Post-detonation pressure and thermal studies of solid high explosives in a closed chamber

648

COMBUSTION OF EXPLOSIVES AND SOLID PROPELLANTS

5. CYBULSKI, W. B., PAYMAN, W., AND WOOD-

Manufacturers and the Institute for the Promotion of Scientific Research in Industry and Agriculture (I. R. S. I. A.), to whom we tender our sincere thanks.

HEAD, ~). W.: Proc. Royal Soc., A197, 51 (1949). 6. DEFFET, L., DE COSTER, M., AND VANDE

WOUWER, P. J.: Fourth Symposium on Combustion, p. 481. Baltimore, The Williams & Wilkins Co., 1953. 7. DEFFET, L., AND BOUCART, J.: Explosifs, 8, 83 (1955).

REFERENCES 1. BERTHELOT, M.: Ann. Chim. Phys., 6, 556 (1885); BERTHELOT, M., AND VIEILLE, P.: Memorial des Poudres, 4, 7 (1891). 2. JONES, H.: Proc. Royal Soc., A189, 415, (1947). 3. EYRING, H., POWELL, R. E., DUFFEY, G. H.,

8. BRIDGMAN,P. W.: The Physics of High Pressure. London, Bell & Sons, 1931; Proc. Am.

Acad. Sci., 76, 1 (1945); Proc. Am. Acad. Sci., 77, 187 (1949). 9. SCHALL,R. : Nobel Hefte, 20, 75 (1954). 10. TAYLOR, J.: Detonation in Condensed Explosives. Oxford, Clarendon Press, 1952.

AND PARLIN, R. B.: Chem. Rev., 45, 69

(1949). 4. COPP, J. L., AND UBELOHDE, A. R.: Trans. Faraday Soc., 44, 658 (1948).

85 POST-DETONATION PRESSURE AND THERMAL STUDIES OF SOLID HIGH

EXPLOSIVES IN A CLOSED CHAMBER By WILLIAM S. F I L L E R Introduction Current methods and principles involved in determining the heat energy released by high explosives have been very well presented by Lothrop and Handriek. 1 Measurements of heat of detonation usually involve small quantities of explosive initiated in a small space within a strong container filled with inert gas. s, 3.4 The quantity of explosive used has been limited by practical considerations of calorimetric technique. Since many explosives will not detonate in small quantities, this limits the type of explosives that may be used. Also, many explosives, although they apparently detonate, do not undergo the same chemical transformation as they do in larger quantities. If an explosive booster is used even in small amounts, its interaction chemically with the test explosive must introduce uncertainties in the measured heat of detonation values. Values for heat of detonation of organic explosives can also be computed if products of detonation are known. The chief drawback of this method is the lack of exact knowledge regarding the products formed on detonation of many explosives. Due to these difficulties of measurement and

computation, heat of detonation for many explosives is not well established, and few published measurements exist. Yet, the energy which can be released by detonation is one of the most fundamental properties of explosives. This paper describes studies of pressures that develop shortly after the detonation of high explosives in a closed chamber and how these pressures may be used to measure heat of detonation of high explosives. I t was found that a hydrostatic pressure developed within the enclosure a few msee after the initial shock wave traversed the chamber space and that the maximum value of this pressure (before decay due to heat conduetion) was in good agreement with simple theory for adiabatic heating of a perfect gas in a fixed volume. If a quantity of heat, H, is added to a gas with a ratio of specific heats, Cp/C~ = % contained in a volume, V, the pressure rise, P, is given by the relation H(~ - 1) P -- - V

(1)

This relationship applies to the ambient gas initially present in the chamber. The correction

POST-DETONATION PRESSURE AND THERMAL STUDIES

for the addition of explosion product gases is small (on the order of 2 per cent) since the chamber initially contained a volume of gas that was large (72.4 cuft) compared with the quantity of gaseous products released by two ounces of explosive. Deviations from the perfect gas law are not significant since the hydrostatic pressure did not exceed two atmos and temperatures did not exceed 500°C. Measured values of maximum static pressure were used in Equation (1) to compute H for a number of explosives detonated in a variety of

649

inside of the chamber for conveniently bolting gauges and other objects into place. PIEZOELECTRIC GAUGE SYSTEM

To provide a detailed picture of the pressure of the explosion shock wave and its reflections from the chamber wall, a tourmaline piezoelectric gauge 5 was used with an appropriate amplifier, cathode ray tube, and photographic recording system, s The gauge system had a time resolution of about 50 psec. This was adequate to produce records which provided a good qualitative picture of the shock phenomena. INDUCTANCE GAUGE SYSTEM

Fro. 1. The chamber facility. •

~,

"

"i

The most useful data were collected by means of a variable inductance gauge of the Bendix type 7, 8 and its associated system. This gauge, although slow responding (1000 cycles/see) can be used to indicate static or slowly changing pressure levels--a task which formed the major part of these investigations and for which the piezoelectric gauge is not suitable. The inductance gauge proper is located outside of the chamber. Gas pressure from the chamber is led to it through a tube mounted in a gland hole. Calibration of this gauge is performed statically by means of a dead weight tester. Two such gauges were used in all work. Absolute differences in measurement between gauges were found to be statistically insignificant. RECORDING METHODS

AA

F I G . 2. G a u g e - c h a r g e

arrangement.

ambient gases. The values so computed were close to values for heat energy measured or computed by other means for comparable explosion conditions.

Apparatus THE CHAMBER FACILITY

The chamber (Figs. 1 and 2) is a 1/~-in.-thick steel cylinder 6 ft long and 4 ft in diameter. The doors are 11/6 in. thick and are secured over the chamber oFening with 24 bolts. A large rubber " 0 " ring is used as a seal. Access holes with rubber sealing glands are arranged about the cylinder walls and doors to accommodate cables, gauges, piping, etc. Brackets are welded to the

Two methods of recording have been used. The first method employed a cathode ray tube and a rotating drum camera. The record consisted of a 10-in. long, 35-mm strip of fihn. In practice two signal scopes and a timing scope, used to calibrate the time base, are photographed simultaneously. Amplitude calibration lines are also put on the film before firing. The drum speed could be set to study pressure fluctuations over a wide range of durations, from incident shock waves of 0.5 msee duration to static pressure decay out to 0.2 sec (later extended to 3.0 sec) in time. Measurements are made from enlargements of the film. The second recording method employs a Brush Instrument Company pen and ink recorder. This recorder is more suitable for static pressure measurements since pressure decay information can be recorded conveniently for as long a time as desired. Its frequency response (100 cycles/ sec) is completely adequate for the purt~ose, and the data reduction is simple and direct.

650

COMBUSTION OF EXPLOSIVES AND SOLID PROPELLANTS

Experimental Design This work was performed in two phases. The object of the first phase was to establish the characteristic features of shock wave and later pressure phenomena from explosions in a confined space and to correlate the later pressure phenomena (static pressure) with theory. The ambient gas was atmospheric air except for exploratory shots in nitrogen. The second phase was a more carefully controlled attempt to measure heat of detonation using the static pressure theory. Argon and nitrogen were used as the ambient gas. Shots were also fired in air and in air diluted with nitrogen. Table 1 is a summary of the experiments performed. EXPLOSIVE

MATERIALS

All explosives were pressed with the exception of pentolite ( P E T N / T N T ) which was cast. The charges were cylinders one in. in diameter with length varying according to the pressing properties of the explosive. I n the center on one end of each charge a l~-in, diameter hole, 1/4 in. deep was drilled for an electrical blasting cap. Materials of sufficient sensitivity or in a sufficiently sensitive form (pressed T N T for example) were used so that reliable detonation might be expected without the use of boosters. Thus contaminating effects of initiating materials was minimized. Comparison of measurements on the initial shock wave with established values was made as a check for proper detonation. These shock wave comparisons were satisfactory and indicated normal detonation was achieved. The detonator was a Hercules special blasting cap, commercially available. In the first phase a weaker detonator, No. 6 Hercules, was used for one shot of each explosive to check for possible variation in measured pressures with the variation in the strength of detonator. None was observed. GASEOUS MATERIALS

Atmospheric air and commercial grade nitrogen--0.2 per cent impm'ity--were employed in phase one. Gas changing was accomplished by evacuation. The estimated total oxygen content in the chamber at the time of firing was 25 g or about 5 per cent of sea level air. An ambient gas purity requirement for phase two was established in terms of a 5 per cent maximum allowable error in heat of detonation values assuming the detonation products utilized

all of the residual ambient oxygen. To meet this requirement for the size chamber being used, the oxygen content of the bottled gas used did not exceed 0.01 per cent. This total oxygen impurity requirement proved extremely conservative in practice, since post-detonation combustion appeared to cut off completely for TNT, an explosive potentially most sensitive to this source of error, even though the chamber contained a TABLE

1. S U M M A R Y OF E X P E R I M E N T S

Explosive Composition

Ambient Gas

No. of. Shots (~/ . . .~o. .. (~A ~o , !N° D~ :a) Data)

Density

g/cc

Phase PETN/TNT(cast) One 50/50 RDX/wax 98/2 RDX/Aluminum/

Air

1.66

N2 Air

1.71

N2

wax

76/22/2 63/35/2 Phase TNT Two 100

RDX/wax 98/2 PETN/wax 98/2

Air N~ Air N2 Air Air & N2 N2 A N2 A Air N2 A

1.83 1.93 1.55

1.67 1.64

TNT = CTHsN~O~ = Trinitrotoluene. RDX = C3H6N606 = Cyclotrimethylenetrinitramine. PETN = CsHsN4OI~ = Pentaerythrite tetranitrate. total of as much as 100 grains of oxygen or about 20 per cent of the concentration of oxygen in sea level air (see Discussion). In the light of this, phase one data for shots in nitrogen may be considered valid in spite of the substantial amountofresidual oxygenpresent in the chamber during those shots. PROCEDURE

All gauges were located near one end of the chamber (Fig. 2); the charge was positioned on the chamber axis 2 ft from the other end. The string and tape used for suspension weighed a

POST-DETONATION PRESSURE AND THERMAL STUDIES

fraction of a g, most of which was left intact after a shot. The charge was then armed by connecting the detonator leads to the firing cable, and inserting the detonator into the hole provided in the charge. The door was then closed and bolted. ~.k~. 0

1

C

'~

|

",

& ~.~,,..~... ~

Fio. 3. Representative pressure-time histories: T.L. = Time Length of signal trace from start of trace. (a) Initial shock and reflections; 1.5 fold magnification of initial portion of 4b. T.L., 3 msec.

(b) (c) (d) (e) (f)

Piezoelectric gauge. T.L., 58 msec. Piezoelectric gauge. T.L., 185 reset. Inductance gauge. T.L., 185 msec. Inductance gauge. T.L., 1.780 sec. Inductance gauge, pen and ink record. T.L., 0.970 sec.

If air was to be the ambient gas, the chamber was ready for firing. If argon or nitrogen was to be the ambient gas, the chamber was evacuated to less than 1 mm, then filled to atmospheric pressure.

Experimental Results QUALITATIVE ANALYSIS OF RECORDS The six records in Figure 3 reveal the nature of the pressure phenomena in the chamber in a progressive manner, timewise, from highly resolved piezoelectric gauge records to highly condensed and smoothed inductance gauge records. Each record is representative of a type obtained at some stage in the work. The charac-

651

teristics discussed in this section are independent of type of charge or ambient gas. Figures 3a, b, c, d, and e are all film records. Figures 3a, b, c, and d are of the same shot. Figures 3a and b show the initial shock wave and later reflections. The initial shock is the first rise and fall. I t is well formed and free from extraneous oscillations until the first reflected shock front arrives. Figure 3c is a simultaneous recording of the same gauge signal at a slower drum speed. The shock fluctuations aI~ compressed and several important new features are apparent. The shock fluctuations are well above the base line rather early in the record. This is the static pressure. Notice also the periodic fluctuation which persists for the length of the record. These features are suggested in Figure 3b, but are clearly brought out in Figure 3c. I n Figure 3d we have an inductance gauge record made at the same drum speed as Figure 3c. The limited frequency response of this gauge results in the smooth appearance of the initial portion of the record, but the character of the remainder of the record is essentially the same as Figure 3c. Figure 3e is an inductance gauge recording highly condensed in time compared with Figure 3d. The relatively broad periodic fluctuations of Figure 3d now appear sharp and narrow, and the static pressure appears in a new light with features of its over-all development and decay directly apparent. Figure 3f is an inductance gauge record obtained with a pen and ink recorder. It differs from Figure 3e in the increased amplitude of deflection and in its smoother appearance due to the low frequency response of the recorder itself compared with that of the inductance gauge system. The oscillation is the same periodic fluctuation observed in Figures 3c and 3d. The sequence of happenings in the chamber which have been illustrated by Figure 3 appears to be as follows: (1) The usual initial shock wave from explosions is propagated to the walls of the chamber. Reflection occurs followed by shock interactions and further reflections. The resultant complex, aperiodic wave-form obtained with the piezoelectric gauges lasts for about 50 msec and, far from being random, is reproduceable from shot to shot in surprising detail. (2) A few msec after the initial shock wave has traversed the chamber, a periodic pressure phenomenon with an amplitude of large magni-

652

COMBUSTION OF EXPLOSIVES AND SOLID PROPELLANTS

rude (several lb per sq in.) develops and persists for several hundred msee. (3) About the same time that the Iceriodic fluctuation develops, a hydrostatic pressure also deveJops and rises to a maximum, and then decays to nearly atmospheric pressure in 20 to 30 sec. The shock and l:eriodic waves, while they last, both overlay the static pressure. QUANTITATIVE

ANALYSIS

OF

RECORDS--I:HASE

ONE

Shock wave measurements were not of direct quantitative interest for the purpose of this paper and will not be discussed further.

,v o D

• .'::.°,"

o

shock waves as the cause of the overshoot. I t is possible that the inductance gauge overshoots in response to the successive reflected shocks irapinging on it. The overshoot could also result from the fact that the gauges will give an integrated response to the many shock reflections reaching it, while at the same time responding to the static pressure. Thus, while shocks persist, the records would show pressure values higher than would occur otherwise. I n either case, the initial overshoot must be ignored in determining a value for maximum static pressure. The sloping portion of the static pressure curve

o

: :

o

-

....

El

,,, -ooo~o " .1" " " "

"

n

! t

SYMBOL o •

z,

40

S H O T G A G E GAS 12 / AIR 12 Z /

AIR

•

16

o •

26 26

/

AIR

X

34

1

34

/ 2

N2

80

18

120

160

~

200 TI ME

240 280 320 (MILLISECONDS)

|

360

400

440

480

520

FIG. 4. Pressure--time scatter plot for one explosive RDX/AI/wax 63/35/2, three shots in air, one shot in nitrogen, two gauges on each shot. Pressure-time histories

All static pressure measurements for phase one were made from records of the Figure 3c type. The characteristic shape of these pressure-time histories may be observed when the traces have been transformed to suitable axes as in Figures 4 and 5. I t consists of an initial overshoot followed by a relatively flat sloping curve for the first 1/6 sec. This is identical to the curve obtained directly in phase two and shown in Figure 3f. An analysis of the overshoot amplitude shows no correlation with static pressure amplitude. However, its duration is nearly the same as the time it takes the shock waves on the piezoelectric gauge records to die out. This points to the

is due to heat loss from the gas to its surroundings. A model which successfully describes this decay consists of a hot gas in turbulent motion and thus always at a uniform temperature. The chamber wall, being of steel and of such high conductivity and specific heat compared with the gas, remains uniformly at atmospheric temperature through the conduction period. Then, if the temperature difference between gas and wall is T and the surface area of the chamber is A, the heat transfer from gas to wall will be at a rate Q = KAT

(2)

where K is a proportionality constant depending

653

POST-DETONATION PRESSURE AND THERMAL STUDIES

on the material of the boundary (rust, paint, stagnant air, etc.). With this understanding of the pressure-time curves, the method used to get maximum static pressure was to draw a straight line through the

RDX 98

MIXTU %- RE AI 0

WAX 2

63

35

2

PETN

TNT

interest. Their amplitude was of the order of several psi. It was assumed, however, that they followed ordinary acoustic relationships and were treated as resonance of the chamber space, considered as a pipe closed at both ends. The ternSYMBOL -X-

~

•

50 so ~ ¢ * NOTE:.FILLED SYMBOLS INDICATE SECOND READIN6 ~30

..~25

W

220 ,DETONATED IN AIR

IO

tr

.-

~

<

--i

¢

a. O0

40

80

120

160

1,1~ ,DETONATED IN NITROGEN

--¢

200 240 280 320 T I M E (MILLISECONDS)

560

400

440

480

520

FIG. 5. Composite pressure--time histories of static pressure for explosives detonated in air and nitrogen. TABLE

2. AVERAGE UNITS

STATIC PRESSURES FROM PHASE ONE Air

IN PSI

N2

Explosive Pressuretime

Resonance .~ ice

Pressuretime

P.esonance

5o/50 . . . . . . R D X (2%

16.0

14.9

7.5

6.0

WaX) ......

15.6

13.5

8.7

6.8

21.3 24.3

20.8 22.1

10.3 12.2

8.0

PETN/TNT

RDX/A1/wax 76/22/2 . . . . . . . 63/35/2 . . . . . . .

Note. These values are uncorrected for volume of gas products.

perature rise of the gas due to the explosion should increase the velocity of sound in the ambient gas and result in an increase of the resonance frequency of the chamber cavity. Since pressure is proportional to the temperature with no change in density, the static pressure could be determined from measurement of this frequency. The equation for velocity of sound, c, in a gas is

c = ~//~°

where Pa is the absolute pressure, 5' is the ratio of specific heats, and p the density of the gas. For a closed tube of length, L, the fundamental resonance frequency has a period

portion of the curve from 100 to 500 msec, extrapolate back, and read pressure at zero time. Values so obtained are listed in Table 2. Chamber resonance measurements

Measurements on the periodic oscillations found on all records proved of considerable

(3)

2L =

- -

(4)

C

If we combine Equations (3) and (4) and solve for the pressure, we have P,-

4pL~ -yr 2

(5)

654

COMBUSTION OF EXPLOSIVES AND SOLID PROPELLANTS

Since'p, L, and 7 are known, Pa may be calculated by measuring r. Average values (corrected for conduction decay) for each explosive listed in Table 2 are close to values obtained directly from pressuretime histories. A surprising fact is that they are about 10 per cent low as a whole, although one would expect waves of such large amplitude to move faster than sound waves, thus resulting in a shorter period and higher pressure values from Equation (5).

from 5 sec out to 20 or 30 sec is much slower but the loss rate still is linear as a function of temperature. This would be consistent with a model comprised of relatively still rather than turbulent hot gases (as in the first 1/~ sec) in a constant temperature enclosure. The conductivity of the gas now dominates the heat loss rate, and since gas conductivity is much lower than the boundary materials which were important during the first 1/4 sec, the heat loss rate would be expected to be lower than that of the earlier nlod61. Discussion

TABLE 3. TYPICAL SET OF STATIC PRESSURE

HEAT ENERGY VALUES

VALUES IN P s i UNITS FROM PHASE T w o TNT 2i itroge_ N n Shot

Gauge 1

T N T i n Argon

Gauge 2

1 16.30

16.51

2 6.39 3 16.18

625 j678

Shot

Gauge i

6

Gauge 2

10.13

9.84

7 980 [ 8 I 079

984 19.70

Heat energies released by the various explosives when fired in the various ambient gases were computed from Equation (1). H in Equation (1) is the total heat released for the weight of charge, W. Since the heat released per gram of explosive, h, is desired, we may write

Note. These values are uncorrected for volume of gas products.

h -

H W

(6)

TABLE 4. H E A T COMPUTATION DATA Air

Nitrogen

Argon

Explosive P

T

W

psi

°C

g

389 3O8

1.368 1.378 3OO 1.378 245 1.384

56.7 56.2 56.6 56.6

6.3 7.3 7.8 7.7

123 144 153 152

410 468

56.4 56.2

10.3 12.2

198 235

19.9 TNT. PETN/TNT 50/50... 15.7 15.3 RDX (2% wax) PETN (2% wax). 12.5 RDX/A1/wax 21.3 76/22/2. 53/35/2 . . . . . . . . . . . . . . . 24.3 QUANTITATIVE

ANALYSIS

OF

1.366 1.359

RECORDS--PHASE

TWO

The maximum static pressure data of phase two were developed from records of the Figure 3f type. A straight edge was placed directly on each record, an average line drawn through the record for the interval from 100 to 500 msec and extended back to zero time where the maximum amplitude was read, just as was done with the phase one composite curves. A typical set of data for one explosive is given in Table 3. Average values for all explosives are listed in Table 4. Analysis of the entire pressure decay curve obtained with the pen and ink recorder revealed that the method of heat transfer changes drastically after the first half second. Heat conduction

T

psi

P g

psi

°C

1.398 1.397 1.396 1.396

56.8 56.3 56.7 56.7

9.7

190

1.66

56.9

12.2 12.3

24O

242

1.66 1.66

56.5 56.8

1.393 1.390

56.9 56.2

°C

Combining Equation using the units cu ft and psi for pressure, weight of explosive is by the expression

g

(6) with Equation (1) and for volume, g for weight, the heat release per unit given in calories per gram

PV

h = 46.64 - W(~ - 1)

(7)

For these experiments the value for V, computed from measurements of the chamber, was 72.4 cu ft. From the U. S. Bureau of Standards 9 7 was obtained in accordance with the measured static pressure and computed temperature, all of which are listed in Table 4. The charge weights are also listed in Table 4. They are average values; individual values varied =t=0.3 g. From

POST-DETONATION PRESSFIRE AND THERMAL STUDIES

this data, heat values were computed (Table 5). Final values, except for the aluminum mixtures, have been corrected for the change in the volume of gas before and after explosion due to addition TABLE

5. H E A T

measurements in air and heat of combustion. Similarly, agreement is found between nitrogen and argon chamber values and heat of detonation. Some exceptions are apparent. P E T N is especi-

ENERGY

VALUES

IN CAL/G

NOL Measurements Explosive

Air

Argon

Heat of combustionlO

1o

860 -1110 1110

3589.5 2782t 2307.2 1974

925 1220 1300 1385

---

3509t 4124?

3220 2500 1770

940 1100 1190~ 1180

76/22/2 ....................

3480 4070

1560 1880

63/35/2 . . . . . . . . . . . . . . . . . . . .

Other Values

Nitrogen

T NT . . . . . . . . . . . . . . . . . . . . . . P E T N / T N T 50/50 . . . . . . . . . RDX (2% wax) . . . . . . . . . . . . PETN (2% wax) . . . . . . . . . . . RDX/A1/wax

2250

655

H e a t of detonation 11

1080 1265t 1260 1450

12"

768 1056t 1239 1343

Note 1. All values are for water in gaseous form. Note 2. Heat of detonation vMues in the Encyclopedia of Chemical Technology ~° were c o m p u t e d ; the Armament Research Establishment ~ used a recently developed calorimeter for approximately 100 g charges ; Tonegutti x~employed 2 g charges in the conventional eMorimetric method. * Original values for water liquid corrected by author to water gas. Obtained by combining values for components. Heat of combustion of aluminum to A1203 was taken as 7039 cal/g. :~ The single value from phase one was not included. 5500

I HEAT OF COMBUSTION

3000

2500

/

o

o~2000

o" ca ~ I50EI

/

/

/

//

I000 ~

d HEAT OF DETONATION

500

o

o

IOO

0

1.38

200 3oo 400 TOTAL OXYGEN IN CHAMBER (SM) 2.76 4.14 5.52 OXYGEN CONCENTRATION (GM/FTS|

500 6,9l

FIG. 6. Chamber heat released by T N T fired in an oxygen-nitrogen mixture of varying proportions at one atmosphere. of product gases. The correction varied from 1.5 to 2.5 per cent. Adjacent to the heat values are heat of combustion and heat of detonation values obtained from other sources. General agreement is found between chamber

ally low. The explanation is not immediately clear. The low value for heat of combustion of T N T suggests the inadequacy of the oxygen concentration of air at one atmosphere to complete the oxidation of explosion products before cooling of the products causes the reaction to cut off. Some light is thrown on this matter by data collected for T N T fired in various concentrations of oxygen obtained by mixing nitrogen with air at one atmosphere. Figure 6 is a plot of that data. It shows the nature of the shift in energy release from 940 to 3,220 cal/g as the ambient gas changes from effectively pure nitrogen to air with a 23 per cent oxygen content (by weight). Notice that the heat energy released is not substantially affected by the increase in total oxygen content until past the 100 g point, although 100 g are more than sufficient to oxidize completely the amount of T N T used. Also note the rather narrow region on the oxygen concentration scale within which the transition occurs. This indicates that oxygen concentration is a critical variable. SIGNIFICANCE

AND

POSSIBLE

EXTENSIONS

OF

THIS TYPE OF MEASUREMENT

The results of these experiments show that efforts to measure the heat of detonation need

656

COMBUSTION OF EXPLOSIVES AND SOLID PROPELLANTS

no longer be restricted to surmounting the difficulties of designing calorimeter bombs to withstand extreme pressures. Also, convenient experimental determinations of heat of detonation values may serve as a guide to the actual detonation products formed and improve the accuracy of computation methods of determining heat of detonation. Although 2 oz charges were used in these experiments, use of larger charges is desirable and chambers may easily be designed for several pounds of explosives. Construction of such a facility is being considered by this Laboratory.

Conclusions (1) The static pressure rise resulting from detonation of high explosives in a closed chamber has been measured and found to be in agreement with simple theory. (2) The theory makes possible the measurement of the energy released by an explosive under various ambient gas conditions. (3) Heat of detonation values for several explosives have been determined and, in the main, are in agreement with values from other SOUrCeS.

(4) I n this method for measuring heat of detonation, temperatures and pressures are relatively low and are easily dealt with in the design of apparatus. Also, this method permits the use of relatively large quantities of explosives in energy measurements.

Acknowledgment This work is an extention of some experiments performed in World War I I by W. E. Gordon for the Office of Scientific Research and Development. REFERENCES 1. LOTHROP, W. C., AND HANDRICK, G. R. : Chem.

Revs., $$, 419 (1949). 2. ROBERTSON, R. : J. Chem. Soc., i19, 1 (1921). 3. ROBERTSON, R., AND GARNER, W. E.: Proc.

Roy. Soc. (London), AI08, 539 (1923). 4. TAYLOR, J., AND HALL, C. R. L.: J. Phys.

Colloid Chem., 51,593 (1947). 5. ARONS, A. B., AND COLE, R. H.: Rev. Sci.

Inst., 21, 31 (1950). 6. COLE, R. H.: Underwater Explosions. Princeton. Princeton University Press, 1948. 7. Bumblebee Telemetering Handbook, Applied Physics Laboratory, Johns Hopkins University, 1949 (see Bendix Model No. TTP-3). 8. Physical Measurements in Gas Dynamics and

9.

10.

11.

12.

Combustion, p. 129. Princeton, Princeton University Press, 1954. U. S. Bureau of Standards Circular 544, Tables of Thermodynamic Properties of Gases, U. S. Gov. Printing Office, 1955. Encyclopedia of Chemical Technology, Vol. 6, New York, Interscience Publishers, Inc., 1951. Armament Research Establishment Memo 1/51: Ministry of Supply, United Kingdom (19511. Tonegutti, M. : Z. ges. Schiess-u Sprengstoffw., 32, 93-97 (1937).

DISCUSSION BY CYRUS C. DUNKLE* Heat of detonation is given by deducting the heat of formation of the explosive, from that of the mixture of products as formed at the Chapman-Jouguet (C-J) point, where they are still hot and at high pressure. Both heats of formation refer to the same standard temperature, e.g., 298°K. Heat of detonation, like heat of explosion, can be corrected to give the standard enthalpy change on formation of the respective products from the explosive. In any explosion calorimeter, the heat measured is that evolved by the time the products have cooled to calorimeter temperature. Interaction beyond the C-J point when possible, as in an oxygen deficient system, is difficult to prevent. We try to, in determining heat of detonation, by strong confinement which keeps pressures higher and thus allows faster cooling; also the products work against resistance by shattering a steel capsule within the bomb while still at high pressure. This too speeds cooling, in hopes of "freezing" the chemical equilibria at what Springall and Roberts term their "high pressure" values. Here there are more C, CO2 and H20 but less CO and It2 than at low pressures. Heats of detonation usually exceed heats of explosion because energy is evolved in forming C, C02 and H20 from CO and Ha at calorinmter temperature. In determining heat of explosion, on the other hand, no such attempt is made. Pressure drops quickly from detonation values, but temperature stays high longer because the products make only a free expansion and do no work. They thus have time to react, and approach their "low pressure" composition before the equilibria freeze. Filler's heats in ambient nitrogen or argon resemble in magnitude our heats of explosion. Those in air resemble heats of combustion in excess of oxygen. AUTHOR'S REPLY

The values presented in this paper are 10w compared with values obtained with cased charges in * Picatinny Arsenal.

DETONABILITY OF ROCKET PROPELLANTS

a calorimeter (see referenceH). Although this might be explained by the Springall-Roberts mechanism described by Mr. Dunkle, it is also possible that factors such as heat losses not yet established, corrections yet to b e applied, and other experimental factors yet to be investigated may also be the cause. Further experiments with larger

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charges may also throw some light on the confinement effect. Although the interpretation of the chamber energies as heats of detonation in the defined sense is not at all established, we believe that further work will demonstrate the probability of this interpretation.

86

THE DETONABILITY OF CERTAIN ROCKET PROPELLANTS ° By JOHN R. SCHOLL, H. W. BYINGTON, AND R. L. POTTER

Introduction The development of rocket engines in this country has been hampered by the lack of a clear understanding of the combustion process. The problem has been particularly severe in the case of rocket engines utilizing fuming nitric acid and jet fuels as propellants. The manifestations of these problems are large chamber pressure oscillations of various frequencies. These are often of sufficient amplitude to destroy the thrust chamber and associated hardware. In the rocket industry this phenomenon is labeled "combustion instability." I n many cases this "combustion instability" has been overcome by design changes. Unfortunately, at this time, there seem to be no definite rules to guide the rocket designer so that this problem can be overcome. The fact that the same propellants can perform so differently in essentially the same rocket engine calls for a theoretical explanation. Several attempts have been made to explain "combustion instability" on a theoretical basis. 1 The physical basis for these theories can certainly be improved, and in fact must be, in order that the rocket designer may benefit from them. If the normal combustion process in a rocket engine can suddenly change to detonation, in some as yet unexplained manner, then qualitatively all the effects of "combustion instability" can be accounted for. Thus "high frequency combustion instability" can be explained away as spinning detonation. The other low frequency instabilities can be considered as detonations of succeeding portions of propellants injected into the thrust • Work supported by Contract W33(038)-ac14169, U. S. A. F.

chamber. Each detonation results in a rise in chamber pressure, thus causing the propellant flows to oscillate severely. The destructive effects follow immediately, since the pressure rise in the gas across such a front is about 45 times the initial pressure. In order to find out if this is indeed possible, it is necessary to find out whether fuming nitric acid and jet fuel can detonate under any conditions, and in particular, whether they can detonate under conditions similar to those encountered in a rocket engine. The experimental portion of this paper is concerned with the first question which has been answered in the affirmative by work at Bell Aircraft Corporation; at this time the second question has not been answered unequivocally, although qualitative arguments may be put forth in support of this idea. A brief discussion of the implications of these experimental results concerning the foundations of present "combustion stability" theories is presented after the experimental section of this paper.

Experimental Equipment and Technique A schematic diagram of the equipment used to determine the conditions under which nitric acid and jet fuels detonate is shown in Figures l a and lb. Figures 2a and 2b are photographs of the test unit. The essential parts of this unit are a stainless steel reaction tube 3.75 cm in diam and 3 m long, separate evaporators for hydrocarbon fuel and acid and the instrumentation required to determine detonation velocities. The apparatus grew slowly as components were added to permit finer control of the acid-fuel