Preliminary Pulsed MPD Thruster Performance

A thrust stand was modified, and a primary calibration technique was developed to evaluate the performance of applied-field pulsed magnetoplasmadynamic thrusters (MPDTs) for 10 kW class solar electric orbit transfer vehicle (SEOTV) missions. The NASA Lewis Research Center (LeRC) 30 kW thrust stand was modified to accept high current pulses delivered to the MPDT. A pendulum system was developed to deliver an in situ primary impulse calibration of the thrust stand. An MPDT was built to accommodate future testing of an externally heated cathode and to allow rapid diffusion of the applied magnetic field. The thruster was driven by a pulse forming network to anode-cathode powers of 350 kW. The thrust stand was calibrated for impulses ranging from 4.8 to 12.2 mN-s, and thruster discharges generated impulses ranging from 2.6 to 4.5 mN-s after subtracting the cold gas flow impulse. Nomenclature Ep =pulse energy, J go =9.81 m/s2 I =impulse bit, N-s Isp =specific impulse, s m =mass, kg Ý m =mass flow rate, kg/s v =velocity, m/s η =thruster efficiency τp =pulse time, s Introduction Performance data and recent systems studies have shown that the use of 10 kW class pulsed applied-field magnetoplasmadynamic thrusters (MPDTs) for primary satellite propulsion may reduce launch costs for spacecraft more than 1000 kg in mass.1-3 Extrapolation of power level trends for communications satellites indicates that next generation satellites can be expected to have power generation in excess of 10 kW, and could cut launch costs through the use of primary electric propulsion.4 Pulsed MPDTs are effectively 10 kW class gas-fed pulsed plasma thrusters (PPTs) since they make use of similar acceleration mechanisms and nearly identical operation philosophies. Each type of thruster is designed for a uniform pulse, and power scaling is accomplished by adjustment of the duty cycle. While PPTs have been flight tested on the LES 6, LES 8/9, and NOVA spacecraft, Japanese researchers have space tested three pulsed MPDT systems including the Electric Propulsion Experiment (EPEX) currently in orbit on the Space Flyer Unit (SFU).5-11 The demonstrated capability and renewed interest in PPTs may serve to bring a spotlight on the benefits of pulsed thrusters. However, performance measurements of a thruster closely approximating the type necessary for primary propulsion must justify further development of pulsed MPDTs. The purpose of this work was to develop a system capable of accurate performance measurements of a pulsed MPDT in order to facilitate testing of a thruster designed to meet orbit transfer requirements. MPDT development in the U.S. has focused primarily on high power (>100 kW) steady-state thrusters with quasi-steady pulsed operation being investigated in the absence of facilities capable of handling the large heat and propellant fluxes of high power thrusters.12,13 Efforts to date have shown the benefits of high power operation and the use of applied magnetic fields.13 Japanese researchers have developed the pulsed MPDT as a propulsion system capable of operation over a wide range of input power, and pulsed applied-field thruster efficiencies in excess of 50 percent using hydrogen and