3. STRUCTURE 3.1 INTRODUCTION The structure of an SPS maintains the separation and orientation of the components with respect to each other. It provides the rigidity through which the attitude control system points the power conversion system at the sun and the microwave transmitter at the earth. Each SPS concept has unique structural requirements. This section uses the silicon planar structure as an illustrative example. Silicon planar was chosen as an example because it has already been well studied, because silicon planar is a very promising concept for lunar SPS design, and because it has a relatively simple structure which makes a good example. The most important requirement for the design is to minimize use of non- lunar materials for construction of the satellite. This requirement minimizes the importance of a low total mass. Assembly in space is another important consideration in design of this space structure. This problem has been studied before (1-9), since launch limitations do not allow launching massive or bulky satellites into space. In addition to these requirements, the structure must have a sufficiently high natural frequency to satisfy pointing accuracy needs and to avoid attitude control interactions. 3.2 DESIGN ALTERNATIVES The configuration of the satellite has to account for stiffness, stability, and power distribution needs. A rectangular shape is one of the common shapes chosen for a silicon array satellite (ref 1 and 10). In this configuration, the structure bears a compression load caused by tension in the solar cell array. The aspect ratio (length/width) of a rectangular structure must be chosen to provide a natural frequency high enough to satisfy the stability requirements. An uncommon SPS option is spin-stabilization, which is often used in smaller satellites. Some spin-stabilized configurations would have advantages. For example, arrays of solar cells could extend radially outward from a spinning central hub. Centripetal force would maintain the rigidity and orientation of the arrays, thus eliminating most structural mass. One drawback of spinning the solar array is that an additional slipjoint is required between the spinning array and the microwave antenna. This drawback is minor: the slipjoint is better than an order of magnitude less massive than the SPS primary structure. Another drawback is that a flexible rotating structure is relatively difficult to maneuver. This may be a minor factor because the primary maneuvering of the solar array is simple: a steady force to offset radiation
RkJQdWJsaXNoZXIy MTU5NjU0Mg==