The burning rate of hydroxylammonium nitrate-based liquid propellants

Twenty-Second Symposium (International) on Combustion/The Combustion Institute, 1988/pp. 1817-1825

T H E B U R N I N G RATE OF H Y D R O X Y L A M M O N I U M LIQUID

NITRATE-BASED

PROPELLANTS

STEVEN R. VOSEN Combustion Research Facility Sandia National Laboratories Livermore, CA 94550

To improve the understanding of the physical processes which occur during the combustion of liquid propellants (LP), a strand burner was used to study hydroxylammonium nitratebased LP flames. From observations of the combustion of LP in such an arrangement, much has been deduced of the physical processes that occur during LP ignition and combustion. Combustion experiments were performed in which mixtures of the salts hydroxylammonium nitrate (HAN) and triethanolammonium nitrate (TEAN) in water were ignited by an electric discharge in a vessel at pressures of up to 30 MPa. The mixtures were contained in a strand burner that permitted visual observation of LP combustion at nearly constant pressure. Images of the combustion were obtained through windows in the pressure vessel by backlit photography, at a framing rate of 60 frames per second, with an exposure time of 100 microseconds. These images clearly showed the movement of a liquid-gas interface and a bright flame during LP combustion. Results of experiments conducted during turbulent combustion are: (1) the combustion of HAN-based LP occurs in two stages in which the liquid phase decomposition of HAN is followed by the decomposition of TEAN, (2) the turbulent combustion rate of LP 1846 decreases with increasing pressure. This variation of apparent burning rate with pressure, a result which is not seen in single component propellant combustion, is attributed to changes in the stability of the liquid-gas interface with pressure, and (3) a model of the combustion of HAN-based liquid propellants as it occurs in bulk has been presented, showing the reaction sequence of the components of HAN-based liquid propellants.

Introduction An understanding of the combustion of liquid propellants (LP) at high pressure is critical to the development of advanced and safer energetic materials. The overall combustion rate for pure monopropellants and explosives has been shown to be governed by the linear burning rate and by liquidgas interface instabilities which increase the surface area. The LP of interest here are more complex, being mixtures of two monopropellants (one oxygen rich, the other fuel rich) in water. Thus the LP may be thought of as a premixed propellant. Of particular interest was an understanding of the chemical and physical processes which occur at the liquidgas interface during combustion at pressures of 6 to 30 MPa. One would expect chemical reactions of the propellant constituents in the liquid phase, and also between the propellants and their decomposition products either dissolved in the liquid phase or in the gas phase. Physical effects that are present during LP combustion, such as the stability of the liquid-gas interface, are a function of the reactant and product viscosities, densities, and surface

tension. Furthermore, the presence of water is expected to have a strong effect on LP combustion, where the pressure range was one third to one and a half times the critical pressure of water. We will examine the combustion of LP 1846, which is currently under consideration for use as a gun propellant. LP 1846 is a hydroxylamiuonium nitrate (HAN) based LP, which is a mixture of HAN and triethanolammonium nitrate (TEAN) in water:

7(NH3OH)NO3 + (CH2OHCHz)3NHNO3 + 12.3H20. Little combustion data (such as flame temperature, burning velocity, species profiles, etc.) are available on this class of liquid propellants. Work by other authors relevant to understanding the combustion of HAN LP have involved the thermal decomposition of HAN, 1-3 the ignition of HAN LP, 4'5 combustion of LP droplets at atmospheric pressnre, ¢~7 and closed-chamber combustion experiments. I1 Burning rate measurements. 8'9A° The results presented here are for combustion in

1817

1818

PROPELLANTS

what is called the turbulent regime, where natural instabilities of the burning liquid cause the liquidgas interface to become highly distorted. The overall burning rate for pure propellants and explosives generally increases with pressure, r~-14 However, the burning rate of a closely related LP (Previous measurements where carried out on LP 1845S--see Table I) reveals an unusual phenomenon of HAN LP combustion not seen in other liquids: the apparent burning rate shows a marked decrease with an increase in pressure. For example, measurements of the apparent burning rate of LP 1845 in 4 mm glass tubes showed an increase in apparent burning rate with pressure up to 10 MPa, and then a steady decrease with increasing pressure up to 100 MPa.BAo To investigate this phenomenon in LP 1846 and to determine the structure and combustion characteristics of HAN LP flames, the combustion of both LP 1846 and HAN-water mixtures were studied in a strand burner at pressures between 6.7 and 30 MPa, a pressure region heretofore unexplored.

~uartz

Ceramic

ilectrode

Electrode

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view

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Electrodes

Propellant

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3

m

;:~'..:~7:~'..: 2 ~..:~7:~..: Bottom

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burner

;:~'..:~7:::..: ,..,..,..,

.~

Side view

Experimental Apparatus

FIG. 1. Schematic of strand burner.

Burner and Pressure Vessel:

To obtain information on the combustion of LP at elevated pressures, experiments were carried out on samples of TEAN-HAN-water mixtures in a strand type burner placed in a large pressure vessel. This experimental capability is unique in its ability to visually observe LP combustion. The open top strand burner (see Fig. 1) had a cross section 5 mm square, and a liquid depth of 30 mm, to produce a volume of 750 × 10-9 m 3. The burner was of a sandwich construction with the central part of the burner being machined from a ceramic (Macor, Corning Glass Works) to give shape to the liquid "strand" and to allow for the placement of ignition electrodes. The outer parts of the burner were constructed of quartz plates to allow for optical access. The burner was located in the center of a .013 m 3 pressure vessel. The vessel had electrical feedthroughs to provide the energy for ignition of the propellant, a pressure transducer for monitoring the dynamic pressure in the vessel, and four 100 mm

diameter Lexan windows for optical measurements (see Fig. 2). The backlit photographs presented here were made with a color CCD camera and a U-Matic recording system which recorded at 60 frames/second. The light source for the photographs was a modulated argon ion laser with a pulse duration of 100/xsec. Since the camera was not shuttered, exposure of luminous objects (such as the flame) was controlled by the 1/60 of a second framing rate, while the exposure of non-luminous objects (such as the liquid-gas interface) was controlled by the laser pulse duration, 100 ;xsec. The burner was filled with propellant, placed in the pressure vessel, and pressurized with an inert gas (either nitrogen or argon). Ignition of the propellant samples was accomplished by an electric discharge through tantalum wires mounted flush with the inside surface of the burner, 30 mm from the burner bottom. The electric capacitive discharge was provided by 20 ~f charged to 200 to 1100 volts.

TABLE I Propellant mixtures

Mixture LP 1845 LP 1846 9.1 M HAN

Weight percent

Concentration (krnole/m 3)

Density kg/m 3

HAN

TEAN

Water

HAN

TEAN

Water

1455 1437 1396

63.2 60.8 62.6

20.0 19.2 --

16.8 20.0 37.4

9.62 9.10 9.10

1.38 1.30 --

13.16 15.95 28.98

LIQUID PROPELLANTS

Pressure

Vessel Be~ Slop Acouslo-Optlc

Modulalor

..~

Fro. 2. Schematic of experimental set-up. Because of the large difference in volume between the pressure vessel and the burner (a ratio of-~17,000:I), combustion proceeded at nearly constant pressure, a 1% pressure rise at 30 MPa was typical. While the effects of rapidly increasing pressure during combustion were largely eliminated, the small pressure rise in the vessel was used as a qualitative measure of the heat release during combustion.

Propellants: The propellants used in this study were LP 1846 and a HAN-water mixture. LP 1846 is 60.8% HAN, 19.2% TEAN, and 20.0% water by weight (lot 503, obtained from the Ballistic Research Laboratory, Aberdeen Proving Grounds, Aberdeen, MD). The 9.1 molar HAN-water mixture used here was chosen to have the same molarity as the HAN in LP 1846. It was prepared from 13 molar HAN (also obtained from the Ballistic Research Laboratory). The HAN-water mixture had a density of 1396 kg/m a. The constituents of the mixtures used in this study are summarized in Table I.

Results LP 1846: A series of single frames taken from a backlit movie of LP 1846 combustion at 30 MPa is shown in Fig. 3a. The flame structure in Fig. 3a is typical of LP 1846 combustion at pressures of 26.7 to 30 MPa. As the combustion wave progresses through the liquid, the first step appears to be the decomposition of the liquid. This is seen as a dark line at

1819

the top of the liquid column. Above the liquid surface is a transparent region in which the motion of a white substance (probably a fog) can occasionally be seen. Above the transparent region is a highly luminous zone. The regions which are most likely to be associated with chemical reactions, the liquidgas interface and the luminous zone, oscillate (both transversely and in the direction of burning) over the entire pressure range, with the luminous zone appearing to be the most unstable. At the conclusion of the experiment less than 1 mm3 of residue is left in the burner. At pressures below 23 MPa, the combustion of LP appears to be quite different. A representative case is shown in Fig. 3b for LP 1846 at 6.7 MPa. A liquid-gas interface moves through the liquid at a rate greater than that which occurs at higher pressures and there is no visible flame. In addition, the region above the interface is transparent at higher pressures while it is opaque at lower pressures. At the conclusion of the experiment, a liquid residue remains in the burner which was determined by FTIR analysis to contain TEAN. In addition, the pressure rise in the vessel dnring the experiment was less than 34 kPa, compared to 340 kPa at 30 MPa. The greater pressure rise in conjunction with TEAN combustion is consistent with chemical equilibrium calculations that predict that approximately 20% of the total heat of reaction results from HAN decomposition, with the remainder being released during reactions between TEAN and the HAN decomposition products. The pressure dependence of the flame structure (HAN decomposition at low pressures, HAN + TEAN cmnbustion at higher pressures) is consistent witb the flame structure deduced by Klein and Sasse15 for the HAN based propellant NOS-365 and with the experiments of Zhu and Law.6 The separation of the LP flame into a region of HAN colnbustion and TEAN combustion can be seen in a series of experiments conducted at 27 MPa as shown in Fig. 4. In each of tbe experiments, the pressure, mixture eompositiun, and electrical energy input were the salne--onlv the initial height of the propellant in the burner was varied (compare first photograph in Fig. 4a and Fig. 4b). The results in Fig. 4a are similar to those of low pressure combustion (see Fig. 3b): no visible flame exists, the pressure rise in the vessel was very small, and TEAN residue was recovered from the burner and vessel. The results shown in Fig. 4b are similar to those of high pressure combustion (see Fig. 3a): a visible flame exists, the expected pressure rise in the vessel was observed, and no liquid residue was present. Whether the effect of the increased liquid depth on complete combustion was caused by an initial pressure surge in the liquid, or by providing an increased residence time necessary to initiate TEAN combustion is not yet known. It can be concluded

1820

PROPELLANTS

!

i

-RESIDUE -BOTTOM TIME 0 sec =

0.25 sec

0.50 sec

0.75 =ec

oo

I

,RESIDUE

i T I M E = 0 sec

0.10 sec

"BOTTOM

0.20 sec

0.30 sec

FIG. 3. Photographs of LP 1846 combustion at a) 30 MPa, and b) 6.7 MPa. from this set of experiments that since the decomposition of LP can occur without the decomposition of TEAN, 1) the first reaction to occur in LP 1846 combustion (and the rate limiting reaction) involves HAN decomposition, and 2) TEAN is responsible for the majority of the heat release during combustion. The importance of HAN decomposition on LP combustion is examined in the next section. The region above the liquid-gas interface is of particular interest. Presumably this region is composed of water, HAN decomposition products, and TEAN, a potentially explosive mixture that controls the energy release rate of the propellant. At higher pressures (see Fig. 3a) it is transparent regardless of the existence of a visible flame (see Fig. 4a, b). The transition from opaque to transparent is accompanied by a change in the liquid-gas interface from corrugated to smooth (within the current resolution of 100 Ixm) and thus this change in opacity may be due to the breakup of the liquid-gas interface as combustion proceeds. 16 The effect of pressure on the liquid-gas interface is apparent in Fig. 3a, b, at the fourth photograph after ignition. Instabilities at 30 MPa are on the order of the width of the burner, whereas at 6.7 MPa the instabilities are as fine as the resolution of the camera. Apparent linear burning rate data were obtained from the photographs (such as Figs. 3 and 4) by determining the displacement of the mean position of the liquid-gas interface. From these data one ean then determine an average apparent linear burning rate by a least squares fit. Figure 5 sum-

marizes the average apparent linear burning rate results for LP 1846 at four pressures. The apparent burning rate decreases as the pressure increases from 6.7 MPa to 30 MPa. To examine possible explanations for this trend it is userid to consider the definition of apparent burning rate. For experiments conducted in a burner of constant cross sectional area, the overall mass burning rate of propellant th(P) is given by fit(P) = pIS(P)Ab, where pl is the liquid density, Ab is the burner area, S(P) is the apparent burning rate and P is the pressure. From a point of view in which the chemical kinetics and the surface area effects are considered separately, the overall mass burning rate is represented as fit(P) = ptS(e)Av(P), where S(P) is the linear burning rate and An(P) is the average area of propellant undergoing decomposition during an experiment (note that A, > Ab). The two definitions of rh(P) can be combined to give: S(P) =

S(P)Av(P)/Ab,

showing the dependence of the measured apparent burning on the linear burning rate and the interface area. The overall combustion rate of LP is thus governed by a combination of LP combustion

LIQUID PROPELLANTS

TIME = 0 sec

0.10 sec

!

TIME = 0 sec

1821

0.20 sec

0.30 sec

0.20 sec

0.30 sec

I

0.10 sec

FIG. 4. Photographs of LP 1846 combustion at 27 MPa a) with a visible flame and b) without a visible [lalne.

chemistry (through S) and the stability of burning propellant surfaces (through An). Thus the observed decrease in rh(P) with increasing pressure could be due to either a decrease in S(P) through chemical effects, or by a decrease in Ap through liquid-gas interface stability effects. In principle, one could measure Ap, but this is not feasible for a highly wrinkled, moving surface. Burning liquid surfaces are known to become unstable as the burning rate increases, but the effect in pure liquid monopropellants is to increase the

surface area (through turbulence) as the pressure increases. The dependence of the linear burning rate on pressure has been measured for several pure liquid monopropellants and was found to increase with pressure.a2 14 It is possible that a decrease in burning rate in LP 1846 might occur as a result of a change in the chemistry of HAN decomposition with pressure. The burning rate may also be affected by changes in decomposition temperature due to concentration of water vapor in the products and the large heat of vaporization of water.

>.

HAN--Water:

300

U O

. *

:>

LP1846 9.1 M HAN

As noted in the previous section, experiments on LP 1846 suggest the importance of the decomposition of HAN on the apparent burning rate of the propellant. To simulate HAN decomposition as it occurs in LP 1846, a HAN-water mixture was prepared with a molarity of 9.1, giving the same fraction of HAN per unit volume as LP 1846 (see Table

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~E

m~

100

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0 0

1 '0

2'0

Pressure

3'0

4 0

(MPa)

FIG. 5. Burning rate data for LP 1846 and 9.1 molar HAN.

A series of experiments was conducted in the previously described apparatus to determine the decomposition rate of 9.1 molar HAN. The results of an experiment at 30 MPa and at 6.7 MPa are shown in Figs. 6a and 6b, respectively. The decomposition of 9.1 molar HAN is seen to progress at a more rapid rate than the combustion of LP

1822

PROPELLANTS

1846 (see Figs. 3a, b). The interface is seen to be corrugated and the region above the interface opaque; this is similar to the combustion of LP 1846 at 6.7 MPa (see Fig. 3b). At the conclusion of the experiment, a greater quantity of liquid, mainly water, remained in the burner than for LP combustion at the same pressure. The depth of the liquid residue for the low pressure run was 5 mm, and the depth of residue from the high pressure run was slightly less than the initial depth of the HAN-water mixture before ignition. The variation of the apparent overall burning rate with pressure is shown in Fig. 5. At the lower pressures (less than 10 MPa) the burning rates of LP 1846 and of 9.1 molar HAN are the same, while at 30 MPa the rate for 9.1 molar HAN is approximately fifty percent greater than that of the propellant. What is important to note is that the trend toward lower apparent burning rates with increased pressure exhibited by LP 1846 is also present in HAN decomposition. While the chemistry and physical properties of the two mixtures are obviously not identical, these experiments do demonstrate the importance of HAN in determining the overall combustion rate of HAN-based liquid propellants. Equilibrium calculations17 for the decomposition of 9.1 molar HAN show the importance of water on the decomposition of HAN, and give an indication as to explanations for the apparent negative pressure exponent of the burning rate. The heat of formation of HAN (taken to be that of aqueous am-

monium nitrate, 87.6 kcal/mol) and the variation of decomposition temperature with pressure were calculated assuming thermodynamic equilibrium. Although the above described experiments are not equilibrium situations, equilibrium calculations serve to indicate trends in the temperature and density of the decomposition products. The decomposition of 9.1 molar HAN was assumed to occur at constant pressure and enthalpy to form any or all of the following: HzO(g), H20(/), H, HO, H202, HO2, H2, N, NO, NO2, NO3, NeO, NH3, N2, O, 02 and HNO3. The mixture was assumed to be ideal. Nonideal gas effects for the water vapor were included but solubility of the gases in liquid water were neglected. For a pressure range of .01 to 40 MPa, with an initial temperature Tu = 298 K, only H20(g), H20(/), N2 and 02 are predicted in appreciable quantities, with NOz at ppm levels at the higher pressures. Given the initial temperature of 298 K, the decomposition temperature was found to increase from 360 K at .1 MPa to 620 K at 40 MPa (see Fig. 7). The reason for the increase in temperature with pressure is due to the presence of increased concentrations of liquid water in the products at higher pressure. The density ratio across the HAN decomposition wave (Pu/Pd = initial density/deeomposition density) is predicted to decrease inversely with the pressure. Predicting the effect of pressure on linear burning rate of the LP is not straightforward. If the principal reactions occur in the liquid phase, the effect of pressure will be minimal, and the in-

'TOP

RESIDUE

i TIME = 0 sec

0.10 sec

0.20 sec

0.30 sec

BOTTOM

oo

-

-TOP

-RESIDUE -BOTTOM

TIME = 0 sec

0 . 0 5 sec

0 . 1 0 sec

0 . 1 5 sec

FIG. 6. Photographs of 9.1 molar HAN combustion at a) 30 MPa, and b) 6.7 MPa.

oo

LIQUID PROPELLANTS

I,,-

2 . 0 2 " 2 ~

1000

1.8

'100

1.6

~_o-, "10

1.4

',2T .1

1 10 Pressure (MPa)

100

FIG. 7. Equilibrium calculation results fbr 9.1 molar HAN decomposition. T/T,, is the decomposition temperature divided by the initial temperature (298 K), PdPg is the density ratio across the decomposition region (the liquid density divided by the gas density). creased temperature with pressure would increase burning rates. However, if gas phase reactions are important, a change in the reaction mechanism with increased pressure could affect the temperature dependence of the linear burning rate. The effect of the expansion ratio on the burning rate is unequivocal; the lower expansion ratio at higher pressure will have a stabilizing effect on the propagating interface, resulting in a decrease of the burning area, Ap(P), with pressure as predicted by theory.16,1s This effect is highlighted by plotting the apparent mass burning rate as a function of the calculated density ratio across the flame (see Fig. 8). The decrease in the overall burning rate as the pressure increases is thus due to changes in the stability of the moving liquid-gas interface. Based on the results of HAN-water decomposition and LP 1846 combustion, the flame structure

~' 400

Conclusions Based on photographs of the turbulent combustion of LP and tlAN-water mixtures, samples of residue in the combustion chamber, and the pressure in the chamber it was found that: 1) As suggested from preliminary experiments using LP 1845, there is a decrease in the average volumetric burning rate of LP 1846 with increased pressure from 6.7 to 30 MPa. The variation of apparent burning rate with pressure, a result which is not seen in single component propellant combustion, is attributed to changes in the stability of the liquid-gas interlace with pressure; 2) The combustion of HAN-based LP occurs in two stages in which the liquid phase decomposition

l

l Products

TEAN Decomposition HANProducts+ TEAN HAN Decomposition (Interface Position)

¢D

ao~ 200 C

|

E

!

m lOO

Liquid Propellant

u}

0

shown schematically in Fig. 9 is proposed. The "flame" portion of HAN-based LP combustion is composed of three regions. In the first, liquid phase reactions involving HAN occur near the liquid-gas interface. Secondly, above the interface, a mixture of gaseous HAN decomposition products, TEAN, and water exist. Depending on the temperature and pressure on the gas side, this mixture could exist in any state from an aqueous foam to a fine mist. Preliminary evidence indicates that this region is two-phase at lower pressures, becoming either a finer mist or possibly single phase at higher pressures. It is not clear at this time whether or not chemical reactions occur in this region• In the final region, which occurs only at higher pressures, TEAN reacts to produce a luminous flame and releases most of the chemical energy of the propellant.

Products

300

m

1823

0

2

4

6

8 10 12 14 PI/Pg

FIG. 8. Burning rate of 9.1 molar HAN vs. the density ratio across the flame (pt/Pg).

I

l

l

Liquid Propellant

FIG. 9. Model of LP 1846 combustion.

1824

PROPELLANTS

of klAN is followed by the decomposition of TEAN. Thus the decomposition of the oxidizing component of the liquid propellant (HAN) governs the overall combustion rate of HAN-based liquid propellants; 3) A model of the combustion of HAN-based liquid propellants as it occurs in bulk has been presented, showing the reaction sequence of the components of HAN-based liquid propellants.

8.

9.

10.

Acknowledgment The author gratefully acknowledges the many helpful discussions with Dr, N. Klein of the Ballistic Research Laboratories, and the assistance of Mr. S. Tucker in conducting the experiments. The work was supported by a memorandum of understanding between the Department of Energy and the Department of Army.

11.

12. 13.

14.

REFERENCES 1. VAN DIJK, C. A. AND PRIEST, R. G.: Comb. Flame 57, 15 (1984). 2. CRONIN, J. T. AND BRILL, T. B.: J. Chem. Phys. 90, 178 (1986). 3. KAUEMAN,J. J. ANO KOSKI, W. S.: "Study of the Hydroxylammonium Nitrate--Isopropyl Ammonium Nitrate Reaction," Technical Report, TID AD-A083646, April 1, 1980. 4. CATTOLICA, R. J. AND KLEIN, N.: Comb. Sci. Tech. 56, no. 4-6, 139 (1987). 5. KLEIN, N., CARLETON, F. B. AND WEINBERG, F. J.: Nineteenth JANNAF Combustion Meeting, Vol. 1, 505, 1982. 6. ZHU, D. L. AND LAW, C. K.: Comb. Flame 70, 333 0987). 7. BEYER, R. A.: "'Atmospheric Pressure Studies of Liquid Propellant Drops in Hot Flows," U.S.

15.

16.

17.

18.

Army Technical Report BRL-TR-2768, October 1986. McBRATNEV, W. F.: " B u r n i n g Rate Data, LPG1845," U.S. Army Memorandum Report ARBRL-MR-03128, August 1981. COMER, R. H.: "Ignition and Combustion of Liquid Monopropellants at High Pressure,'" Sixteenth Symposium (International) on Combustion, p.1211, 1976. CHIU, D. S., BanctJTl, A. J., HuI, P. AND BOT TEl, L,: Twenty-fourth JANNAF Combustion Meeting, CPI Publication 476, 2, 79 (1987). TRAVlS, K. E., KNAerON, J. D . , STOR~E, I. C. AND MORRISON, W. F.: "Closed Chamber Tests on Liquid Propellants," U.S. Army Technical Report BRL-TR-2755, September 1986. WHITrAKER, A. G., DONOVAN, T. M. AND WILLIAMS, H.: J. Chem. Phys. 62, 908 (1958). ANDREEV, K. K., GLAZKOVA,A. P. AND TERESHKIN, I. A.: Russian J. of Phys. Chem. 35, no. 2, 204 0961). BELYAEV, A. F., BOBOLEV, V. K., KOROTKOV, A. I., SULIMOV, A. A. AND CHUIKO, S. V.: "'Transition from Deflagration to Detonation in Condensed Phases," Translated from the Russian by R. Kondor, NTIS Number TT 74-50028, Moscow 1973. KLEIN, N. AND SASSE, R. A.: "'Ignition Studies of Aqueous M o n o p r o p e l l a n t s , " U.S. Army Technical Report ARBRL-TR-02232, April 1980. ARMSTRONG, R. C. AND VOSEN, S. R.: Twentyfourth J A N N A F Combustion Meeting, CPI Publication 476, 2, 11 (1987). REYNOLDS, W. C.; "Implementation in the Interactive Program STANJAN,'" Department of Mechanical Engineering, Stanford University, January 1986. ZELDOVICH, YA. B., BARENBLATT,G. I., LIBROVlCH, V. B. AND MAKHVILADZE, G. M.: "'The Mathematical Theory of Combustion and Explosives," Consultants Bureau, New York, 1985.

COMMENTS R. J. Priem, Priem Consultants, USA. Why don't you report your burning rate data as a burning velocity normal to the surface which can be obtained by dividing the bulk rate of burning by a surface area. Author's Reply. High speed photography of the burning liquid surface shows that the surface is not planar, or even smooth, but consists of instabilities of at least 50 Ixm and possibly smaller. It is possible that the smallest scale instabilities cannot be resolved photographically, and thus it is not possible

to determine the motion of the surface normal to itself.

W. Waesche, Atlantic Research Corp., USA. It is known that Han reacts rapidly below the surface of composite solid propellants; it is likely that han decomposition is controlling here, as well. How does the han decomposition temperature compare with propellant meniscus/surface temperatures at different pressures?

LIQUID PROPELLANTS

Author's Reply. O n e of the m a i n conclusions of this work was that H A N d e c o m p o s i t i o n controls t h e b u r n i n g of t h e propellant. An e x p e r i m e n t a l d e t e r ruination of t h e t e m p e r a t u r e of t h e liquid surface is currently being undertaken.

T. A, Litzinger, Pennsylvania State Univ., USA. H a v e you tried e x p e r i m e n t s with larger d i a m e t e r s ? If so, does t h e b u r n i n g rate b e c o m e i n d e p e n d e n t of p r e s s u r e for larger d i a m e t e r s ? W o u l d this value be a m o r e m e a n i n g f u l m e a s u r e o f b u r n i n g rate?

Author's Reply. E x p e r i m e n t s h a v e b e e n p e r f o r m e d in b u r n e r s r a n g i n g in size from 1.0 × 1.0 m m to 7.0 × 7.0 ram. For b u r n e r s larger t h a n 2

1825

m m ~ in area, t h e b u r n i n g rate shows an increase o[" b u r n i n g rate with d i a m e t e r , p r e s u m a b l y indicative of an increased surface area with an increase in b u r n e r size. For larger b u r n e r s , t h e r e is a possibility that the b u r n i n g rate will b e c o m e p r e s s u r e i n d e p e n d e n t if cellular b u r n i n g occurs. T h e r e is e v i d e n c e of c u s p s f o r m i n g on t h e liquid surface at sizes larger than 5.0 ram, so it m a y be possible to obtain b u r n e r size i n d e p e n d e n t m e a s u r e m e u t s ~ r b u r n e r s larger than 10 cm or so. C u r r e n t salbty considerations prohibit m e from d e a l i n g with t h e quantities of propellant n e c e s s a r y to c o n d u c t t h e s e experiments. While b u r n e r size i n d e p e n d e n t m e a s u r e m e n t s may b e m o r e m e a n i n g f u l from a f i m d a m e n t a l s t a n d point, t h e d y n a m i c s o f propellant b u r n i n g on the smaller l e n g t h scale is i m p o r t a n t from a practical standpoint.