Thomas Haag

The characterization of a miniaturized ionic liquid electrospray thruster for nanosatellite applications is presented. The thruster investigated features an emitter array of 480 emitter tips per square centimeter and a propellant tank with an entirely passive propellant supply; it is operated at a power level of less than 0.15 W. The paper presents energy- and mass-resolving beam spectroscopy of the packaged thruster system, as well as two independent thrust measurements. This allows derivation of thruster performance parameters under realistic firing conditions, including individual thruster efficiency contributions, specific impulse, and thrust. The total thruster efficiencies of 36%; specific impulse of , including all losses; and thrust of are presented at emission currents of for a device of . The current emission data without current decay of are presented with a maximum of 172 h.

A thrust stand for use with magnetoplasmadynamic (MPD) thrusters operated at powers up to 250 kW steady state has been built and tested. The stand was based on an inverted pendulum configuration which resulted in large displacements and high resolution. Up to 50 mm of deflection was observed under a force of 5 N. This large range of displacement significantly reduced the effects of facility induced vibrations on thrust measurements. A remotely operated system was provided for in situ calibration of the thrust stand prior to and immediately after data were obtained. Calibrations showed that thrust measurements were linear and repeatable to within a fraction of 1%. Structural distortions of the vacuum facility due to pumpdown were detected with an inclinometer located in the thrust stand base. Slope deviations as small as 10 arcsec could be compensated using a remotely controlled leveling motor. Early problems with magnetically induced tares caused by the thruster discharge current were reduced by rerouting high-current cables to decrease stray fields. Tares due to discharge current were on the order of 26 mN at 3000 A, and those due to an applied field current were 63 mN at 1400 A. The thrust stand was used with a water-cooled, applied field, steady-state MPD device at power levels up to 125 kW. Hot thruster firings as long as 1 h were performed. By precisely maintaining a level thrust stand base, thermal drift was held to about 2% of the full scale reading over this period. The remaining thermal drift could be subtracted from the thrust measurement to further reduce systematic error. Tares caused by the applied magnetic field were similarly removed. By subtracting tabulated discharge current magnetic tares, thrust measurement uncertainty was reduced to approximately 2% of the measured value.

The accurate, direct measurement of thrust or impulse is one of the most critical elements of electric thruster characterization, and it is one of the most difficult measurements to make. This paper summarizes recommended practices for the design, calibration, and operation of pendulum thrust stands, which are widely recognized as the best approach for measuring micronewton- to millinewton-level thrust and micronewton-per-second-level impulse bits. The fundamentals of pendulum thrust stand operation are reviewed, along with the implementation of hanging pendulum, inverted pendulum, and torsional balance configurations. The methods of calibration and recommendations for calibration processes are presented. Sources of error are identified, and methods for data processing and uncertainty analysis are discussed. This review is intended to be the first step toward a recommended practices document to help the community produce high-quality thrust measurements.

A torsional-type thrust stand has been designed and built to test pulsed plasma thrusters in both single shot and repetitive operating modes. Using this stand, momentum per pulse is determined strictly as a function of thrust stand deflection, spring stiffness, and natural frequency. No empirical corrections are required. The accuracy of the method was verified using a swinging impact pendulum. Momentum transfer data between the thrust stand and the pendulum were consistent to within 1%. Following initial calibrations, the stand was used to test a Lincoln experimental satellite (LES-8/9) thruster. The LES-8/9 system had a mass of approximately 7.5 kg, with a nominal thrust to weight ratio of 8.0×10−6. A total of 34 single shot thruster pulses was individually measured. The average impulse bit per pulse was 266 μN s, which was slightly less than the value of 300 μN s published in previous reports on this device. Repetitive pulse measurements were performed similar to ordinary steady-state thrust measurements. The thruster was operated for 30 min at a repetition rate of 132 pulses/min and yielded an average thrust of 573 μN. Using average thrust, the average impulse bit per pulse was estimated to be 260 μN s, which was in agreement with the single shot data. Zero drift during the repetitive pulse test was found to be approximately 1% of the measured thrust.

NASA’s Glenn Research Center has been selected to lead development of NASA’s Evolutionary Xenon Thruster (NEXT) system. The central feature of the NEXT system is an electric propulsion thruster (EPT) that inherits the knowledge gained through the NSTAR thruster that successfully propelled the Deep Space 1 to asteroid Braille and comet Borrelly, while significantly increasing the thruster power level and making improvements in performance parameters associated with NSTAR. The EPT concept under development has a 40 cm beam diameter, twice the effective area of the Deep-Space 1 thruster, while maintaining a relatively-small volume. It incorporates mechanical features and operating conditions to maximize the design heritage established by the flight NSTAR 30 cm engine, while incorporating new technology where warranted to extend the power and throughput capability.