Of the systems discussed above, the MPD-arc jet system looks quite attractive; however, as mentioned above, this thruster is in such an early stage of development that the weight and performance values have considerable uncertainty. The conventional thermal arc jet system is also attractive, especially from the standpoint of inert weight. The electrolysis system has the advantage of drawing very little power, but has large resupply requirements. The ion systems have low resupply requirements but high inert weights and large power requirements. The trade-offs among these systems are influenced by transportation costs, assembly schemes, and cost amortization as well as many other factors. For example, should impulse requirements drop significantly or installed thrust level increase, the ion systems would appear less attractive than under the current assumptions. The MPD arc jet has been selected for the reference configuration primarily on the basis of low initial and total weights. The small number of thrusters should enhance reliability and is also a favorable consideration. L. E. Livingston Spacecraft Design Div. e. RCS Operation During Eclipse Eclipse by the earth for a maximum duration of 75 minutes (section IV-A-3) imposes a large energy storage requirement on the first three systems in table IV-B-4-1. Using an optimistic battery power density of 110 W-hr/kg, battery mass is 11400 kg per MW average power. On this basis, the MPD arc jet system at average power would require 261,000 kg of batteries. Since a 30-year life is very unlikely, periodic replacement would be necessary. As an alternative, an Og/^ (water electrolysis) system can be used during eclipse, eliminating the storage requirement for RCS electrical energy. Since the satellite is eclipsed only one percent of the time and half of the total estimated impulse requirement is for orbit corrections (which need not be performed during eclipse), the propellant required during eclipse is 1/200 of that given in table IV-B-4-1, or 14,000 kg per year. Furthermore, because the eclipses occur in two groups six months apart, only 7,000 kg maximum gaseous storage capacity is required. The electrolysis can be done continuously at a low rate to minimize electrolysis cell weight. The estimated dry weight of such a system is 34 M.T., with annual propellant requirements of 16 M.T., including tanks. At the low angular velocities contemplated for the SPS (on the order of 0.25 deg/hr), delaying an attitude control pulse until after an eclipse would not seriously enlarge the deadband about the X and Z axes. If this is done, eclipse propellant can be reduced some 40 percent, to about 4200 kg per six months.
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