High pressure combustion characteristics of RDX based propellants

Twenty-Second Symposium(International)on Cornbustion/The CombustionInstitute, 1988/pp. 1835-1842

H I G H P R E S S U R E C O M B U S T I O N CHARACTERISTICS OF R D X BASED PROPELLANTS BAO-CHANG ZHAO AND ZHI-JIAN ZHAO

Chemical Engineering Department East China Institute of Technology, Nanjing People's Republic of China The characteristics of RDX (cyclotrimethylene trinitramine) based propellant eombustion over a wide range of pressure (5-430 MPa or 49-4250 atm) were studied experimentally. The data obtained from a strand burner and a closed bomb system show the influence of RDX concentration, particle size, the addition of NGD (nitroguanidine) and different binders used on the burning rate characteristics. It was found that the use of very fine RDX powder (2 3 pma) and the addition of a small concentration of NGD help to eliminate the pressure exponent shifts. The thermal analysis of the RDX based propellants with a Perkin-Elmer Corp. Diflbrential Scanning Calorimeter, Model DSC-2C, and SEM examinations of quenched propellant sampies were conducted to study high pressure combustion mechanism of the nitramine propellants. The results revealed that the pressure exponent shifts of the RDX based propellants are related to the difference of decomposition of RDX and its binders, physical structure variation, burning surface change and chemical activation variation of N20 with pressure.

Introduction The combustion of RDX based propellants has recently received more attention in research studies because of the interest in developing energetics and smokeless solid propellants .which are considered for rocket propulsion and gun applications. Extensive studies have been conducted to determine the burning rate characteristics and to understand combustion mechanism of the propellants. 1-a In their studies, Cohen and Strand4 have found that there exist the burning rate exponent shifts of the nitramine propellants. They attributed this phenomenon to difference of the burning rates and decomposition behaviors between RDX and its binder. The reason that the high pressure exponent appears is that the burning rate of the nitramine powder exceeds that of the binder. Other explanations for the phenomenon involve physical effects, burning surface structure change (from melt layer to a eratered surface) and the ignition delay for the nitramine crystals. Nevertheless, due to the difficulties in experimental investigations, the high pressure combustion mechanism of nitramine based composite propellants is not yet fully understood. Over low pressure range, several studies have shown that addition of RDX to double base propellant lead to a decrease of the burning rate with

the slight increase in the pressure exponent. The pressure exponent increases when the RDX concentration increases. The burning rate appears weakly dependent upon nitramine particle size, but the use of fine nitramine powder helps to remove the slope break phenomenon to higher pressure. However, few studies have been devoted to high pressure combustion characteristics of RDX based composite and composite modified double base (CMDB) propellants. This research is to determine experimentally some high pressure combustion characteristics of RDX/ CMDB and RDX/polymer-binder propellant, and to contribute to a better comprehension of the burning mechanism.

Experimental In the first part of this study we examine the influence of the principal parameters that may af feet the r(p) laws, such as RDX mass fraction, BDX particle size, the presence of small concentration of NGD and energy content of the binder. The second part is devoted to the burning mechanism of the RDX based propellants as deduced from dif ferential scanning calorimetry of the propellants and by examining the extinguished samples tinder a scanning electron microscope (SEM).

1835

1836

PROPELLANTS

Apparatus: The apparatus for burning rate measurements include a closed bomb system and the strand burner5 in which pressure can be kept constant during propellant combustion in a wide range of pressure (5430 MPa or 49-4250 atm). The apparatus for extinguishing burning propellant is a vented combustor with a port and a pressure gauge. The rupture disk was selected to make the port open at our desired pressure. The instrument for DSC analysis of the propellant is a Perkin-Elmer Corp. Differential Scanning Calorimeter, Model DSC-2C. As available from the manufacturer, the instrument is capable of making a rapid and accurate response to chemical heat generation with a sensitivity of 0.1 mcal/see and working at temperatures over the range from 98 K to 998 K (-175-725 ° C), Our experimental conditions were as follows: scanning temperature range: 323 K-673 K; heating rate range: 10-80° C/min; pure nitrogen flow rate: 50 ml/min.

Samples Used in the Experiments: The nitramine used in this study was finely crystallized RDX (cydotrimethylene trinitramine). The mean diameter (weighted average value) of the RDX particles were 2.6 Ixm, 14.5 Ixm, 27.7 t~m and 32.2 txm respectively. Two types of binders were prepared: 1. a DB matrix (marked NNC2 in this paper) which consists of 53.3% nitrocellulose (NC), 44.2% nitroglycerine (NG) and 2.5% ethyl centralite (C2); 2. an inert polymer called ethyl cellulose (EC). The detailed chemical compositions of the propellants are listed in Table I.

The manufacturing prooesses of propellant samples included mixing, plasticating, shaping and drying. First, all ingredients of one sample were mixed with an amount of solvents, followed by plastication of the mixtures. Then the plasticated mass was extruded through dies into desired shapes-spaghetti-like strands with 5 mm diameter and 30 mm long for strand burner measurements and single-perforated grains for dosed bomb tests and other experimental analyses. Finally, drying removed water and organic solvents. All the samples listed in Table I were made by the same processes.

Results and Discussion

Effect of RDX Concentration and Binder Type: Figure 1 shows the burning rates of different RDX based propellants and the DB matrix (NNC2). It is evident that addition of RDX to the DB matrix leads to an increase of the burning rate at high pressures and the slight decrease at low pressures. The effect of RDX concentration on the burning rate is also related to pressure range: in low pressure range (p < about 60 MPa), the increase of the RDX concentration decreases the burning rate; in high pressure range, however, it increases the burning rate. All the RDX based propellants shown in Fig. 1 are observed to have pressure exponent shifts. In the range of middle pressure the pressure exponents are relatively high. The type of binder used in the propellants affects markedly the burning rate characteristics, The propellant with polymer binder has higher pressure

TABLE I Chemical compositions of the propellants used in this study Prop, RDN-1 RDN-2 RDN-3 RDN-4 RDN-5 RDU-4 RDU-4-2 RDU-5 RDU-6 ERDN *80% RDX NGD DNT TA

RDX (size I z m ) 60.0 20.0 43.0 43.0 43.0 32.5 32.5 30.0

(14.5) (14.5) (14.5) (27.7) (32.2) (2,6 + 14.5") (14.5) (2.6 + 14.5") 25.0 (2.6 + 14.5") 72,0 (14.5)

NNC2

DOP

35.0 75.0 52.0 52.0 52.0 52,0 52.0 54.0 40.0

5.0 5.0 5.0 5.0 5.0 1.5 1.5 6.0

2.6 ~m mixed with 20% 14.5 Ixm. = cyclotrimethylene trinitramine = nitroguanidine = dinitrotoluene = triacetine

NGD

8.0 8.0 10.0 17.5

8.0

DOP NC EC C2

= = = =

NC

DNT

EC/101

TA/C~

6.0 6.0 10.0 5.0

dioctyl phthalate nitrocellulose ethyl cellulose ethyl centralite

6.o/1.5 lO.O/5.o

RDX BASED PROPELLANTS

z,0[

I,75

LOG U CM/S

1,5 LNO PRO, n I XRDN ,84.1,21.I 1,25 2 RDN-I ,84,1,21, 3 RDN-2 ,BG,I,14, 1,8 4 NNC2 ,92

1837

1,80 I 67 LOG U CMiS No P o, I RI)N-3 2 El)H-4

1 54 1 41

~=~

.~

..."}¢"

3/~I)N-5 ;~

128

8,75 8,5

...~;;" NO n ~.,.~. 1 , 84,1,85 , 91

8, 89

3,96,1,15,94 -, 25

3/z/'~1

-8,5 " /~ 8,5 ~0,94 1,38 1.82 2,26 2,7 LOG PRESS MPA

8,63 t0.2 8,5

~

i

,,

1

1

I

i

i

i

1,6 1,82 2,84 2,26 2,48 2,7 LO~ PRESS MPA

FIG. 1. Burning rates of RDX based propellants and the DB matrix.

FIG. 2. Effect of RDX particle size on burning rates.

exponent and more break points in the burning rate vs pressure curve than the propellant with the DB matrix, as illustrated in Fig. 1. The break point near 260 MPa is noticeable. Taylor6 and Boggs 7 have found that the pressure exponent of pure HMX (cyclotetramethylene tetranitramine) deflagration is about 0.93. Since the burning rate curve of RDX is similar to that of HMX, s we can deduce that at very high pressure, RDX is predominent in controlling the burning rate of the RDX propellant with polymer binders.

than the propellant RDU-4-2, and that the pressure exponent in high pressure range of RDU-4 is smaller than that of RDU-4-2. Therefore, not only the RDX particle size but also the size distribution have great effects on high pressure combustion of the RI)X based propellants.

Effect of RDX Particle Size: Burning rate plots for three RDX/CMDB propellants containing 43.0% RDX of different particle size are presented in Fig. 2. The results indicates that at pressure below 30 MPa, the dependence of the burning rate on the RDX particle size is insignificant. As pressure increases, the effect of the particle size emerges. It is interesting to note that over the range of pressure from about 190 MPa to 225 MPa, all the burning rate curves shown in Fig. 2 change their slope. The slope break becomes more apparent with the RDX particle size increasing. Figure 3 presents burning rate plots for two RDX/ NGD/CMDB propellants containing different particle size distribution. It is seen that at high pressure the propellant RDU-4 has lower burning rate

Effect of NGD Addition: The incorporation of small concentrations (8.0%17.5%) of NGD into RDX/CMDB propellant can alter the burning rate, as illustrated in Fig. 4. In contrast to the RDX/CMDB propellant, the RDX/ NGD/CMDB propellants show no evident slope break in their burning rate curves with pressure exponents between 0.7-0.93 over the range of pressure from 30 MPa to 280 MPa. Increasing NGD concentration decreases the pressure exponent.

DSC Analyses: To understand the mechanism of the pressure exponent shifts of the nitramine propellants, it is important to analyze the difference of thermal behaviors of RDX and its binders, and to study correlation between decomposition and deflagration of the propellants. At the heating rate of 10° C/rain, the thermal decomposition behavior of RDX, NGD and the DB

PROPELLANTS

1838

10.00

2,~

~ml~le h~ot l~t,l.(cal/g) RI~ 274.0 /430 |17.6

NtlRPIAI IZED

LOG II CM/S

St2q~ ~ITE

/ 477.7 | ~

U.t3t)~ m i n '

NI'F',2

3E~.2

1,6 NO PRO,

II ~.~.o

n

/ /

~

52fi.8 L

5.00

1,2 ~,8

47~.2 0.130 2fO.O ,~n.t)

420.0

hSt}.O h/l~.O

5~0.5 510.0

560.0

5~.t)

'l'l:/tl'£gg'l'Ul~I¢ ( g )

0,4 I

I

I

I

i

I

t

I

1,69 1,88 2,87 2,26 2,45

1,5

LOG PRESSMPA FIG. 3. Effect of RDX particle size distribution on burning rates. matrix is different, as illustrated in Fig. 5. The thermal proeess of RDX has two stages: liquefaction occurs over the temperature range from 475.8 K to 480.0 K, followed by exothermic decomposition of which the initial temperature is 42 K higher than that of the DB matrix. Fig. 5 shows that there are

2,8

1,2

DSC

FIG. 5. DSC charts of RDX, NGD and the DB matrix (NNC2). I

1,6

6"]0.0

LOG Im CM/S NO PRO, n I RDN-3 1,05,,91 2 RDU-40,88

I

,.~.~

3RDu-5 ,83 4

0,8

J.LI~ ROI~I.~LIZED S~/I v~l~: 15.C0aeg/nn 7

1t,4 L

1,5

three stages in the NGD thermal process: first, an exothermic peak spans temperatures from 471.1 K to 490.9 K in which phase change of the NGD would take place; second, an endothermic peak which resuits from liquefaction of the NGD; and finally, the exothermic decomposition occurs after 526.8 K that is 47 K higher than the initial decomposition temperature of the RDX. In the RDX/CMDB propellants, the RDX crystals have an ignition delay at low heating rate. Fig. 6 shows DSC charts of the RDX/CMDB propellants with different RDX concentrations. One common characteristic of these charts is the appearance of two exothermic peaks. As RDX concentration increases (or correspondingly, the mass fraction of NNC2 reduces), the first peaks lower while the second peaks rise. It is evident that at low heating rate (10° C/min, for example), there are two basic stages in the thermal decomposition of the RDX/CMDB propellants--the first stage is mainly NNC2 (the DB matrix) decomposition, and the second, the RDX decomposition. However, the RDX and the DB matrix interact in propellant decomposition. Table II lists the measured kinetic parameters for RDX, NNC2 and three samples. The analysis of these data led to a conclusion that RDX is a catalyst of DB matrix decom-

I

i

L

i

I

I

i

~SI$-Ij.~ 52

i

1,69 1,88 2,87 2,26 2,45

LOGPRESSMPA FIG. 4. Effect of NGD on the burning rate of RDX/CMDB propellants.

t~.ll/ 3~.0 ~a'~.O410.0 ~ , 0

4~,0

~JO.O 5~0,0 5~t%.0 ~ , 0

'I~4PE,RATUK~ (K)

DSC

FIG. 6. DSC charts of RDX/CMDB propellants.

RDX BASED PROPELLANTS TABLE II Kinetic parameters measured at the heating rate of 10° C/min Sample NNC2 RDX RDN-1 RDN-3 RDN-2 RDN-1 RDN-3 RDN-2

Temp. K

Log~oA s-~

E kcal/mol

434-465 480-509 470-481 434-478 435-475 490-516 494-509 500-510

17.71 18.63 9.73 19.11 24.40 16.59 29.34 32.79

42.6 47.0 27.1 46.5 57.0 38.0 71.9 79.7

position, to some extent, the DB matrix inhibited RDX decomposition on the other hand. As the heating rate increases, the initial decomposition temperature of propellant is becoming higher, while the maximum decomposition rate is getting larger. Fig. 7 shows DSC charts of RDU-5 and its DB matrix under different conditions of heating rate. When the heating rate is 10, 20, 40, 80° C/min respectively, the ratio of the corresponding maximum decomposition rate is 2 . 6 : 3 . 7 : 8 . 3 : 8 . 5 for the DB matrix, and 1.8:3.7:20.4:21.3 for the nitramine propellant. The results revealed that the exothermic chemical reactions associated with RDX decomposition and deflagration are sensitive to temperature variation. The

NORHALIZED NNC2

/

567.7

4~.3

~

18:39

high pressure exponent of the RDX/CMDB propellant would be related to the behavior of the reactions since the near-surface temperature in the combustion wave varies with pressure. An important species associated with these chemical reactions is N20 which is decomposed from RDX. Although the shock-tube measurement by Fifer9 showed that the oxidation-reduction reactions involving N20 were slow compared to the HCHO + NO2 reaction at low pressure, they would be dominant in the near-surface flame at high pressure. It is argued that the chemical activation of NzO varies with pressure--it could be inert at low pressure and active at high pressure. The decrease of N20 mass fraction in nitramine propellants appears to be helpful to diminish the pressure exponent shifts. Fig. 8 shows that addition of NGD to the RDX/CMDB propellant has lowered the second decomposition peak in the DSC chart. Therefore, the reason why the RDX/NGD/ CMDB propellants show no evident pressure exponent shifts is the difference of decomposition behavior between two types of propellants, i.e., RDX/ NGD/CMDB and RDX/CMDB. Compared with the latter, the RDX/NGD/CMDB propellants have little N20 species if nitramine contents are the same for the two types of propellants, because the decomposition products of NGD do not contain NzO. Moreover, the middle product of NGD decomposition was found to be a melted residue which can combine the middle product of RDX decomposition--hydroxymethyl-formylamide, a catalyst of RDX decomposition, to form a melted matter. This is effective in inhibiting great autoacceleration of RDX decomposition in propellant. SEM Observations:

7.~o

.....

I0

351.1

83

:)~3.9

~

.~'--"

\~

\~6.9

O.C~

~.0

~2)

410.0 440.0 ~,~.0 '~120.9 5Ut).O 5~).0 5'J).O TB"~P~A~RE (K)

NORHALIZED $~DtJ-$

.........

-%~.

.m.:~..q-~ ~ ,~,-~,-

The microscopic examinations of the extinguished propellant samples have two aspects: interior structure and burning surface. Figure 9 presents views of the interior structure of the RDX/CMDB and the RDX composite propellant before and after combustion. It is seen that there are different physI +(XJ

e/~,14RAIE: I{),tT} tk,~il~

~.~.~.

f).T)

80

~t,O.7

,.~N'.~ , ! i

~.2

grL6

T~J~I F~RAIURF. ( Z )

FIG. 7. Comparison of DSC charts of the nitramine propellant and its DB matrix under different scanning rates.

~,n

~.0

~.0

450.0

~0.0

510.0 5~.0

TDIPERATURE (R)

57o.o

~.0 eSC

FIG. 8. Comparison of DSC charts of two types of propellants.

1840

PROPELLANTS

RDU-4 propellant before combustion

'2O rm' RDU-4 propellant quenched at 66 MPa

'50Inn' ERDN propellant before combustion

'20 pm ERDN propellant quenched at 66 MPa

FIG. 9. Views of interior structures of R D X / C M D B and after quenched.

and R D X

composite propellant before combustion

ical structures--one is homogeneous, and the other is heterogeneous. Defects existed in the RDX/CMDB propellant grains before combustion. Figure 9 shows that during high pressure combustion, cracks which may originate from the defects appear, allowing hot highpressure gases to penetrate the larger cavities, thereby providing additional surface area for combustion. It should be mentioned here that the pressure at which samples were quenched is near the slope break point of the RDX/CMDB propellant. Therefore, an important reason why the slope of apparent burning rate vs pressure curves changes at high pressure is the physical structure variation. Nothing but agglomerates are seen in the RDX composite propellant (ERDN), as shown in Fig. 9. The material strength measurement for ERDN showed very low compressive strength and plasticity. So it is deduced that powder crush is an important factor affecting high-pressure combustion of RDX composite propellant. The burning surfaces of the RDX based propellants are seen to be irregular and porous. Figure 10 shows dry burning surfaces without melted covering, in contrast to the circumstance at low pressure as illustrated by Cohen-Nir. z Cavities on the burning surfaces were observed to be deeper and larger at higher pressure. It is evident that high pressure combustion of the RDX based propellants are also affected by the variation of burning surface. Figure 10 also shows that the use of fine RDX

particle size has improved the burning surface structure of the propellant. In fact, the physical structure and mechanical properties of the RDX based propellant have close relationship with RDX particle size and its distribution in propellant. Table III lists our experimental data of mechanical properties of propellant RDU-4 and RDU-4-2. It is seen that the use of fine RDX particle size improved mechanical properties of the RDX based propellant, which in turn improved high-pressure combustion characteristics of the propellant, as illustrated in Fig. 3. Our SEM observations also revealed that defects are fewer in the RDX/NGD/CMDB propellants than in the RDX/CMDB propellants. The addition of NGD to RDX/CMDB propellant improves the physical structure of the propellant.

Conclusions The difference of decomposition of RDX and its binders, physical structure variation of propellant, burning surface change and chemical activation variation of NzO with pressure were found to be responsible for the pressure exponent shifts of the RDX based propellants over a wide range of pressure. RDX concentration is one of the most important parameters considered in determining the highpressure combustion characteristics of RDX pro-

1841

RDX BASED PROPELLANTS

"5~m' fine RDX (2.6 um), RDX/CMDB type

"5~n' coarse RDX (14.5 um), RDX/CNDBtype

'5 }am' coarse RDX (14.5 um), RDX composite type FIG. I0. Views of the quenched burning surfaces of R D X / C M D B M P a showing the absence of melt.

pellants. Increasing it decreases the burning rate at low pressures, but increases markedly the burning rate at high pressures. The DSC analysis revealed that RDX in propellant has an ignition delay at low heating rates and an autoacceleration of decomposition at high heating rates, and that the heat released from the exothermic reactions associated with RDX decomposition and deflagration are sensitive to temperature variations. The SEM observations revealed that the physical structure of the RDX based propellants can be altered by RDX content. Therefore, RDX has great effects on high-pressure decomposition and deflagration of the nitramine propellants. The effect of RDX particle size on propellant combustion is significant in high pressure range, although the burning rates appears weakly dependent upon it in low pressure range. This can be

and R D X

composite propellant at 66

attributed to the effect of RDX particle size on physical structure and mechanical properties of the propellants. Under the same processing conditions, optimizing RDX particle size distribution in propellant is helpful to improve high-pressure combustion characteristics of RDX based propellants. The incorporation of NGD into RDX/CMDB propellant helps to eliminate the pressure exponent shifts due to its improvement in physical structure and thermal behavior of the propellant. Furthermore, to control effects of possible chemical activation of N20 on the near-surface flame of propellant, and to improve mechanical performance of propellant, careful selection of propellant binders is very important.

Acknowledgment We thank Mr. An-Fang Lu who provided the strand burner experiments.

TABLE III Compressive strength and plasticity of two propellants Prop. RD U-4-2 RD U-4

Com. Stre. N/m 2

Max. Stra. %

3.796E 7 5.045E7

48.61 49.33

REFERENCES 1. BENREUVEN, M., CAVENY, L. H., VICHNEVETSKY, R. J., AND SUMMERFIELD, M.: Sixteenth Symposium (International) on Combustion, p. 1223, The Combustion Institute, 1977. 2. COHEN-NIR, E.: Eighteenth Symposium (Inter-

1842

PROPELLANTS

national) on Combustion, p. 195, The Combustion Institute, 1981. 3. KUBOTA, N,: E i g h t e e n t h Symposium (International) on Combustion, p. 187, The Combustion Institute, 1981, 4. COHEN', N. S.: Nitramine Smokeless Propellant Research, ADA054311, 1977. 5. Z~tAO, Z.-J., Lu A.-F. AND ZHAO, B.-C.: High Pressure Burning Rate Measurement of Nitramine Solid Propellants, Paper presented at the 1988 Central States Meeting of the Combustion Institute, Indianapolis, IN, May 1988.

6. TAYLOR, J. W.: Comb. Flame 6, 93 (1962). 7. BOGGS, T. L., PRICE, C. F., ZURN, D. E., DERR, R. L., AND DIBBLE, E. J.: AIAA paper 77-859, (1977). 8. BOGGS, T. L.: Fundamentals of Solid-Propellant Combustion (Kuo, K. K. AND SUMMERFIELD, M., Eds.), Chapt. 3, p. 161, American Institute of Aeronautics and Astronautics, Inc., New York, 1984. 9. FIFER, R. A.: Proc. Tenth International Shock Tube Symposium, p. 613, Shock Tube Research Society, Japan, 1975,