A numerical investigation of high-explosive grain size effects on the performance of boosters

COMBUSTION

AND

FLAME

70:61-64

(1987)

61

A Numerical Investigation of High-Explosive Grain Size Effects on the Performance of Boosters P I E R K. T A N G Los A/amos National Laboratory, N M 87545

Initiation of insensitive high explosives requires use of a boosting system with a more sensitive and typically more energetic explosive. However, problems arise if the booster material is too energetic. The initiability of some insensitive but not so energetic high explosives can be enhanced by lowering the density and decreasing the grain size; thus those explosives can be used as booster materials. A class of experiments, commonly called onionskins, has been used to detect the divergence pattern of the detonation wave and determine the performance of the booster. With reactive burn model in a hydrodynamic code. numerical simulations can be performed to obtain the same objectives. Examples are presented using low-density superfine and ultrafine triaminotrinitrobenzene (TATB) as booster explosives to initiate plastic-bonded TATB-base high explosive.

INTRODUCTION

BOOSTER PERFORMANCE

The transformation o f high explosives (HE) from the initial state to the final product under shock conditions is very complex; the physical aspects are as important as the chemical ones in controlling this behavior. In general, the reaction is accomplished in a short time and within a short distance from the shock front so detail can be ignored. This is the case when the detonation is fully developed and the process is insensitive to the surroundings. However, the situation becomes more complicated in the shock-to-detonation transition, which is no longer considered instantaneous. The run distance to detonation as a function o f initial shock pressure, generally known as Pop plot [1], has been widely used as a measure o f the sensitivity o f the explosive under shock conditions. The Pop behavior depends on grain size, as evident in a study [2] using an explicit hot spot model [3, 4] which treats the explosive reaction with a multistep process, including hot spot excitation, decomposition, and propagation o f reaction outside the hot spots. The application o f the model in booster performance evaluation is next.

Initiation o f insensitive high explosives such as PBX-9502 (95 % T A T B , 5 % Kel-F 800) ~ requires a boosting system in which the explosive used should be more sensitive and usually quite energetic, for example, PBX-9404 (94% H M X , 3% NC, 3 % CEF). 2 The booster size depends on the acceptor explosive as well as the booster. The selection o f booster explosive and size is usually through experiment. A class o f experiments, commonly known as onionskins, has been extensively conducted to examine the divergence o f the detonation pattern in the acceptor HE. Good spherical divergence implies good initiation o f the acceptor by the booster for that particular booster explosive and size. Experimental works have recently been reported [5]. With better modeling o f high explosive behavior, it is now possible to perform numerical simulations and to predict the outcome o f certain physical experiments. In some applications, however, the energetic aspect o f PBX-9404 for booster use is not desir-

Copyright © 1987 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue. New York, NY 10017

t PBX: Plastic-Bonded Explosive; KeI-F 800: Binder. 2 HMX: Octogen; NC: Nitrocellulose; CEF: Binder.

O010-2180/87/$03.50

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PIER K. TANG

able since it might drive the systems too hard. resulting in some unwanted effects. T A T B at density of 1.8 g/cm ~ (gencrally referred to as low density) has better shock sensitivity in gap tests than regular density TATB (1.9 g/cm~); and ultrafine grain (arithmetic mean diameter, 10 gin) is much more sensitive than superfine (arithmetic mean diameter, 20 p.m) 16J. The Pop plot behavior of those two grain sizes of low-density TATB has been replicated using thc hot spot model [21 and is given in Fig. 1. Again, at high shock pressure level, because of the larger hot spot mass fraction effccted by the greater grain surface area, uhrafine TATB is more sensitive than superfine; however, the cooler hot spot temperature associated with the smaller grain of ultrafine TATB leads to lower sensitivity when the shock is weak. This crossing behavior is even more pronounced under cold conditions [71. Since low-density TATB is less energetic than PBX-9502, it should not have the shortcoming of overdriving the system as PBX9404 would. The use of low-dcnsity TATB for booster high explosive seems quite attractive. Figure 2 shows the configuration for numerical simulation, common in onionskin experiments and in many applications. Only one-half of the hemispherical region is shown, with the vertical axis as line of symmetry. The booster size is 30 mm in diameter, and the previously discussed low-density TATB is the booster HE, using two different grain sizes: superfine and ultrafine. A layer or PBX-9502 with 10-ram thickness ,~'nvelops the booster. The explicit hot spot model and HOM 100

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Fig. 2. Confi~uralion for onionskin ,',,imulation: b(~)stcr diameter, 30 ram: ~cccptor lhickncs,,,. I0 ram.

equation o f state [8] are used for those two booster HEs and the acceptor. The booster is initiated by a small detonator, 8-mm diameter and 2-mm thickness. The reaction model used for this detonator is called programmed burn and is based on constant detonation velocity with a traction of surface used for initiation on the bottom of the detonator. The detonator explosive is PBX-9404 with the JWL equation of state used [91. For computational convenience, the HE system is bound by layers of Plexiglas to provide some pressure boundary and to prevent excessive mesh distortion. Computation is done on DYNA2D code 1101. Figures 3a through 3g show a sequence of burned mass fraction contours at different times with superfine TATB booster on the left and ultrafinc TATB on the right. Recall that ultrafine TATB is the finer of the two. A detonation front is represented by closely packed contour lines, whereas the extended reaction zone is depicted with widc line separation. At 1 p.s, Fig. 3a, the detonation front is well defined except for the i

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A BOOSTER PIERFORMANCE STUDY

Superfine

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Fig. 3d. Burned mass fraction contours plot at 2.5 p.s, superfine and ultrafine.

Superfine

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Fig. 3b. Burned mass fraction contours plot at 1.5 #s, superfine and ultrafine.

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Fig. 3a. Burned mass fraction contours plot at I ,us. superfine and ultrafine.

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Fig. 3e. Burned mass fraction contours plot at 3 ,us, superfine and ultrafine.

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Fig. 3c. Burned mass fraction contours plot at 2 p,s, superfine and ultrafine.

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Fig. 3f. Burned mass fraction contours plot at 3.5 ,us, superfine and ultrafine.

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Fig. 3g. Burned mass fraction contours plot at 4/zs, superfine and ultrafine.

64 slight rarefaction on the side in the ultrafine booster. On the other hand, the detonation propagates mainly along the axis of symmetry in a planar fashion in the superfine booster, reflecting the effect of the detonator without significant divergence. There is shock-induced reaction but no significant sideways detonation. This behavior persists in Figs. 3b and 3c at 1.5 #s and 2 p,s. A dead zone is clearly shown in the ultrafine booster. Figure 3d shows that at 2.5 p,s the detonation front is well inside the PBX-9502 explosive with which ultrafine HE is used. For superfine booster, only a small fraction of the PBX-9502 is detonated. However, a second front emerges at 3 ~s when the shock intensity is high enough, as seen in Fig. 3e. The reaction still continues inside the superfine booster but not as a detonation, whereas the detonation continues to propagate quite nicely with ultrafine grain. With superfine, the two detonation fronts gradually merge at 3.5 #s, as shown in Fig. 3f, but the propagation pattern is far from desirable; meantime, the detonation front with ultrafine is about to complete in the acceptor region. At 4 p,s, the reaction process is essentially over for that boosting system (Fig. 3g) even though a dead zone remains and a few isolated spots appear. Those spots are caused by the deficiency in shock pressure calculation when the condition is marginal. The superfine case shows more definite dead regions on the booster surface, and somc spots caused by desensitization [4] are clearly shown. A large dead zone will remain inside the booster even at later time.

PIER K. TANG a clue to the performance of booster high explosive, at least in some qualitative sense. Flash radiography is even better for the investigation of the reaction region at selected times. However, this numerical study has demonstrated that grain size can have significant impact on booster performance, in particular the improvement of initiability, by using smaller-grain HE. Ultrafine low-density TATB can be a candidate as booster HE for the initiation of insensitive high explosives such as PBX-9502.

REFERENCES I.

2.

3. 4.

5.

6.

7,

8.

CONCLUSIONS No experimental evidence appears in the literature to support the numerical modeling result at this time, but the onionskin-type experiment is not difficult to perform. The streak record that shows the breakout of the detonation front should provide

9. 10.

Ramsay, J., and Popolato. A.. Proceedings o f the Fourth Symposium on Detonation, Office of Naval Research, ACR-126, 1965, p. 233. Tang, P., Forest, C., Johnson, J., and Seitz, W., Proceedings o f the International Symposium on Intense Dynamic Loading and Its Effects, Science Press. Beijing, China, 1986, p. 207. Johnson, J., Tang, P., and Forest, C., J. Appl. Physics 57:4323 (1985). Tang. P., Johnson, J., and Forest, C., Proceedings o f the Eighth Symposium (InternationaO on Detonation, preprint, 1985, p. 375. Bahl. K., Bloom, G., Erickson, L., Lee, R., Tarver, C., Von Holle, W., and Weingart, R., Proceedings o f the Eighth Symposium (International) on Detonation, prcprint. 1985. p. 729. Hon~xlcl, C., Humphrey, J., Weingart, R., Lee, R., and Kramer, P., Proceedings o f the Seventh Symposium (International) on Detonation, Naval Surface Weapons Center. NSWC MP 82-334. 1981, p. 425. Seitz. W., Shock-Wave in Condensed Matter, Procecdings of the American Physical Society Topical Conference, 1983, p. 531. Mader, C., Numerical Modeling o f Detonations, University of California Press, Berkeley, 1979. Lee, E., Hornig, H., and Kury, J., Lawrence Livermore National Laboratory Report UCRL-50422, 1968. Hallquist, J., Lawrence Livermore National Laboratory Report UCID-18756. 1982.

Received 17 November 1986; revised 24 April 1987