IV-E. OPERATIONS Ideally, SPS power output would remain uniform at all times. In reality, however, there will be variations from several causes as illustrated in figure IV-E-1-1. When the solar array is oriented perpendicular to the orbit plane, as has been found desirable from a weight standpoint (see section IV-B-4), the solar energy collected varies as the cosine of the sun's declination, producing the six-month cycle at the top of figure IV-E-1-1. Total variation is about 450 MW. Orbit eccentricity will cause a cyclic fluctuation in satellite- to-rectenna distance. For an expected eccentricity of 0.04, this results in a daily power output variation of about 100 MW superimposed on the six-month variation. Eclipses by the earth (see section IV-A-3) will cause total shutdowns daily around midnight for about six weeks in the spring and fall. Maximum duration is about 75 minutes. Eclipses by other SPS will cause shutdowns at about 6 a.m. and 6 p.m. for several days, also in the spring and fall, with a maximum duration of about 15 minutes. Total shutdowns will also be required at times for maintenance. Duration is expected to be a few weeks, and frequency about once every five years. The primary operational problems will arise from the eclipses. Starting and stopping a 5-GW standby generator system to fill these brief gaps will be difficult. At 0.5° spacing, as many as 38 SPS will be in the earth's shadow simultaneously. These will, in general, all be transmitting to rectennas in the same range of longitudes, and will tend to be the only SPS servicing these rectennas. Power sharing to spread the impact of these outages will therefore involve relatively long transmission distances for the shared power; this may limit the usefulness of power sharing to overcome this problem. A shifting, non-equatorial orbit that is never eclipsed has been examined and found to be impractical (see page IV-A-3-22). No entirely satisfactory solution to the eclipse problem is apparent, and further study is required. Maintenance shutdowns will be of much longer duration and less frequent, and should consequently present less of an operational problem. The daily and six-month cyclic variations in power output can be eliminated if necessary. A circular orbit avoids the daily fluctuation, but at a cost in orbit maintenance propellant of some 200 M.T. per year (I_n = 10,000 Ib-s/lb). The six-month cycle can be eliminated by b p L. E. Livingston Spacecraft Design Div.
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