chemical mechanism for secondary flash suppression

Eighteenth Symposium (International) on Combustion The Combustion Institute, 1981

C H E M I C A L M E C H A N I S M F O R S E C O N D A R Y F L A S H S U P P R E S S I O N

ARTHUR COHEN LEON DECKER

USA Ballistic Research Laboratory, ARRADCOM, Aberdeen Proving Ground, MD 21005

Chemical suppression of secondary muzzle flash is expected to depend on reactions which inhibit H 2 / Oz explosions. Shock tube experiments to determine the effect o f potassium hydroxide (KOH), potassium sulphate (K2SO4) , and potassium nitrate (KNO~) on H2/O ~ explosion delays have been performed at temperatures between 900-1500 K and at pressures near 250 KPa. Inhibition has been observed in experiments with KOH aerosols but has not been observed in experiments with K2SO 4 and KNO~ particles. The results of a light scattering experiment indicate that appreciable vaporization of KOH particles takes place prior to explosion. The inhibition is attributed to homogeneous gas phase reactions and the results are consistent with a mechanism in which a radical recombination reaction is catalyzed by KOH.

Introduction

Suppression of secondary flash, which is responsible for most of the radiation from gun plumes, continues to be a problem for charge designersJ It is due to combustion of shock heated muzzle gas after mixing with ambient air. z Both mechanical and chemical methods are used for flash suppression. Mechanical methods affect the thermodynamic states of the muzzle gas by either preventing formation or diminishing the strength of the shock struc- ture responsible for increasing both the pressure and temperature of this gas. Chemical methods are presumed 3'4 to affect the combustion mechanism by introducing species which slow down reaction rates making ignition more difficult. Alkali metal salts are known to be effective flash suppressants. Potassium nitrate (KNO3) and potassium sulphate (K~SO4) are used routinely in propellant formulations for this purpose. Empirical methods exist for helping charge designers select the quantity of salt necessary for flash suppression under different fir- ing conditions, but are not always successful. ~ These methods are based on the results of shock tube experiments in which the ignition limits of muzzle gas/air mixtures were determined. However, the effectiveness of flash suppressants was determined from observations of gun firings using propellant formulations with varying amounts of alkali salts, so that details of the inhibition mechanism could only be inferred. Equilibrium thermodynamic calculations indicate that the combustibles in propel-

lant gases consist primarily of CO and H2 and that a large fraction of the potassium compounds appear as potassium hydroxide (KOH). CO is not expected to greatly affect H~ combustion rates. 5 Therefore, if the ratio of the concentration of CO to H 2 is not too large, then it is expected that the ignition limits of the muzzle gas/air mixtures will depend on the H J O 2 chain branching mechanism.

Shock tube experiments have shown o'r'8 that ini- tiation of explosions and the subsequent formation of detonation waves in this system can occur in two different modes depending on initial state of the mixture. At temperatures (T) above 1100 K and reactant pressures (P) less than an atmosphere, "strong" ignition is observed. The delays (t) are generally less than 0.5 ms and their activation energy (E) ~ 18 kcal/mole. Both agree quite well with predictions based on the explosion mechanism, s'r Under these conditions, t depends primarily on the rate of the chain branching reaction:

H + O~ = OH + O. (1)

Therefore, it is to be expected that reactions which decrease radical concentrations will inhibit explosions. At lower temperatures and higher pressures, "mild" ignition is observed. Both t and its temperature dependence increase, in agreement with predictions of the explosion mechanism. This is a result of a decrease in active radical concentrations due to the recombination reaction:

225

226 PROPELLANT COMBUSTION

H + 02 + M = HO 2 + M (2)

becoming competitive with reaction (1). Under these conditions explosions still occur as long as the thermodynamic state of this system is within the ignition limits. Since the temperature dependence of t is greatly increased under mild ignition conditions (E - 50 kcal /mole at T - 1000 K), attempts to inhibit explosions by decreasing radical concentrations through recombination reactions, which are highly exothermic, may not be successful. Therefore, it is possible that the effectiveness of a chemical flash suppressant will depend on conditions in the muzzle blast field. Recent measurements 2 show that secondary flash from a 7.6 mm caliber rifle is due to ignition near the muzzle gas/air interface where the pressure can be several atmo- spheres and the temperature near 1800 K. This suggests that ignition takes place under "strong" conditions. At present, the measurements of species concentrations in the blast field, which are required to predict ignition, have not been made. This represents a serious limitation on modeling ~ efforts to determine the effectiveness of flash suppressants.

There is little direct evidence and much specula- tion on the manner in which alkali metal salts act as flash suppressants. They are known to inhibit many hydrocarbon flames io.11.12 and are assumed to act similarly in the secondary flash region." 34. However, the flame inhibition mechanism is not completely understood. It appears that both physical and chemical processes are involved. 11 Chemical inhibition is attributed to reactions which increase rates of radical recombination reactions thus lower- ing radical concentrations and thereby reducing chain branching rates. Conflicting views exist con- cerning the identity and state of the species responsible for flame inhibitors. Van Tigglen H favors a heterogenous mechanism in which catalysis of the recombination reactions occur on the salt surface while others 1~ favor homogeneous gas phase reactions involving either the metal atom or its hydroxide. Experiments with alkali hydroxide vapors have not been reported previously due most likely to difficulties in handling these compounds. However, KOH is suspected of being responsible for inhibitions since some experiments indicate that potassium alone is not an effective inhibitor. 13

Quantitative effects of alkali salts on propellants flames have not been reported. McHale ~4 measured emission intensities from rocket motor exhaust plumes and found a good correlation between flash suppression and calculated equilibrium KOH vapor concentration. Inhibition was attributed to the fol- lowing gas phase reactions:

H + KOH = K + H 2 0 (3)

OH + KOH = KO + H20. (4)

Jensen 15 attributes inhibitions to a catalytic mechanism in which reaction (4) is replaced by a three body recombination reaction,

K + O H + M = K O H + M, (5)

although Friedman 13 suggests that the expected rate constant will be too small for this reaction to be important. In view of the highly speculative nature of the role of potassium salts as inhibitors, it appeared worthwhile to determine the effects o1 potassium hydroxide as well as potassium sulphate and potassium nitrate salts on the explosive reaction in the H 2 / O 2 system.

Experimental

A reflected shock technique was used to measure explosion delays in mixtures of H 2 and 02 diluted in Ar. The technique has been described previously. 7 It requires that the global activation energy of the explosive reaction be sufficiently large so that a range of shock strengths exists in which reaction occurs only in the reflected shock region. The time for explosions to occur in this high temperature, stag- nant gas is determined from pressure measurements by a piezoelectric transducer flush mounted in the end wall of the shock tube. or from emission measurements by a photomultiplier tube (PMT) located near the end wall. The time constant of the PMT electronic circuit is approximately 1 Ixs. The temperature (T) and pressure (P) of the reflected shock gas are obtained from solution of the 1-D gas phase conservation equations using the incident shock velocity and the initial shock tube conditions. The velocity is determined by electronic counters trig- gered by shock arrival at pressure transducers located within 2 m of the end wall. The transducers also started oscilloscopes which recorded the end wall pressure and emission signals.

Aerosol Experiments

Experiments with KOH were conducted in a 76 mm I.D. glass shock tube by forming an aerosol from the vapor. Figure 1 is a schematic of the test section and the technique used to generate the aerosol. The PMT was an RCA 1P28 (S-5 response) which was located adjacent to a 1 mm slit taped to the shock tube, 11 mm from the end wall.

The combination of high boiling point (1600 K) and reactivity makes working with KOH difficult. In order to get sufficient KOH into the shock tube and to avoid reactions at the high temperatures required for vaporization, it was necessary to line the heated portion of the sillimanite furnace tube (356 mm long, 25 mm I.D.) with silver foil. KOH

CHEMICAL MECHANISM FOR SECONDARY FLASH SUPPRESSION 227

FURNACE PRESSURE [ F ' ' ' J L"-"3

o' L I PMT SILLIMANITE / I I

(silver foil lined) / RESERVOIR

11 mm SILVER BOAT WlKOH

Fn(;. 1. Schematic of the glass shock tube test section and aerosol generating equ ipmen t

(certified ACS, Fisher Science Co.), in a silver boat, was placed at the center of the tube which fit snuggly in a muffle furnace (203 mm long, 35 mm I.D.) and was heated to 1018 K (vapor pressure of KOH = .277 KPa). The furnace tube was connected to a hole in the shock tube end wall and to the reservoir with Cu and tygon tubing. The whole system could be pumped down to 5 Pa. Due to combust ion in the furnace, premixed H 2 / O 2 mixtures could not be used to form aerosols. The procedure used to obtain the test mixture consisted of first passing 0 2 / A r through the furnace, evacuating the fill line and then proceeding with H 2 from the reservoir. The total flow time was approximately one minute. The final shock tube pressure was 10.7 KPa and the composition, assuming complete mixing, was .2 H z, .1 0 2, .7 Ar. This procedure was carefully followed for all experiments. Five minutes were allowed for diffusion mixing before starting the experiment. After the first minute of the waiting period, the mixture became cloudy and condensat ion appeared on the glass surface. This is at tr ibuted to coagulation and settling of the larger aerosol particles. No further visible changes were evident. Estimates 16 of mass transfer rates from KOH in the furnace using the temperature dependence of vapor pressure given by Dubois, ~7 and a diffusion coeffi- cient of 4 cm ~/s indicate that the KOH mole fraction in the shock tube <1 • 10 -3, The amount of hydroxide which condensed overnight on the shock tube wall was determined by carefully wiping the wall with wet towels. Titration of the solution obtained by rinsing the towels with distil led water gave a value of 3.5 • 10-5 for the number of moles hydroxide in the tube and indicates that the average KOH mole fraction was .6 X 10 -3 (shock tube vol- ume ~- 14.4 l). For such small values, the effect of vaporization and the difference between gas phase and two-phase shock parameter calculations can be neglected. '~ Estimates of aerosol particle size were made from consideration of settling rates. Calcula- tions based on Stokes equation, ~9 indicate that KOH particles with diameters (D) >2 ttm will settle out during the five minute waiting period.

Figure 2 contains end wall pressure and PMT traces from two experiments with approximately

5V

1V

P ~ 248 KPa T ~ I142K V = .803 mm/ps

I 688 us I

V - - 959 us

i:J

5V

IV

P ~, 261 KPa T ~, I177K V = .818 mm/~s

m in _7"-_~

FIG. 2. Two oscilloscope records containing the end wall pressure (lower) trace and PMT (upper) trace. Gain settings (volts/cm) are shown to the left of the traces. Records (a) and (b) correspond to absence and presence, respectively, of KOH in the furnace. V is the extrapolated end wall incident shock velocity. T and P are calculated from conservation equations based on the nominal mixture composition (.2 H z, .I 0 2 , .7 Ar) and V. The initial jump in pressure indicates the arrival of the shock at the end wall. In (a), the second jump in pressure and the emission detected by the PMT are due to an explosion and the ensuing detonation near the end wall. t (explosion delay) is the time between pressure jumps (959 IJ, s). The delay to initial emission (688 Ixs) < t and is believed due to onset of exothermic reactions. The maximum emission coincides with explosion. Inhibi t ion by KOH is evident in (b) by the absence of both the second pressure jump and emission.

equal shock velocities, showing the effect of KOH on H 2 / O 2 explosion delays under "mi ld" ignition conditions.

Figure 3 contains end wall pressure and PMT traces from experiments in the glass shock tube with

228

2V

1V

PROPELLANT COMBUSTION

P ~. 356 KPa 62 ps T ~, 1408K

- ~ ~ V = . 9 0 8 mm/us

approximately constant shock velocities showing the effect of KOH in H J O 2 explosion delays under "'strong" ignition conditions.

The results of the explosion delay measurements, in the nominal (diffusion mixed) .2 H 2, .1 0 2, .7 Ar mixture, with and without KOH in the oven are shown in Fig. 4. Also shown for comparison, are the results of experiments with the oven off to prevent combustion, in which the gases were premixed to give a known stoichiometric mixture of .2 H 2, .1 0 2, .7 Ar.

These results show that the presence of KOH increases delays at high temperatures and prevents

2V

IV

P ~, 377 KPa 122 ~s T "~ 1457K

V = .927 m / u s

.05V

1V

P ~, 383 KPa i 500 us g T ,, 1472K

�9 V = .933 ram/us

FIG. 3. Three oscilloscope records containing the end wall pressure (lower) and PMT (upper) trace. Gain settings (volt/cm) are shown to the left of the traces. Record (a) was taken without KOH in the furnace. Records (b) and (c) were taken with KOH in the furnace. V, T, and P correspond to the velocity, temperature, and pressure as explained previously in Fig. 2. Inhibition under these conditions is evident by the increase in explosion delays (despite somewhat higher temperatures) for experiments with KOH. In (c), the pressure trace indicates that after shock reflection, the initial explosion generated a combustion wave (instead of a detonation wave) leading to a slow rise in pressure.

i

moo•

1000 ' • 8 0 0 -

+

o ~ 7

8o 2 . a

60 1 •

lo r .6

i i

i I J

i / o o~ C>; /

i

/ l I

/ /

1 I

7 / t i

I i

i I i i

r

i l

i t

/ tVAP

# t r

, 3 , D : 4 , m

, ,'D = 2/Win i

/ /

s rEX P

I

I

/ i

I

t / MIX OVEN KOH WAVE

j DIF ON + None o DIF ON + C

I / ~ DIF ON "- D

/ O DIF ON O

/ &. PRE OFF - O

/

/

.7 .8 .0

I03,,T ~K ' I I

Ftc. 4. Plot of log t vs 103/T for experiments with

h , I I

1.0 I.I 1.2

(+) and without ( - ) KOH and with the oven ON and OFF. DIF = diffusion mixed gas. PRE = premixed gas. Also noted is the type of wave (C = combustion, D = detonation) initiated by the explosion. The line labeled t~,p is obtained from calculated explosion delays using the kinetics and explosion criterioh given by Voevodski. 6 The lines labeled t~a p are estimated times, based on unsteady (Fick's law) di f fus ionY for KOH particles with diameter D to vaporize completely. In order to show the results in the absence of wave formation, a delay of 4.5 ms which is the observation time, was assigned to these data. It appears that presence of KOH at high temperatures increases delay and at low temperatures prevents explosion.

CHEMICAL MECHANISM FOR SECONDARY FLASH SUPPRESSION 29.9

explosions at low temperatures. For the same T (i.e., incident shock velocity) delays with the diffusion mixed gases without KOH are much greater than those with premixed gases. Therefore, the former are not stoichiometric but near the end wall are probably fuel-rich. For these experiments, T represents maximum values.

The calculation of t~a p indicates that for D < 4 Ixm there is sufficient time for KOH particles in the reflected shock region to vaporize completely before explosions occur.

In order to determine the time required for evaporation of KOH particles, light scattering experiments were undertaken. Figure 5 is a schematic of the shock tube test section for these experiments. The 441 nm line from a 10 mW continuous HeoCd laser (Liconix Model 401) was passed through the center of the shock tube, 49 mm from the end wall. Scattered light was collected perpendicular to the beam by a lens (focal length - 5 cm, diameter - 4 cm) and focussed onto a slit in front of a PMT (Hammamatsu R136, S-20 response) through an interference filter (Optics Technology filter ~436). Emission was restricted to an approximately 40 mm wide area centered about the beam by means of masks. The result of the initial experiment in which the aerosol was generated using a premixed gas (.2 H2, .1 N 2, .7 Ar) is shown in Fig. 6. The flow time through the furnace to reach the initial pressure of 10.7 KPa was approximately one minute. A five minute waiting period was employed before starting the experiment to achieve a KOH particle distribution similar to that obtained with diffusion mixed gases.

The light scattering trace shows that vaporization takes place behind the incident shock where the temperature is 684 K. The long rise times observed during shock compressions (~35 and 60 Ixs for the incident and reflected shocks, respectively) indicates that some multiple scattering occurs. Although, at present, the reflected shock temperatures in the diffusion mixed gas experiments cannot be determined precisely, it is fairly certain

~( 49 i,ml ~W

i(40 ~n )~ PRESSURE - - SENSOR

22 ~ LASER BEAM (I ~0

[ ~ - LENS

70 ~IL - - FILTER - - - SLIT

FIG. 5. Schematic of the test section for light scattering experiments.

P ", 248 KPa T -, 1146K V = .802 mm/~s

.05V

200 us

F1G. 6. Oscilloscope record containing the end wall pressure (lower) trace and light scattering (upper) trace. The aerosol was generated using a premixed gas (.2 H ~,. 1 N 2,'7 Ar). The initial pressure = 10.7 KPa. The dashed line corresponds to the approximate DC signal level before incident shock arrival. The two maxima in the light scattering trace are the result of compression and subsequent vaporization behind the incident (T = 681 K, P = 67 KPa) and reflected (T = 1146 K, P = 248 KPa) shocks.

that they are greater than 684 K. For those experiments in which inhibition was observed, KOH vapor was present prior to explosion. Assuming Rayleigh scattering and a uniform particle distribution, calculations of particle concentration from scattering intensity indicate that approximately 12% of the particle mass vaporized during the observation time in the incident shock region (~140 p.s) and approximately 50% vaporized within ~320 I~s after reflected shock passage. Although these results do not rule out a heterogeneous mechanism they do show that gas phase reactions Of KOH can be responsible for inhibition of the H 2 / 0 2 explosions.

Dusty Gas Experiments

Experiments with K2SO a, KNO 3 and charcoal particles were conducted in a 100 mm I.D. stainless steel shock tube by producing a dusty gas mixture near the end wall. Figure 7 is a schematic of the shock tube test section.

The solids were dispersed into a region near the end wall by an electric primer which drives a piston into an air filled cylinder. At the end of the cylinder flush mounted in the end wall is a plug with a hole which is filled with ~100 mg of the powder. The shock waves and ensuing flow generated by the moving piston, propel the powder into the shock tube. Framing camera photographs of the operation in a 3" I.D. glass tube show that a fairly uniform powder distribution is obtained over a 200 mm distance from the end wall within 0.7 ms after activating the primer. Most of the particles remain

230 PROPELLANT COMBUSTION

PRESSURE DISPERSER ]~OOmm I \ \ rPRIMER SCHLIEREN

PMT

K 200mm >K57 mm >1 Fu;. 7. Schematic of stainless steel shock tube

showing the disperser used for generating a dusty gas and the location of Schlieren windows and PMT.

suspended for almost 10 ms so that the average concentration is ~ .1 g / c m 3.

Premixed gases (.2 H 2, . 1 0 z, .7 Ar) were used in these experiments. The initial pressure was 10.7 KPa which resulted in a reflected shock pressure near 250 KPa. K2SO4 was obtained from military flash reducer bags. KNO 3 (reagent grade) and activated charcoal were obtained from Baker Chemical and Merck and Co., respectively. The salts were passed through a ~400 sieve (37 Ixm). A ~ 100 sieve (74 Ixm) was used for charcoal in order to get sufficient quantities for the experiments. For shock tube operation, the primer was activated 1-2 ms before the expected arrival of the shock at the end wall. Since for these experiments t < 1 ms, this insured that the induction zone reactions and explosion occurred in the dusty gas. Schlieren photographs help clarify the gas dynamic processes which occurred in the test section and show that weak blast waves generated by the disperser precede the particles into the shock tube. These disturb the initial mixture conditions so that it is difficult to calculate, with confidence, the reflected shock conditions from the measured incident shock velocity. Also, the presence of particles in such large concentrations would require two-phase shock calculations. In order to avoid these difficult calculations, experiments with charcoal (which is not an effective flash suppressant) and without a primer (blank) were performed to determine the effect of the disperser operation on t and to serve as a standard against which behavior with potassium salts could be compared. In order to present the results and make comparisons with data taken without particles, reflected shock temperatures have been calculated from the measured incident shock velocities in the usual manner (neglecting effects of blast waves and the presence of particles). Figure 8 contains the results of the experiments with K2SO4, KNO3, charcoal and without particles (blank). The agreement of blank data with the calculations is not particularly good. The charcoal data show somewhat better agreement but this may be fortuitous. The difference between blank and charcoal data is not great which suggests that either the effects of the

1 i

1000 -- 800; / 600 ~" o {>D. / aO0 • 0 ; / >

Z I > o o : -

"nO ~ ~0 7" o

6; I / 0 BLANK

T / ~ ~zso,

.8 .9 1.0 103T.

o / o

1.I

Flo. 8. Plot of log t vs 10'~/T for explosions in .2 H2,. 1 O 2, .7 Ar mixture in the presence of K 2 SO 4, KNO 3, and charcoal particles and without particles (blank). The line represents calculated values for the blank data (line t,,xp in Fig. 4).

blast waves and the presence of particles on gas temperatures are not great or nullify each other. Comparison of the K 2 SO 4 data with both charcoal and blank data indicates that inhibition does not occur. A similar comparison of data with KNOz which decomposes at T = 673 K (103/T = 1.5) indicates that some increase in t may occur at higher temperatures. However, it is believed that this increase is within the scatter of the data and that the presence of potassium salts does not inhibit H ~ / O z explosions.

Conclusion

The presence of the solids K~SO4 and KN03 at concentrations near. 1 g / cm ~ does not inhibit explosions in stoichiometric H ~ / O z mixtures under both "mild" and "strong" ignition conditions. The presence of KOH in fuel rich mixtures does inhibit explosions under both conditions. The results of a light scattering experiment indicate that appreciable vaporization of KOH particles takes place prior to explosions. It is believed that inhibition occurs through homogeneous gas phase reactions. The KOH

CHEMICAL MECHANISM FOR SECONDARY FLASH SUPPRESSION 231

mole fraction appears to be <10 -3 which suggests that inhibition involves a catalytic mechanism. In order to confirm these conclusions and to determine the details of the inhibition mechanism, experiments in which simultaneous measurements of light scattering and absorption by K and OH species will be made in both reactive and unreactive mixtures. The relation of these results to muzzle gas/air explosions will be determined by measurements in H2/Oz mixtures containing CO and N2.

REFERENCES

1. MAY, I. W. AND EINSTEIN, S. I., "P red ic t ion o f Gun Muzzle Flash," CPIA Publication 292, December 1977, Vol. 2, pp. 229-239.

2, KL1NGENBERG, G.: Combustion and Flame 28, 1 (1977).

3. YOUNC, H. H., ED., Smoke and Flash in Small Arms Ammunition, Midwest Research Institute, Contract No. DA-23-072-ORD-769, 1954.

4. CARFAGNO, S. P. AND RUDYJ, O. N., Relationship Between Propellant Composition and Flash and Smoke Produced by Combustion Products, Franklin Institute, Contract No. DA-36-034- 501-ORD-78RD, 1960.

5. LEwis, B. AND VON ELRE, G., Combustion, Flames and Explosions of Gases, Chapter III, Academic Press, New York and London, 1961.

6. VOEVOI)SKI, V. V. ANn SOLOUKmN, R. I.: 10th Symposium (International) on Combustion, pp. 279, The Combustion Institute, 1965.

7. COHEN, A. ANn LAaSEN, ].: Explosive Mechanism of the H2-O ~ Reaction Near the Second Ignition

Limit, BRL Report No. 1386 (AD 667362), De- cember 1967.

8. MEYER, J. W. ANn OPPENnEIM, A. K.: 13th Sympo- sium (International) on Combustion, p. 1153, The Combustion Institute, 1971.

9. YOUSEFIAN, V. ANn MAr, I. W.: Prediction of Muzzle Flash Onset. Paper presented at the 16th JANNAF Combustion Meeting, Monterey, CA, September 1979.

1O. ROSSER, JR., W. A., INAMI, S. H., ANn WISE, H.: Combustion and Flame 2, 102 (1963).

11. DEW1TTE, M., VREBOSCH, J., AND VAN TIGGLEN, A.: Combustion and Flame 8, 257 (1964).

12. IYA, K. S., WOLLOWITZ, S., AND KASHAN, W. E.,: Combustion and Flame 22, 415 (1974).

13. FRIEDMAN, R. ANn LEVy, J. B.: Combustion and Flame 7, 195 (1963).

14. McHALE, E. T.: Combustion and Flame 24, 277 (1975).

15. JENSEN, D. E. ANn WERB, B. C.: AIAA Journal 7, 947 (1976).

16. ROrlSENOW, W. M. AND Cnol, H. Y.: Heat, Mass and Momentum Transfer, Chapter 15, Prentice- Hall, Inc., 1961.

17. Dunols, J. AND MILLET, J: C. R. Acad. Sci. C 269 (22) p. 1336, 1969.

18. RUDINGER, F.: Nonequilibrium Flows, (P. P. Wegener, ed.), Vol. 1, Chapter 3, Marcel Dekker, New York and London, 1969.

19. FucHs, N. A.: The Mechanics of Aerosols, Chap- ter III, The MacMillan Co., New York, 1964.

20. BIRD, R. B., STEWART, W. S., AND L1GHTFOOT, E. N.: Transport Phenomena, p. 595, John Wiley & Sons, New York, London, 1962.

COMMENTS

N. Kubota, Iapan Defense Agency, 1apan. We examined the secondary combustion flame of dou- ble-base propellant rocket motors. The secondary combustion flame which has a continuous spectrum could be entirely eliminated by the addition of KNO 3 or K2SO 4 (--1%) in the propellants. I presume that the emission of the secondary flame comes from the carbon, possibly soot produced in the secondary combustion zone. Did you examine the spectrum of muzzle flash? I would appreciate if you would

comment on the similarities of muzzle flash and the secondary combustion flame of rocket motors.

Author's Reply, We have not made any spec- troscopic observations. Measurements made by other investigators indicate that the flash consists primarily of continuous radiation with some resonance radiation from Ca and Na. The species responsible for the continuous is at present unknown.

chemical mechanism for secondary flash suppression

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