Ram accelerator operating characteristics at elevated fill pressures

An experimental investigation of the starting and operational characteristics of a 38-mm bore ram accelerator with propellant till pressures greater than 5 MPa is in progress. Successful starts with 60 - 124 gm projectiles have been achieved using methane/oxygen/niuogen propellants at fill pressures ranging from 6 to 15 MPa. At fill pressures of 8.5 MPa and above, it was found that projectiles having the nominal throat diameter (29 mm) required a minimum entrance velocity of 1250 m/s, which is about 100 m/s faster than the minimum needed to successfully start the ram accelerator at fill pressures below 7.5 MPa. Projectiles with a reduced throat diameter (25 mm) and a solid magnesium nose cone were successfully started at 611 pressures up to 15 MPa with entrance velocities around 1300 m/s. The average accelerations achieved using high pressure stages were in general less than predicted for thermally choked operation by the Boltzmann equation of state.