A Model for Predicting Transition in Railgun Fiber Brush Armatures

The PEGASUS railgun at the French-German Research Institute of Saint-Louis is capable of launching 0.3-kg projectiles with muzzle velocities greater than 2500 m/s without mechanical failure of the sabot which is made of fiber-reinforced plastics. Unfortunately, for these high velocities, the metallic fiber brush armature shows contact transition. One of the main long-term objectives is to improve the railgun performance by delaying this armature transition, leading to an increase of the overall railgun efficiency and a reduction of the rail erosion. In order to obtain a better understanding of the transition mechanisms, several series of PEGASUS shots were examined with projectile masses ranging from 250 g up to 2 kg. As a result, a model was developed in order to predict the transition. It determines the mass loss of the brush armature due to electrical heating, mechanical shearing, and viscous heating. It can be shown for the low-weight high-velocity projectiles that friction heating becomes a predominant transition mechanism. For the 13-stage PEGASUS railgun, the transition model was implemented in a lumped parameter simulation code which takes into account the time-dependent armature voltage drop and the armature mass loss. The comparison of the computed transition time with measured values shows a very good agreement.