Performance of Quasi-Steady MPD Thrusters at High Powers

Abstract

T familiar axisymmetric MPD thruster is considered with a conical tungsten tip cathode, (-|, |, and f in. diam), and a copper plate anode-orifice, (2, 3, and 4 in. diam). The performance of such thrusters is investigated, with millisecond pulses, over a wide range of experimental conditions. The five noble gases are used as propellants, at flow rates in the range 0.1 to 100 g-sec. MPD currents 5 to 50 ka are employed and the corresponding input powers are in the range 0.1 to 10 Mw. Thrust is determined from impulse measurements with wellknown current waveforms, while instantaneous measurements are made for all other variables: m, J, and V, flow rate, current and voltage. A detailed analysis of the experimental results shows that the MPD performance is limited by a critical value of (J/m), which depends only on MPD geometry and on propellant properties. MPD operation, beyond the critical point, becomes increasingly objectionable on account of instabilities, sharp rise of the voltage, erosion and participation of spurious propellant. At the limit of credible MPD operation, the specific impulse depends on propellant properties only, and varies from about 3000 sec for lighter propellants, to about 400 sec for heavier propellants. In the same range, the maximum credible MPD efficiency varies from about 30% to about 10%. However, the efficiency depends on other MPD conditions also.