Contact pressure distribution and transition in solid armatures [EM launchers]

Previous theoretical predictions of the transition velocity of solid armatures based on the velocity skin-effect (VSE) usually assume that adequate rail/armature contact pressure exists at the rear armature surface. It has also been shown that the rear angle (φ) of a triangular armature contact region should be minimized but that for φ<30° it is probably not possible to achieve the required contact pressure distribution at “start-up” when magnetic forces are small. Transition velocities are derived for conditions where contact pressure does not initially exist (or is inadequate) at the armature rear for axial distances from 0 to >10 mm and for values of φ from 12° to 90°. It is shown that the absence of adequate contact pressure at the rear armature surface results in significant reductions in the transition velocity in reasonable agreement with experimental results reported previously. An armature concept to overcome these reductions in transition velocity is discussed in which the rear armature contact regions (with φ<15°) are supported by the addition of a rear support attached to but insulated from the armature trailing arms. It is estimated that the transition velocity of such an armature, of 90 mm diameter and 4 kg launch package mass, would be about 2.0 km/s