Power loss is less than 0.1 percent of annual output; however, these eclipse conditions must be considered in integrating satellite-generated power with surface systems. 4. Configurations In an attempt to minimize structural weights, the concept illustrated in figure IV-6(a) (column/cable) was developed utilizing compression columns and supporting cables (see "SECS Structure," sec. IV-B-3). With this configuration, the transmitting antenna has to be mounted on the north or south end of the solar array to be able to view the Earth continuously. However, microwave recoil from the antenna (about 5 lb) causes a constant disturbing torque, and the offset in the center of mass also produces a solar radiation pressure torque. To eliminate these disturbances, the solar array area was doubled and an antenna was mounted on each end. The resulting configuration is essentially two 5-GW satellites sharing a commmon structure. For a given total power requirement, this approach has an additional advantage in that the number of satellites is halved and consequently the distance between satellites doubled. This simplifies traffic control and maintenance and reduces the impact of eclipse by other satellites. This configurational approach did result in a very low structural weight, as will be seen in the subsequent presentation of mass properties; however, it should also be noted that the SECS structural weight is not a large percentage of the satellite total weight, ranging from 1 percent (minimum, column/cable) to 6 percent (maximum, truss) for the cases considered. The column/cable configuration has the potential disadvantage of being incompatible with a mission mode that involves construction of the satellite, or modules thereof, at low-Earth orbit, which then provides solar energy to propel the satellite to geosynchronous orbit. A second configuration was also considered in some depth (fig. IV-6(b)). It is referred to as the "truss" configuration. Like the column/cable, it has two 5-GW antennas and a "double-size" solar array, although a single 5-GW system with central antenna is also possible. It is built up as a three-dimensional truss and may be easier to construct in geosynchronous orbit than the column/cable configuration. It can also be built in modules at low-Earth orbit. Several other configurations were considered briefly; they were adaptations of the two concepts described previously and did not appear to offer any overriding advantages. The sizes of the configurations as presented in figure IV-6 are related to "probable," or nominal, efficiencies of conversion and transmission as discussed in section IV-A-1. The relationship between efficiency, array size, and mass will be presented in the next section.
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