Unsteady Effects on Ram Accelerator Operation at Elevated Fill Pressures

Experiments show that as the projectile acceleration of the ram accelerator is increased, by increasing the propellant fill pressure or reducing the projectile mass, its performance begins to deviate significantly from that predicted by the widely used quasi-steady control volume model. At high fill pressures, experimental velocitydistance data are overpredicted by the quasi-steady model for thrust determination when using a real-gas equation of state for the combustion products. The primary reason for this behavior is that the mass of the propellant accumulating in the control volume at high fill pressure approaches the mass of the projectile itself. A revision to the control volume model to account for unsteady flow effects indicates that the thrust coefficient vs Mach-number profile obtained for high-pressure conditions is consistently lower than that obtained with the quasi-steady model. This deviation correlates with experimental results obtained in a 38-mm-bore ram accelerator at fill pressures in the range of 15-20 MPa. The best agreement with high-pressure experimental data is obtained using the unsteady modeling approach in conjunction with the heat release vs Mach-number profile calculated using the Boltzmann equation of state.