsoAIAAPaper 2003-4810.

[12] Shalom, A., Aped, H., Kivity, M., and Horowitz, D., “The Effect ofNanosizedAluminum onComposite Propellant Properties,” 41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Tuscon,AZ, July 2005;alsoAIAA Paper 2005-3604.

[13] Galfetti, L., Severini, F., DeLuca, L. T., and Meda, L., “Nano-Propellants for Space Propulsion,” 4th International Spacecraft

PropulsionConference [CDROM], Cagliari, Italy, edited byA.Wilson,European Space Agency SP 555, June 2004, pp. 125.1–125.6.

[14] Dokhan, A., Price, E. W., Seitzman, J. M., and Sigman, R. K., “TheEffects of BimodalAluminumwithUltrafineAluminumon theBurningRates of Solid Propellants,” Proceedings of the Combustion Institute,Vol. 29, No. 2, 2002, pp. 2939–2945.doi:10.1016/S1540-7489(02)80359-5

[15] Gilbert, P. D., Zaseck, C. R., Nazartio, L. R., and Son, S. F., “AnInvestigation of NovelMetal Complexes as Composite Propellant BurnRate Modifiers,” 44th AIAA/ASME/SAE/ASEE Joint Propulsion

Conference and Exhibit, Hartford, CT, July 2008;alsoAIAAPaper 2008-5260.

[16] Ma, Z., Li, F., and Bai, H., “Effect of Fe2O3 in Fe2O3=AP CompositeParticles on Thermal Decomposition of AP and on Burning Rate of theComposite Propellant,” Propellants, Explosives, Pyrotechnics, Vol. 31,No. 6, 2006, pp. 447–451.doi:10.1002/prep.200600060

[17] Kapoor, I. P. S., Srivastava, P., and Singh, G., “NanocrystallineTransition Metal Oxides as catalysts in the Thermal Decomposition ofAmmonium Perchlorate,” Propellants, Explosives, Pyrotechnics,Vol. 34, No. 4, 2009, pp. 351–356.doi:10.1002/prep.200800025

[18] Wang, Y., Xia, X., Zhu, J., Li, Y., Wang, X., and Hu, X., “Catalytic

Activity of Nanometer-Sized CuO=Fe2O3 on Thermal Decompositionof AP and Combustion of AP-Based Propellant,” Combustion Science

and Technology, Vol. 183, No. 2, 2010, pp. 154–162.doi:10.1080/00102202.2010.507561

[19] Patil, P. R., Krishnamurthy, V. N., and Joshi, S. S., “DifferentialScanning Calorimetric Study of HTPBBased Composite Propellants inPresence ofNanoFerricOxide,”Propellants, Explosives, Pyrotechnics,Vol. 31, No. 6, 2006, pp. 442–446.doi:10.1002/prep.200600059

[20] NANOCAT Brochure, Mach I, Inc., King of Prussia, PA, http://www.machichemicals.com/pdf/SFIO.pdf.

[21] Wei, L., and Huah, C., “Synthesis and Characterization of Cu-Cr-ONanocomposites,” Journal of Central South University of Technology,Vol. 14, No. 3, 2007, pp. 291–295.doi:10.1007/s11771-007-0057-5

[22] Ünver, A., Dilsiz, N., Volkan, M., and Akovali, G., “Investigation ofAcetyl Ferrocene Migration from Hydroxyl-Terminated PolybutadieneBased Elastomers by Means of Ultraviolet-Visible and AtomicAbsorption Spectroscopic Techniques,” Journal of Applied Polymer

Science, Vol. 96, No. 5, 2005, pp. 1654–1661.doi:10.1002/app.21624

[23] Gore, G. M., Bhatewara, R. G., Tipare, K. R., Nazare, A. N., andAsthana, S. N., “Studies on Ferrocene Polyglycol Oligomer—Burning-Rate Modifier for Composite Propellants,” Journal of Propulsion and

Power, Vol. 20, No. 4, 2004, pp. 758–760.doi:10.2514/1.11460

[24] Munson, W. O., McGee, L. R., and Walker, R. B., “Burning RateModifier Effects on a High Rate Tactical Propellant,” 13th AIAA/SAE

PropulsionConference, Orlando, FL, July 1977;alsoAIAAPaper P-77-928.

[25] Fonblac, G., and Herran, B., “The Maturity of Butacene BasedComposite Propellants,” 30th AIAA/ASME/SAE/ASEE Joint Propul-

sion Conference, Indianapolis, IN, 27–29 June 1994;alsoAIAAPaper 1994-3194.

[26] Gerards, A. B., Neidert, J. B., and Pledger, K. L., “High-PerformancePropellant Development and Demonstration,” 41st AIAA/ASME/SAE/

ASEE Joint Propulsion Conference and Exhibit, Tuscon, AZ,July 2005;alsoAIAA Paper 2005-3616.

[27] Swarts, J. C., Lamprecht, G. J., and Greyling, S. E., “FerroceneDerivatives as High Burning Rate Catalysts in Composite Propellants,”South African Journal of Chemistry, Vol. 50, No. 4, 1997, pp. 208–217.

[28] Rudy, T., Butacene Critique, Mach I, Inc., 2009, pp. 2, 4–6.[29] Raynal, S., and Doriath, G., “New Functional Prepolymers for High

Burning Rates Solid Propellants,” 22nd AIAA/ASME/SAE/ASEE Joint

Propulsion Conference, Huntsville, AL, June 1986;alsoAIAAPaper 1986-1594.

[30] Doriath, G., “Energetic Insensitive Propellants for Solid and DuctedRockets,” Journal of Propulsion and Power, Vol. 11, No. 4, 1995,pp. 870–882.doi:10.2514/3.23912

[31] Talawar, M. B., Sivabalan, R., Asthana, S. N., and Singh, H., “NovelUltrahigh-Energy Materials,” Combustion, Explosion, and Shock

Waves, Vol. 41, No. 3, 2005, pp. 264–277.doi:10.1007/s10573-005-0031-1

[32] Korobeinichev, O. P., Paletsky, A. A., Tereschenko, A. G., and Volkov,E. N., “Study of Combustion Characteristics of AmmoniumDinitramide/Polycaprolactone Propellants,” Journal of Propulsion

and Power, Vol. 19, No. 2, 2003, pp. 203–212.doi:10.2514/2.6119

[33] Agrawal, J. P., “Some New High Energy Materials and theirFormulations for Specialized Applications,” Propellants, Explosives,

Pyrotechnics, Vol. 30, No. 5, 2005, pp. 316–328.doi:10.1002/prep.200500021

[34] Husband, D. M., and Ferguson, D. C., “Development andDemonstration of a Very High Burn Rate Alternate Propellant for theManeuver Propulsion Assembly (MPA) Used in the Anti-Satellite(ASAT) Missile,” JANNAF Propulsion Meeting, New Orleans, LA,Vol. 3, Aug. 1986, pp. 673–679;also Paper 1987-0747.

[35] D’Andrea, B., Lillo, F., Faure, A., and Perut, C., “ANewGeneration ofSolid Propellants for Space Launchers,” Acta Astronautica, Vol. 47,Nos. 2–9, 2000, pp. 103–112.doi:10.1016/S0094-5765(00)00049-7

[36] Kubota, N., Sonobe, T., Yamaoto, A., and Shimizu, H., “Burning RateCharacteristics of GAP Propellants,” 24th AIAA/ASME/SAE/ASEE

Joint PropulsionConference andExhibit, Boston, July 1988;alsoAIAAPaper 88-3251.

[37] Nagayama, K., and Oyumi, Y., “Combustion Characteristics of HighBurn Rate Azide Polymer Propellants,” Propellants, Explosives,

Pyrotechnics, Vol. 21, No. 2, 1996, pp. 74–78.

MANSHIP, HEISTER, AND O’NEIL 1397

Dow

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EN

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| http

://ar

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aa.o

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DO

I: 1

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14/1

.606

18

doi:10.1002/prep.19960210205[38] Hori, K., and Kimura, M., “Combustion Mechanism of Glycidyl Azide

Polymer,” Propellants, Explosives, Pyrotechnics, Vol. 21, No. 3, 1996,pp. 160–165.doi:10.1002/prep.19960210310

[39] Selim, K., Ozkar, S., and Yilmaz, L., “Thermal Characterization ofGlycidylAzide Polymer (GAP) andGAP-BasedBinders for CompositePropellants,” Journal of Applied Polymer Science, Vol. 77, No. 3, 2000,pp. 538–546.doi:10.1002/(SICI)1097-4628(20000718)77:3<538::AID-APP9>3.0.-CO;2-X

[40] Shen, S. M., Wang, S. W., Chiu, Y. S., Chen, S. I., Chang, F. M., andHuang, C. C., “Thermal Decomposition of Cured GAP-AP PropellantsContaining Catocene,” Thermochimica Acta, Vol. 216, March 1993,pp. 255–266.doi:10.1016/0040-6031(93)80396-R

[41] Lusby, C. A., Ferguson, D. C., and Husband, D. M., “Friction andImpact Sensitivity of Formulations Containing Glycidyl AzidePolymer,” Propellants, Explosives, Pyrotechnics, Vol. 20, No. 1, 1995,pp. 27–31.doi:10.1002/prep.19950200108

[42] Min, B. S., “Characterization of the Plasticized GAP/PEG and GAP/PCL Block Copolyurethane Binder Matrices and its Propellants,”Propellants, Explosives, Pyrotechnics, Vol. 33, No. 2, 2008,pp. 131–138.doi:10.1002/prep.200700241

[43] Vasudevan, V., and Sundararajan, G., “Synthesis of GAP-PB-GAPTriblock Copolymer and Application as Modifier in AP/HTPBComposite Propellant,” Propellants, Explosives, Pyrotechnics, Vol. 24,No. 5, 1999, pp. 295–300.doi:10.1002/(SICI)1521-4087(199910)24:5<295::AID-PREP295>3.0.-CO;2-8

[44] Kasikçi, H., Perkel, F., and Özkar, S., “Curing Characteristics ofGlycidyl Azide Polymer-Based Binders,” Journal of Applied Polymer

Science, Vol. 80, No. 1, 2001, pp. 65–70.doi:10.1002/1097-4628(20010404)80:1<65::AID-APP1076>3.0.CO;2-J

[45] Manu, S. K., Varghese, T. L., Mathew, S., and Ninan, K. N.,“Compatibility of Glycidyl Azide Polymer with Hydroxyl TerminatedPolybutadiene and Plasticizers,” Journal of Propulsion and Power,Vol. 25, No. 2, 2009, pp. 533–536.doi:10.2514/1.38145

[46] Bluestone, S. R., “Development of Composite Solid Propellants Basedon Dicyclopentadiene Binder,” M.S. Thesis, Aeronautics andAstronautics Engineering Dept., Purdue Univ., West Lafayette, IN,2010.

[47] Corpening, J. H., Palmer, R. K., and Heister, S. D., “Combustion ofAdvancedNon-Toxic Propellants,” International Journal of Alternative

Propulsion, Vol. 1, Nos. 2–3, 2007, pp. 154–173.doi:10.1504/IJAP.2007.013018

[48] Bluestone, S. R., Heister, S. D., Son, S. F., and Austin, B. J.,“Development of Composite Solid Propellants Based on Dicyclo-pentadiene Binder,” 46th AIAA/ASME/SAE/ASEE Joint Propulsion

Conference and Exhibit, Nashville, TN, July 2010;alsoAIAAPaper 2010-6589.

[49] Hinkelman, M. C., “Development and Testing of DicyclopentadieneBased Solid Composite Propellants,” M.S. Thesis, Aeronautics andAstronautics Engineering Dept., Purdue Univ., West Lafayette, IN,2011.

[50] Abou-Rachid, H., Lussier, L., Ringuette, S., Lafleur-Lambert, X.,Jaidann, M., and Brisson, J., “On the Correlation between Miscibilityand Solubility Properties of Energetic Plasticizers/Polymer Blends:Modeling and Simulation Studies,” Propellants, Explosives,

Pyrotechnics, Vol. 33, No. 4, 2008, pp. 301–310.doi:10.1002/prep.200700211

[51] Ninan, K. N., Balagangadharan, V. P., and Catherine, K. B., “Studies onthe Functionality Distribution of Hydroxyl-Terminated Polybutadieneand Correlation with Mechanical Properties,” Polymer, Vol. 32, No. 4,1991, pp. 628–635.doi:10.1016/0032-3861(91)90474-W

[52] Dilsiz, N., and Ünver, A., “Characterization Studies on AgingProperties of Acetyl Ferrocene Containing HTPB-Based Elastomers,”Journal of Applied Polymer Science, Vol. 101, 2006, pp. 2538–2545.doi:10.1002/app.23918

[53] Sekkar, V., “Comparison Between Crosslink Densities Derived fromStress-Strain Data and Theoretically Data Evaluated Through the �-Model Approach for a Polyurethane Network System Based onHydroxyl Terminated Polybutadiene and Isophorone-Diisocyanate,”Journal of Applied Polymer Science, Vol. 117, No. 2, 2010, pp. 920–925.doi:10.1002/app.31643

[54] Moss, J., “Development of Tools for Experimental Analysis of ErosiveBurning in/near Slots in Solid Rocket Motors,” M.S. Thesis,Aeronautics and Astronautics Engineering Dept., Purdue Univ., WestLafayette, IN, 2008.

[55] Manship, T. D., “High Burn Rate Solid Composite Propellants,” M.S.Thesis, Aeronautics andAstronautics Engineering Dept., PurdueUniv.,West Lafayette, IN, 2010.

[56] Gilbert, P. D., “Multifunctional Additives for Composite SolidPropellant,”M.S. Thesis,Mechanical Engineering Dept., PurdueUniv.,West Lafayette, IN, 2008.

E. KimAssociate Editor

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