Development of an Expedient Two-Stage Gas Gun

Abstract

The ram accelerator is a hypervelocity launcher that accelerates projectiles through a tube filled with gaseous propellant. Combustion is initiated by the passage of the projectile and will overtake the projectile unless it enters the tube at a sufficiently high Mach number. With economical propellant mixtures such as methane/air, the projectile must be launched into the tube at approximately 1000 m/s. This limit on ram operation restricts projectile launch duties in smoothbore ram accelerators to powder guns and light gas guns.The baffled-tube ram accelerator (BTRA) retards the travel of the combustion-driven shock front with a series of baffles, preventing combustion from overtaking the projectile at low Mach numbers. Recent experiments with low velocity starting in the University of Washington’s BTRA have confirmed that successful ram operation can occur at a projectile entrance velocity of 650 m/s in a BTRA operating with methane/air propellant. These results open the door to using poorer-performing launchers than have previously been contemplated for ram projectile launch. A subscale two-stage gas gun was constructed to determine the feasibility of ram starting with a gun that achieves these low entrance velocities through pressure and temperature multiplication of an extremely low-cost propellant, such as nitrogen or compressed air. Projectile launch conditions were predicted with the isentropic relations and gun performance was compared to a model of a single-stage infinite-breech gas gun. A high-speed camera was used to directly measure muzzle velocity and to infer pressure at the nozzle. The measured ballistic efficiency of the launcher indicates that substantial increases in propellant temperature and pressure were achieved, in line with predictions.