second mode, payload retrieval, has a payload capability of 19000 lb (8607 Kg). The final mode involves deploying and retrieving a payload of equal weight to an orbit (roundtrip), and Fig. 3.1-7 lists 12500 lb (5662 Kg) as the capability. 3.1.2.3 Solar Electric Propulsion System (SEPS) Ion propulsion system performance for both Plan 1, which calls for SSPS delivery from LEO to geosynchronous equatorial orbit, and Plan 2 which requires a similar delivery from 7000 n mi is dependent on SEPS thrust and SSPS weight. Figure 3.1-8 presents SEPS in-plane performance for a transfer from a 190 n mi circular orbit to geosynchronous orbit (at 28.5°) for various thrust-to-weight ratios. The figure shows that approximately one year is required to reach mission orbit with the lowest thrust-to-weight ratio being considered: this traversal spends 120 days in the Van Allen radiation region, a period during which exposed solar cell effectiveness will be degraded by approximately 40%. This degradation will be accounted for when sizing the solar array which provides power for the ion propulsion system. 3.1.3 Altitude Selection The issue of altitude selection is tied to both the Shuttle payload/altitude capability and air drag effects. The trades involved with selecting a LEO assembly altitude (Plan 1), or a high earth orbit assembly altitude (Plan 2) are centered around the consequences of supplying a Tug fleet for Plan 2 or an Orbit Keep/Altitude Control Module (OK/ACM) for assembly in LEO. Ultimately, the selection becomes one of cost and mission complexity. This subsection reports the effects of air drag on the SSPS in LEO, and will discuss the sizing of an OK/ACM system required to maintain the SSPS at the selected altitude. Investigation of air drag effects on a satellite is dependent on the value of the satellites ballistic coefficient (). Throughout this analysis, two values of have been investigated, 0.175 and 1.75. These values were selected by assuming a total weight of value of 2, and an order of magnitude difference in the area into the wind (A). The ballistic coefficient value of 0.175 assumes that the SSPS solar cells are covering the structure (as they would be in actual use) and that the SSPS has its edge into the wind. The ballistic coefficient value of 1. 75 assumes that the solar cells are stored in a rolled window shade fashion, and the effective area is 10% of the nominal area. A one degree peak-to-peak oscillation about the center of the SSPS is also assumed. Since orientation of the SSPS edge perpendicular to the orbital velocity vector (edge into wind) follows a sinusoidal pattern, a mean area into the wind was computed. The computation considered the centroid of the
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