Army Research Laboratory 

Aberdeen Proving Ground, MD 2 1005-5066 

ARL-TR-2444 April 2001 

On the Flame Structure of RDX 

Barrie E. Homan, Martin S. Miller, and John A. Vanderhoff 

Weapons and Materials Research Directorate, ARL 

. 

Approved for public release; distribution is unlimited. 
Abstract 

In an attempt to refine the database for the evaluation of detailed-chemistry 

combustion models, absorption spectroscopy has been applied to low pressure, selfdeflagrating RDX flames. Semi-empirically determined production rates of reactants 

from the solid propellant surface, together with a detailed gas-phase elementary reaction 

set, were used to develop a model that minimizes the effect of uncertainties in the 

description of solid phase processes. The spatial profiles of two low concentration, 

highly reactive, short-lived diatomic species, CN and NH, were obtained at pressures 

varying from 1 to 2 atm in air. Two major species, NO and OH, were also profiled 

using this technique. The CN and NH profiles agree well with previous measurements 

and the current model. NO and OH profiles are also in good agreement with the 

models. During the course of these spectroscopic measurements, burning rates for RDX 

over a pressure range of l-2 atm have also been determined. These values range from 

0.23 rnmk at atmospheric pressure to 0.50 mm/s for 2 atm and are noticeably lower than 

some of the other published measurements. Calculations stemming from the proposed 

model predict a burning rate of 0.29 mm/s and 0.54 mm/s for the same pressure levels. 

ii 
Acknowledgments 

We thank Dr. Anthony Kotlar for developing the least squares fitting programs and the 

necessary molecular database files for OH, NO, CN, and NH. 

. . . 

111 
iv 
Table of Contents 

Page 

Acknowledgments .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

. . . 

111 

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii 

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

ix 

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

2. Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

3. Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

4. Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

5. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

5.1 Absorption ........................................................................................................... 16 

5.2 Video Records ..................................................................................................... 24 

5.3 Burning Rate ....................................................................................................... 25 

6. Discussion .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 

7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 

8. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 

Distribution List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 

Report Documentation Page .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 

V 
vi 
List of Figures 

Fiaure 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

17. 

Experimental Setup ................................................................................................ 

Triple-Pass Beam Path ............................................................................................ 

0.794~mm Diameter, 1-atm Air Self-Deflagrating RDX .......................................... 

Pyrolysis Law ......................................................................................................... 

Specifk Heat of RDX ............................................................................................. 

RDX Species Model Profiles .................................................................................. 

OH Concentration Profile at 1 atm .......................................................................... 

OH Concentration Profile at 2 atm .......................................................................... 

1-atm Temperature Profile for RDX ....................................................................... 

2-atm Temperature Profile Obtained From OH Spectra .......................................... 

CN Spectrum for 1 atm, 0.7 mm From Sample Surface ........................................... 

CN Concentration Profile at 1 atm .......................................................................... 

NH Profiles in 1-atm Air ......................................................................................... 

High Resolution Profiles of CN and NH at 1 atm .................................................... 

High Resolution NO Species Profiles ...................................................................... 

Temperature Profile From NO Spectra at 1 atm ...................................................... 

Burn Rate of RDX as a Function of Pressure .......................................................... 

p&&g 

3 

5 

8 

11 

13 

15 

17 

18 

18 

19 

19 

21 

22 

23 

24 

25 

26 

vii 
. . . 

Vlll 
List of Tables 

Table 

1. 

2. 

3. 

4. 

Semi-Empirical Model Parameters .......................................................................... 

Comparison of CN and NH Concentrations Obtained in the Self-Sustained 

Combustion of RDX ............................................................................................... 

Video Measurements for RDX Burning in Air ........................................................ 

Comparison of NO Concentrations at 1-atm Air ..................................................... 

page 

14 

22 

25 

29 

ix 
X 
1. Introduction 

Over the last several years, propellant combustion models, including detailed gas-phase 

chemistry, I have been developed for cyclotrimethylene trinitramine (RDX) and 

cyclotetramethylene tertranitramine (HMX) propellants [l-7]. These models attempt to describe 

the generation of gas phase reactants as a combination of evaporation from the surface and 

subsurface molecular decomposition [2, 4, 8, 91. While the usual combustion diagnostic data, 

such as temperature and species concentration profiles, are predicted by these calculations, the 

more important parameter they strive to predict is the rate of burning as a function of pressure. 

Burning rate measurements for RDX exist in the literature, and current models show that 

excellent agreement can be achieved. Unfortunately, some of the necessary physical and 

chemical parameters and mechanisms used to describe the solid gas-phase transition are not 

known with a high degree of certainty. Hence, “excellent” agreement should be viewed with this 

in mind. A new semi-empirical model that minimizes the need for a detailed description of the 

condensed phase processes has been developed; it will focus on the chemistry of the gas phase. 

In addition to burning rate measurements for RDX, there is also an accumulation of other 

quantitative combustion diagnostic measurements. Thermocouple [7, 91, mass spectrometric 

[ 10, 111, and optical measurements [12-161 constitute the majority of measurements which 

produce temperature and concentration profiles for RDX combustion. Over the last eight years, 

the U.S. Army Research Laboratory (ARL) has been using absorption spectroscopy in an effort 

to expand and refine the combustion diagnostic database for propellants and propellant 

ingredients. In some early studies [ 17, 181, concentrations of CN, NH, and OH species were 

determined from absorption spectra taken during the self-sustained combustion of the solid 

propellants M-30 and HMXl.* The measurements were reported with the caveat that these 

concentrations should not be considered maximum values due to uncertainties in path length. 

Previous experiments [12, 131, as well as modeling results [ 1, 2,9] for RDX, have indicated that 

the spatial extent of CN, NH, and NO is within approximately 1 mm from the surface of the 

*Composition of HMXl is 73% HMX, 17% Trimethylolethanetrinitrate, and 10% polyester binder. 

1 
propellant at atmospheric pressure and decreases with increasing pressure. Therefore, 

measurements of CN, NH, and NO require a high degree of spatial resolution for the detection 

system. In addition, the combustion process must exhibit a high degree of one-dimensionality if 

species profiles are to be measured with a line-of-sight technique. Spatial coincidence of the thin 

combustion region containing these short-lived species and the probing beam is critical, as any 

misalignment can reduce the apparent measured concentration. OH does not suffer from these 

limitations as the concentration slowly varies in the secondary flame region. 

In these early studies, the propellant was ignited by a hot wire placed at the center of the 

sample. Thus, burning propagates from the sample center. A shroud gas flowing around the 

propellant to’eliminate side burning and to keep the absorption path length free of contaminants 

also provided an unwanted edge cooling effect. A visual examination of some propellant 

samples that self-extinguished when burning near the low pressure deflagration limit revealed 

that the burning surface was not flat, but rather concave. This surface feature was not detectable 

with the video camera arrangement employed for those experiments. The resultant nonplanar 

burning was thought to be the major factor resulting in low measured concentrations. For these 

reasons, subsequent experiments focused on propellants with large dark-zone regions [ 19-211, 

where the spatial resolution requirements are much less. 

Recently, attention has again been turned to obtaining CN, NH, NO, and OH concentration 

measurements. The RDX monopropellant was chosen for the study because of recent 

community interest in modeling this system. An additional factor considered was that this 

monopropellant forms a liquid layer during combustion, and this phase would give a more planar 

burning surface. In this report, concentration profiles and burning rate measurements, together 

with new model predictions, are compared to results obtained from the literature. 

2. Experiment 

The optical absorption experiment previously described [17-211 has been modified and 

improved significantly. The new experiment setup is shown in Figure 1. Originally, a hot wire 

2 

4 

intensities were increased about two orders of magnitude by applying a 75-V pulse to the 

simmering arc lamp, which provided the greatest benefit at shorter wavelengths. Increased 

emission light gathering, inherent in the snapshot experimental setup, was further addressed by 

introducing a shutter into the optical path prior to the combustion chamber. Emission spectra 

obtained with the shutter in the closed position are then used to correct the absorption spectra. 

The path length in the vessel, located outside the combustion region, was reduced by arms that 

extend into the vessel. These arms are adjusted to be slightly larger than the RDX sample 

diameter, and they trim the absorption light to a vertical sheet. Quartz windows glued to the 

ends of these arms seal the vessel while allowing transmission of UV-visible light. Another 

modification, shown in Figure 2, involved mounting these windows at a slight angle to the beam 

and mirroring a portion to produce multiple passes through the sample area within the pressure 

vessel. Vacuum deposition of Al on partially masked quartz substrates allowed for the triple 

pass arrangement and increased the absorption path length by about a factor of three. Initially, 

the mirrored side of the quartz windows faced inside to minimize interface losses. 

Unfortunately, these delicate surfaces were degraded by the combustion process and required 

replacement every two or three runs. In the present configuration, the windows have been 

flipped so that the mirrored surface is protected by the quartz substrate, thus eliminating the need 

for frequent replacement. Upon exiting the chamber, the light is imaged onto the spectrometer 

entrance slit with spherical lens L2 (Figure 1). The addition of a cylindrical lens L3 further 

focussed the light into a line coincident with the spectrometer slit. An appropriate bandpass filter 

is also included in the light path to provide more stray light rejection. 

Because of the large temperature gradient near the propellant surface, beam steering is a 

concern for optical diagnostic techniques in combustion processes [23]. This effect may cause 

the beam to sample different regions in the flame as well as steer the output off of or onto 

different regions of the detector collection area. A fiber optic ribbon cable was included in the 

collection optics for one experiment to provide a way to isolate the cross talk of adjacent detector 

regions. The custom-made fiber consisted of 20 fibers with 200+1-n cores, 280-p center to 

center. One end of the fiber was placed within = 1.5 cm from the center of the propellant 

sample, thereby significantly reducing the effective lever arm of the collection optics. 

overlap the 1-ms light pulse of the xenon arc lamp. This detector is a two-dimensional (2-D) 

array with 578 x 385 pixels oriented such that the 578 pixels are along the horizontal 

(wavelength) axis. The snapshot experiments have the 385 pixels in the spatial direction divided 

into regions either 16 or 32 pixels wide, giving 24 or 12 spectra, respectively. Thus, the distance 

covered and the spatial resolution depends on the number of vertical pixels summed and the 

magnification of the optics to the detector. This magnification is controlled by the distance 

between lens L2 and the sample. In this case, the range covered is 0.25 cm., and the spatial 

resolution is 0.021 cm. The spatial resolution is checked by moving a razor blade through the 

vertical absorption light sheet at the position that the sample would occupy and recording the 

micrometer drive settings at the point where the count rate drops to one half for each of the 

strips. Care was taken to ensure that cross talk between adjacent strips was minimal Multiple, 

spatially distinct absorption spectra are recorded for each pulse of the absorption lamp. In 

reality, the initial position of the RDX sample is adjusted to block one or more of the strips 

during the first lamp pulse. This configuration assists in locating the propellant surface. In the 

case of the single probe beam absorption measurements, the detector had all of the vertical pixels 

summed into one super pixel. The same spatial resolution measurement apparatus is used here, 

except the profile of the single beam is obtained. A Gaussian function was fitted to the 

derivative of this profile to yield full-width-half-max (FWHM) values of 0.006-0.009 cm. 

The 0.8-cm diameter RDX sample could not be made to reliably burn in a 1-atm nitrogen 

environment. Ulas et al [16] also reported this behavior for their O&l-cm diameter RDX 

samples; the lowest reported nitrogen pressure for which RDX combustion data was taken was 

1.7 atm. The 1-atm measurements reported here were performed in air to overcome this 

limitation and to take advantage of the larger reaction zone. Present measurements (discussed in 

the results section) indicate that RDX has a burning rate of 0.023 cm/s in 1-atm air. The time 

required to obtain the spatially resolved absorption spectra is the duration of one pulse, or 1 ms. 

During this time period, the propellant would move less than 1 pm. However, if more than one 

pulse is to be used for data, then movement of the propellant surface should be taken into 

account. A full data collection cycle consists of exposure, plus the time required to read the 

detector. For the 12-strip configuration, this time is -150 ms. Furthermore, if an emission 

6 
spectrum is required for each absorption spectrum, the cycle time is doubled, effectively limiting 

the pulse rate of the arc lamp to 3 pulses/s. In this situation, the propellant surface moves about 

80 p between pulses. In high-resolution mode, the collection cycle is reduced to -130 ms total, 

with the sample surface regressing approximately 30 m between spectra. 

Emission lines from Mercury and Neon pen lamps were used to calibrate the wavelength 

scales and determine the spectral resolution for the system. Measured spectral resolutions are 

determined for each experimental configuration. As an example, one configuration using a 

100-p spectrometer entrance slit had a measured spectral resolution (FWHM) of 0.35 nm. These 

values, incorporated in a Lorentzian lineshape, were input parameters for least-squares fitting the 

absorption spectra. 

For these studies, cylindrical samples of RDX were prepared by pressing recrystallized RDX 

powder to about 540 MPa, or 96.9% of theoretical maximum density (TMD) into a cylindrical 

dye of 0.79&m diameter. A drop of methanol in the powder mixture allowed intact extraction 

of the pressed RDX pellets. A diamond saw cut these pressed RDX samples to the final 

thickness, typically 0.2 cm. These pellets were then put in a vacuum dessicator for 12 or more 

hours. In the past studies [ 17-211, an adjustable flowing nitrogen purge gas maintained constant 

pressure and provided a shroud gas flow around the burning propellant sample. To approximate 

a constant pressure, the propellants were either burned in open air or in a combustion vessel 

connected to a gas cylinder of sufficiently large volume (44 liter). Since there is no gas flow, a 

coating must be applied to the sides of the RDX samples to inhibit edge burning. A clear 

fingernail polish (Wet ‘n’ Wild) was used. As seen in Figure 3, this technique produced a flat 

burning surface with a rounding of the edges. 

Video recordings were made for each experimental run. A Cohu Model 82 10 color video 

camera operating in standard VHS or SVHS modes, connected to a Panasonic Model AG-1830 

video cassette recorder and a Sony Model PVM-1341 color monitor, provided visual and tape 

recordings of each experiment. A Nikon 105~mm microlens coupled to the camera magnified 

the burning RDX samples to about 15X. A representative video frame of RDX combusting in 

7 

I(Y), the transmitted intensity of light at frequency v is attenuated along a path of length L, 

according to 

where I, is the initial intensity, kV is the absorption coefficient of the molecule of interest, and 

B(v) is a baseline correction factor for any broadband attenuation along the path length. For the 

present conditions where the light source and the spectrometer bandwidths are much larger than 

the width of the absorption line, an instrument function, S(v,v,), centered at vO, is introduced to 

give 

It = hv,)I(Wv, (2) 

where It is the integrated light transmitted. The instrument function for the detection system is 

obtained from low-pressure lamps described in the experimental section, and the molecular 

transition lineshape is approximated by a delta function. Molecular parameter and transition 

information have been tabulated previously [ 181 with appropriate references. The calculation of 

fractional absorption for CN in this reference considered only the (0,O) and (1 ,l) vibrational 

transitions occurring in the B*Z - X*x electronic system. In the present calculations, the (2,2), 

(3,3), and (4,4) transitions are also included. Franck-Condon factors for these transitions are 

taken from Suchard [25], where q22 = 0.64884, q33 = 0.57392, and q4 = 0.52453. Effects of the 

perturbation for the (4,4) transition of CN are not considered. Using initial guesses for 

temperature and molecular concentration for a given species of interest, It can be calculated from 

equation 2. These calculations are adjusted and compared with the experimental measurements 

of the transmitted light in a nonlinear, multivariate, least-squares fitting program in which the 

concentration, temperature, wavelength calibration, and baseline parameters are allowed to vary. 

Soon after propellant ignition, the flame begins to migrate down the sides of the sample 

(Figure 3), despite the application of a nail polish (cellulose acetate) inhibitor. The narrow profile 

of the intermediate species near the dark zone closely follows the surface contour. As shown in 

9 
equation 1, the product of the absorption coefficient, kV (proportional to concentration), and the 

path length L, is constant for a given transmission. The rounding of the edges in an otherwise 

flat combustion region results- in a reduced .effective path length available for a line-of-sight 

technique. To correct this, the mole fraction obtained by the fit, which is linearly dependant on 

the path length, was increased by the ratio of uncorrected to corrected path lengths. The 

corrected path length in the dark zone combustion region was determined by measuring the flat 

portion of the violet emission from CN. Assuming that this emission delineates the horizontal 

extent of the combustion zone for other species in this region, the effective path lengths for NO 

and NH were also taken to be the same as that of CN. For the triple-pass experiment, the 

correction can be as high as 50%. Because the concentration of OH peaks much further into the 

final flame zone, the path length was taken as the width of the propellant sample. 

4. Model 

We have developed a new burning-rate model [26], which may be particularly suited to 

testing the gas-phase reaction mechanism by comparing experimental and theoretical species 

profiles. This model treats the gas phase at the level of complex elementary-reaction networks. 

In the condensed phase, the linear surface regression rate (r) is assumed to be related to the 

surface temperature (T,) by the following semi-empirical relationship: 

-2 

r=A,e “S. (3) 

This expression, known in the propellant-combustion literature as the pyrolysis law, can be 

derived for surface regression due to condensed-phase ractions in certain limiting cases [7,27]. 

Zenin [28] has shown that this relation gives a good account of his data for double-base 

propellants, for which the regression mechanism is probably reactive. A linearized least-squares 

fit of this relationship to the data of Zenin [7] for RDX is shown in Figure 4; the good fit 

illustrates that it also works well when the regression mechanism is evaporative. We assume in 

10 
I I I 

1.1 1.3 1.5 1.7 1.9 

l/T, (10” K-l) 

Figure 4. Pyrolysis Law. 

this model that there are no reactions occurring in the bulk liquid, even though both exothermic 

and endothermic surface gasification mechanisms are permitted. These two mechanisms 

distinguish themselves in the enthalpy of the products relative to the starting material. 

The pyrolysis law specifies the rate at which reactants for the gas phase are formed. The 

identity and mole fractions of these reactants, as created at the surface, must also be known. 

Liau and Yang [2] and Liau [9] modeled two condensed-phase reactions by which RDX may 

decompose, but their calculations showed that little decomposition of RDX actually occurred 

there. Therefore, we assumed that the gas-phase reactant leaving the surface was all vapor-phase 

RDX. 

The boundary conditions on the chemical species mass fractions (Y) at the liquid/gas 

interface are as follows: 

IilYii” = lilYi + p*Yi+wiO, (4) 

11 
where i is the index denoting the ith chemical species, k0 denotes the condensed (-) and gas (+) 

phase boundary conditions, Vi is the diffusion velocity, ps is the gas phase mass density, and 

m denotes the mass flux related to the linear burning rate and surface density by . 

rk = pSre (5) ’ 

Molecular diffusion in the condensed phase is neglected. In terms of this notation, our previous 

assumption that the “product” of pure, liquid-phase RDX “decomposition” is vapor-phase RDX 

is expressed as 

Y-O =l. FtDX (6) 

Assuming no condensed-phase molecular diffusion, the boundary condition on energy flux at 

this interface is 

to 

- xrhY~“h+o - &Y;“V;oh+o. (7) 

i i 

The thermal conductivity in the condensed arid gas phases at the surface is represented by h, and 

h,, and hi is the specific enthalpy of the ith species. By further assuming there are no bulk liquid 

reactions, the species boundary conditions can be combined with the energy conservation 

equation in the condensed phase. By integrating over the interval (-00, -0), the energy boundary 

condition at the interface between the gas and condensed phases becomes 

= ri~~(Y~-~h’s-y~--h;‘). 

i 

(8) 

This form of the boundary condition is very useful because the thermophysical properties of 

the condensed phase at any temperature other than the initial temperature need not be known, at 

12 
least for purposes of finding the burning-rate eigenvalue. Of course, if the temperature profile in 

‘the condensed phase is desired, these properties at all temperatures between TO and the surface 

temperature T, must be known. 

The enthalpy of the condensed phase at the initial temperature was determined as follows. 

The specific heat of solid RDX was obtained by a least-squares fit of a linear temperature 

function to the data of Shoemaker et al. [29] and Miller [30]. The fit is shown in Figure 5 and 

parameters are in Table 1. This functional form of specific heat was integrated over temperature 

to determine the enthalpy of solid RDX using the value at 298 K to obtain the integration 

constant. This treatment differs from that of Liau and Yang [2] and Liau [9] in that their model 

assumes a constant value of specific heat. 

150 200 2.50 300 350 400 450 500 550 

Temperature (K) 

Figure 5. Specific Heat of RDX. 

Finally, the detailed reaction mechanism occurring in the gas phase needs to be specified, 

along with the associated thermodynamic and transport parameters. The reaction mechanism, 

thermodynamic data, and transport adopted here are the same as those used by Liau and 

13 
Table 1. Semi-Empirical Model Parameters 

AS WR Ps 

(ca$K) 

HFCDX 

(crw 6) Wcm3> (caYmo1) 

3/695 x lo5 9,330 1 .66a 0.03604 + 7.105 x 10m4 T 14,690 at 298 K 

This value was reported by Zenin [7] for his pressed specimens. 

Yang [2] and Liau [9]. In both models, the gas-phase conservation equations are solved using 

adaptations of the PREMIX code [31] developed at Sandia National Laboratories. 

Some discussion of the difference between the two models is in order. The Liau and Yang 

model [2,9] is a more fundamental treatment than the semi-empirical model, in the sense that the 

regression mechanism is treated explicitly. This mechanism is mostly evaporative, with some 

small part due to RDX condensed-phase reactions. It is also more fundamental because it does 

not rely on measurements made at every pressure for which a calculated burning rate is desired. 

The semi-empirical model requires a measurement of surface temperatures at all pressures, and 

these measurements are known to fail at sufficiently high pressures due to thermocouple 

response errors. The condensed-phase reactions and the evaporation description (to some 

degree) are uncertain, however, and these uncertainties map into the model results in an 

unknown way. Our semi-empirical model, on the other hand, at the expense of giving less detail 

about the condensed phase (e.g., liquid-layer thickness), compensates empirically for these 

uncertainties in underlying fundamentals. Hence, it may provide a better test of the gas-phase 

reaction mechanism. This is true, at least to the extent that the surface temperature data is 

accurate, and our assumption of no condensed-phase reactions is warranted. In any event, 

Figure 6 shows that the major species profiles computed by the two models, as well as the 

temperature profiles, do not differ to any significant degree. It can be inferred from this 

agreement that the Liau and Yang description of the condensed-phase and surface-regression 

mechanism is in substantial agreement with the net physical effects of the condensed-phase 

processes, as expressed by the experimentally calibrated pyrolysis law. However, it is also true 

that since the condensed-phase reactions appeared to play a small role in the RDX combustion, 

the detailed description of the condensed-phase processes was not stringently tested. For 

14 
H2 

500 loo0 1500 2ooo 

Distance from Surface (pm) 

500 1000 1500 

Distance from Surface (pm) 

Figure 6. RDX Species Model Profiles. 

purposes of comparison, it is reassuring that the results from such different condensed-phase 

treatments are so close. 

There is an important discrepancy between one of the assumptions of this model and an 

inference made by Zenin [7] based on his microthermocouple data. We believe that all of the 

RDX models [l-4] are similarly discrepant. Zenin found that the net enthalpy change at the 

surface decreased monotonically with increasing pressure. This enthalpy change was computed 

to be +84 Cal/g at 1 atm, +61 Cal/g at 5 atm, -58 Cal/g at 20 atm, and - 140 Cal/g at 90 atm. 

Positive enthalpy changes are consistent with an endothermic process such as evaporation, but 

15 
negative enthalpy changes are not. Thus, the possibility exists that the good agreement between 

models and experimental burning rates shown later in Figure 17 is merely fortuitous. Zenin’s 

inference [7] that the surface enthalpy change decreases with increasing pressure depends on the 

accuracy of both the measured surface temperature and the numerical derivative of the 

temperature at the surface. These are notoriously difficult measurements, and Zenin 173 gave no 

analysis or discussion of experimental uncertainties. Therefore, there was no way to determine 

how sensitive the validity of the inference was to the experimental uncertainties. At this time, 

there appears to be no way to resolve this issue, but future work should be directed toward 

understanding this inconsistency. 

5. Results 

5.1 Absorption. In Figure 7, OH concentration profiles in 1-atm air measured with the 

snapshotechnique are compared to experimental values obtained by Parr and Hanson-Parr [ 151 

and Ulas et al. [ 161. Our model predictions and those of Liau and Yang [2] and Liau [9] and 

Melius and Yetter in Liau [9] are also presented in this figure. Parr’s values were obtained by 

absorption spectroscopy-calibrated planar-laser-induced fluorescence of RDX in 1 -atm air. 

Attempts to conduct self-deflagration measurements in 1-atm nitrogen were unsuccessful, 

confirming observations by Ulas et #al. [ 161 who conducted their experiments at 1.7 atm. At 

distances greater than 2 mm from the surface (well into the luminous flame zone), the measured 

concentration of OH approaches the adiabatic value of 0.024-mol fraction (MF), as calculated 

using the NASA-Lewis thermochemical code. OH species concentrations also approach the 

adiabatic values for 2-atm air, as shown in Figure 8. In this case, the concentration reaches a 

steady-state value closer to the surface. This is supported by visual evidence that the offset of 

the luminous flame zone decreases with higher pressure. The quantity of OH and its vibrational 

structure is very sensitive to temperature. As the temperature decreases toward the surface, the 

amount of OH is significantly reduced. This results in degraded signal-to-noise ratios, thus 

yielding unreliable temperatures. Because of the difficulty of obtaining precise measurements 

directly from the CN and NH spectra, temperature profiles were obtained by combining our 

measured OH temperatures in the flame with OH and thermocouple (TC) values from the 

16 
0.04 

g 0.03 

.d 

E 

2 0.02 F4 

0 

z 

2 0.01 

l Data cl Parr [15] A Ulas [ 161 - - Model 

- Melius in [9] l . . Yetter in [9] -e- Liau [2] 

.- 

L! 

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 

Height 

Figure 7. OH Concentration Profile at 1 atm. 

literature. Data from Zenin [7] and Hanson-Parr and Parr [ 121 at 1 atm, combined with 

temperature data from OH spectra, are shown in Figure 9. At higher pressures, the OH 

concentrations (and temperature) near the propellant surface are increased, thereby allowing for a 

more reliable temperature fit. Temperatures obtained from OH spectra at 2 atm are shown in 

Figure 10. At this point, it was assumed that the temperature profile for 1.5 atm fell midway 

between the l- and 2-atm profiles. The temperature was therefore removed as a variable from 

the multivariate fits for NH and CN. 

Figure 11 shows a spectrum indicating CN absorption from the B21E’ -X2x’ system around 

386 nm, together with the accompanying fits. This snapshot spectrum was taken 0.7 mm from 

17 
0.030 1 

O-025 Adiabatic 

I . . . . . . . . . . . . . . . . 0 . . l . . . . . . . . . . . . 

-3 0.020 

ii 

& 0.015 

g 0.010 

I 

3500 

3w 

1000 

500 

0.0 0.2 0.4 0.6 0.8 1.0 1.2 

Height (mm) 

1.4 1.6 1.8 2.0 

Figure 8. OH Concentration Profile at 2 atm. 

0 zellin 171 

0 p=[141 

v Data 

VW 

- Fit 

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 

Height (mm) 

Figure 9. 1-atm Temperature Profile for RDX. 

18 
. 

3500 

1500 

1000 

0 

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 

Height (mm) 

Figure 10. 2-atm Temperature Profile Obtained From OH Spectra. 

0.72 

0.70 

0.68 

g 0.66 

.w 

-8 0.64 

m 

ii b 0.62 

0.60 

0.58 

0.56 

-[ 

7 

. . 

. ’ : . . . 

. : 

. . . 

375 380 385 390 395 

Wavelength (nm) 

Figure 11. CN Spectrum for 1 atm, 0.7 mm From Sample Surface. 

19 
the surface of RDX while it was undergoing self-sustained combustion in 1-atm air. The dashed 

line represents a fit where only the (0,O) and (1,l) vibrational transitions are included. Including 

the (2,2)-(4,4) bands (solid line) significantly improved the fit to the data. Although a narrower 

instrument resolution would seem warranted for this figure, results from the more reliable 

calibration technique that uses a low-pressure pen lamp were used to fix this parameter. The 

solid line fit to the data results in a CN mole fraction of 175 ppm. According to the procedure 

described previously to determine the path length, an 11% correction to this data resulted in a 

CN mole fraction value of 195 ppm. 

Figure 12 shows three CN mole fraction profile snapshots in 1 -atm air for two different RDX 

combustion runs. In addition, Gaussian functions are plotted as a simple representation of the 

two data sets. For reference, the temperature profile for 1-atm RDX combustion in air is also 

plotted on the same graph. Within experimental uncertainty, a Gaussian adequately represents 

the experimental data. For one of the runs, a fiber optic was substituted for the collection lens in 

an attempt to determine possible effects of beam steering on the CN concentration profile. The 

propellant sample was positioned to initially cover some of the measurement strips. Therefore, 

the zero of the height scale (or distance from the propellant surface) was determined by the onset 

of the transmission signal balanced against visual evidence from the video camera. Only small 

differences occur in both maximum CN concentration and the position of maximum CN 

concentration. These differences are well within the uncertainty associated with run-to-run 

variations and demonstrate that beam steering is not compromising the data at this level of 

spatial resolution. 

Snapshot CN profiles of RDX combustion were also conducted at higher pressures. In 

1.5-atm air, the position of maximum CN concentration decreased from -0.82 mm (1 atm) to 

0.59 mm, and the FWHM width also decreased from 1.1 mm (1 atm) to 0.78 nun. A decrease in 

the maximum CN mole fraction was also observed. The width was again 0.78 nnn for the 2-atm 

case. However, the position of maximum CN concentration increased to 0.71 mm, while the 

20 
250 3000 

2800 

2600 

24002 

2200 I? 

2 

2000 & 

E 

18OOg 

1600 

-r 

1400 

1200 

0.0 0.4 

&light (2) 

1.6 2.0 

Figure 12. CN Concentration Profile at 1 atm. 

maximum CN mole fraction decreased by about a factor of two. Here, the data were more 

scattered, and a Gaussian-like shape was not well defined. 

Snapshot NH concentration fraction profiles for RDX combustion in 1-atm air are shown in 

Figure 13. There was considerably more scatter in the data due to the reduction in S/N, inherent 

for NH measurements. Hence, a Gaussian-function representation of this data was only applied 

for consistency. The determined position of a maximum NH mole fraction (0.81 mm) was the 

same as that for CN (0.82 mm) at 1-atm air pressure. 

Snapshot profiles of CN and NH were also conducted at higher pressures and are 

summarized in Table 2. The position and widths of the concentration profiles generally 

decreased above atmospheric pressure, as would be expected from the visual evidence. Due to 

the limited spatial resolution of this technique, both peak positions and widths became less 

certain as the pressure was increased to 2 atm. For both CN and NH, there is a monotonic 

decrease in the measured concentration with increased pressure. 

21 
t 

2800 

2600 

2 

- 2400; 

5 

- 2200% 

$ 

- 2000 g 

z 

- 1800 

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 

Height (mm) 

Figure 13. NH Profiles in 1-atm Air. 

Table 2. Comparison of CN and NH Concentrations Obtained in the Self-Sustained 

Combustion of RDX 

. 
Higher resolution 1-atm CN and NH concentration profiles (Figure 14) were obtained with 

the single-beam absorption experimental setup. Spatial information was gathered in this case by 

allowing the sample to burn by the probe beam. The measured concentration peak position and 

model for both CN and NH agree well with values measured by Parr and Hanson-Parr [141 and 

predicted by Liau and Yang [2] and Liau [9]. Although our measured species mole fraction 

compared to within a factor of two, the ratio of CN to NH of -1.2 was consistent. 

A NH 

-o- CN Parr [14] 

- NHParr[14] 

- Model 

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 

Height (mm) 

Figure 14. High Resolution Profiles of CN and NH at 1 atm. 

The NO species concentration profile of burning RDX in 1-atm air is compared to data from 

Parr and Hanson-Parr [141 in Figure 15. In this case, the Parrs used absorption spectroscopy due 

to the large self-absorption that adversely effects LIP measurements of this species. Current 

modeling results and predictions from Liau and Yang [2] and Liau [9] also appear in this figure. 

Data was taken by conventional single beam absorption spectroscopy because of the greater 

spatial resolution afforded this technique. At distances greater than 0.5 mm from the surface, 

23 
0.30 

0.25 

0.20 

g ‘3 

3 i: 0.15 

g 

0.10 

0.05 

0.00 

-I- 

. 

T 

- .- Liau and Yang (model) [2] 

0.0 0.2 0.4 0.6 0.8 1.0 1.2 

Height (mm) 

Figure 15. High Resolution NO Species Profiles. 

well into the final flame zone, measured species concentrations were higher than those predicted 

by the models. Jn the same region, the temperature of NO (Figure 16) is less than that measured 

by the OH diagnostic or predicted by the models. This higher concentration is most likely 

caused by the presence of cold NO molecules lying in the absorption path, but outside of the 

flame region. This effect becomes prevalent as the sample deflagrates due to the sequential 

spectra collection scheme, with early (closer to the surface) measurements being least affected. 

5.2 Video Records. In addition to the information obtained from analysis of absorption 

spectra, a video record exists for each experiment. A typical video frame for RDX combustion is 

shown in Figure 3. Both burning rates and flame structure have been determined from these 

video records. Table 3 contains distance estimates for the dark zone and the CN reaction zone 

lengths. These distance estimates come from visible light emission, as seen by the Cohu CCD 

color camera. The thickness of the violet-colored region above the cornbusting RDX surface is 

taken as the FWHM thickness for the CN concentration. The nonluminous region between the 

24 
4000 

3500 

3000 

2500 

2000 

1500 

1000 

500 

0 

l 

--- 

--- : 

..; . . . . .~~-, - 

l l 

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 

Height 

Figure 16. Temperature Profile From NO Spectra at 1 atm. 

Table 3. Video Measurements for RDX Burning in Air 

Pressure Dark Zone Length 

(at.@ 0-m-N 

1.0 0.3 

1.5 0.2 

2.0 0.1 

CN Reaction Zone Length 

(mm> (-l-vHm 

0.5 

0.3 

0.2 

. 

RDX combusting surface and the start of the violet region is taken as the measure of the dark 

zone. These values were compared with absorption data and other published work in the 

discussion section. 

5.3 Burning Rate. In Figure 17, the results of the two model calculations of burning rate 

are compared to a number of sets of experimental rate measurements. Both models agree well 

with the data. Liau and Yang [2] and Liau [9] noted that over the pressure range displayed, the 

endothermic condensed-phase reaction dominated the exothennic reaction, and did so to a 

25 
1 

h 

2 

E 0 V 0.1 

L 

0.01 

0 Zenin [7] 

o Miller 1311 

v Data - - 

v Atwood [33] 

m Ulas 1161 

n Zimmer 1321 

+ Brewstei [3$ 

0 Parr [I21 

- Liau [9] 

- Model 

& Present 

/ d 

/ 

l 

c( v 

v 

0.1 1.0 

P (MPa) 

10.0 

Figure 17. Burn Rate of RDX as a Function of Pressure. 

greater extent at the lower pressure. This may explain the slightly higher pressure exponent 

evident with that model. 

Burning rates as a function of pressure are given in Figure 17. Other published 

measurements are also plotted on the same figure for comparison. Measurements of Zenin [7], 

Ulas et al. [16], Zimmer-Galler [32], and Atwood et al. [33] use nitrogen as the pressurizing gas. 

Brewster and Schroeder’s [34] results, Parr and Hanson-Parr’s [14] results, and the present 

results are measurements for an air environment. The burning rate for RDX combusting in 2-atm 

nitrogen gas was also measured. Well within experimental uncertainty, we found the burning 

rates for the air and nitrogen environments to be the same, even though the gas-phase structure 

appears different close to the edges where air can be entrained into the combustion process. A 

more pronounced rounding of the edges is attributed to this entrainment. Experimental burning 

rate measurements for RDX vary by a factor of two at atmospheric pressure, and our result of 

0.23 mm/s represents the lowest value. Zenin’s values are the highest. These present 

measurements at atmospheric pressure, within experimental uncertainty, agree with Brewster and 

26 
Schroeder [34] and Parr and Hanson-Parr [ 141. Our burning rate values for 1 .O, 1.5, and 2.0 atm, 

line up best with the results of Ulas et al. [ 161. A video observation of the PDX combustion 

shows that convex surfaces can occur, especially for the smaller diameter samples. Moreover, 

small and large bubbles form in the melt layer, giving rise to occasional surface movement being 

driven by buoyant effects. Sample preparation, impurities, and density can also effect the 

measured burning rate of the propellant. 

6. Discussion 

Measurements of the highly reactive intermediate species, CN and NH, require a high degree 

of spatial resolution, as these species are formed and consumed over a small spatial extent. 

Furthermore, this spatial extent is pressure dependent, decreasing as the pressure increases. 

Consequently, the maximum spatial resolution for a line-of-sight technique (absorption) is 

severely challenged when applied to making measurements across a burning, moving propellant 

surface. Any nonplanar burning and/or misalignments of the absorption light beam with the thin 

region, where short-lived radical species exist, leads to degradation of the apparent spatial 

resolution. A quick calculation shows an 18% drop in apparent concentration for CN, and NH is 

possible with as little as five degrees of misalignment. The harsh reality that the combustion of 

propellants does not produce ideal, 1-D systems, is indeed a concern that current models and 

absorption spectroscopy techniques ignore. However, much effort has been expended to produce 

a large area on the top of the sample that is flat to within the resolution of the visual record 

(cl00 p). There is a small region on the edge of the combustion zone that is not in line with 

this flat region but follows the curved edge of the sample. If the combustion wave follows the 

topography of the sample, the line-of-sight path length in this curved region is on the order of the 

spatial extent of the species being measured. Because of the narrow profiles of NO, CN, and 

NH, the absorption path length in this outside region of the combustion zone is small compared 

to that over the flat portion. Furthermore, convolution of the finite absorption beam diameter 

(assumed circular) with the calculated species profiles (assumed Gaussian) produced only small 

effects in the final profile (~5%). Unfortunately, there must be an assumption made about the 

shape of the actual profile to deconvolute the experimental data. For the reasons already 

27 
mentioned, the best results should be for RDX burning in 1-atm air (the lowest pressure data, 

where the spatial resolution requirements are the least stringent). A comparison with other 

published results for 1-atm conditions are discussed, followed by an interpretation of the higher 

pressure data. 

Table 2 provides a summary of the CN and NH peak concentrations, widths of the species 

profile, and peak standoff distances obtained for self-sustained RDX combustion at pressures 

ranging from 1 to 2 atm air. For a comparison, the Parr-s are the only known investigators who 

have published quantitative results for species concentrations in the self-deflagration of RDX at 

1-atm air pressure. In their more recent measurements, Parr and Hanson-Parr [ 141 report peak 

CN mole fractions of 3 10 ppm for self-sustained combustion of RDX in 1-atm air. These 

measurements were made for 9.5~mm diameter pressed RDX propellant samples. In this same 

work, they report peak NH mole fractions of 260 ppm. Looking at column four of Table 2, 

which is a measure of the spatial extent of the profiles, in all cases the Parr data gives a smaller 

spatial extent (by about a factor of two) than our measurements taken with the snapshot 

technique. However, data from the high-resolution experiment more closely matches the 

position and widths of these two species. It should be noted that the video estimates (Table 3) of 

the FWHM values also support the high-resolution data. 

As the measured concentration profile and models of NO reach a maximum close to the 

surface, the peak position and the height from this peak to half max (HM) provides an indication 

of the extent of NO and is reported in Table 4. Our measurements of substantial NO at the 

surface (0.15 MF) agree well with the predictions of the models. In contrast, Parr and HansonParr [14] measured very little NO at the surface, with the peak concentration occurring a factor 

of two further into the dark zone. 

As far as the pressure dependence of the CN and NH profiles taken with the snapshot 

technique, the behavior suggests a spatial resolution limitation at the elevated pressures. The 

peak standoff distance, as well as the extent (FWHM) of the profile, is expected to decrease 

28 
Table 4. Comparison of NO Concentratiods at 1-atm Air 

Half-Max Standoff 

Peak Concentration Peak Standoff From Peak Position Author 

(mm) (mm> 

II 0.25 I 0.1 I 0.4 I Present work, high resolution 

II 0.17 0.4 0.1 Parr and Hanson-Parr [143 

II 0.26 I 0.09 I 0.33 I Present work (model) 

II 0.21 I 0.13 1 0.34 1 Liau and Yang [2] (model) 

with increasing pressure. Only a slight indication of this trend is present from the absorption 

data of Table 2, while estimates from video records (Table 3) suggest more pronounced 

decreases in peak standoff and FWHM with increasing pressure. For propellant diagnostics of 

small samples with short temporal constraints, or samples with large reaction zones and reduced 

spatial needs, the snapshot absorption technique is useful. 

Species profiles in RDX flames have been obtained during 1-atm self-deflagrating 

combustion. Although the peak concentrations of CN and NH differ by a factor of two, both the 

model and other researcher’s measurements [ 141 and the positions and concentration ratio 

measured by the high-resolution technique are very close. The factor of two difference in 

concentration is not alarming, as CN and NH are minor concentration species existing in a small 

region close to the combusting surface. Because NO is a major species both in concentration and 

importance to the combustion process, the differences between the profiles measured in this 

work and that of Parr and Hanson-Parr [ 141 for the same combustion conditions are more of a 

concern. Parr and Hanson-Parr [141 used a military grade of RDX that contained up to 5% HMX 

(assayed), whereas our samples were recrystallized to remove such impurities. Some of the 

difference in concentration measured at the surface may be due to this impurity since it is 

thought that HMX produces less NO at the surface. The choice of absorption path length will 

also effect the measured concentration. In this work, the path length was chosen to coincide with 

the visual extent of the CN region (see Figure 3). Parr and Hanson-Parr [141 chose a longer path, 

29 
which was determined by the presence of a brown gas (N02) with a diameter larger than the 

propellant. This brown gas was not apparent in our measurements. Further evidence of the high 

concentration of NO near the surface came from recent measurements of Hanson-Parr and 

Parr [35] using spontaneous Raman spectroscopy as the diagnostic tool. Their measured value of 

0.26 MF NO within 0.1 mm of the surface is in good agreement with our measured and predicted 

values. 

Current model predictions are in good agreement with the concentration profiles for all four 

species measured in this work. The gas-phase mechanism used to model RDX combustion has 

been able to predict species profiles under various experimental conditions [2, 4, 91. However, 

evaporation from the surface, RDX subsurface decomposition mechanisms, and thermophysical 

properties of the liquid layer are all avenues for further research. 

7. Conclusions 

RDX has become the principal prototype in the study of the detailed combustion mechanisms 

of energetic materials. Experimental verification of these mechanisms is, therefore, of central 

interest to this research. The combined experimental and modeling efforts presented in this 

report are an attempt to clarify and extend the limits of the present understanding. 

Calculations of the burning rate are the result of integration over the fundamental chemical 

and physical mechanisms; for this reason, successful model predictions of the measured burning 

rates are not a particularly sensitive test of the validity of model assumptions. A more stringent 

test is afforded by comparing model and experimental species profiles. In this work, new 

measurements are presented for four gas-phase species profiles, OH, NO, CN, and NH, arising 

from the steady-state combustion of RDX at pressures of l-2 atm. Under these same conditions, 

there has been only one other group to measure the species profiled in this report. In that effort 

[ 12-141, diagnostic techniques different from those used in this work were applied to the 

measurement of the OH, CN, and NH species, and more recently NO [35]. The independent and 

complementary nature of these two sets of measurements are important in building a reliable 

30 
experimental database with which to test the models. The two sets of measurements are 

generally consistent with differences detailed’in the discussion section. 

The experimental species profiles are compared with predictions of two models of RDX 

combustion that attempt to follow the elementary gas-phase reactions in detail. Like the two sets 

of experimental measurements, these two models are somewhat complementary in that they both 

use the same gas-phase reaction mechanism, but treat the condensed-phase and surface 

gasification mechanism differently. One model was previously developed by Liau and Yang [2] 

and Liau [9], and the other is presented in this report. Due to the new model’s use of an 

empirical relationship between burning rate and surface temperature (pyrolysis law), it is less 

speculative of the condensed-phase processes. But the Liau and Yang model gives a more 

detailed description of the condensed-phase processes. Because the two models predict virtually 

the same burning rates and species profiles, despite their differing treatments of the condensedphase processes,’ comparisons between experimental and theoretical profiles in the gas phase 

should be a meaningful test of the reaction mechanism there. Agreement between our new 

species profiles and the corresponding model predictions are judged to be within our 

experimental uncertainty. This favorable comparison, coupled with the good agreement between 

measured and predicted burning rates, adds considerable confidence to the validity of the gasphase reaction mechanism. 
32 
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Steady Laminar One-Dimensional Premixed Flames.” SAND85-8240, Sandia National 

Laboratories, 199 1. 

32. Zimmer-Galler, R. “Correlations Between Deflagration Characteristics and Surface 

Properties of Nitmmine-Based Propellants.” AUA Journal, vol. 6, no. 11, pp. 2107-2110, 

1968. 

33. Atwood, A., P. 0. Curran, C. F. Price, and J. Wiknich. “Burning Rate, Radiant Ignition, and 

Global Kinetics of Cyclotrimethylene Trinitramine (RDX).” Proceedings of the 32nd 

JANNM Combustion Meeting, CPIA Publication 638, vol. 1, pp. 149-159, 1995. 

34. Brewster, M. Q., and T. B. Schroeder. “Experimental Study of Steady and Unsteady 

Combustion of RDX.” Proceedings of the 32nd JANNAF Combustion Meeting, CPIA 

Publication 638, vol. 1, pp. 85-93, 1995. 

35. Hanson-Parr, D., and T. Parr. “Spontaneous Raman Spectroscopy Applied to Nitramine 

Flames.” Proceedings of the 35th JANNAF Combustion Meeting, in press December 1998. 

35 
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REPORT DOCUMENTATION PAGE Fom Approved 

OMB No. 07044188 

On the Flame Structure of RDX 

man, Martin S. Miller, and John A. Vanderhoff 

Aberdeen Proving Ground, MD 210055066 

11. SUPPLEMENTARY NOTES 

12a. DlSTRlBUTlONlAVAlLABlLlTY STATEMENT 12b. DISTRIBUTION CODE 

Approved for public release; distribution is unlimited. 

13. ABSTRACT(Maximum 200 words) 

In an attempt to refine the database for the evaluation of detailed-chemistry combustion models, absorption 

spectroscopy has been applied to low pressure, self-deflagrating RDX flames. Semi-empirically determined production 

rates of reactants from the solid propellant surface, together with a detailed gas-phase elementary reaction set, were used 

to develop a model that minimizes the effect of uncertainties in the description of solid phase processes. The spatial 

profiles of two low concentration, highly reactive, short-lived diatomic species, CN and NH, were obtained at pressures 

varying from 1 to 2 atm in air. Two major species, NO and OH, were also profiled using this technique. The CN and 

NH profiles agree well with previous measurements and the current model. NO and OH profiles are also in good 

agreement with the models. During the course of these spectroscopic measurements, burning rates for RDX over a 

pressure range of l-2 atm have also been determined. These values range from 0.23 mm/s at atmospheric pressure to 

0.50 mm/s for 2 atm and are noticeably lower than some of the other published measurements. Calculations stemming 

from the proposed model predict a burning rate of 0.29 mm/s and 0.54 mm/s for the same pressure levels. 

14. SUBJECT TERMS 

RDX, combustion, diagnostics, burning rate, modeling, absorption, NO, CN, NH, OH 

15. NUMBER OF PAGES 

39 

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