The 30 cm ion thruster system referred to in table IV-B-4-1 represents a relatively modest advance in current technology. The 100 cm ion thruster system represents an advanced system which is projected to be attainable in the time frame of the SPS. Compared to the 30 cm thrusters, the advanced ion thrusters have a much higher specific impulse and thrust, as well as a higher thrust to weight ratio. However, the higher projected power conditioned weights for the advanced thrusters result in a net increase in dry weight for the same installed thrust. The MPD-arc jet is a thruster in the early stages of development that appears to have the potential (compared to ion thrusters) for higher thrust levels, higher thrust/weight ratios, and lower power conditioning requirements. Unfortunately, at this time, the ISp, weights, and power requirements are considered highly speculative. The thermal arc jet is a relatively wel1-developed device; large arc jets have been used for materials testing and entry heating studies for years. Ammonia was chosen as the propellant, although hydrogen would give approximately twice the ISp. It was felt that the penalties associated with the hydrogen storage and transfer (both from volumetric and temperature considerations) would greatly reduce if not eliminate the advantage associated with Isp. The final system listed is an electrolysis system, in which oxygen and hydrogen produced from water are combusted in a more-or-less conventional rocket engine. The primary advantage of this system over a conventional LO2/LH2 rocket engine system is the storage efficiency and handling ease of water compared to cryogenic oxygen and hydrogen. The "system dry weight" in table IV-B-4-1 includes thrusters, power processing equipment, and propellant tankage (for one year's supply of propellant). System weight items that are not included are radiators required for heat rejection from the thrusters and electrical wiring to supply power. The "SPS penalty" weight is calculated as 4.5 kg/kW of average power required by the thrusters. The system weights represent the summation of the dry weight, SPS penalty, and the indicated number of years of propellant supply. At a 6.4 percent duty cycle, a 30-year satellite life would require an average thruster life of 23 months. Of the systems discussed above, the ion thrusters have demonstrated 24-month lifetimes; experience with the other thrusters is considerably short of the requirement. Life predictions for an MPD thruster are difficult, but it may be in the same range as a thermal arc jet, which is to say in the range of 3 months. For the electrolysis system, a thruster life of some 6 months appears reasonable, with the electrolysis cells themselves having an essentially indefinite life with periodic refurbishment. Fortunately, the thrusters without sufficient life expectancy have a high thrust/weight ratio, so that either installed redundancy or periodic replacement seems feasible.
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