Investigation of Baffled Shock Tubes for Low Velocity Ram Accelerator Start

Abstract

The ram accelerator is a hypervelocity launcher in which a projectile is injected into stationary tubes filled with premixed fuel and oxidizer. Combustion is initiated as the projectile passes, creating a high pressure base that travels with it to produce thrust. To start the ram accelerator process, the projectile must be injected into the mixture at a minimum Mach number. Preliminary investigations from previous work indicate successful ram acceleration can be achieved by accelerating the propellant towards the projectile, ultimately reducing the projectile’s entrance velocity while keeping the same in-tube Mach number. Accelerating propellant towards an oncoming projectile for a baffled tube ram accelerator configuration was investigated. Experimental shock wave propagation data were collected from a baffled shock tube and a computational fluid dynamics simulation was constructed in ANSYS Fluent. Experimental data provided estimates for pressure ratios, diaphragm burst times, and diaphragm scoring configurations for a low velocity start configuration. Both experimental and numerical results showed strong shock wave attenuation throughout the baffles, with a 75% strength reduction in 13 diameters compared to a smooth bore case. The contact surface velocities from the simulation provided a wave arrival time in the driven section used to time with a projectile bursting the entrance diaphragm. The results of thisinvestigation designed an experiment to establish initial proof of concept for low velocity start in methane-oxygen with carbon dioxide and methane-air-oxygen mixtures.