vector. The analysis was based on the large trough concentrators with an array of elliptical planform and 2 to 5 GW antennas. For each case a concentrator angle and an array planform length/ width ratio were chosen which gave the minimum total program cost. The optimum concentrator angle for the X-POP cases was larger than for the Z-solar case due to the lower average solar incidence. Also the optimum length/width ratio for X-POP was larger. As shown in Figure 7-30 the dry weight is less for the Z-solar concept. However, the propellant requirements are substantially greater resulting in a larger total 30 year mass (Table 7-9). Initial cost for Z-solar is less than X-POP primarily due to the smaller array area. However, the total cost over the 30 year lifetime accounting for the time value of money is slightly less for the X-POP case with an average output of 10 GW. The significance of these alternate orientation fuel mass trades has a great impact on the antenna pointing control system complexity as shown in Figure 7-30. TABLE 7-9. PHOTOVOLTAIC SPS ORIENTATION COMPARISON 7. 1. 6. 6 THRUSTER MODULE As mentioned in subsection 7.1. 5. 3, the SPS will require a propulsion system to satisfy control and station keeping requirements while in LEO and GEO. Attitude control will also be required during the transfer from LEO to GEO. While in LEO, attitude control and station keeping of the SPS will be necessaiy during and after assembly, and the demands on a propulsion system to satisfy these requirements are expected to be considerable. Atmospheric drag makeup is expected to be the single greatest contributor to this problem. The problem of determining the requirements and sizing a propulsion system for controlling and station keeping the SPS in the LEO is complicated and is still under investigation. Therefore, a discussion primarily of the propulsion requirements for controlling and station keeping the SPS in GEO is contained here.
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