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ORIGINAL ARTICLE

Radiation-induced pyrolysis of solid fuelsfor ramjet application

Trevor D. Hedmann

Naval Air Warfare Center Weapons Division, China Lake, CA 93555, USA

Received 19 December 2014; accepted 19 April 2015Available online 26 May 2016

KEYWORDS

Hydroxyl-terminatedpolybutadiene;Pyrolysis;Ramjet;Boron;Aluminum

tional Laboratory foe (http://creativecom

16/j.jppr.2016.04.00

26.

vor.hedman@navy.

esponsibility of Natia.

Abstract A wide variety of hydroxyl-terminated polybutadiene (HTPB) based fuels areexperimentally assessed in anaerobic reaction. In this study HTPB pyrolysis is investigatedusing a CO2 laser as the energy source. The formulation of the solid fuel samples issystematically changed to isolate the effects of carbon black, metal fuel additives, and smallamounts of oxidizer. In addition, chemical changes to the fuels including curative type andbase polymer are varied. Rates of pyrolysis reaction are reported for a wide range of solid fuelsapplicable to ramjet application. Processes involving the sintering together of metal particles,accumulation of carbon black, and formation of a melt layer are found to affect the reactionrate. It is determined that the surface composition is the most influential factor influencing theregression rate of HTPB based fuels.& 2016 National Laboratory for Aeronautics and Astronautics. Production and hosting by Elsevier B.V.

This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

The solid fuel ramjet (SFRJ) continues to be an attractivepropulsion system because it offers a good balance ofsimplicity, specific impulse, speed, and safety [1]. Researchon the SFRJ gaseous oxidizer-solid fuel combustion

r Aeronautics and Astronautics. Produmons.org/licenses/by-nc-nd/4.0/).

2

mil

onal Laboratory for Aeronautics

environment has been conducted for decades and continuesto be active today. Netzer et al. [2,3] led an effort to build asmall scale SFRJ combustor for the development ofpredictive numerical tools [4]. Similar studies over theyears have utilized sub-scale burners to investigate theflame stability [5,6], inlet design [7], and development ofhigh fidelity fluid dynamic numerical tools [8,9].

Sub-scale SFRJ motors have also been used to investi-gate high density fuel formulations comprised of polymerwith imbedded metal particles [10,11]. The most commonlyused fuel is hydroxyl-terminated polybutadiene (HTPB).

ction and hosting by Elsevier B.V. This is an open access article under the

Figure 1 A depiction of the combustion environment of a solid fuelramjet (SFRJ). Due to boundary layer formation, much of theregression of the fuel grain is driven by pyrolysis. A diffusion flameforms between the oxidizer flow and the pyrolysis products.

Trevor D. Hedman88

This polymer is important to the propulsion community dueto widespread application in rocket motors, hybrid systems,and solid ramjet applications. The prevalent use of HTPB isa result of desirable mechanical properties at a wide rangeof temperatures, even when highly loaded with particles.These advantages are described in greater detail elsewhere[12,13]. For an HTPB based fuel with metal additives, thecombustion environment in a typical SFRJ is depicted inFigure 1. Here a solid fuel is shown with embedded metalparticles added to increase density and energy content. Theblue lines above the fuel grain represent the high velocitycross-flow of the oxidizer stream. This flow rapidly forms aboundary layer above the fuel surface, immediately follow-ing expansion near the inlet shock system [14]. Sinceoxidizer penetration of this boundary layer is limited bythe flow characteristics, radiation from the flame zone (red)accounts for much of the heat transfer back to the fuelsurface. The pyrolysis products diffuse toward the flamezone, often percolating through accumulated metal particlesat the surface. The formation of a boundary layer and adiffusion flame with a standoff distance results in anincreased importance on the radiative heating of the fuelsurface and the resulting pyrolysis [15]. Condensed phaseconduction also becomes increasingly important since sur-face has higher thermal conductivity due to the accumulatedmetal. The absorption and dispersion of heat through thesolid fuel is complex because radiation, conduction, andconvection all significantly contribute.Both the widespread use of HTPB in various propulsion

systems and the vital role of pyrolysis have motivated manystudies on the subject. Many of these studies have beenreviewed by Beck [16]. In short, various techniques havebeen used to rapidly heat fuel samples to measure theirdecomposition [17] and pyrolysis rates [18,19]. Thisinformation is typically used to determine chemical kineticparameters or reaction mechanisms [20,21]. Esker andBrewster [22] carried out experiments to quantify thepyrolysis rate of IPDI cured HTPB. A CO2 laser was usedas the heat source and anaerobic regression rates weredetermined using a micro-force transducer for heat fluxesbetween 50 and 500 W/cm2. It was found that 3 wt%carbon black decreases in the pyrolysis rate of up to 50%.

Pyrolysis rates have been reported previously for HTPB,however, typically this is done for a single fuel andattention is focused on the technique rather than systematicchanges to the fuel.

The current study seeks to expand previous HTPB pyrolysisstudies by examining the effect of various additives, curative,and changes to the polymer. This is motivated by the fact thatisolated HTPB pyrolysis studies often cannot be compared dueto chemical differences in the fuel and by a lack of dataregarding the effect of additives on pyrolysis rate. The goal ofthis study is to determine the effect of the curative, polymer,added carbon black, and metal fuel additives on the pyrolysisrate and surface behavior.

2. Experimental methods

2.1. Fuel preparation

The solid fuels prepared in this study are all HTPB based,systematically modified to study the effects of curative type,polymer type, carbon black, metal fuels, and oxidizer. In mostof the fuel formulations, the HTPB used is an R45-M(604045, RCS RMC) cured with: isophorone diisocyanate(IPDI, Sigma Aldrich MFCD00064956) or a polymethylenepolyphenylisocyanate (PAPI 94, Dow Chemical 94 MDI). Forthose formulations containing a large amount of micron-sizedadditives, the PAPI 94 curative was used as the cure times arevery short compared with IPDI, typically about 20 minutes inthe ratio used. The advantage of the short cure time is that thecolloidal mixture becomes tacky and increasingly viscousquickly, severely limiting settling of added particles.

Additives to the polymer based fuels were carbon black(Cancarb N991), aluminum (Valimet, H-3), boron (Mach I,Boron 95), a magnesium boron mixture (Mach I, 60 wt% Mg),and ammonium perchlorate (AP, Sigma Aldrich 208507). Theparticle sizes for these additives are listed in Table 1 along withformulations for each fuel. For details regarding specificingredients, the reader is directed to the manufacturer, as eachingredient is commercially available. In the preparation of eachsample, the HTPB was degassed under vacuum for 15 minprior to mixing. Ingredients were added and mixed by handfollowing more exposure to vacuum to release air voids andevolved gas bubbles from the mixture. Teflon containers of thefuels were cured in an oven at 65 1C.

Three different polymers were used in the fuel formulations:HTPB R45M, HTPB HTLO (R-45HTLO, Cray Valley), andKrasols (HLBH P-3000, Cray Valley). Both HTPB R45Mand HTLO have a molecular weight near 2800 g/mol anddiffer only by the higher polydispersity of HTLO. A highermolecular weight of 3100 g/mol is reported for HTPBKrasols, a fully hydrogenated hydroxyl-terminated polyole-fin. Details regarding the physical and chemical properties ofthese polymers can be found in Ref. [23]. All measurementsreported for each formulation were made using samples fromthe same mix and cast.

Table 1 A name and description of each HTPB based fuel is provided below.

Fuel designation Description

HTPB-1 90 wt% HTPB R45M cured with 10 wt% PAPI 94HTPB-2 91 wt% HTPB R45M cured with 10 wt% IPDI, 0.3 wt% maleic anhydride and 0.3 wt% triphenyl bismuthHTPB-3 HTPB-1 with 1 wt% carbon black (carbon black nominally 250 nm)HTPB-4 HTPB-2 with 1 wt% carbon black (carbon black nominally 250 nm)HTPB-5 HTPB-3 with 30 wt% aluminum (nominally 10.5 mm diameter)HTPB-6 HTPB-3 with 30 wt% boron (nominally 95 mm in diameter)HTPB-7 HTPB-3 with 30 wt% boron/magnesium mixture (nominally 95 mm diameter)HTPB-8 HTPB-3 with 5 wt% AP (nominally 20 mm AP)HTPB-9 HTPB-3 with 10 wt% AP (nominally 20 mm AP)HTPB-10 90 wt% HTPB HTLO cured with 10 wt% PAPI 94HTPB-11 90 wt% Krasols cured with 10 wt% PAPI 94

Figure 2 The experimental system used to make fuel pyrolysismeasurements is shown schematically.

Radiation-induced pyrolysis of solid fuels for ramjet application 89

Note that the fuel formulations listed in Table 1 aresystematically changed to study effects of various ingredi-ents. Fuels HTPB-1 and HTPB-3 represent a baseline for allof the formulations containing metal particles and are usedto examine the effects of carbon black addition. The twofuels designated HTPB-2 and HTPB-4 use a differentcurative and can be compared with IPDI cured blends inopen literature. The remaining fuels were formulated tostudy the effects of metal particles, oxidizer, and basepolymer.

2.2. Pyrolysis rate measurements

The pyrolysis rate measurement was performed using aCO2 laser (Coherent Diamond E-400) as the heat source forenergy flux values of 50, 100 and 200 W/cm2. The CO2

laser is a radio frequency excited, liquid cooled, pulsedsystem. Experiments were conducted with a pulse fre-quency and duration of 20 Hz and 30 ms respectively.Power meters were used to measure the effective heat fluxdelivered to the sample. For each flux value, the exposureduration was at three seconds, set using a delay pulsegenerator (Stanford Research DG535). In the case of slowregressing fuels/conditions, additional measurements weretaken with a laser pulse of five seconds for easier highspeed video analysis. The HTPB based fuels are known tobe sufficiently opaque to the 10.6 mm radiation [22].Neglecting reflection, the laser imparts 48, 95, and 190 Jonto the sample per test at 50, 100 and 200 W/cm2

respectively. However, carbon black was added in somefuel formulations to replicate common practice in SFRJ andhybrid applications.

The experiments were completed in a Plexiglas chamberwith positive pressure supplied by a nitrogen flow. Eskerand Brewster [22] performed a study of pyrolysis rates ofHTPB based fuels using a CO2 laser and found that plumetransmission of the beam was strongly dependent on samplesize. The large plume of pyrolysis products from one cmsquare samples was found to absorb the majority of theincident radiation. However, the plume generated bysmaller samples 3 mm in diameter was found to transmit80–90% of the 10.6 mm radiation. Samples used in the

present study were 7 mm in diameter. In order to minimizethe effects of plume absorption, the samples for the presentstudy were oriented horizontally, with the length of thecylinder parallel to the surface of the optical table. In thisway, the hot plume is carried upward by buoyancy, awayfrom the fuel surface. For clarity, the reader is directed toFigure 2 where the fuel sample configuration is displayedrelative to the incident radiation. To further negate theeffects of plume absorption, the entire fuel sample wasplaced into a nitrogen curtain for each measurement.Ambient temperature nitrogen gas enveloped the sample,flowing upward with a flow rate of 30 L/min and carryingany pyrolysis products out of the path of the laser radiation.The nitrogen curtain ensured pyrolysis since oxygen wasdisplaced and also carried the plume of product gases out ofthe path of the laser radiation. In a few cases, despite thenitrogen curtain, a faint flame appeared far from the fuelsurface, prompting additional replication.

Measurements of the pyrolysis rate were made using twomethods: direct observation of the sample using high speedcinematography (Photron SA5) and mass loss measure-ments. High resolution images were recorded at a rate of500 fps and each sample was weighed before and after eachexposure to the laser radiation. Camera resolution wasmeasured to be 12 pix/mm and regression rates were

Trevor D. Hedman90

determined by tracking and averaging the fuel surfacemovement at three locations over time. Based on the massloss, exposure time, and sample density, the pyrolysis ratewas calculated using r¼ (mi�mf)/(ρAt). Here r is the fuelregression rate, mi and mf and initial and final samplemasses, ρ is the sample density, A is the sample areaexposed to the laser, and t is the laser pulse duration. Thesamples were cylinders, sized using a coring tool to adiameter of 6.35 mm and cut to 12 mm lengths. Sampledensity and area were calculated for each sample based onmeasured dimensions and masses prior to laser exposure.In general, reasonable agreement was noted between the

measurement methods. These methods were validatedagainst micro-force transducer measurements reported inRef. [22], for an IPDI cured HTPB without carbon black orother additives. The comparison is shown in Figure 3. Notethat both methods used in this study compare well withpreviously reported pyrolysis rates for flux levels of50–200 W/cm2, especially considering the data scatter.Discrepancies in the data sets likely result from slightdifferences in the fuel composition, plume transmissioncorrection [22], and frequency/pulse duration of the deliv-ered radiation.

2.3. Differential scanning calorimetry (DSC)

Thermal behavior in a nitrogen environment was alsoinvestigated at a much slower heating rate. Small, approxi-mately 5 mg samples of HTPB-1 through HTPB-11 wereanalyzed using DSC. These samples were loaded into 40 mLaluminum pans for analysis in a Perkin Elmer DSC (DSC8500). Open pan experiments were performed with ultra-high purity nitrogen at 20 ml per minute. The samples wereheated from room temperature to 500 1C at a rate of 5 1C/min.

Figure 3 Comparison of measurements made in current study withRef. [22] for an IPDI cured HTPB fuel.

3. Results and discussion

3.1. Curative comparison

Measurements were made on the laser-induced pyrolysisof HTPB (R45M) polymers cured with both PAPI-94 andIPDI curatives. Note that although the curative was changedbetween the fuel blends, the cure temperature and durationwere constant. The results of these measurements are shownin Figure 4 for flux levels of 50, 100, and 200 W/cm2. Eachpoint in the figure is an average of six measurements; threefrom the high speed video recording and three based onmass loss. The error bars represent the standard deviation. Itwas observed that the calculated regression rates based onmass loss and those based on visual measurements using thehigh speed camera were in good agreement. The HTPB-1(PAPI 94) fuel exhibited faster pyrolysis rates than HTPB-2at all flux levels. For example, at a flux level of 200 W/cm2,the change in curative resulted in a 9% difference inregression rate. This disparity, although statistically sig-nificant at only the highest flux, is thought to be due to adifference in the degree of curative-induced cross-linking,and consequently the energy required to disrupt the polymermatrix.

This comparison illustrates the difficulty in comparingreported pyrolysis rates for different formulations in theliterature. For example, Arrhenius kinetic parameters weregenerated for radiation-induced pyrolysis of HTPB and sixother binder materials by Cohen et al. [24]. The curativeused in the HTPB blend was not reported, but pyrolysisrates were compared with Arrhenius kinetic parametersestimated using other techniques [22,25,26]. Results of thecurrent study suggest that altering the curative can effectsignificant changes in pyrolysis rate, and influence theestimated reaction kinetics.

3.2. Effect of carbon black

Carbon black is typically added to make fuel formula-tions opaque to the flame radiation in the combustionsection of a SFRJ. Radiation is estimated to account forat least 15% of the total heat transfer back to the surface, asreported by Metochianakis et al. [27]. However, thisestimate may be low for fuel surface regions shielded bya thick boundary layer that blocks convective heat transfer.Fuel formulations both with and without carbon black werecompared using the laser pyrolysis experimental system.The pyrolysis rates of HTPB-3 (PAPI 94, carbon black) andHTPB-4 (IPDI, carbon black) were measured and arepresented in Figure 5. Note the decrease in pyrolysis ratedue to the carbon black addition. The decrease in pyrolysisrate is shown more clearly in Figure 6, a bar chartcomparing HTPB-1 (PAPI 94 curative without carbonblack) and HTPB-3 (PAPI 94 curative with 1 wt% carbonblack). A decrease in rate was not observed in one instance;comparing HTPB-2 and HTPB-4 at 200 W/cm2.

Figure 5 Fuels HTPB-1 and HTPB-2 (no carbon black) are comparedwith HTPB-3 and HTPB-4 (1 wt% carbon black). Each point is the averageof six measurements and error bars represent standard deviation.

Figure 6 Average regression rates for fuel samples with and withoutcarbon black in the formulation. The addition of carbon black slowsthe regression, especially at the lower laser fluxes.

Figure 4 A comparison of laser-induced pyrolysis rates for HTPBcured with IPDI (HTPB-1) and PAPI 94 (HTPB-2) is shown above.The data were recorded at 50, 100, and 200 W/cm2; data is offset fromthese values to elucidate the scatter. Error bars represent standarddeviation.

Radiation-induced pyrolysis of solid fuels for ramjet application 91

The decrease in pyrolysis rate of the fuel formulationscontaining carbon black can be attributed to the particleaccumulation on the surface. Images of the surface beforeand after laser exposure were recorded using a digitalKeyence 3D microscope (VH-S5). In the case of the fuelsamples without carbon black, it was found that the surfaceis dominated by a melt layer. This is evidenced by bubblesand signs of flow into low points on the surface. A typicalimage of the surface of HTPB-1 is shown in Figure 7A.This image was taken after laser exposure at a flux level of100 W/cm2. With the addition of 1 wt% carbon black, thesurface becomes marked with accumulated carbon. This isshown in Figure 7B, an image of the surface of HTPB-3recorded after exposure at 100 W/cm2. The white arrowsindicate the locations of the carbon buildup, as seen bycircular areas approximately 200 mm in diameter. As theflux level was increased, the carbon accumulation becamemore uniform and covered entire regions of the surface. InFigure 7C, recorded after surface exposure at 200 W/cm2,an area of extensive carbon accumulation is seen. While the

entire fuel surface was not covered in accumulated carbonblack at the highest flux level, it did appear more uniformlydispersed over large areas. This difference in surfacecomposition may explain the less pronounced effect ofcarbon black addition at 200 W/cm2 (see Figures 5 and 6)where a thin layer is present rather than thick circularregions. It is thought that the more uniform distribution ofcarbon black observed at 200 W/cm2 causes a morehomogeneous surface temperature and in this case mayeven augment the regression rate due to increased heatconductivity (compare HTPB-2 and HPTB-4).

Figure 7 illustrates the changing surface compositionwith the addition of carbon black. Without it, the surface ischaracterized by bubbles, formed by the rapid volatilizationof the HTPB polymer. When added, a portion of the laserenergy is absorbed by the accumulated inert carbon on thesurface rather than the fuel. The circular regions are thoughtto form due to migration of the carbon black duringregression, made possible by the melting/bubbling behaviorof the polymer. Although added in only 1 wt%, theregression rate was reduced by 27% on average over thethree second beam exposure. Esker and Brewster [22] noteda 30–40% decrease in the pyrolysis rate of HTPB whencarbon black was added in 3 wt%. While the regression ratedecreases in pyrolysis, the benefit of adding carbon black ina SFRJ application is two-fold: increased absorptivity at thepropellant surface in the combustion chamber and broad-band emission from the hot soot particles in the gas phaseback to the surface. For the pyrolysis reaction, resultspresented here suggest that carbon black is in excess whenadded in 1 wt%. However, experiments done in a combus-tion environment are required to verify that carbon blackadded in smaller quantities will yield the desired benefits.

3.3. Effect of metal additives

It is desirable to add metal particles to the SFRJ grain toincrease the fuel density, energy content, and Isp. The metal

Figure 7 Image A shows the surface of HTPB-1 (PAPI 94 curative) after exposure to a laser flux of 100 W/cm2. Images B and C are post-laserexposure surfaces of HTPB-3 (HTPB-1 with carbon black) with flux levels of 100 W/cm2 and 200 W/cm2 respectively.

Figure 8 Data collected from both the high speed camera and massloss are shown together for the metalized HTPB fuels. Note that theamount of scatter in the data tends to increase with laser flux.

Trevor D. Hedman92

particles greatly increase the thermal conductivity of themixture; for example, HTPB-5 (30% aluminum) is somefifty times more thermally conductive that HTPB-1 basedon a volume average and over one hundred times moreconductive based on mass averaging [28]. This is becauseof the large disparity in thermal conductivity for metal suchas aluminum (205 W/mK) and an HTPB (1.98 W/mK), athermal insulator. In radiation-induced pyrolysis, a higherconductivity tends to spread the heat through the samplerather than concentrate it at the surface, thus slowing theregression. This is coupled with the fact that the metal doesnot react in pyrolysis, but accumulates at the surface. Thusthe measured rates are expected to distinctly decrease fromthat of neat HTPB.Laser pyrolysis of the metallized fuel samples was

complicated by the porous network of metal particles thataccumulate on the surface. Compared to the previousmeasurements, there was more scatter in the data becausethe metal structures that form on the surface duringexposure to the CO2 laser energy distort the amount ofvisually observed regression. This resulted in slightly lowervalues for the visual method of measurement and highervalues for the gravimetric based method. The amount ofscatter in the data is shown in Figure 8.All the metalized samples regress below 0.2 mm/sec at

the lowest laser flux of 50 W/cm2. However, the aluminizedfuel (HTPB-5) exhibited much faster pyrolysis rates withincreasing flux, especially compared with the other metal-lized fuels. In fact, the average rate measured for HTPB-5 isonly slightly lower compared to HTPB-3 (non-aluminizedvariant). An explanation for this difference is developed byexamination of the surfaces after exposure to the CO2 laser.Photographs of the surfaces of HTPB-5, HPTB-6, andHTPB-7 are shown in Figure 9A, B, and C, respectively.These images were recorded using the digital microscopedescribed previously after laser exposure at 100 W/cm2. Itwas found that the aluminum particles tend to sintertogether and form ridges exposing areas of unreactedHTPB. In contrast, images of the surface of HTPB-6 andHTPB-7 show more uniform coating of accumulated metalparticles with only small pockets of HTPB exposed. Thus

the fuel is better able to absorb energy from the laser andpyrolyze in the aluminized fuel (HTPB-5) compared toHTPB-6 and HTPB-7.

3.4. Fuel rich propellant

The flammability limits of solid fuels used in a ramjetapplication are an important consideration due to the highlyvariable oxidizer flow field conditions in the combustor.Robust fuels that will not easily extinguish in low-oxidizerenvironments are desired. One method for achieving such afuel is to add a small percentage of oxidizer to theformulation [24]. This approach decreases the specificimpulse of the fuel, but still may be desirable based onmission requirements. Two formulations with ammoniumperchlorate (AP) were evaluated using the CO2 laserexperimental system: HTPB-8 and HTPB-9 containing5 wt% and 10 wt% AP, respectively.

Again, at least six measurements of pyrolysis rate wererecorded for each fuel at three flux levels: 50, 100,200 W/cm2. The rates were averaged for each conditionand are plotted in Figure 10. The error bars represent thestandard deviation of the measured rates. With an addition

Figure 9 Images collected post-laser pyrolysis at 100 W/cm2. Images A, B and C show the surfaces of HTPB-5 (aluminum), HTPB-6 (boron)and HTPB-7 (boron/magnesium) respectively.

Figure 10 The pyrolysis rates for HTPB based fuels with andwithout AP are shown above. Each column represents an average ofsix measurements with the CO2 laser experimental system. Error barsare included to show the standard deviation of the measurements.

Radiation-induced pyrolysis of solid fuels for ramjet application 93

of 5 wt% AP, the regression rate decreased approximately27% on average. At 10 wt% AP, the fuel regression ratedecreased over 50% on average. These large decreases werenot expected, especially considering the fact that the AP hasthe potential to change the reaction mechanism frompyrolysis to combustion. Given the measured rates, itappears that the AP does not burn with the HTPB, butrather absorbs heat that would otherwise have caused HTPBpyrolysis. This was confirmed by the absence of flames inthe high speed video recordings.

The surface of the fuel rich formulations was examinedafter exposure to the laser. The surface appeared to be amixture of melted AP and carbon black deposits. A meltlayer was made apparent by the presence of translucent,glossy areas. On the surface of HPTB-3 (without AP,1 wt% carbon black) these areas were not present. Theevidence of the melted AP is shown in two images inFigure 11 that were recorded after exposure to the laser at aflux level of 100 W/cm2. Image A is the surface of HTPB-8(5 wt% AP, 1 wt% carbon black) and image B is the surfaceof HTPB-9 (10 wt% AP, 1 wt% carbon black). The glossy,high contrast areas denote the melted AP that has notreacted, but mixed with the accumulated carbon black.These areas were increasingly common when comparingHTPB-9 (10 wt% AP) to HTPB-8 (5 wt% AP). Theabsorption of heat and melting of the AP is thought to bethe cause of the sharp decrease in regression rate.

3.5. Polymer type

The effect of HTPB type was also investigated using theCO2 laser experimental system. Three polymers wereexamined: HTPB RM45M, HTPB HTLO, and Krasols.Average regression rates measured in pyrolysis are shownfor all three formulations in Figure 12. The error barsrepresent standard deviation of eight measurements for eachcondition. Eight measurements were made to study polymertype to limit the uncertainty; this required completion oftwo additional measurements for HTPB-1. It was found thatHTPB-1 (R45M) and HTPB-10 (HTLO) blends exhibitednearly identical pyrolysis rates for all the CO2 laser fluxesexamined. Krasols, a polymer of higher viscosity andmolecular weight compared to HTLO and R45M, pyrolyzed

at lower rates at every flux. Both high speed video and post-laser exposure microscopy showed a thick, highly viscousmelt layer that formed on the HTPB-11 samples. This layerwas much more prominent in the HTPB-11 samples whencompared directly with HTPB-1 and HTPB-10. Chemically,the principal difference between Krasols and the R45Mand HTLO fuels is a more linear chain structure due to thelack of C-C double bonds. Krasol is a fully saturatedmolecule. It appears that this difference in chemicalstructure causes a statistically significant drop in thepyrolysis rate. This is thought to be due to its propensityto melt rather than gasify, as a thick melt layer wasobserved during experimentation on the surface of HTPB-11 samples. On average, this reduction for HTPB-11 isnearly 20% when compared with HTPB-1 at 200 W/cm2.

3.6. Pyrolysis mechanism

Heat traces recorded using the DSC described earlier arepresented in Figure 13. The heat traces chosen for this plotshow the trends observed in all fuels investigated in thepresent study. HTPB-1 is a baseline material with PAPI 94as the curative. Fuels HTPB-4,7, and 10 are also cured withPAPI 94, but contain carbon black, metal particles, and

Figure 11 The surfaces of fuel rich propellants are shown after laser pyrolysis. Images A and B are the surfaces of HTPB-8 and HTPB-9,respectively. The lighter colored areas (shiny/reflective) are a mixture of AP particles and carbon black.

Figure 12 A comparison of pyrolysis rates for various cured HTPBpolymer blends is shown.

Figure 13 Heat traces measured on a DSC for several of the HTPB-based, PAPI 94 cured, fuels studied are shown. The traces for HTPB-4,7, and 10 are offset for reader clarity by 25, 50, and 80 mWrespectively. Endotherm is downward.

Trevor D. Hedman94

ammonium perchlorate, respectively. In each case thedecomposition of the HTPB was accelerated, evidencedby the earlier reaction temperature for the upper three heattraces compared to the lowest (HTPB-1). The exothermicityobserved in the heat traces from 350–450 1C can be

attributed to HTPB depolymerization [25]. Smallerexotherms preceding this temperature range are due tovolatilization of the diisocyanate curative. The fuels withparticle additives begin to exotherm some 20–30 1C earlierin the heating profile. Also, the additives cause the heatrelease to be more gradual, in contrast with the abruptexothermic behavior seen near 430 1C for HTPB-1.

Although the DSC heats the fuel samples at rates ordersof magnitude slower than the CO2 laser system, a compar-ison of heat traces shows that there is no retardation of thedecomposition/pyrolysis reactions of these fuels due topresence of additives. The lower reaction rates observedin the laser pyrolysis experiments cannot be attributed toslower chemical breakdown caused by additives. Thus, thesurface composition and laser radiation absorption mechan-ism are thought to account for the slowed regression ratesobserved in the fuels formulated with carbon black, metals,and oxidizer particles. This finding aligns with the results ofCohen et al., who studied pyrolysis of a wide range ofpolymers with an arc lamp. It was found that the regressionrate could not be predicted based on chemical structure, butthat it was strongly dependent on the boiling and meltingbehavior of the polymer [24].

As particles are added to the base HTPB fuel, physicalproperties of the material are substantially altered. Forexample, the thermal conductivity is changed by orders ofmagnitude. Since pyrolysis rates were collected for a widerange of solid, HTPB based fuels, it was desired to examinethe effects of density and thermal conductivity. Thesephysical properties were calculated for each fuel based onmass averages using measurements reported by Hanson-Parr and Parr [28]. It was found that both thermalconductivity and density could not be correlated with theregression rate. The most influential factor is the surfacebehavior, or macroscopic processes that occur duringexposure to the CO2 laser.

4. Conclusions

There are recognizable differences between the combus-tion environment in a SFRJ and the CO2 laser experimentdescribed here. Some of these include a reduced flow rate

Radiation-induced pyrolysis of solid fuels for ramjet application 95

over the fuel surface, reduced size/different geometry, andnarrow-band radiation rather than heat feedback fromburning metal particles. However, the SFRJ combustionenvironment relies on gasification of the fuel to form adiffusion flame above the surface. Radiation from the flamezone is a critical mechanism for surface regression in aSFRJ. This is especially the case when a boundary layer ispresent, resulting in a large standoff distance for the heatfeedback from the diffusion flame. Therefore, the CO2 laserexperiment is thought to be a good tool for the study oframjet fuels on a small scale.

Measurements recorded using the CO2 laser system werevalidated by direct comparison with literature [22]. Thesystem was used to assess the pyrolysis rate of ten HTPB-based solid fuels at various flux levels. The curative,presence of carbon black, polymer type, addition of metaladditives, and addition of oxidizer were investigated. Basedon the findings, it can be concluded that small variations inthe fuel formulation can impart substantial changes inpyrolysis rate. For example, carbon black, added at only1 wt% caused a 33% decrease to the pyrolysis rate at100 W/cm2. This was also the case for the curative (PAPI94 and IPDI) where the formulation change caused at least a9% difference. Disparities were also measured when vary-ing the chemical structure of the polymer. This findinghighlights the need for caution when comparing previousHTPB ramjet combustor and pyrolysis studies with recentformulations.

It can be concluded that macroscopic processes such asmelting, boiling, and metal accumulation control the pyr-olysis rate of HTPB based fuels. In every case where theregression rate significantly changed, there was an observedchange in surface composition or rheology that could accountfor the change. Also, while the thermal conductivity anddensity of the HTPB based solid fuel samples vary widely,they are not well correlated with the regression rate. Carbonblack, metals, and even AP were found to accumulate on thesurface and impede the incident laser radiation from imping-ing on the base fuel. For fuels containing metals, the surfacemorphology was observed to change depending on the metaladded and affect the rate of pyrolysis. The fact that thereaction rate is controlled by macroscopic processes ratherthan gas phase reactions or bulk physical properties makes a-priori predictions difficult. Studies generating empirical dataon the response of fuels in ramjet environments will berequired for model development.

Acknowledgments

The authors wish to thank the NAVAIR ILIR program,managed at ONR by the N-STAR program (Naval Research– Science and Technology for America's Readiness) admi-nistered by Scott Munro. Additionally, the Naval AirWeapons Center Warfare Division at China Lake is thankedfor internal support of this study through Ronald Schultz.

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