cell array is thus provided short load paths for inertial loadings. The limits of dynamic stability for the overall array establish the planar truss compressive element design. This is a less efficient structural concept from the standpoint of structural weight alone; however, potential advantages exist for the power distribution system and for assembly efficiency due to the repetitiveness of the configuration. This is similar to the structural design approach used in the A. D. Little SSPS study (reference 1) and forms the basis for the "truss" configuration shown in figure IV-B-3-3, Although weight is a prime cost driver for the SPS structure, the column/cable and truss configuration structural weights are respectively less than 1 percent and 3 percent of the total system weights. Therefore, it is plausible that total system costs may be lower with a less efficient but more versatile structure. Requirements of transportation, assembly, quality control, manufacturing, etc. may add structural weight but lower overall costs. The technology relationship between SPS structure and aircraft structure can be likened to the relationship between aircraft structure and the structure of a bridge. The structure for a solar power station will be totally different from conventional aircraft or spacecraft design. However, there are no major technological problems in the array structure which cannot be solved through development and test programs demonstrating construction, assembly, and material qualifications prior to SPS initiation. b. Loads, Environment, and Dynamics There are three primary natural loads to consider for a large satellite in geosynchronous orbit: Gravity gradient forces on a system oriented perpendicular to the orbit plane cause a torque about the axis normal to the orbit plane which is cyclic over a 12-hour period. For a truss configuration sized for a 10 GW ground output, this torque would have a maximum magnitude of about 1.2-million N-m over the period, requiring a maximum control force of about 100 N applied out at the corners of the array. The gravity gradient also produces a 12-hour cycle of tensile force in the array which peaks at about 50 N. Attitude control is simplified if the moments of inertia about orthogonal axis are equal. The gravity gradient tends to stabilize the configuration in the direction of the maximum moment of inertia. If it turns out that initial construction in a lower orbit is less costly than geosynchronous construction, gravity gradient torques are more troublesome if allowed to exist, since torques are about 230 times as great at 500 km as they are at geosynchronous altitude. Consequently, the station would probably be held horizontally during construction, minimizing or eliminating torques from this source. Solar radiation pressure causes an evenly distributed force of about 600 N on a satellite of this size, but it results primarily in a pertubation to the orbit, tending to make the orbit slightly eccentric. However, because of the tremendous difference in area/mass ratios of the solar array and antenna, solar pressure would cause a shear force of about 90 N maximum between the array and the antenna. In addition, substantial torques can be created if the center of mass and the center of pressure are not coincident in the system. Solar and lunar gravity and the earth's equatorial ellipticity cause substantial orbit perturbations but do not create any significant structura1 loads. Atmospheric drag at geosynchronous altitude is negligible. Drag,
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