The Velocity and Efficiency Limiting Effects of Magnetic Diffusion in Railgun Sliding Contacts

The authors identify and characterize a velocity and efficiency limiting effect called contact velocity skin effect (VSEC) which occurs in the interface of a railgun's sliding contact. Despite enormous contact forces, the armature will remain separated from the rail by a thin adsorbed layer, on the order of 10 to 15 angstroms in this investigation. Electrical conduction through the layer is via quantum tunneling which is more resistive than classical conduction through the conductor. A comprehensive theory to account VSEC is presented and is applicable to all constant gradient electromagnetic launchers. The theory not only accounts for the velocity-limit effect, but also predicts a maximum efficiency. Theoretical parameters include the inductance gradient, system resistance, projectile velocity, and sliding contact area. Theoretical predictions are compared to experimental data from a conventional and an augmented railgun. The augmented railgun is a 3-turn augmented system with 40 mm bore x 750 mm length typically operating at 250 kA peak current with a projectile velocity of 300 m/s. The conventional railgun is 40 mm bore x 4000 mm length which typically operates at 850 kA with a projectile velocity of 1600 m/s. The characterization of VSEC predicts velocity saturation and efficiency roll-off in electromagnetic launchers and provides new insights into EML operation and physics especially with regard to the armature and its contacts.