nitramine propellants

Twenty-Third Symposium (International) on Combustion/The Combustion Institute, 1990/pp. 1331-1337

BURNING

RATE

CATALYSIS

OF AZIDE/NITRAMINE

PROPELLANTS

N. KUBOTA AND T. SONOBE Third Research Center, Technical Research and Development Institute Japan Defense Agency Sakae 1-2-10, Tachikawa, Tokyo 190, Japan In order to improve the burning rate characteristics of azide/nitramine propellants composed of energetic aside polymer and energetic crystalline nitrarnine particles, the effect of the addition of catalysts on the combustion wave structure was studied. Glycidyl azide polymer (GAP) was used as an energetic azide polymer and cyclotetrametylene tetranitramine (HMX) was used as an energetic nitramine particles. The burning rate of GAP/HMX propellant was increased significantly by the addition of 2.0% lead citrate (PbCi) and 0.6% carbon (C). The measurements of the flame structure and the temperature distribution in the gas phase were conducted with microphotographs and microthermocouples. The results indicate that the luminous flame was produced some distance above the burning surface and the flame standoff distance was increased when the catalyst was added. However, the reaction rate to produce the luminous flame was not affected by the catalyst. Both the heat flux transferred back from the first-stage gas phase zone just above the burning surface and the heat released at the burning surface were determined to be increased by the addition of the catalyst.

Introduction Nitramines such as HMX (cyclotetramethylene tetranitramine) and RDX (cyclotrimethylene trinitramine) are energetic materials which are used as ingredients of solid propellants for rockets and guns. Nitramines are characterized with N--NO2 bond and produce heat by the oxidation reaction with the remaining hydrocarbon fragments. 1-5 Since the nitramines are crystalline particles, nitramine propellants are formed as a mixture of the nitramine particles and polymeric inert binders such as polyether or polyester type polymers. 3'6'7 However, the specific impulse of this class of nitramine propellants is low due to the mixing of inert binders. In addition,'though the decomposition products of RDX and HMX are stoichiometrically balanced and the adiabatic flame temperature is high above 3000 K, the burning,rate is reported to be low. 6 Accordingly, the burning rate of nitramine propellants becomes very low. In order to increase the specific impulse and burning rate, energetic azide polymers are used as polymeric binders of nitramine propellants. The mixture of azide polymer and nitramine particles is termed azide/nitramine propellants, which produces smokeless and reduced infrared signature combustion products because of the reduction of the concentration of HzO and COz. Azide polymers are unique energetic materials which generate heat by decomposition, s Since azide polymers contain a lit-

tle concentration of oxygen atom, the heat released by the combustion is not due to the oxidation reaction, but due to the decomposition of C--N3 bond structure to form C - - N bond structure and N2. Though the specific impulse of nitramine propellants is increased by the use of azide polymers, 9 the burning rate of azide/nitramine propellants is still low compared with that of conventional nitrate ester based propellants, such as nitrocellulose/nitroglycerin double-base propellants. It has been reported that lead compounds are effective in increasing the burning rate of double-base propellants. i0- 15 The nature of this increased burning rate is the so called "super-rate burning." Lead compounds act on the decomposition process of the nitrate esters and also increase the gas phase reaction just above the burning surface of the propellants. 13-1~ Similarly, in this study, attention is given to lead compounds which may act as catalysts on the decomposition and combustion reactions of azide/nitramine propellants.

Physicochemical Properties of Azide/Nitramine Propellants Glycidyl azide polymer, the so called "GAP," is a typical energetic azide polymer which burns very rapidly even though the adiabatic flame temperature is 1465 K at 5 MPa. The azide/nitramine propellants tested in this study consisted of 20% GAP

1331

PROPELLANTS

1332

and 80% HMX, which was termed CAP/HMX propellants. The GAP was cured with 12.0% hexamethylene diisocyanate (HMDI) and crosslinked with 3.2% trimethylolpropane (TMP) to formulate GAP binder. The HMX was finely divided crystallized 13 HMX of bimodal particle sized distribution (70% of 2 Ixm in diam and 30% of 20 }xm in diam). The physicochemical properties of GAP and HMX are shown in Table I. The burning rate catalyst evaluated in this study was the mixture of 2.0% lead citrate (PbCi), Pba(C6HsOv)2"xH20, and 0.6% carbon (C). The propellant consisted of 19.4% GAP, 78.0% HMX, 2.0% PbCi, and 0.6% C was used as a reference propellant in this study. In addition, the effect of the weight fraction of HMX (6) mixed within CAP/ HMX propellants on the burning rate catalysis was examined. In order to differentiate the mode of the action of the catalysts, the burning rates of both GAP and HMX with and without catalysts were also measured. The HMX samples were pressed pellets made of the same bimodal particle sized distribution of the HMX used to formulate GAP/HMX propellants. The size of the pellets was 8 mm in diam and 7 mm in length.

Experimental The measurements of the burning rate and flame structure were conducted with a chimney type strand burner which was pressurized with nitrogen. The burning surface and the luminous flame produced above the burning surface were recorded by a highspeed video camera through a transparent window attached on the side of the strand burner. The temperature distributions in the combustion wave were

measured with microthermocouples (5.0 ~m and 12.5 Ixm in diam of Pt-Ptl0%Rh wires) which were embedded within the propellant strands. The detailed experimental techniques used in this study are described in Ref. 16.

Experimental Results and Discussion Burning Rate: The burning rates of GAP and HMX are shown in Fig. 1. GAP burned much higher rate than HMX even though the energy contained within the unit mass of GAP was less than that of HMX. The difference of the burning rate between GAP and HMX became small in high pressure regions tested in this study. Figure 2 shows the burning rate characteristics of GAP/HMX propellants with and without catalysts. The burning rate of the propellant containing 2.0% PbCi or 0.6% C remained unchanged. However, when GAP/HMX propellant was catalyzed with 2.0% PbCi and 0.6% C, the burning rate was approximately tripled at low pressure region as shown in Fig. 2. The effect of the addition of the catalyst diminished as pressure increased. It is important to note that the burning rate was increased only when PbCi and C were mixed together within GAP/HMX propellant. In order to understand the mode of the action of the catalyst, two types of samples, GAP with 2.0% PbCi and 0.6% C and HMX with 2.0% PbCi and 0.2% C, were prepared. As shown in Fig. 3, the burning rates of both GAP and HMX remained unchanged when the catalyst was added. Furthermore, the burning rates of GAP with 2.0% PbCi or with 0.6% C, and of HMX with 2.0% PbCi, or with

TABLE I Physical and chemical properties of GAP and HMX

Chemical formula:

GAP

HMX

C3H~NaO

C4HsNaOa

Molecular structure: l i

Density ( x l O ~ kg/ma): Heat of formation (kJ/kg): Flame temperature at 5 MPa (K):

HI C I H--C--H I N3

1.30 +957 1465

O/

n n = 2O

NO2 J HzC--N--CH2 I L OzN--N N--NOz b I H2C--N--CHz I NO2 1.90 +253 3255

BURNING RATE CATALYSIS

20LI

II I I

I

!

I

I

I

20

IJ

1333

1 I I I I

~*

o2~

I,Jll

i

1

0.5

I

l

I I 2 PRESSURE, MPa

i 4

m i I 7

FiG. 1. Burning rate characteristics of GAP and HMX showing that the burning rate of GAP is higher than that of HMX even though the energy contained within the unit mass of GAP is lower than that of HMX. 0.6% C were examined. The results indicated that these catalysts act neither on the combustion process of GAP nor HMX. The addition of 0.6% C within GAP rather decreased the burning rate of GAP. The catalysts act effectively to promote the

I

! It

PbCi+0.6% C I

I

I

2 PRESSURE, MPa

I

I

4

FIG. 3. Burning rate characteristics of GAP and HMX with and without catalysts showing no effect of the addition of the catalyst on burning rate.

burning rate only when the catalyst (2.0% PbCi and 0.6% C) is added within the mixture of GAP and HMX. The burning rate ratio of the catalyzed and noncatalyzed GAP/HMX propellants is shown in Fig. 4 as a function of ~. The catalyst acted effectively in the region 0.7 < ~ < 0.9 at low pressures. The catalyst effect diminished as pressure increased. The maximum burning rate ratio was approximately 3.2 at ~ = 0.8 (p = 0.5 MPa).

3,5

I

~-

I

I

I

~ 0.5 MPa o 5.0 MPa

3.0

2.5

o

x

I

.0.6% c

o

1 I IIII 0.5

I

o2.0% PbCi

/~ 1

I

2.0

0.6

GAP/HMX PROPELLANT-~ without CATALYSTS'~ .2.0% PbCi

." : ~ ~

A0.6% C

Lx2.0% PbCi+O.6% C

-

0.3 l 0.3

i

i J Ill

0.6

I

l

I

I

2 PRESSURE, MPa

i

;

4

FIG. 2. Burning rate characteristics of catalyzed GAP/HMX propellants (~ = 0.8) with 2.0% PbCi, 0.6% C, and 2.0% PbCi + 0.6% C showing that the burning rate is increased only when the propellant is catalyzed with 2.0% PbCi + 0.6% C.

~

1.5 1.0

F-

0.5 0

I I I I 0.2 0.4 0.6 0.8 WEIGHT FRACTION OF HMX,

FIG. 4. Burning rate ratio of catalyzed and noncatalyzed GAP/HMX propellants as a function of the weight fraction of HMX showing that the catalyst acts effectively in the region 0.7 < ~ < 0.9.

PROPELLANTS

1334

5

Flame Structure:

Observations of the flame structure revealed that GAP burned without luminous flame accompanied with carbonaceous fragments. The temperature increased relatively smoothly above the burning surface and reached the maximum temperature at a little distance from the burning surface. On the other hand, HMX burned with a luminous flame which was produced just above the burning surface of the HMX pellet. As shown in Fig. 5, the luminous flame of GAP/HMX propellant (~ = 0.8) was produced at a short distance above the burning surface. The flame standoff distance was increased significantly by the addition of the catalyst. The non-luminous zone which separated the luminous flame zone and the burning surface was a preparation zone for generating the luminous flame zone. As shown in Fig. 6, the flame standoff distance decreased as pressure increased for both catalyzed and noncatalyzed GAP/ HMX propellants. However, it should be noted that the luminous flame zone stands at a further distance from the burning surface even though the burning rate is increased significantly. The reaction rate in the gas phase to produce the luminous flame can be represented by

pgUg =

[~

(ogdx

(1)

If one assume that the reaction rate is constant throughout the preparation zone, the overall reaction rate in the preparation zone is expressed as Cog =

pguJLg=

ppr /L~

(2)

The mass continuity equation at the interface of the

l

I

3

I

1

I

~a,

I

I

5

zx/

3

o/\

2

~

g o x

I

2

~

-

X

zx

~2

zx

-----

o/ I

I 0.7

0.7

c~

O

0.5

,.o

:-

0.5

~

0.3

O \

O.2

0.3 0.2

O noncatalyzed A catalyzed 0.1 .2

I 0.3

*

= ' .... 0.5 0.7 PRESSURE, MPa

I

0.1 1.5

FIG. 6. Flame standoff distance of noncatalyzed and catalyzed (2.0% PbCi + 0.6% C) GAP/HMX propellants (~ = 0.8) as a function of burning rate and pressure. gas phase/condensed phase is used to obtain Eq. (2). The reaction rates of catalyzed and noncatalyzed GAP/HMX propellants (~ = 0.8) were calculated using the experimental values shown in Fig. 6. Both reaction rates increased linearly in a log Cog versus log p plot as shown in Fig. 7. The reaction rate in the preparation zone remained relatively unchanged when the catalyst was added. In other words, there was no effect of the addition of the catalyst on the reaction in the preparation zone even though the burning rate of the propellant was increased drastically. The results indicate that the heat flux transferred back from the luminous flame zone to the burning surface has a negligible effect on the burning rate of GAP/HMX propellant. Thermal Structure:

NONCATALYZED

CATALYZED I

t I 0 mm p = 0 . 5 MPa

FIG. 5. Flame photographs of GAP/HMX propellants (~ = 0.8) with and without the catalyst (2.0% PbCi + 0.6% C).

Since the temperature profile gives considerable information about the heat transfer mechanism of the combustion waves, the thermal structure of both noncatalyzed and catalyzed GAP/HMX propellants (~ = 0.8) were studied. The measured temperature profile showed a two-stage temperature rise in the gas phase. The first steep temperature rise occurred near the burning surface, i.e., the first-stage

BURNING RATE CATALYSIS 10

I

I

I

I

I

I II

7 5 E

!

X F--

1 o

-

c~ 0 . 7 0.5 O noncatalyzed 0.30.2 0.2

A catalyzed I

!

I

I

'

0.3 0.5 0.7 PRESSURE, MPa

='' I

.5

FIG 7. Reaction rate in the preparation zone of noncatalyzed and catalyzed (2.0% PbCi + 0.6% C) GAP/HMX propellants (~ = 0.8) showing that the reaction rate remains relatively unchanged when the catalyst is added. reaction zone. This steep temperature rise flattened out some distance from the surface. The second steep temperature rise was observed at the boundary between the preparation zone and the luminous flame zone, i.e., the second-stage reaction zone. A typical set of the temperature profiles of the noncatalyzed and catalyzed GAP/HMX propellants at 0.5 MPa is shown in Fig. 8. The heat balance at the burning surface can be represented by t7 r = a~b/r

(3)

r = (dT/dx)~,g

(4)

where

= T~ - To - Qs/cp oq = Xg/pvc p

(5) (6)

In order to differentiate the site and mode of the action of the catalyst on burning rate, the values of

1335

the parameters in Eq. (3) were determined from the results of the temperature profiles. Though the scatter in the data was high because of the scatter in the qb and Ts values, the averaged burning surface temperature for both noncatalyzed and catalyzed GAP/HMX propellants was determined to be approximately the same, 695 K at 0.5 MPa. The temperature gradient was higher for the catalyzed propellant (4.3 • 10~ K/m) than that for the noncatalyzed propellant (2.3 • 106 K/m) at 0.5 MPa. Thus, the heat flux transferred back from the gas phase to the burning surface was determined to be 360 kW/m 2 for the catalyzed propellant and 190 kW/m z for the noncatalyzed propellant at 0.5 MPa. The heat released at the burning surface was obtained from the data Ts, qb, and r shown in Fig. 6. The Qs was determined to be 468 kJ/kg for the catalyzed propellant and 369 kJ/kg for the noncatalyzed propellant at 0.5 MPa. In the computations of the heat flux transferred back from the gas phase to the burning surface and the heat of reaction at the burning surface, the physical parameter values used were: Pp = 1.77 • 103 kg/m 3, cp = 1.30 kJ/ kgK, and hg = 8.4 • 10 -6 kW/mK. The results indicate that the heat flux transferred back from the gas phase to the burning surface is increased approximately 89% and the heat of reaction at the burning surface is increased 27% at 0.5 MPa by the addition of 2.0% PbCi and 0.6% C where the burning rate is increased 210%. It has been reported that the initiation of the decomposition of HMX occurs with the scission of the N - - N 0 2 bond and the primary reaction yields NOz, N20, N~, CHzO, and other products. 1-3 Since NOz reacts quite rapidly with CH20 and produces NO, CO, COz, and H2, an exothermic reaction involving NO2 and CHzO occurs on and just above the burning surface of the HMX pellet and the temperature increases rapidly. In the later stage of the gas phase reaction, NO and N20 react slowly with H2, CO, and remaining hydrocarbon fragments, and produce a luminous flame zone. No excess oxidizer fragments are remaining in the combustion products of HMX. On the other hand, the initiation of th e decomposition of GAP occurs with the scission of the N--N2 bond and the primary reaction yields N2, H2, C(s), CO, and remaining hydrocarbon fragments at the burning surface of GAP. 8 Accordingly, when GAP is mixed with HMX, the decomposition product of GAP is considered to act as a relatively nonreactive gas on the decomposition product of HMX. The observed two-stage flame structure of GAP/ HMX propellant is considered to be caused mainly by the reduction of NO2 to NO in the first-stage reaction zone and by the reduction of NO to N2 in the second-stage reaction zone, which is similar to the two-stage flame structure of double-base propellants. 13-r~,1~ Though a direct examination of the

1336

PROPELLANTS

- - - NONCATALYZED CATALYZED 1800

SECOND-STAGE REACTION ZONE

surface and in the succeeding gas phase reaction zone, i.e., first-stage reaction zone are responsible for the burning rate increase of catalyzed GAP/HMX propellants.

Nomencla~re 1400 v

l- -

,,'S

c L p Q r T u x ct

ioOO6oo Z

~w

rs

200

I

I 1

FIRST-STAGE REACTION ZONE

0

I

1

I

I

I

2

BURNING DISTANCE, x 103 m

FIG. 8. A typical set of the temperature profiles in the combustion wave of GAP/HMX propellants (6 = 0.8) with and without the catalyst (2.0% PbCi + 0.6% C) at 0.5 MPa. observed burning rate catalysis has not yet been completed, the increased burning rate was found to be caused by the increased reaction rate in the firststage reaction zone just above the burning surface and by the increased heat release at the burning surface.

Conclusions The burning rate of GAP/HMX propellant is increased by the addition of 2.0% PbCi and 0.6% C. The effect of the addition of the catalyst is dependent on the mixture ratio of GAP and HMX. The maximum catalyst effect on burning rate is obtained at approximately ~ = 0.8. The catalyst acts neither on the burning rate of GAP nor of HMX. The catalyst acts only when the catalyst was added within the mixture of GAP and HMX. The flame structure of GAP/HMX propellant consists of a two-stage reaction zone. The luminous flame stands some distance above the burning surface. The flame standoff distance is increased by the addition of the catalyst. However, the reaction rate in the preparation zone to produce the luminous flame remains relatively unchanged. The temperature gradient at the first-stage reaction zone just above the burning surface is increased when the catalyst is added. Thus, the heat flux feedback to the burning surface is increased. The heat of reaction at the burning surface is also increased. This implies that the catalytic reactions at the burning

h p to

specific heat, kJ/kgK flame standoff distance, m pressure, MPa heat of reaction, kJ/kg burning rate, m/s temperature, K gas flow velocity, m/s distance, m thermal diffusivity at the burning surface defined in Eq. (6), mZ/s thermal conductivity, W/mK density, kg/m 3 reaction rate in the first-stage reaction zone defined in Eq. (1), kg/m3s

Subscripts 0 g p s s,g

initial condition gas phase condensed phase burning surface gas phase just above the burning surface REFERENCES

1. COSGBOW,J. D. AND OWEN, A. J.: Comb. Flame 22, 13 (1973) and 22, 19 (1973). 2. BOGCS,T. L.: Fundamentals of Solid Propellant Combustion (K. K. Kuo and M. Summerfield, Eds.), Progress in Aeronautics and Astronautics, Vol. 90, p. 121, 1984. 3. FIFEB, R. A.: Fundamentals of Solid Propellant Combustion (K. K. Kuo and M. Summerfield, Eds.), Progress in Aeronautics and Astronautics, Vol. 90, p. 177, AIAA, 1984. 4. MITANI, T. AND WILLIAMS, F. A.: Twenty-first Symposium (International) on Combustion, p. 1965 The Combustion Institute 1987. 5. KUBOTA,N. AND SAr~MOTO, S. : Propellants, Explosives, Pyrotechnics 14, 6 (1989). 6. KUBOTA, N.: Eighteenth Symposium (International) on Combustion, p. 187, The Combustion Institute, 1981. 7. KUBOTA, N.: Twenty-First Symposium (International) on Combustion, p. 1943, The Combustion Institute, 1986. 8. KUBOTA, N. AND SONOBE, T.: Propellants, Explosives, Pyrotechnics 13, 172 (1988). 9. KUBOTA, N., SONOBE, T., YAMAMOTO, A., AND SHIMIZU, H.: Burning Rate Characteristics of GAP Propellants, AIAA Paper No. 88-3251,

BURNING RATE CATALYSIS

1O. 11. 12.

13. 14.

AIAA (1988), or to be published in J. of Propulsion and Power. PnECKEL, R. F.: ARS J. 31, 1286 (1961), and AIAA J. 3, 346 (1965). HEWKIN, D. J., HICKS, J. A., POWLING, J., AND WATt'S, H.: Comb. Sci. Tech. 2, 307 (1971). KUBOTA,N., OHLEMILLER,T. J., CAVENY, L. H., AND SUMMERFIELD, M: Fifteenth Symposium (International) on Combustion, p. 529, The Combustion Institute, 1975. KUBOTA,N., OHLEMILLER,T. J., CAVENY,L. H., AND SUMMEaFmLD, M: AIAA. J. 12, 1709 (1974). KUBOT^, N.: Seventeenth Symposium (International) on Combustion, p. 1435, The Combustion Institute, 1979.

1337

15. LENGELLE, G., BIZOT, A., DUTERQUE, J., AND TnUBEnT, J. F.: Fundamentals of Solid Propellant Combustion (K. K. Kuo and M. Summerfield, Eds.), Progress in Aeronautics and Astronautics, Vol. 90, p. 361, 1984. 16. KUBOTAN., OHELEMILLEn,T. J., CAVENY,L. H., AND SUMMERFIELD, M.: The Mechanism of Super-Rate Burning of Catalyzed Double Base Propellants, AMS 1087, Department of Aerospace and Mechanical Sciences, Princeton University, March 1973, or AD-763786. 17. KUBOTA, N.: Fundamentals of Solid Propellant Combustion (K. K. Kuo and M. Summerfield, Eds.), Progress in Aeronautics and Astronautics, Vol. 90, p. 1, 1984.