Performance criteria for EM rail launchers with solid or transition armatures and laminated rails

Velocity limitations of electromagnetic (EM) rail launchers with solid armatures due to the velocity skin-effect are assessed for a range of homogeneous materials (copper, aluminium, molybdenum, and tungsten). Performance criteria, as limited by armature melting in the region adjacent to the rail/armature contact surface and by rail-surface melting, are deduced analytically based on defined electrothermal reference conditions. The performance criteria predict the reference transition velocity at which the solid contact surface changes to mainly a melted contact surface, which is shown to be a fundamental property of the materials. The analysis shows that the critical factors in obtaining a high-transition velocity are a relatively high-resistivity armature material combined with high-temperature rail-surface coatings. For example, a transition velocity of >2.0 km/s for a molybdenum armature and a tungsten rail-surface coating on a copper rail are predicted, compared with <0.5 km/s for the aluminium/copper combination frequently used. Such increases in transition velocity could be important in the development of a solid armature as the first stage of a multistage launcher with transition, hybrid, and possibly plasma armatures as the later stages.<
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