increasingly focused on electrostiatic devices which are more suitable than MPD devices for the very high energy planetary missions. In addition, basic MPD theory is not well understood, and important anomalies within test results remain essentially unresolved. Consequently, MPD thruster and propulsion system characteristics and programmatic factors cannot be confidently predicted. The proposed thruster, which ionizes argon propellant by means of an electric arc, accelerates the plasma both by MPD action and by the thermal energy generated in the plasma by the arc. Thus, the dual operation, MPD-arcjet thruster is designed to optimize total combined performance from both accelerating forces. Figure VI-D-1-15 indicates very poor overall efficiencies resulting during the very limited past laboratory testing of argon as a propellant. Note, however, that 1967 Electro-Optical Systems Company (EOS) lithium propellant tests resulted in much improved performance and that theoretical frozen flow losses (a major determinate of total efficiency) for argon are actually less than for lithium. These factors have generated considerable agency and industry optimism that argon performance can be improved to acceptable levels with additional laboratory research. Table VI-D-1-7 shows predicted MPD-arcjet chracteristics, with a range of performance uncertainties just discussed. As with the ion thruster, the designer has full flexibility to adjust the operating specific impulse within the full design range. Important factors, inherent in MPD-arcjet thrust devices, which significantly impact selection for the COTV function, include the fol lowing: o Moderate thrust levels and thrust to weight ratios. o Possibly high specific impulse. o Simple thruster hardware with nominal amounts of power conditioning. o Uncertain overall efficiency and corresponding thermal control considerations. o Uncertain operating life without refurbishment. o Definite requirement for additional, laboratory hardware research and state-of-the-art advancement. In summary, the MPD-arcjet thruster holds promise of being the optimum selection for the COTV propulsion function; however, even its basic feasibility cannot be determined without additional laboratory research.
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