Projectile Initiation of Explosives

Held observed that his data could be correlated by assuming that the critical jet value for explosive initiation was the jet velocity squared times the jet diameter (d). If the velocity is in millimeters per microsecond and the jet diameter is in millimeters, he reported that the critical V d was about 5.8 for Copper jets initiating 60/40 RDX/TNT at 1.70 g/cc. 

The jet was described as a cylinder with a radius of 10 or 20 cells and 40 cells long and with the appropriate initial velocity. The cell size was. 01,. 02, or. 005 cm, depending on the jet radius. The maximum burn pressure in the Forest Fire burn was set to 1.5 Mbar to permit overdriven detonations, and the rate was limited to The viscosity coefficient used for PBX-9502 was 0.25 and for PBX-9404 was 0.75. Sufficient viscosity to result in a resolved burn was necessary, just as in the detonation failure calculations described earlier in this chapter. 

An uncertainty in the calculations is the approximation of the jet as a cylinder of uniform velocity colliding end-on with the explosive. Although the jet is actually many small metal pieces, the calculations indicate that the critical conditions are determined by the first piece of about the same length as its radius. Side rarefactions dominate the flow after the reflected shock wave travels one diameter length back into the jet. 

The proper initial conditions for the hot jet must be used. Because it was formed by being shocked and then rarefied, it will have a residual temperature greater than ambient and a density less than the initial density of the jet material. 

Estimates of the Copper residual state can be made by assuming the material is at the same state as if it had achieved its final velocity by a single shock and then had rarefied to one atmosphere. The residual temperature of the Copper initially shocked to 830 kbar and then rarefied to a free-surface velocity of 0.3 cm/fisec is 768 K the residual density is 8.688 g/cc, comparable to the initial density of 8.903. Calculations were performed with Copper initial conditions of 8.903 g/cc and 300 K and of 8.688 g/cc and 768 K. Since the changed equation of state results in only slightly changed explosive shock pressure, the calculated results were insensitive to the Copper jet initial conditions. 

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James, H.R. Critical energy criterion for the shock initiation of explosives by projectile impact. Propellants, Explosives, Pyrotechnics 13, 35-41 (1988). 

M.A. Cook etal, TrFaradSoc 56, 1028-38(1960 Promotion of shock initiation of detonation by metallic surfaces) 36a) Andreev Belyaev (i960), 265-68 (Starting impulse and mechanism of initiation) 268-70 (Initiation by heat) 270-73 (Initiation by flame) 273-86 (Initiation by shock or friction) 287-89 (Initiation of expln in projectiles on hitting a target) 36b) J. Favier C. Fauquignon, MP 42, 65-81(1960) (Initiation of expls. and transmission of detonation) 37) D.B. Moore J.C. Rice, Detonation of Secondary Explosives by Lead Azide , SRI (Stanford Research Institute), Poulter Laboratories, Technical Report 004-60(1960) 37a) S.J. Jacobs, AmRocket-... 

Fuzes, a term often used synonymously with fuse, are devices which initiate high explosives (detonating fuze) or low explosives (igniting fuze) in projectile ammunition or pyrotechnics after launch, or which activate the main charge of a mine, bomb, or other nonprojectile ammunition. In addition to an explosive, fuzes incorporate some kind of mechanical or electronic initiation device ... 

The explosive charge is placed into the firing chamber of the mortar. The projectile is placed into the mortar too. The rider on the graduated scale that records the maximum swing is adjusted to zero position. Finally, the initiation of the explosive charge is performed. 

Figure 4 shows experimental threshold data (8) for steel rods impacting bare and covered PBX9404. Shock pressure in the explosive (P) is plotted against the effective diameter of the projectile. The effective diameter is either the diameter of the projectile for bare explosive or the diameter of the initial shock in the explosive for covered explosive. It can be seen that to a first approximation the relationship between P and Dq is the same as that between P and D. ... 

An experimental study of the projectile impact initiation of detonation in cased explosives has revealed both shock and non-shock regimes. The shock regime can be described by an empirical critical energy criterion for detonation. However, a full description of the processes leading to detonation or to violent reaction cannot be based on such a criterion and requires a full description of hot spot formation and growth mechanisms. 

The penetration velocities of projectiles interacting with explosives initiated by the projectile have been found to be much lower than the penetration velocities of inerts of the same density. Studies of projectile penetration dynamics in inert and reactive targets have been performed using the Eulerian reactive hydrodynamic code 2DE described in Appendix C. 

It should be noted that a detonator, by itself, cannot initiate a large quantity of an insensitive HE (TNT, PA or Comp B), which is used as a filler for projectiles, bombs, mines, etc. Such HE s must not be too sensitive in order not to create hazard in handling and transportation. They could be,however, initiated by a very powerful detonator, such as contg a large quantity of a sensitive explosive such as LA, LSt or MF. This would also be undesirable because handling and transportation of large quantities of such expls is very hazardous... 

The hot gases from the percussion primer permeate thru the baffle and initiate the BkPdr delay pellet, which burns under the reproducible pressure conditions which are obtained within the delay element housing, with a delay time of 0.035 sec. When the BkPdr element has burned thru, a spit of flame impinges on a detonator loaded with LA a true detonation develops and progresses successively thru the Tetryl-loaded lead-out, the Tetryl-loaded booster lead-in, the Tetryl-loaded booster and Explosive D (Ammonium Picrate) -loaded main (or bursting) charge of the projectile. 

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