limit-cycle sinusoid, and resulted in reducing the effective area by 60% over that of holding a fixed offset into the wind. Figure 3.1-9 illustrates the effect of air drag on SSPS altitude over a 24 month period. The ballistic coefficient of 0.175 represents the SSPS with solar cell fully deployed. A nominal dynamic (Jacchia) mid-1979 atmosphere and a 95 percentile atmosphere were separately assumed. Considering the 95 percentile atmosphere and an initial altitude that Shuttle can reach with 65000 lb (29445 Kg) payload, the SSPS re-enters (assumed to be 75 n mi) after only one month in orbit. The nominal atmosphere assumption merely adds another month to the SSPS orbit life and indicates that an orbit keep module must be added to the SSPS if assembly is to be performed at 190 n mi. The figure shows that SSPS’s with initial orbits of 250, 300, and 400 n mi will not re-enter within a year under nominal atmospheric conditions. Figures 3.1-10 and 3.1-11 illustrate the wide variation in orbit lifetime which exists for vehicles with the two different ballistic coefficient values mentioned earlier. Figure 3.1-10 presents orbit decay characteristics for the SSPS at an initial 190 n mi altitude. Orbit lifetimes which differ by almost an order of magnitude result when the SSPS solar cells are fully deployed ( ) as compared to the case where they are stored in rolls. Storage of the cells then shows two advantages; first, air drag is reduced and secondly, solar cell degradation is reduced during Van Allen belt transit. Figure 3.1-11 shows similar information for an initial altitude of 250 n mi, and illustrates the distinct advantages of assembly at higher altitudes. The question of atmospheric density at 250 n mi becomes academic if a ballistic coefficient of approximately 1.75 can be assured. For these cases SSPS assembly could extend several years without even having to consider the addition of an orbit-keeping module to the SSPS. Unfortunately, Shuttle payload capability (on integral OMS) to 250 n mi is less than half of what it is to 190 n mi (see Fig. 3.1-4). If the present Shuttle is baselined as the SSPS launch vehicle, then fleet size and Shuttle traffic considerations dictate that 190 n mi be selected as the assembly altitude. The selection presupposes that an orbit-keeping module, which uses a reasonable amount of propellant over the assembly period (1 or 2 years), can be sized to maintain the 190 n mi altitude. The orbit-keeping module has to supply a force equal in magnitude (and opposite in direction) to the air drag force. Figure 3.1-12 presents the forces required to compensate for air drag in low earth orbits. A constant force of 11 lb would maintain the SSPS at 190 n mi during the assembly period. The fact that the structure buildup will be progressive over the assembly period has been ignored. Rather, the conservative assumption which has
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