1.3 ELECTRICALLY-POWERED COTV (LOW Isp) The electrically-powered stages take advantage of the satellite's assumed capability to supply power during transit; by doing so, the COTV avoids the penalty of providing its own power generation equipment. Although an electrical COTV with an independent power supply is technically feasible, the incentive for exploitation of the satellite's capability (for power levels as high as these systems require) was felt to be irresistible. In contrast to the chemical stage, where a final T/W of 10“3 was selected, the electrical stages were sized using a final T/W of 10“4 where possible. The effect of this is to change the trip time from 15 days (for the chemical stage) to about two months. The types of electric thrust systems covered are electrochemical, resistojet, and arcjet systems. The electrochemical concept is a hybrid electrical/chemical system; it utilizes an electrolysis unit to produce gaseous O2 and from water. These propellants are then burned in a more-or-less conventional rocket engine. A resistojet thruster operates by transferring heat from an electrically-heated element to the propellant, which is then expanded in a nozzle. For this study, only was considered for use in the resistojet. The thermal arcjet thruster operates on the same basic principle as a resistojet, but is capable of much higher temperature operation because, instead of a metallic heating element, an electrical arc is used as the heat source. To gain some insight into the effects of density and Isp, ammonia (NH3) and H2 propellants were considered for the arcjet. 1.3.1 CONCEPT DESCRIPTIONS A schematic of an electrochemical propulsion system is shown in figure VI-D-1-6. It consists of the electrolysis unit, gaseous propellant accumulators, and the pressure-fed gas/gas thrusters. The potential advantages of an electrolysis thruster system over a conventional LO2/LH2 rocket engine lie in the cost, handling, storage, and transport of water rather than L02 and LH2. The disadvantages include dependence of external power (from the SPS), high thruster assembly weights at high power levels, and lower Isp (due to operation at the stoichiometric mixture ratio of 8:1). When an attempt was made to size a system for a 60-day trip time, the system weights were very high; by bringing the thrust level down a more reasonable weight was attained. Operation at the stoichiometric mixture ratio results in a small rise in combustion temperature (less than 200°F) and a 4 percent decrease in Isp. Details on the electrochemical system are shown in table VI-D-1-3. In the numbers shown, electrothermal pumping was used to raise the pressure of propellants and the engines are pressure-fed. Further work on this system could show that it would be more desirable from a weight standpoint to operate the electrolysis unit a low pressure and utilize electric-powered compressors to raise the pressure of the propellants.
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