Pulsed plasma thrusters for orbit transfer

Abstract

Introduction T HE use of electric propulsion for near earth missions has recently received increased attention with the recognition of possible long-duration missions involving payloads that require substantial levels of electrical power. If most of the mass of the power supply (including heat rejection) can be ascribed to the mission payload, then the optimum exhaust velocity can increase substantially above levels accessible with chemical thrusters. From classic mission analyses for electric propulsion, the optimum exhaust speed is given by u= (kectT)' where k=l-2 depending on the ratio of u to the necessary velocity increment At>; e is the efficiency of thrust power from total power, a. is the specific power associated only with the electric propulsion system, and r is the mission time. If the total power system has a specific power of 30 W/kg for example, but 80% of this system is needed by the mission, then a =150 W/kg. A thirty-day mission (with k = 2 and e = 0.5) would then have an optimum specific impulse of 2000 s. There are several electric propulsion concepts that could satisfy this specific impulse requirement. This Note discusses some aspects of a particular approach, based on a system that has already performed successfully (albeit modestly) on long term space missions, the pulsed plasma microthruster (PPT). The principal interest is the introduction of electrical propulsion in a manner that can evolve as electrical power levels increase, drawing on actual flight experience while maintaining the physical processes in the thruster. Such introduction would benefit from experience with the PPT design. That is, optimization of thruster performance can be sacrificed to system simplicity in order to match available power supplies and minimize risk. For example, the PPT uses a Teflon fuel block fed into the electrode region by a negator spring, and thus avoids repetitive valve operation and cryogenic fuel storage. Furthermore, arc discharges tend to operate at voltages that are independent of the charging voltage of the current source. The energy from a pulsed plasma thruster might vary slightly from shot to shot, but the requirement for voltage regulation of the power supply is much less severe than for other types of electric thruster. This Note reviews some aspects of mission performance using pulsed plasma thrusters. Recent experimental work suggests the possibility of a single thruster that could match increasing space-electric power levels.