Performance and lifetime assessment of magnetoplasmadynamic arc thruster technology

Abstract

A summary of performance and lifetime characteristics of pulsed and steady-state magnetoplasmadynamic (MPD) thrusters is presented. The technical focus is on cargo vehicle propulsion for exploration-class missions to the moon and Mars. Relatively high MPD thruster efficiencies of 0.43 and 0.69 have been reported at about 5000-s specific impulse using hydrogen and lithium, respectively. Efficiencies of 0.10 to 0.35 in the 1000-to 4500-s specific impulse range have been obtained with other propellants (e.g., Ar, NHa, NI). Electrode power losses of less than 20% at megawatt power levels using pulsed thrusters indicate the potential of high MPD thruster performance. Extended tests of pulsed and steady-state MPD thrusters yield total impulses at least two to three orders of magnitude below that necessary for cargo vehicle propulsion. Performance tests and diagnostics for life-limiting mechanisms of megawatt-class thrusters will require high-fidelity test stands, which handle in excess of 10 kA, and a vacuum facility whose operational pressure is less than 4 x 10 ~2 Pa. Nomenclature e = electronic charge, 1.6 x 10 ~1 9 C g = gravity acceleration, 9.81 m/s 2 H 0 = unheated propellant enthalpy, J/kg A P -specific impulses, s J = arc current, A (J 2 /m) c = onset parameter, A 2 -s/kg k = Boltzman's constant, 1.38 x 10~2 3 J/K m = mass flow rate, kg/s P = input power, W P a = power loss to anode, W P e = input electric power, W P L = power to water cooling system, W T = thrust, N T e = electron temperature, K V a = anode voltage drop, V AK = velocity increment required for a mission, m/s v = average exhaust velocity, m/s v\ = thrust efficiency f/ th = (P -P L )/P, thermal efficiency (j > = work function of anode material, V

High Power MPD Thruster Performance Measurements

“…For the self-field baseline thruster operating with an argon mass flow rate of 0.25-g/s, the specific impulse reaches a value of nearly 7000 s before the occurrence of onset. At 0.5-g/s, the maximum specific impulse is approximately 6300 s, while at 0.75-g/s the maximum specific impulse before onset is around 6000 s. These I sp values are substantially higher than prior experimental data obtained with other self-field MPD thruster geometries operated with argon propellant, 12,13,26 and should be repeated to confirm their validity.…”

“…12,13,27,28 Although not used in the self-field performance data reported below, this option is available through the use of an un-cooled magnet consisting of insulated 4-AWG copper cabling wound on a 0.2-m (8") OD pvc pipe. The insulated cabling is continuously wound in 7 layers, with 18 turns per layer.…”

“…As such, thrusters operating with noncondensable propellants are also being developed at various research facilities to better understand and improve gas-fed thruster performance. [19][20][21][22][23][24][25] Although gas-fed MPD thruster efficiencies have typically been lower than devices operated with lithium, 12,13,26 recent numerical models indicate that significantly improved performance can be achieved through the use of modified electrode geometries and the proper application of expansion nozzles and/or applied magnetic fields. 7,27,28 The use of non-condensable gas propellants mitigates possible ground handling and spacecraft contamination issues associated with lithium propellant, and may provide efficient thruster performance over a wide range of operating conditions.…”

“…For a detailed history of MPD thruster development, the reader is referred to a number of review papers that chronicle the progress and challenge of this promising in-space propulsion technology. [11][12][13] A variety of thruster geometries have been operated using different propellants, with lithium vapor propellant providing the most efficient performance to date. Lithium-fed MPD thrusters developed by the Moscow Aviation Institute in Russia have reported efficiencies of around 45%, with specific impulse (I sp ) values approaching 5000 s at power levels exceeding 100-kW.…”

A Survey of Propulsion Options for Cargo and Piloted Missions to Mars

“…In general, however, results have been in the 10 -35% in range at 1000 -4500 s specific impulse using Ar, NH 3 , and N 2 as propellants [26].…”