  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
insensitive to uncertainties in the material properties, and reproduced
measurements of the projectile. 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 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. 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. If the explosive is initiated with a
plane wave lens, the tensile stress is lower than 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 to verify the operation of explosively-launched projectiles should
attempt to measure porosity induced in the projectile: arrival time
measurements may be insensitive to porous regions caused by damaged or
recollected material.
