Energy efficiency trends in a coaxial gun plasma engine system.

Abstract

The efficiency of converting capacitor stored energy into energy in the exhaust stream of a repetitively pulsed two-stage coaxial plasma engine has been determined calorimetrically for various capacitances, voltages, gas densities and density profiles, and gun geometries. Observed efficiency trends have led to empirically determined optimum gun lengths, electrode . radius ratios, and energy storage voltages. Energy efficiencies of 63% have been measured so far, without having reached a leveling off of the efficiency increase trend. The largest gains in efficiency have been realized by a new approach to gas pulse injection, which provides more complete filling of the gun barrel with propellant by axial rather than radial injection prior to discharge. The most efficient gun configuration resulting from the calorimetric studies was placed on a thrust balance along with its associated capacitor energy storage system, and thrust data were obtained. Propellant mass flow rate also was measured, and the results were combined with the thrust data to determine over-all efficiencies (770 = T 2/2raP) and specific impulses (/sp = T/mg). The over-all efficiencies, so measured, were typically significantly lower than the energy efficiencies measured calorimetrically, since a large fraction of the injected propellant was not utilized in the discharge. Adjustment of r?o and /sp, taking into account the unused mass, gave rise to significantly higher 770 but at undesirably high jTsp. Additional steps taken to improve the engine system performance are discussed.