Analysis of the Mechanically Choked Ram Accelerator

Abstract

The ram accelerator is a hypervelocity launch concept which uses a ramjet-like propulsive cycle to accelerate a projectile to hypersonic velocities. Research surrounding this technology has thus far been primarily focused on the thermally choked mode of operation, which relies on acceleration to sonic conditions of subsonic flow after the diffuser via constant area heat addition in order to bind the end of the combustion zone. However, early work suggested the possibility of using a nozzle to mechanically choke the flow, allowing the flow to be accelerated to greater than Mach one at the exit. Numerical work done with this mode demonstrated a substantial increases in thrust, sometimes double the non-dimensional thrust produced by the thermally choked mode at the same non-dimensional heat release value. Additionally, due to the mode’s ability to have the flow exit supersonically, the mode’s flight Mach number was shown to be able to exceed the Chapman-Jouguet detonation velocity, expanding the operable Mach number range. In order to validate these claims, a set of projectiles was manufactured and fired in the University of Washington’s ram accelerator laboratory. Over the course of these experiments the projectile showed a consistent and meaningful increase in both thrust and acceleration as compared to their thermally choked counterparts by an average of 50.1% and 20.5% respectively. This was in spite of increased mass accrued from the tail sections. The mode showed difficulty in starting in the first baffled tube when entering below 900 ms and showed consistent structuralfailure between 1150 - 1200 ms . Due to system limitations, these results were only shown through the baffled tube section of the system, and railed tube operation, which would be needed to fully validate theoretical claims, was unable to be successfully achieved. Regardless, the test series showed promising results and further testing will be required to validate the mode’s effectiveness as compared to theory.