IV-B-3. Structure a. Introduction Objective consideration of the concept of solar power stations in space involves many scientific principles, and requires an awareness of current and feasible technology, experience in the practical aspects of accomplishing major endeavors, and the perspectives of future energy demands and the economic validity of alternative approaches. In considering the structure for a solar power station, it is essential to dispose of preconceived ideas of structures based on terrestrial experience, and to focus instead on concepts which address the design requirements. Some background information is available from studies performed by Arthur D. Little Inc., the Boeing Company, and the JSC Pilot Plant Study (reference 1-3). The geosynchronous orbit environment is characterized by a hard vacuum, the energy and mass fluxes from the sun, and the earth's gravitational field, magnetic field and thermal radiation. Operating loads on the primary structure are markedly low thereby increasing the significance of transportation, assembly and maintenance loads. The large scale of a solar power station emphasizes potential dynamic characteristics which must be addressed in the design of its structure. In addition, since local temperatures are established almost entirely by radiation exchange, the design of structural elements and their thermal control are intimately coupled. The general requirements of the structure are: (1) maintain the overall and local integrity of the configuration to collect or focus the relatively diffuse solar energy flux (1.4 GW/km2) and (2) minimize capital investment through a lightweight system which offers an ease of construction for both the structure and the array. This leads to an ironical statement of the structural requirements: structural stiffness and conceptual flexibility. The lifetime of the structure is a materials requirement to withstand the ultraviolet and hard radiation environment and the thermal transients associated with occultation. The vast size of the solar power station array and the general structural requirements lead directly to an open structure for light-weight and to a three-dimensional structure for stiffness. Within this category, two structural concepts have been investigated: one providing a maximum concentration of load paths and a second possessing a minimum concentration of load paths. Both concepts achieve a light-weight structure within the limitation of large scale, low loads, minimum gauge considerations and high structural efficiency. The first and lightest structural concept makes maximum use of the most efficient structural element, the cable, and a minimum use of buckling limited compression members. The lightest structural configuration using this concept to provide three-dimensional stiffness (over a given cross-sectional area) is a tetrahedron formed by tension lines and held in place by four compression members extending from the centroid to each vertex. This configuration is shown schematically in figure IV-B-3-1. This structural concept and the idea of minimizing gravity gradient torque has led to the column/cable configuration shown in figure IV-B-3-2. The secondary structure for this configuration is conceptually a three-dimensional spider web of tension lines to maintain local configurational integrity, overcome electrical current interaction forces, and to provide sufficient membrane stress for dynamic stability of the array. The second structural concept possesses a uniform pattern of structural "hard points" characterized by a "planar" truss. The distributed solar R. C. Ried, F. Stebbins, J. D. Medlock/ Structures and Mechanics Division
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