Ignition dynamics of high explosives

The laser ignition of the explosives HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine C₄H₈N₈O₃, β-phase and δ-phase), PBX 9501 (95% HMX, 2.5% Estane, 2.5% BDNPA/BDNPF), TATB (1,3,5-triamino-2,-l, trinitrobenzene, C₄H₈N₈O₆ and PBX 9502 (95% TATB, 5% Kel-F) and aged PBX 9502 has been conducted with the intent to compare the relative ignition characteristics of those explosives and to investigate the effect of beam profile, binder addition, and porosity. It has been found that there was little difference between a clipped gaussian beam and a top hat profile on the laser ignition of HMX. It has been observed that the addition of binder in the amounts present in PBX 9501 resulted in longer ignition delays than that of HMX. In contrast to HMX, the addition of binder to TATB in PBX 9502 showed no measurable effect. Porosity effects were considered by comparing the ignition of granular HMX and pressed HMX pellets. Porosity appears to increase ignition delay possibly because of an increased effective absorption scale or increased gaseous product advection. This porosity effect also yielded longer ignition delays for δ-phase (porosity of 12%) than for β-phase (porosity of 6%) HMX. In order to simulate ignition in voids or cracks, the standard ignition experiment was modified to include a NaCl window laced at variable distances above the sample surface. When ignition experiments were performed at 29 W/cm² P and 38 W/cm² a critical gap distance was observed of 6 ± 0.4 mm below which ignition was severely inhibited. This result underscores the importance of gas phase processes in ignition and illustrates that conditions can exist where simple ignition criteria such as surface temperature are inadequate.