The effect of acceleration and exit velocity on hypersonic projectiles launched by a ground-based railgun

In this paper, the effects of projectile acceleration profile on interior and transition ballistics are investigated in order to design HVP (Hyper-Velocity projectile) for railgun. Our railgun facilities accelerate projectile to 600 – 2501 m/s in order to evaluate the effect of exit velocity on sabot separation. However, sabot melt prevents us from observing detail sabot separation and shock wave interactions. Thus, in this paper, CFD-RBD method is applied to calculate the projectile acceleration and sabot separation motion due to unsteady aerodynamic force. In the interior calculation, acceleration behavior is approximated by the cubic spline interpolation using our experimental result in V_exit = 658, 1268, 1875, 2048, 2501 m/s. Firstly, in the interior ballistic calculation, the projectile and sabot induce compression waves due to acceleration. In 0.6 ms, the single normal shock wave stands ahead of projectile. The precursor shock wave reaches the end of tube and propagates outside of tube. When the precursor shockwave propagates outside of tube, Mach disk stands just before the precursor shock wave due to expansion. Especially, the cases of V_exit ≥ 1875 m/s show similar pressure distribution and precursor shock wave propagation outside of tube. On the other hand, supersonic exit velocity, in V_exit = 1268 m/s, increases distance between precursor shock wave and Mach disk. Also, in V_exit = 658 m/s, although Mach disk is close to the muzzle, precursor shock wave propagates far away from Mach disk, which generates completely different pressure distribution. Secondary, in transition ballistic calculation, sabot separation motion does not depend on projectile exit velocity in our railgun configuration. From this result, the design of HVP need not to consider sabot trajectory variation even if exit velocity changes from supersonic (V_exit = 658 m/s) to hypersonic (V_exit = 2501 m/s) regime.