First, there is no known way of testing a full-scale SPS structure, as it is currently envisioned, on the ground. The structural requirement is one of stiffness which requires a zero gravity environment to achieve experimental verification of structural performance. Even component testing would be quite limited since some of the structural concepts would not support their own terrestrial weight. This implies a design predicted on analysis of the performance. A logical way to achieve confidence in this analysis capability is through scale model similitude testing and component testing. If the analysis predicts the data on a scaled system test, confidence would be obtained in the prediction of full-scale system performance. Second, the design and performance of the SPS structural system is closely coupled to the design and performance of the attitude control and pointing systems. Major load path characteristics and dynamic inputs to the structure depend on the control system design and operation. Third, the thermal and structural analyses of the SPS can only be decoupled if either a low coefficient of thermal expansion structural material is used or an active structure is employed. Fourth, the construction of the SPS structure and the assembly of all the systems is a major consideration in the structural design and in the development of the overall system configuration. Structural Configurations - An initial consideration of a structural configuration for the photovoltaic SPS array used the relatively massive power distribution system as a prime structure "mast", with a solar cell support truss made up of minimum gauge aluminum "Venetian blinds" (ref. Ila). This approach afforded adequate structure for the minimal loads and afforded an elastic buckling mechanism to accommodate local off-design loads. An initial MPTS structural configuration (ref. 23) employed a two-tier truss structure. The prime structure afforded overall stiffness, while the secondary structure accommodated the radiating subarray elements. The significance of thermal distortion to achievable flatness was recognized (ref. 23) along with the attractive features of a low coefficient of thermal expansion material such as a graphite composite. Studies of thermal engine concepts led to different structural concepts due to the large concentrated masses in modular units (ref. 11b).
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