Investigation of low velocity starting techniques for the ram accelerator

Abstract

I. Introduction An investigation of transition from the conventional gun to the ram accelerator, at velocities below 800 m/s, has been carried out at the University of Washington. Attempts to repeat the ram accelerator performance achieved at low entrance velocities in some of the very first experiments, in which a gunpowder system was utilized, were made with projectiles that carried no pyrotechnics. This investigation used propellants consisting of CH 4/O2/CO2 and C2H6/O2/ CO2 at 25 and 20 atm, respectively. Various projectile geometries were fired at an entrance velocity of 780 m/s to examine the effects of nose cone angle, projectile length, and projectile-to-tube diameter ratio. The potential of using a short ignitor stage to facilitate ram accelerator operation in propellants that would not otherwise ignite was also explored. It was found that a small change (-2%) in CO2 dilution would dramatically change the results from a unstart to falloff' in a distance of ~1 m, regardless of the projectile geometry, when using the CH4-based propellant. This same result was attained with the short stage, even though the total diluent content had to be increased to observe a wave fall-off. Experiments in the C2H6based propellant showed similar sensitivity to small variation in CO 2 dilution, however, a successful start was achieved with a projectile having a larger diameter. The results of this investigation confirmed that low velocity ram accelerator operation is feasible without external systems, even though the process is very sensitive to propellant composition and projectile geometry. The ram accelerator is a hypervelocity launcher in which a projectile, similar in shape to the centerbody of a ramjet, travels supersonically through a tube filled with premixed gaseous fuel and oxidizer.1 A schematic of the idealized flowfield of the thermally choked ram accelerator propulsive mode is presented in Fig. 1. The propellant flowing over the nose of the projectile is ram compressed by a series of reflected shocks, passes the projectile throat (minimum flow area), and expands supersonically relative to the projectile behind the throat. The propellant then encounters a normal shock system followed by a subsonic combustion zone which releases the chemical energy to the flow. The combustion-supported normal shock system generates a high base pressure which accelerates the projectile down the tube. The thermally choked ram accelerator propulsive mode is observed at projectile velocities below the Chapman-Jouguet detonation velocity of the combustible gas mixture.2'3 The investigation described here lies completely within this subdetonative velocity regime.