Ram Accelerator Operating Limits, Part 1: Identification of Limits

Operational limits of the thermally choked ram accelerator are investigated. A quasisteady, one-dimensional model of the ram accelerator predicts it should be able to operate when the projectile Mach number is sufficient to maintain supersonic flow past the projectile throat and the heat release is sufficient to stabilize a normal shock upon the projectile body, but not enough to drive the shock over the projectile throat. These limits to operation can be expressed as relations of Mach number and heat release Q, and together they define a theoretical envelope of operation in the Q-M plane. The corresponding experimental envelope was investigated by injecting projectiles at different Mach numbers into methane/oxygen/ nitrogen and hydrogen/oxygen/methane mixtures at 25 and 50 atm, respectively. The results indicated a broad range of mixtures that were able to accelerate the projectile through the Chapman-Jouguet (CJ) detonation speed of the mixture. In the more energetic mixtures, the normal shock wave surged forward and immediately unstarted the projectile as it entered the test section. Unstarts were also observed when the projectile was accelerated beyond the CJ detonation speed, but because of the projectile's long intube residence time, these unstarts are believed to have been structural, not gasdynamic in nature.