A worst case impulse requirement for controlling and station keeping a 1.16 x 108 kg photovoltaic SPS in GEO is calculated to be 8. 0 x 109 kg-s per year. Thus, the yearly propellant mass required will be large. To minimize this amount of propellant, and subsequently the cost of yearly resupply, electric propulsion appears necessary for controlling and station keeping the SPS. Assuming that SPS end mounted thruster modules are baselined for the LEO to GEO transfer, these modules can also be used to provide pitch and yaw control as well as station keeping. In both cases, the number of thrusters necessary is much smaller than the number required for transfer. The remaining thrusters are considered to be spares. SPS roll control, however, will have to be provided by smaller separate modules located on the Y-axis of the spacecraft. A graphic representation of the location of the thruster modules on the SPS was shown previously in Figure 7-26. Thrusters required to produce torques about the X-axis are at four locations on each side of the SPS, while the proper thrust direction for producing torques about the Y- and Z-axes results from rotating the end mounted thruster modules. Using each of the three electric thruster concepts discussed in subsection 7.1. 5. 3, a propulsion system was sized to satisfy the requirements for controlling and station keeping the SPS. The results of this analysis are summarized in Table 7-10. With gravity gradient torques being the design driver, the total number of thrusters required to counteract the disturbance torques about the appropriate axis was determined. It is realized that thruster contingency will be required for the modules used to provide roll control. The propulsion system mass is based on the specific mass of each propulsion system type, i. e., 3. 66 kg/kW for the Boeing ion, 1. 9 kg/kW for the Boeing MPD, and 1. 35 kg/kW for the JPL MPD. The propulsion system mass consists of the sum total of the masses of such items as power conditioning units, control systems, cabling, vaporizers, actuators, isolators, thrusters, neutralizers, support structures, etc. Masses attributed to such items as propellant, propellant tank structure, propellant expulsion, valves, plumbing associated with tankage, residuals, reserves, etc., are not included. The propellant mass is based on a control and station keeping total impulse requirement of 8. 0 x 109 kg-s per year (worst case). In this case, not only is the orbit period of the SPS continuously adjusted because of the influence of solar pressure, but the SPS is maintained over one point on the Earth at all times. The propellant tank mass was calculated assuming a tank mass fraction of 0. 93 and that the required propellant was contained in a single tank. Propellant contingency was not taken into consideration. Items such as propellant expulsion devices, plumbing, insulation, valves, etc., are included in the propellant tank mass.
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