Analysis of the Mechanically Choked Ram Accelerator Propulsive Mode

At the inception of the ram accelerator concept, two projectile geometries were put forward for operation: thermally-choked and the mechanically-choked modes. Experimentation with the ram accelerator has thus far mainly used thermally-choked projectiles which utilize full-tube heating. This mode of operation loses out on potential thrust available from expansion of the combustion products while in contact with the tail cone of the projectile. The mechanically-choked variation features a smaller diameter section in between the inlet spike and the tail cone which functions as a combustion chamber similar to traditional ramjet engines. This allows combustion to be fully completed along the length of the projectile, which in turn means that expansion of the post-combustion gases can take place on the tail cone. Using an analytical model of this mode of operation, we tested a variety of different gas compositions to solve for the ideal projectile geometry for each mixture. This allowed us to find the non-dimensional thrust for each geometry and compare the result with the equivalent thermally-choked projectile results. We found that, consistently, the mechanically-choked mode of operation produced higher exit Mach numbers, leading to higher thrust across many different gas mixtures. The results from this study will be used to manufacture mechanically-choked projectiles for experimental validation in the ram accelerator in the future.