A transitioning hybrid armature concept [for EM launchers]

Hybrid armatures have potential advantages over solid and plasma armatures because they could have a reduced axial length and consequently a lower mass than the former and lower resistive losses than the latter. However it is expected that their use could result in excessive arcing erosion damage to the rails at low velocity and increased forces in the rail/armature gap due to partial confinement of the arcing energy losses in this region. In this transitioning hybrid armature concept, it is proposed to overcome these problems by having initially a solid armature contact region, armature arcing surfaces of a low erosion rate metal and an “open” armature geometry. The latter allows the rapid expansion of arcing and erosion products radially inwards from the small rail/armature gap to the much larger bore volume. The armature will be mainly of aluminum alloy with the solid contact region being located at the armature rear with dimensions limited to achieve a predetermined transition velocity of about 1.0 km/s. The armature surface forward of the solid contact region is not in contact with the rails and is “clad” with a high temperature low erosion rate metal such as a tungsten/copper/nickel alloy to minimize the armature arcing erosion depth and consequently the are voltage and energy losses. The rail launcher electrical efficiency and performance limits are assessed for launch packages of 4 kg mass, 90 mm diameter, kinetic energy of 10 MJ and exit velocity of 2.3 km/s. It is estimated that this hybrid armature could result in an effective launcher efficiency about 4% higher than expected for a similar transitioning solid armature including the effect of the reduction in armature parasitic mass