Initial operation, modeling and optimization of a low-velocity augmented railgun

An electromagnetic launcher is being studied for ejection of 350-g projectiles at velocities of up to 140 m/s. The electromagnetic approach is desirable because of the variable ejection velocity, high payload fraction, and lack of pyrotechnics. The low velocity of the projectile allows the use of low-voltage, solid-state switching in the pulsed power, but makes obtaining high-efficiency more difficult. The launcher is a 4.5-cm square bore, 80-cm long fully augmented system, with copper inner rails and aluminum augmenting rails. The launcher is driven by 2, 5000 /spl mu/F, 5 kV capacitor banks, with a thyristor switch and diode crowbar for each bank. The banks are connected to the launcher with copper buswork. The diagnostics include the current in each bank; B-dot probes for the position of the projectile, a laser time-of-flight velocity measurement at the muzzle and the armature voltage, measured from the muzzle. The armature voltage measurement requires substantial corrections to remove the induced voltages from the augmenting and armature fields. The projectile is a Delrin body with A1100 wires as the armature. Six to ten, 0.32-cm diameter wires are used. The wires run through the projectile in separate holes, and then are bent into a staple shape in grooves along the sides of the projectile. The wires are then sanded to obtain a flat surface for contact to the rails. The JxB forces that propel the projectile forward also force the wire sides into the rails to maintain good electrical contact. The design goals of 350-g and 140 m/s are achieved, but at an overall electrical efficiency of only 7%. The electrical parameters of the various circuit elements are measured, and the early-time skin resistances of several elements lead to the low efficiency. Reconfiguring these elements to have lower early time resistance is predicted to improve the efficiency to the 15% range. Results of circuit measurements and models will be presented.