Thermal decomposition of RDX/BAMO pseudo-propellants

Thermal Decomposition of RDX/BAMO Pseudo-Propellants

YOUNGJOO LEE,*‡ CHING-JEN TANG,† and THOMAS A. LITZINGER‡

Department of Mechanical Engineering and Fuel Science, Pennsylvania State University,University Park, PA 16802, USA

Measurements of gaseous species and temperature profiles for RDX/BAMO pseudo-propellants wereperformed to study their decomposition and the chemical and physical effects of their binder ingredient,3,39-bis-azidomethyl-oxetane (BAMO), on the base ingredient, 1,3,5-trimethylene trinitramine (RDX). Thepropellants were made from a physical mixture of RDX and BAMO in weight ratio of 80:20. Experiments wereconducted at atmospheric pressure in argon with heat fluxes of 100 and 400 W/cm2 delivered by a CO2 laser.Gaseous samples were extracted through the use of quartz microprobes and analyzed by a triple quadrupolemass spectrometer (TQMS). Temperature profiles were measured using micro-thermocouple techniques toinvestigate surface and gas-phase reaction zones identified by the species measurement. Results of species andtemperature measurements showed chemical and physical interactions between the two ingredients. From thespecies measurements, products of each ingredient, RDX and BAMO, were found to exist simultaneouslythroughout the gas phase; however, primary reaction chemistry in the gas phase was dominated by RDX. Threedifferent categories of gaseous products were identified in the species measurements: species common to bothBAMO and RDX; from RDX or BAMO only; and those which can not be attributed to RDX or BAMO alone.Surface temperatures were ;640 and ;670 K at 100 and 400 W/cm2, respectively, while that of RDX was ;610K for both heat fluxes. The temperature profiles showed the existence of an isothermal region in the gas phase,with the temperature of ;1200 K at 100 W/cm2 and ;1500 K at 400 W/cm2. The effect of BAMO inRDX/BAMO was clearly evident in the change of surface mole fractions and profiles of major species,expansion of reaction zones, and the constant temperature profile in the gas phase. © 1999 by TheCombustion Institute.

INTRODUCTION

For an increased specific impulse at the reducedflame temperatures, nitramines such as 1,3,5-trimethylene trinitramine (RDX) (Fig. 1) andcyclotetramethylene tetranitramine (HMX)have been widely used as energetic materialsfor the propellant ingredients in applicationssuch as gun and rocket motors. However,nitramines have low burning rates coupledwith a relatively high and undesirable burningrate exponent. Therefore, in recent years,there has been a move toward the use of thenitramine explosives as fillers in an oxidizer/binder matrix propellant. In the search fornew binder materials for the nitramine pro-pellants, a great deal of interest has recentlycentered on azido compounds, such as 3,39-bis-azidomethyl-oxetane (BAMO) (Fig. 1).These polymers draw considerable interestsince they have potential as energetic bindersand plasticizers in nitramine-based propel-lants, where relatively low flame temperature

and high mass impetus are desirable. Thisnitramine/binder combination propellant ischaracterized by high velocity of detonation,low vulnerability, and good thermal stability.The characteristics are especially useful asrocket propellants, since these provide highspecific impulse while generating minimumsmoke.

Two configurations of the oxidizer/binder ma-trix have been used in fundamental studies ofnitramine/BAMO propellants: heterogeneousmixture and sandwich configuration. Althoughthe heterogeneous mixture type is the final formof the propellant to be used in actual appli-cations, the study of their behavior and de-composition may be too complicated for fun-damental studies due to thin reaction regionsand the heterogeneity of ingredients. A strat-egy to alleviate these experimental difficultiesis to study combustion of geometrically simplesystems such as an oxidizer/binder sandwich;many such studies have been performed tounderstand the diffusion flame of ammoniumperchlorate (AP)-based formulations [1]. Lim-ited research has been conducted on the com-*Corresponding author. E-mail: [email protected]

COMBUSTION AND FLAME 117:795–809 (1999)© 1999 by The Combustion Institute 0010-2180/99/$–see front matterPublished by Elsevier Science Inc. PII S0010-2180(98)00131-X

bustion and decomposition characteristics ofnitramine/binder(s) sandwich systems, whichinclude HMX/CTPB (carboxyl-terminated poly-butadiene) [2], HMX/HTPB (hydroxyl-termi-nated polybutadiene) [3], HMX/BAMO [3], andRDX/BAMO [4]. However, the previous studiesshowed that nitramines had no diffusion flamewith nonenergetic binders, whereas they indeedhad very weak diffusion flames with energeticbinders at very low pressures. From the previousmeasurements [3, 4], no explicit evidence of theflame interaction over the two laminae ofnitramines and BAMO was found because themajor species measured over each ingredientwere almost identical to those of the purematerial. Since no diffusion process was in-volved in the flames, chemical mechanism inthe gas phases of BAMO was substantiallyidentical to that in the absence of the nitra-mine. Therefore, it could be concluded thatthe study of sandwich configurations betweennitramines and BAMO did not provide muchinformation on their chemical interaction andeffects of each ingredient to the other. Thisconclusion directed the research efforts re-ported in this paper more on nitramines/BAMOheterogeneous mixtures.

Thermal decomposition of HMX/BAMOpropellants was studied by Oyumi et al. usingisothermal thermogravimetric analysis (TGA)and differential scanning calorimetric analysis(DSC) with a helium flow rate of 150 ml/min [5].The binder used in this study was a mixture ofBAMO and tetrahydrofuran (THF) in molefraction of 60/40, where THF was used as across-linking agent to make the copolymer athree-dimensional network structure using theinertness of THF. The decomposition of theHMX/BAMO propellant in the study showedoverall two-step first-order kinetics; the first-step reaction was considered to be the decom-position of BAMO binder itself, since the rateof the first-order reaction was similar to that ofBAMO binder. The HMX decomposition waspredominant in the second-step reaction and wasactivated by the binder decomposition. Fouriertransform infrared (FTIR) spectroscopy and gaschromatography (GC) were also applied to mea-sure the effects of cross-link ratio on the acceler-ated aging of the HMX/BAMO propellants,where the accelerated aging was conducted at 347K for several weeks [5]. The propellants with avery low cross-link ratio made cavities betweenHMX and BAMO by the evolution of N2, CO2,and H2O during the accelerated aging. The burn-ing rate of the propellant was larger than that ofthe pure BAMO binder or HMX. However, thepropellant could not maintain combustion at pres-sures below about 30 ATM, where its burning ratewas equal to that of BAMO binder.

The same research group investigated burn-ing rate augmentation of BAMO-based propel-lants with HMX and HMX/AN (ammoniumnitrate) mixtures, in which the BAMO used wasan azide copolymer of BAMO/AMMO (7/3,molar ratio) [6]. According to their theoreticalcalculation, it was reported that the BAMO/HMX propellants showed a relatively low flametemperature (1210–1320 K) and smokelessproperties. The burning rate of the propellantwas augmented by the addition of lead stearateand AP, which resulted in a decrease of theburning rate exponent. Results for the BAMO-based propellants showed an endotherm ofphase transition and an exotherm between 500and 540 K that exactly coincided with rapidweight loss. From these results, it was concluded

Fig. 1. Chemical structures of RDX and BAMO.

796 Y. J. LEE ET AL.

that the heat generated by the azide binderdecomposition initiated and accelerated theHMX decomposition.

Although some research on HMX/BAMOhas been done in the field of nitramine/BAMOcomposite propellants, decomposition chemis-try of the propellants is not yet well understood.Moreover, no experimental investigation ofRDX/BAMO propellants has been reported todate, except for a decomposition study per-formed in this laboratory [7]. Although RDX isan excellent and more widely used propellantingredient than HMX, no research work hasbeen reported on combustion chemistry ofRDX/BAMO pseudo-propellants. Since RDXis the ingredient with much cheaper cost ofproduction and almost equivalent thermal per-formance to HMX, more research is needed forRDX/BAMO propellants. Therefore, the maingoal of this paper is to investigate the combus-tion chemistry of RDX/BAMO propellants; pri-mary efforts will be focused on the thermaldecomposition and mutual interaction of thetwo ingredients in the RDX/BAMO heteroge-neous mixture propellants. The experimentalresults of RDX/BAMO obtained in this studywill be also compared with the previous resultsfor BAMO [8, 9] and RDX [4, 10] in thislaboratory.

EXPERIMENTAL APPROACH

Experiments for RDX/BAMO were con-ducted at a pressure of 1 ATM and at heatfluxes of 100 and 400 W/cm2 during laser-assisted combustion with a high-power CO2laser. The schematic diagram of the experimen-tal setup and detailed features have been de-scribed elsewhere [4, 10]; therefore, only a briefexplanation directly relevant to this study will begiven here.

A heterogeneous mixture of RDX andBAMO ingredients was used in this study tofabricate RDX/BAMO pseudo-propellants. ForRDX/BAMO in this study, the idiom “pseudo”was used to emphasize that the propellant usedis a heterogeneous physical mixture of the twoingredients, not a chemically homogeneous mix-ture. The propellants were made of a mixture ofRDX and BAMO powders in a weight ratio of

80:20; selection of the weight ratio between thetwo ingredients was made based on the amountsof binders contained in widely used compositepropellants such as XM39 and M43. The pow-der mixture was pressed into a pellet-type sam-ple with a 0.64-cm (0.250) diameter by an in-house, pellet presser, with the same pressureand procedure used for RDX propellants [4].For a test, the propellant sample was glued to asmall sample holder, angled at 45° to the inci-dent laser beam so that the sampling micro-probe could approach the sample perpendicularto its surface center. During the test, the samplewas pushed toward the sampling probe by alinear positioner to obtain species profiles ver-sus height from the sample surface. Gas sampleswere extracted through a quartz microprobeand analyzed using a triple quadrupole massspectrometer (TQMS) [4, 10]. Using parent anddaughter modes of the TQMS and careful cali-bration, it was possible to differentiate andquantify all species at the same mass-to-charge(m/z) value. Direct images were acquired usinga Pulnix video camera with a macro lens fordirect image photography of the flame abovethe samples during combustion and for theidentification of sampling heights. Magnifica-tion of 30 to 40 times was normally employed torecord the images and obtain the samplingheight with the spatial resolution of ;20 mm.Sensitivity coefficients of most stable specieswere calibrated directly with gas mixtures ofknown concentration; those of the species forwhich standards were not readily available wereestimated by the ionization cross-sectionmethod [11]. For all of the calibrations andactual tests, an ionization energy of 22 eV wasused to minimize fragmentation of moleculesand to get acceptable intensities.

Temperatures at the surface and in the gasphase were measured using 25-mm diameterplatinum/rhodium (Pt/Rh) and tungsten/rhe-nium (W/Re) thermocouples, according to themethods suggested by Zenin [12] with somemodifications [4, 13]. Surface temperatureswere measured separately from gas-phase tem-peratures to obtain more consistent surfacetemperatures. The setup for the measurementof surface temperature is shown in Fig. 2. Thejunction of a thermocouple was placed at the

797THERMAL DECOMPOSITION OF RDX/BAMO

center of the sample surface, and two smallweights (W in the figure) were used to createadequate tension to hold the thermocouple onthe surface as the sample burned. In the setup,the weight chosen was in the range of 0.8–2.0 g,and the angle (f) which the wire makes withthe lateral edge of the propellant in the figurewas 10 –30°. The measurement of a completetemperature profile was performed using theconfiguration of a “side-by-side-divided” sam-ple [4, 13]. Each half of the propellant samplewas pressed separately, and the thermocouplewas embedded in one half of the sample.Then, a small amount of glue was applied atthe ends of the two thermocouple wires on thesample edges, and the other half of the samplewas attached to it without moving the thermo-couple. For measuring complete gas-phasetemperature profiles, W/Re thermocoupleswere usually used to reduce the meltingchances of thermocouples during measure-ments. The measured thermocouple signalswere amplified by a wide-band preamplifierand recorded on a Nicolet NIC-310 digitaloscilloscope. The scope has two input chan-nels—one for thermocouple signals and theother for a photodiode signal (PDS). The pho-

todiode signals were recorded simultaneouslywith the temperature signals to synchronize thevideo images of the flame to the temperaturemeasurement.

RESULTS AND DISCUSSION

The burning behavior of RDX/BAMO was sig-nificantly different from that of pure RDX; theRDX/BAMO actually showed an intermediatecharacter between RDX and BAMO flames.Figure 3 shows snapshots of burning behaviorduring species (Fig. 3a) and temperature (Fig.

Fig. 2. Schematics of the temperature measurement setupfor surface temperature measurements.

Fig. 3. Burning behavior of RDX/BAMO pseudo-propel-lants during (a) species and (b) temperature measurements.


3b) measurements; the same behavior was ob-served in both measurements. The burningRDX/BAMO showed an uneven surface andintermittent flames during combustion, al-though the burning surface was more even thanthat of pure BAMO. The intermittent flameswere numerous “localized” glowing spots, someof which made burning streaks through the gasphase as in Fig. 3. No luminous flame wasobserved throughout typical measurements at100 W/cm2. However, unstable luminous flameswere sometimes observed for short time dura-tion right after propellant ignition, and thosemay be related to nonuniform mixture near thesurface of the propellants. Luminous flameswere also observed more frequently around theend of experiments at 400 W/cm2; however, theywere distributed in the gas phase so that theydid not exhibit a flame sheet similar to RDX[10]. The evolution of these unstable flamesseems to be directly related to BAMO in RDX/

BAMO. At 1 ATM, burning rates increasedfrom ;0.2 and ;1.0 mm/sec in RDX at 100 and400 W/cm2, respectively, to ;0.35 and ;1.4mm/sec in RDX/BAMO. The burning rates forBAMO were ;3.4 and ;11.0 mm/sec for therespective conditions.

Identification and Differentiation of theMeasured Species

Results of species identification for RDX/BAMO pseudo-propellants are given in Table 1;the same results were obtained for both 100 and400 W/cm2. Major species shown in the tableexhibit explicit evidence for the coexistence ofdecomposition products from the two ingredi-ents. Some major species obtained during thedecomposition of pure RDX [4, 10] and pureBAMO [8, 9] were also measured in this study:H2O at mass 18; HCN at mass 27; CO and N2 atmass 28; and H2CO at mass 30. However, the

TABLE 1

Daughter Mass Ions for Each Major Parent Mass of RDX/BAMO Pseudo-propellantsat 1 ATM

ParentMassa

Daughter Masses inHigh Collision-energy Mode

Daughter Masses inLow Collision-energy Mode Remarks

27 — 26 .. 25 HCN .. C2H3

28 12, 14, 15 27, 26 .. 25 CO, N2, C2H4

18 — 17 .. 16 H2O44 16, 28, 30 .. 29, 27,

1543 .. 42 N2O, CO2 .. CH3CHO

30 14, 16 .. 13 29 .. 28, 27 NO and H2CO42 14, 15 .. 27, 13, 26,

2843 .. 42 CH2CHNH (a fragment from

mass 43)b

29 14, 13, 15, 12 28, 27 H2CO and H2CNH43 15, 29, 30, 28, 27,

1642, 42 HNCO, CH3CHNHc

46 14, 30 .. 29, 28 45e NO2 and X (unidentified)d

41 14, 15, 27, 28, 26,13, 29

40,e 39 CH3CN2 and/or CH2CNHf

39 13, 18, 14, 16, 28 38, 37 a fragment from big mass/speciesg

26 — 25 propellantC2H2 .. CN

a List is in descending intensity.b Daughters are different from those of mass 42 in BAMO.c Identified species are the same as that in BAMO.d Daughters that do not result from NO2 clearly existed, but the stable species (X) which satisfy(-fies) the measured

daughters could not be found.e Mass 40 signal was contaminated by argon, the environmental gas, but the mass signal was still found after deducting the

argon signal.f This stable fragment came from CH3CHNH, mass 43.g Data are from BAMO since the expected daughters are the same as those in BAMO.


relative order of their intensities was identical tothat from pure RDX, which suggests that theprimary reaction chemistry is dominated byRDX. Moreover, the fact that masses 46, 44,and 29, which were at most minor species forpure BAMO, were measured as major signalsalso supports the dominance of RDX chemistryin the gas phase. This observation may be simplyexplained by the fact that the sample is com-posed of 80% RDX by mass. However, anotherimportant finding in Table 1 is that masses 43,42, 41, and 39 were measured as major signals,which is true only in pure BAMO [8, 9]. There-fore, in the thermal decomposition of RDX/BAMO, two distinctive reaction chemistriesthat belong to each ingredient seem to coexist.Other masses that had very low intensities werealso found to exist; those include masses 15, 31,45, 47, 52, 53, 54, 55, 56, 69, and 81. Most ofthese masses seem to be related to BAMO [8,9], except masses 45, 47, and 81 that are solelymeasured in RDX [4, 10]. However, the signalintensities were relatively low compared withthose of the major species; therefore, they were

not considered in this study for the identifica-tion and quantification of species. However, it isnot clear how the two reaction mechanismsinteract with each other in the gas phase. Theauthors are not aware of similar research resultsfor the RDX/BAMO pseudo-propellants; there-fore, comparisons to the work of others are notpossible.

Because of BAMO, masses 27 and 28 inTable 1 included some amount of C2H3 andC2H4, respectively, since those masses were onlycomposed of HCN (at mass 27), CO, and N2 forRDX [4, 10]. C2H3 is a fragment from C2H4 inmass 28. Mass 44 consists of N2O, CO2, and asmall amount of CH3CHO; this reflects themixed effects from both ingredients since onlyN2O and CO2 were found as major mass signalsin pure RDX [10]. CH3CHO at mass 44 was notmeasured from either RDX or BAMO; there-fore, despite its small amount (see Figs. 4 and5), it can be claimed that CH3CHO results fromreactions among gaseous products or interme-diates from RDX and BAMO. It is worthwhilenoting that CH3CHO was found in XM39 and

Fig. 4. Species profiles and element fraction profiles for deflagration of RDX/BAMO pseudo-propellants at heat flux of 100W/cm2 in argon at 1 atmosphere. The dashed lines in the element fraction represent equilibrium balances from the chemicalformula of RDX/BAMO.


M43, RDX-based composite propellants, un-der the same experimental condition [14].However, they included some other ingredi-ents that could easily produce CH3CHO. Thepresence of C3H8 in mass 44 was excludedsince C3H8 is a saturated hydrocarbon and is avery unlikely product from BAMO. The ex-clusion of C3H8 from the products was alsosupported indirectly by the absence of C3H6,which has a less saturated structure, inmass 42.

NO and H2CO constitute mass 30, but thecalibration results of both species suggest thatthe major portion is NO [4]. This also reflectsthe fact that the RDX decomposition domi-nantly drives RDX/BAMO reactions, because

H2CO constituted almost all of the mass 30 inBAMO [8, 9]. Mass 42 was also measured inBAMO decomposition and its identified daugh-ter masses were identical to those from RDX/BAMO in Table 1; however, the order of daugh-ters measured for parent mass 42 in eachpropellant was substantially different. From thecalibration of CH3CN [4] and the low collision-energy daughter masses at mass 41 in BAMO[8, 9], mass 42 in RDX/BAMO is expected tohave the same functional groups, CH3 and N forthe respective daughter masses 15 and 14 as inCH3CN. Therefore, based on the daughterions in Table 1, the following structure (on theright) can be suggested for mass 42 of RDX/BAMO:

Fig. 5. Species profiles and element fraction profiles for deflagration of RDX/BAMO pseudo-propellants at heat flux of 400W/cm2 in argon at 1 atmosphere. The dashed lines in the element fraction represent equilibrium balances from the chemicalformula of RDX/BAMO.

H H H H

YC2C5N2H H2C2C5NY

H H

Mass 42 (from RDX/BAMO) Mass 42 (from pure BAMO)


The structure previously suggested in BAMOdecomposition for mass 42 (on the left in theupper figure) may not be excluded; however, thepossibility of its presence is low due to the lowintensity of daughter mass 27, which was amajor daughter of mass 42 in BAMO [8, 9].Although both result from species fragmenta-tion in the ionizer, the species on the right wasnot measured or reported previously in anyliterature on RDX or BAMO. Therefore, theexistence of CH2CHNH in mass 42, along withCH3CHO in mass 44, suggests possible reac-tions between the products from RDX andBAMO. H2CNH and CHO could be assignedfor mass 29, which were also found in BAMOand RDX. Mass 43 was composed of HNCOand CH3CHNH in approximately equalamounts based upon the daughter mass distri-bution in Table 1. HNCO was previously iden-tified in the RDX study as an important initialspecies [4, 10].

Unexpected daughter masses were obtainedfor parent mass 46; those were mass 14 and 45in the high and low collision-energy modes,respectively. Since the only daughter mass fromNO2 found during calibration is mass 30 (NO),there must be another species that provides Nor CH2 for daughter mass 14 and that can losea single H to give mass 45. However, no singlechemical structure was found to satisfy thedaughter mass information, whether it is astable species or a radical. Therefore, the datasuggest that two species are contributing to thesignal at mass 46. From the fact that the trendof daughter mass 30 is very different fromthose of other daughter masses in mass 46, butsimilar to the trend for RDX at the samecondition [4, 10], the daughter mass 30 inTable 1 may be reasonably considered tototally evolve from NO2.

Daughter masses and their order of mag-nitude for parent mass 41 were a little differ-ent from those in BAMO [8, 9]. From theprevious calibration result [4], species formass 41 can be suggested as CH3CN, aceto-nitrile, and CH2CNH, a stable fragment fromCH3CHNH. However, the relative amount ofeach species cannot be estimated from theavailable information. Mass 39 measured onlyin BAMO is also observed in RDX/BAMO.However, no structure can be suggested for

the mass since no matching stable species orfragments were found where they were foundfor BAMO [8, 9]. Mass 26 emerged as asignificant signal in RDX/BAMO; while it wasfound to be only a small signal in BAMO andwas only possible as a fragment, CN, fromHCN. However, in RDX/BAMO, mass 26 isfound to consist primarily of C2H2 with somefragments from some higher molecular weightspecies. The source of C2H2 appears to beBAMO, since C2H2 was not detected in RDXcombustion.

From the identification and differentiation ofspecies measured in RDX/BAMO, it can beconcluded that three different categories ofgaseous products exist at the surface and in thegas phase of RDX/BAMO propellants. Theyare (1) species common to BAMO and RDXsuch as N2, CO, H2CO, and HCN; (2) thosesolely from RDX such as NO2 and N2O, orBAMO such as C2H4 and CH3CN; and (3)those which can not be attributed to RDX orBAMO alone, such as CH3CHO andCH2CHNH. Therefore, existence of the thirdspecies group suggests that some reactions areoccurring in the condensed and/or gas phases toproduce those species.

Chemical Structure

Mole fractions of the species evolved in thegas phase of RDX/BAMO pseudo-propellantsare shown in Fig. 4. The measurement wasmade during laser-assisted combustion at thecondition of 1 ATM and 100 W/cm2 in theargon environment. Multiple species includedin masses 28, 29, 30, and 44 were differenti-ated for species quantification using theTQMS. Most of the major species were iden-tical to those in pure RDX: H2O, NO, HCN,H2CO, N2, NO2, N2O, and CO. H2, a notori-ously difficult species to measure, is notshown here due to its highly fluctuating be-havior throughout the gas phase. Its molefraction was estimated to be 1–2% at thesurface. When this species measurement re-sult was compared with that of RDX at thesame experimental condition [4, 10], someunique characteristics of the gas-phase pro-cesses of RDX/BAMO propellants werefound.


From the comparison of surface mole frac-tions of the species measured in RDX/BAMO(Figs. 4 and 5) and RDX [4, 10] at the samecondition, the chemical effects of BAMO inRDX were found to be significant. First, at 1ATM and 100 W/cm2, the mole fractions ofsome major species at the surface changedsignificantly. In Table 2, where surface molefractions of RDX/BAMO and its ingredientsare compared, the surface mole fractions ofNO2 and CO2 substantially increased from0.04 and 0 in pure RDX to 0.14 and 0.03,respectively. If no chemical interaction is as-sumed between the two ingredients in thecondensed phase, the values should be theweighted arithmetic averages of the respectivevalues measured in RDX and BAMO withweighting factors for the molar mixing ratio.Although RDX and BAMO were mixed in80:20 in mass, the molar-mixing ratio in RDX/BAMO resulted in 74:26 due to their molec-ular weight differences. The increases of NO2and CO2 can not be easily explained sinceneither species was found from BAMO de-composition. The weighted average values forNO2 and CO2 are 0.039 and 0 (last row inTable 2), thus they suggest chemical interac-tion. A reversed trend was found for HCNand N2 since their respective mole fractionsdecreased to 0.14 and 0.10 from 0.23 and 0.06in pure RDX and 0.22 and 0.41 in pureBAMO at 100 W/cm2 (Table 2). For HCN andN2, the mole fractions of 0.23 and 0.15 areexpected in RDX/BAMO without chemicalinteraction, if no interaction occurs in thecondensed phase. Therefore, it is clear thatthere is a chemical interaction between the

two ingredients in the condensed phase. Moredirect evidence of the chemical interactionmay be found from the existence of CH3CHOand CH2CNH at the surface. Since the effectof BAMO is reflected complicatedly on thespecies trends at the surface of RDX/BAMO,it is not clear at this moment how BAMOchemically affected the increase or decreaseof these species. The data in Fig. 4 definitelyshow the effect of BAMO on the reactions ofRDX/BAMO propellants, although it is notclear whether the production came from theinteraction between the two ingredients oronly from BAMO decomposition. Consider-ing that none of the measured species, exceptNO2, are decreasing near the surface in Fig. 5,the source of the production of CH3CHO andH2CO may be related to higher molecularweight species which are not being measuredin the present study.

The influence of BAMO was also clear in thegas phase: however, it was manifest in compli-cated ways. In Fig. 4, NO2 and H2CO that wereprevalent in the primary flame of RDX [4, 10]exist up to ;2 and ;4 mm, respectively, whileNO and HCN did not show consumption up to;4 mm. Therefore, the primary reaction zonenear the surface was stretched out more thanfour times compared to that of RDX at thesame condition (Fig. 6) [4, 10]. The primaryreaction zone was identified by the end pointof NO2 consumption and the starting point ofNO consumption. H2CO decreased along withNO2 in RDX [4, 10]; however, this is not thecase in RDX/BAMO. The source of HCNproduction in Figure 4 is also not clear;

TABLE 2

Comparison of Surface Mole Fractions of RDX, BAMO, RDX/BAMO in the ActualMeasurement, and RDX/BAMO Without Chemical Interaction at ATM

and 100 W/cm2

NO2 HCN N2O NO N2 H2CO CO CO2 H2O Tsa

RDX 0.04 0.23 0.09 0.21 0.06 0.03 0.08 0 0.22 610BAMO 0 0.22 0 ;0 0.41 0.18 0.03 0 0.06 1,050RDX/BAMO 0.13 0.14 0.08 0.13 0.10 0.10 0.06 0.02 0.17 640RDX/BAMO without

chemical interaction0.039 0.23 0.088 0.206 0.067 0.033 0.079 0 0.217 617

a Ts 5 surface temperature (°K).


however, it may be related to higher molecu-lar weight species that were measured, but notquantified in the present study. The mostprobable candidate for the source of HCNproduction might be mass 81, and it is be-lieved to be C3H3N3 (triazine), the same as inRDX [4].

In Fig. 4, the production of NO and HCNsustained up to ;3 mm, while no practicalchange of mole fractions in N2 and CO wasmeasured. Also, a high mole fraction of H2CO(;0.05) was found at ;3 mm and beyond.Compared with the species profile of RDX atthe same condition in Fig. 6 [4, 10], despite thedifference of their mole fractions, the conditionabout at 3 mm in Fig. 4 seems similar to thecondition of RDX at 0.25–0.5 mm, where(NO 1 HCN) reaction is ready to occur toproduce major products such as CO, N2, andCO2. This fact is also identified from the relatedtemperature profile (Fig. 7). The gas-phasetemperature at the same condition was mea-sured to be constant at an average of ;1100 K.The temperature is similar, perhaps coinciden-tally, to the gas-phase temperature of BAMO

[8, 9], although much higher fluctuations wereobserved in BAMO. Considering that no visibleflame was observed throughout RDX/BAMOmeasurements at this condition, this low tem-perature is quite reasonable. Therefore, al-though RDX is found to control primary reac-tions of RDX/BAMO, the species andtemperature measurements at the surface andin the gas phase up to ;3 mm demonstrate theeffect of BAMO on the chemical structure ofRDX/BAMO. The effect of BAMO on thecondensed-phase reactions will be investigatedin detail in the next section, “Thermal WaveStructure,” with regard to species and temper-ature.

Mole fractions of the species evolved fromRDX/BAMO propellants at 400 W/cm2 areshown in Fig. 5. Most of the measured speciesare the same as those at 100 W/cm2 (Fig. 4);however, the mole fractions of some majorspecies are very different. As a result of theincrease of heat flux from 100 to 400 W/cm2,mole fractions of HCN and N2 at the surfaceincreased from 0.14 and 0.10 to 0.19 and 0.17,respectively. At the same condition, the molefractions of CO2, H2O, and H2CO decreasedfrom 0.03, 0.17, and 0.10 to 0.01, 0.21, and 0.02,respectively. In RDX [4, 10], the increase ofheat flux was primarily reflected in the surfacemole fractions of NO2 and NO, with an increaseof NO2 and decrease of NO, without any signif-icant change of other major species. However,for RDX/BAMO, the change of heat flux wasreflected in most of the major species measured.This effect of heat flux is also seen in compari-son of surface temperature. For RDX, thesurface temperature was almost constant ;610K for both heat fluxes, despite the change of themole fractions of NO2 and NO [4, 10]. However,for RDX/BAMO, the surface temperaturechanged from ;640 to ;670 K with the changeof heat flux from 100 to 400 W/cm2. This changeof surface temperature might affect condensed-phase reactions in RDX/BAMO, causing thechanges of some major species. The reasons forthe mixed trends of species changes resultingfrom the increased heat flux are not clear at thistime.

One important observation can be drawnfrom the comparison of surface mole fractions

Fig. 6. Profiles of some selected species of RDX at thecondition of 1 ATM and 400 W/cm2 in argon environment.

Fig. 7. Temperature profile of deflagrating RDX/BAMOpseudo-propellants at 1 atmosphere and 100 W/cm2 in anargon environment.


between RDX and RDX/BAMO in Table 3 at400 W/cm2. In Table 3, the mole fractions ofmost species from the RDX/BAMO measure-ments are more similar to those of RDX/BAMO without chemical interaction than thesame comparison at 100 W/cm2. In the table,the only significantly changing species was N2;N2 shows a significant increase of mole fractionfrom 0.04 in RDX [4, 10] to 0.17 in RDX/BAMO. This fact suggests that the effect ofBAMO became less significant at high heatingrates so that surface (or condensed-phase)chemistry resembles that of RDX. Therefore,the heating rate may also be an important factorin determining the level of effect of BAMO inRDX/BAMO propellants. Despite this similar-ity of surface mole fractions, the chemical andphysical interactions of the two ingredients inRDX/BAMO propellants were also seen fromthe species profiles at 400 W/cm2. The expan-sion of reaction zone was clear: the end posi-tions of NO2 consumption and NO productionincreased from ;0.7 mm in RDX to ;2.0 mmin RDX/BAMO. Reaction zones near the sur-face were stretched out more than two timescompared to those of RDX at the same condi-tion. Therefore, like the result at 100 W/cm2,the interaction between RDX and BAMO isboth chemical and physical; however, the inter-action level was less than that at 100 W/cm2.One may argue that the gas-phase absorption ofheat flux from the CO2 laser also contributes tothe chemical and physical differences betweenthe two heat fluxes. Some previous studies wereperformed experimentally [15] and theoretically[16] to investigate this topic; however, the re-sults were mixed. Although no conclusion canbe drawn at this moment, the heat-flux absorp-

tion by gas-phase species in this study is be-lieved to be minimal from the experimentalresult [15].

As in 100 W/cm2, the expansion of reactionzones due to BAMO was also identified at 400W/cm2. In Fig. 5, two major final products N2

and CO are constant up to ;1.5 mm. Theconsumption of NO2 and production of NO alsoends around the same distance. Therefore,considering that the distance for the sametrends existed at ;0.5 mm for RDX [4, 10],the primary reaction zone increased aboutthree times due to the presence of BAMO.After 1.5 mm from the surface, the gas-phasereaction mechanism shows primarily the char-acteristics of RDX: consumption of HCN andNO, and production of N2, CO, and H2. Al-though there still existed some species related toBAMO decomposition in the region, such asNH3, CH3CN, and some hydrocarbons, theamount of such species was too small to influ-ence major reactions in the reaction zones. Theonly major exception is the change in the rela-tive magnitudes of N2 and CO throughout thegas phase; this may be attributed to high pro-duction of N2 from BAMO. In RDX at thesame condition, the quantity of N2 was alwayssmaller than that of CO throughout the gasphase [4, 10], whereas the opposite was true inRDX/BAMO.

Thermal Wave Structure

Other evidence of the role of BAMO in RDX/BAMO and possible interactions between thetwo ingredients, BAMO and RDX, may bededuced from temperature profiles. Typical

TABLE 3

Comparison of Surface Mole Fractions of RDX, BAMO, RDX/BAMO in the ActualMeasurement, and RDX/BAMO Withoug Chemical Interaction at ATM and

400 W/cm2

NO2 HCN N2O NO N2 H2CO CO CO2 H2O Tsa

RDX 0.16 0.23 0.08 0.14 0.04 0.03 0.07 0 0.22 610BAMO 0 0.22 0 ;0 0.42 0.15 0.02 0 0.05 1,050RDX/BAMO 0.14 0.19 0.09 0.13 0.17 0.02 0.06 0.01 0.21 670RDX/BAMO without

chemical interaction0.157 0.23 0.08 0.14 0.048 0.032 0.069 0 0.217 617

a Ts 5 surface temperature (°K).


temperature profiles for RDX/BAMO areshown in Figs. 7 and 8 for 100 and 400 W/cm2,respectively. For each condition, multiple testswere performed for obtaining a representativeprofile, and all of them showed similar behav-iors both temporally and spatially. As in Fig. 3,video recording of the flame behavior duringthe temperature measurements showed onlynumerous “localized” burning spots on the de-flagrating surface from which bright flamestreaks evolved during combustion, without theflame sheet of pure RDX [4, 10]. The temper-ature profile measured in the time domain isalso shown in Fig. 8 at 400 W/cm2 to illustrate itstemporal behavior along with the PDS whichwas measured simultaneously. The PDS repre-sents the brightness and stability of the flameduring temperature measurements and was al-ways set to the lowest voltage scale for easyidentification of the change of flame behavior.The relatively small but rapid fluctuations of thePDS represents a typical combustion behaviorof RDX/BAMO propellants: numerous “local-

ized” burning spots on the deflagrating surfacefrom which bright flame streaks evolved duringcombustion. The same burning behavior wasobserved during temperature measurementsat 100 W/cm2; this behavior did not have aneffect on the consistency of the temperatureprofiles. However, for 400 W/cm2 as in Fig. 3a,a bright, relatively unstable flame emergedafter ;2 sec, abruptly during combustion,which corresponds with the temperature re-gion between 2100 and 2300 K in the figure.The later brighter and hotter flame, which wasnot initially present, was distributed through-out the gas phase, without the stable flamesheet as was observed in RDX flames [4, 10].The luminous flame at 400 W/cm2 did not occurin every test and, therefore, was not a typicalphenomenon. However, it provided a good es-timation of a possible final flame temperature ofRDX/BAMO.

Although their general trends are similar,there are significant differences between thetwo profiles at 100 and 400 W/cm2. First, surfacetemperatures of RDX/BAMO were measuredas 640 6 20 K at 100 W/cm2 and 670 6 20 K at400 W/cm2. The uncertainty of temperature ineach condition was calculated with the averagesurface temperatures measured from multipletests. The position of the thermocouple junctionon the propellant surface could be monitored bythe video camera in each test because the liquidzone thickness was small, in the range of ther-mocouple diameter (;25 mm). Although noseparate investigation was conducted on theeffect of the weight (W in Fig. 2) and angle (f)for the temperature measurement in this study,a previous study found that no significant errorswould be produced in the surface temperaturemeasurement for the ranges of the two variablesselected in this study [17] (see the section,“Experimental Approach”). There seems to bea relationship between the change of speciesprofiles and temperatures in RDX/BAMO atthe surface. In an attempt to obtain a plausiblerelationship between species and temperatureat the surface, an analysis on the species mea-sured at the surface was undertaken based onexperimental results for RDX by Brill andBrush [18], in which a decrease of the N2O/NO2

ratio was found as temperature increases in the

Fig. 8. (a) Temperature profile of deflagrating RDX/BAMO pseudo-propellants at 1 atmosphere and 400 W/cm2

in an argon environment with photo-diode signal, using 25mm W3Re/W25Re thermocouples. (b) The same tempera-ture profile (solid line) via height above the surface with thatof pure RDX (dotted line) at the same condition [4, 10].


condensed phase. Assuming that the con-densed-phase reaction of RDX/BAMO is dom-inantly governed by RDX, which constitute 80%of RDX/BAMO in mass, the same trend ofN2O/NO2 might also be expected in this study.From previous RDX decomposition studies un-der laser-assisted combustion in this laboratory[4, 10], the following two global reactions couldbe attributed to the production of N2 and NO atthe surface [4, 19, 20]:

H2CO 1 N2O3 H2O 1 CO 1 N2 (R1)

2HONO3 H2O 1 NO 1 NO2 (R2)

The ratio of N2O/NO2 in this study could bereformulated as [N2O 1 N2]/[NO2 1 NO] withthe consideration of N2 and NO productionfrom N2O and NO2 through (R1) and (R2),respectively, where a shift reaction existed be-tween the two product species in (R2): NO2 7NO 1 1/2O2 [21]. The ratio was calculated fromthe data in Tables 2 and 3 to be 0.69 and 0.96 at100 and 400 W/cm2, respectively. This result iscontrary to what would be expected: a decreaseof N2O/NO2 ratio as the surface temperatureincreases. Therefore, the condensed-phasechemistry of RDX/BAMO does not appear tobe dominated by RDX decomposition; the ef-fect of BAMO seems to be important. Thisfinding may be justified from the fact that thetwo-phase (liquid and vapor) region which wasusually found in RDX was not observed inRDX/BAMO and that no significant reactionwas found to occur in the condensed phase ofRDX [19]. This result also agrees with theresults of previous HMX/BAMO studies inwhich BAMO decomposition was found to firstinitiate decomposition and then accelerate thatof HMX [5, 6].

The temperature profiles at both conditionsessentially had the same trend; they changedfrom a surface temperature through a near-surface region of temperature increase into anisothermal temperature region. This region wascalled “isothermal region” in this study because,despite its fluctuating nature, temperature wasessentially constant throughout the region, andalso may be called “dark zone” from the tradi-tional definition of composite propellant com-bustion [22]. The temperatures in this regioncould be approximated ;1100 and ;1500 K at

100 and 400 W/cm2, respectively. This largetemperature difference at the isothermal regionbetween the heat fluxes was not measured forother RDX-based composite propellants suchas XM39 and M43 [14]. Therefore, a simpleanalysis was performed to identify whether theincrease resulted solely from the difference oflaser heat flux or from convective heat transferterm in the energy equation at the thermocou-ple junction. The temperature difference due tothe heat flux increase from 100 to 400 W/cm2

was measured to be ;300 K in a separate test atatmospheric pressure in a stagnant air environ-ment. However, the actual environmental con-dition was very different; the thermocouple is inthe flow of a high-speed burnt gas mixture.Therefore, it was assumed in the analysis that aW/Re thermocouple with a spherical junction,25 mm in diameter, was placed in the worstexperimental condition: 300 W/cm2 heat fluxincrease; no heat flux attenuation by gaseousproducts; and fully developed gas-mixture flowaround the thermocouple. From the calculation,it was revealed that, even at this worst condi-tion, the magnitude of convective heat transfer(;80%) is much higher than that of radiativeheat flux from laser (;20%). Therefore, itturned out to be reasonable to assume that mostof the temperature increase directly resultedfrom convective heat transfer from environmen-tal gases.

An abrupt temperature increase was ob-served in some tests during temperature mea-surements at 400 W/cm2, e.g., ;2 sec in Fig. 8,associated with the appearance of the distrib-uted luminous flame. After the rise, the temper-ature reached another plateau at 2200 6 100 K.This later brighter and hotter flame was notinitially present and was distributed throughoutthe gas phase, without a flame sheet which wasobserved in RDX [4, 10]. In RDX/BAMO,unlike pure BAMO [9], highly reactive speciesprimarily from BAMO (H2CO and HCN) foundoxidizing species from RDX (NO2, N2O, andNO) at the first isothermal temperature (;1500K). Thus, reactions between the two speciesgroups seem to produce energies enough toproceed into further reactions which result inthe higher temperature (;2200 K) and lumi-nous flame. It is noteworthy that the high tem-peratures from the distributed flame serve only


to give an estimate of a final flame temperature.Considering that the adiabatic flame tempera-ture of RDX/BAMO with the present formula-tion is calculated to be ;2570 K, this finaltemperature measured seems to be still lowcompared to the maximum possible value dueto incomplete combustion.

SUMMARY AND CONCLUSIONS

Measurements of gaseous species and temper-ature profiles were performed to study thermaldecomposition of the RDX/BAMO pseudo-propellant and mutual interaction of its ingre-dients, RDX and BAMO. Experiments wereconducted at 1 ATM in argon with heat fluxes of100 and 400 W/cm2 delivered by a CO2 laser.The results of species measurements revealedthat the products of each ingredient, RDX andBAMO, were found to exist simultaneouslythroughout the flame; however, the primaryreaction chemistry in the gas phase was domi-nated by RDX. Three different categories ofgaseous products were identified in the speciesmeasurement of RDX/BAMO propellants: spe-cies common to BAMO and RDX; speciesunique to BAMO or RDX alone; and speciesthat can not be attributed to RDX and BAMOand apparently result from the interactions ofRDX/BAMO chemistries. Compared to pureRDX, the thickness of primary and secondaryreaction zones in the gas phase of RDX/BAMOsignificantly changed between the two heat-fluxconditions. Due to BAMO in the pseudo-pro-pellants, the temperature profiles showed dis-tinct characteristics which were very differentfrom that in pure RDX. The surface tempera-tures increased to ;640 and ;670 K at 100 and400 W/cm2, respectively, from that of RDXwhich was ;610 K for both heat fluxes. Tem-perature profiles in the gas phase showed anexpanded isothermal region with the tempera-ture of ;1100 and ;1500 K at respective con-ditions. Most of the difference in temperatureof the isothermal regions between the two con-ditions resulted from the thermal condition atthe measurement position. No luminous flamewas observed in any typical species and temper-ature experiments.

This work has been supported by the Office ofNaval Research, Mechanics Division under ONRContract No. N00014-93-1-0080 and through theCaltech Multidisciplinary University Research Ini-tiative under ONR Grant No. N00014-95-1-1338.The support and encouragement of Dr. R. S.Miller are greatly appreciated.

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Received 16 June 1998; accepted 2 September 1998


Thermal decomposition of RDX/BAMO pseudo-propellants

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