The influence of density gradient of driving gas on projectile launching velocity

The most common hypervelocity propulsion systems are light gas guns. Especially, the ability of two-stage light gas guns is suitable to accelerate projectile at velocities ranging from 2 km/s to 9 km/s. However, velocities higher than 10 km/s are demanded eagerly for ballistic limit equations on on-orbit impacts and meteoroids. In order to enhance the launch performance of the light-gas guns, a concept of using density-gradient gas as the driving gas instead of single helium or hydrogen gas has been proposed. An analytical acceleration model of the projectile in the launch tube with constant cross-sectional area is deduced. The launch process can be divided into three stages. The first stage is the projectile driven by the first shock wave. The second stage is the projectile driven by shock waves reflected on the gas interface. The last stage is the projectile caught up by the rarefaction wave created by the suddenly stop of the piston. The comparison in launch performance between neon-helium density-gradient driving gas and helium driving gas is made, and the influences of parameters of the gradient gas on the launch performance are studied. Results show that the neon-helium density-gradient driving gas can improve the launch velocity by about 0.4−1.4 km/s or lower the maximum base pressure by about 0.2−0.9 GPa. The biggest influential factors for the launch velocity and the maximum base pressure are the density of high density gas and the piston velocity, following by the initial gas pressure and the gaseous polytrophic index. High density gas with both high density and high gaseous polytrophic index would be the prior choice due to the reason that higher gaseous polytrophic index could make the maximum base pressure lower. The launch velocity has little correlation with the ratio of high density gas. However, low ratio of high density gas could lower the maximum base pressure.