Table VI-D-1-7 shows predicted characteristics for two sizes of argon thrusters, with the 30 centimeter size described at the lower end, and the 100 centimeter size at the higher end of the specific impulse range. This range of about 7,500 to 20,000 seconds is limited on the low end by a rapid loss of efficiency and at the high end by high voltage constraints. Within the range, however, the designer has full flexibility of specific impulse selection by proper choice of design and operating parameters. Particularly important characteristics, inherent in electrostatic acceleration thrust devices and which significantly impact the selection and application of the ion thruster, include the following: o Low thrust level, an inherent current density limitation due to the exhaust space charge, dictates large numbers of heavy thrusters to achieve an adequate total propulsive force for transfer times of 6 months or less. o Low thrust to weight ratio due to complex hardware and the low thrust level. o Extremely high exhaust velocity (or specific impulse), limited primarily by accelerating voltage breakdown levels, allows a tremendous reduction in total propellant mass. o High input power to thrust ratio, a penalty of achieving high specific impulse, dictates a correspondingly large electrical power source. o High overall efficiency reduces waste heat/thermal control problems. o Requirement for very stable, multilevel voltages dictates expensive, complex, heavy power conditioning. Although little doubt exists as to the basic feasibility of developing an ion/argon engine, available data is insufficient to define exact characteristics within the ranges shown in Table VI-D-1-7. A best estimate at this time, however, indicates a multiple cathode device of at least 100 centimeter size and a specific impulse of about 10,000 seconds. This value is well above the efficiency roll-off point and yet does not seriously push into extremely high exhaust velocity ranges. Based on the mercury engine, a 24 month operating life is probably realistic. 1.4.2.3 MPD-ARCJET CHARACTERISTICS At the present time , MPD type thrusters are receiving limited United States research attention, primarily by Princeton University, but have received little attention since the late 1960's when emphasis
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