On high explosive launching of projectiles for shock physics experiments

Abstract

The hydrodynamic operation of the "Forest Flyer" type of explosive launching system for shock physics projectiles was investigated in detail using one and two dimensional continuum dynamics simulations. The simulations were numerically converged and insensitive to uncertainties in the material properties; they reproduced the speed of the projectile and the shape of its rear surface. The most commonly used variant, with an Al alloy case, was predicted to produce a slightly curved projectile, subjected to some shock heating and likely exhibiting some porosity from tensile damage. The curvature is caused by a shock reflected from the case; tensile damage is caused by the interaction of the Taylor wave pressure profile from the detonation wave with the free surface of the projectile. The simulations gave only an indication of tensile damage in the projectile, as damage is not understood well enough for predictions in this loading regime. The flatness can be improved by using a case of lower shock impedance, such as polymethyl methacrylate. High-impedance cases, including Al alloys but with denser materials improving the launching efficiency, can be used if designed according to the physics of oblique shock reflection, which indicates an appropriate case taper for any combination of explosive and case material. The tensile stress induced in the projectile depends on the relative thickness of the explosive, expansion gap, and projectile. The thinner the projectile with respect to the explosive, the smaller the tensile stress. Thus if the explosive is initiated with a plane wave lens, the tensile stress is lower than that for initiation with multiple detonators over a plane. The previous plane wave lens designs did, however, induce a tensile stress close to the spall strength of the projectile. The tensile stress can be reduced by changes in the component thicknesses. Experiments verifying the operation of explosively launched projectiles should attempt to measure porosity induced in the projectile: arrival time measurements are likely to be insensitive to porous regions caused by damaged or recollected material.

Explanation of anomalous shock temperatures in shock-loaded Mo samples measured using neutron resonance spectroscopy

“…Proton radiographs were subsequently obtained of a similar Forest Flyer design, initiated with a plane-wave lens, showing curvature of the projectile in close agreement with the simulations in the region of the projectile affecting the W-doped Mo. 5 The simulations were used to estimate the variation of compression, temperature, and particle velocity in the doped Mo as a function of time. In principle, the time-dependent fields could be used to simulate the neutron attenuation with the time-dependent neutron spectra.…”

“…The latter is generally not designed to be triggered externally with a short latency or high a priori precision in delivery of the diagnostic pulse. For the trial NRS experiments, the projectile was accelerated by detonating solid chemical explosive, using the "Forest Flyer" design, 5 which gave an uncertainty in impact time from the trigger signal of o͑100͒ ns. The Forest Flyer produced a nonisotropic distribution of high speed fragments.…”

“…According to continuum dynamics simulations, the design used for the Mo NRS experiments was subject to hydrodynamic flows which deform and damage the projectile. 5 In particular, the case profile is likely to produce curvature of the projectile. The projectile would also be accelerating on impact.…”

Shock and release temperatures in molybdenum: Experiment and theory

“…The high explosive launcher used the Forest Flyer design [22]. With this system, the projectile was slightly dished on impact, though this should not affect the pyrometry measurement significantly.…”

Laser produced thin metallic planar mini-flyer generation using fiber optic plate

“…The ability of a flyer to investigate the compressed as well as impacted material under cold conditions (without pre-heating), makes it an ideal tool for such research. Laser produced mini-flyers offers several advantages over the other techniques such as gas gun, exploding wires (Burr, 1967;Neff et al, 2009) and chemical explosives (Swift et al, 2007) for investigating the dynamic properties of materials. In particular, the small dimensions involved (of millimeter order) and the low cost of laser shots offers advantages such as good repetition rate, reduction in experimental length and time, high shot throughput, simplified experimental apparatus and reasonably moderate energy (few hundreds of mJ) requirement to achieve flyer velocities of few km/s.…”