Scale Effects on Solid-Propellant Coaxial Magnetoplasmadynamic Thruster Performance

An experimental investigation of solid-propellant, nonsteady (pulse durations of ∼1 ms), magnetoplasmadynamic (MPD) thruster operation as a function of thruster scale (size) is qualitatively discussed. Measurements include the electrical characteristics, impulse bit (thrust stand), exhaust velocity (time-of-flight Langmuir probe system), and ablated propellant mass for each scaled thruster. The same number of thruster shots is used to collect data at each operating point. The baseline thruster is found to operate in one of two electrical modes (high and low), in a random fashion, at a fixed energy. As the absolute energy level is increased, the probability of high-mode operation is found to increase. The scaled thrusters (33% smaller and 25% larger based on nozzle exit radius) operate in only one electrical mode. Furthermore, these tests show that the smaller thruster's operating characteristics at equal thruster energy exhibit higher currents, impulse bits, and propellant consumption, whereas the other two thrusters show the effective Maecker's law coefficient b e f f increasing as a function of the time integral of the MPD discharge squared current Ψ, as well as the thruster efficiency increasing with thruster energy. These data imply that the smaller thruster operates in a different regime than the larger thrusters. This operational regime difference is likely related to the smaller thruster's higher current densities at constant thruster energy.