Characterization of Gas Dynamic Effects in Baffled Tube Ram Accelerators

Abstract

The ram accelerator is a hypervelocity launch technology which uses a ramjet-like propulsion cycle to accelerate payloads to velocities in the kilometers-per-second range. The smooth bore ram accelerator (SBRA) has shown impressive performance at high velocities, drawing attention for its potential as an orbital launch device, as well as defense, emergency response, and mining possibilities. Recent developments have extended the capability of the system in the low-velocity regime. The most novel development is the inclusion of baffles within the launch tube, creating the baffled tube ram accelerator (BTRA). These baffles serve to isolate combustion regions within the launch environment, and have been proven to allow stable operation with propellants of higher energy densities. This work investigates secondary effects which arise from the more complicatedgasdynamic environment of the BTRA. Testing of the BTRA system has confirmed that predictions from quasi-one-dimensional SBRA theory are not always accurate in this more complicated geometry. In conjunction with computational efforts from other works, this study characterizes the impact of varying baffle dimensions in inert gas and combustive environments. Inert gas phenomena were investigated pertaining to spacing between baffles, baffle wall thickness, and projectile-to-bore variation, and using a a novel shadowgraph imaging technique. Performance phenomena were characterized by variation of baffle inserts within the BTRA test system, with projectile velocity and thrust or drag measured for both inert and non-combustive configurations. The discovery of starting capabilities below established minimum limits permitted easier injection of projectiles into BTRA systems, with profound implications on large-scale implementation of the technology. Additionally, gasdynamic environments were imaged for projectiles of differing bore occlusion, suggesting the development of a distinct operating mode at high occlusion percentages which explains previous difficulties and inconsistencies in experiments with near-full-bore projectiles. Finally, it was found that both the spacing between baffles and the baffle chamber radius had direct effects on projectile thrust environment, permitting increased performance at the expense of energy density tolerance, and suggesting that optimization of the BTRA design may be a multi-dimensional effort with promising potential.