the reflectors have both sides or one side coated with high emissivity material. During nominal operation, the only obvious influence of the solar flux is the potential ultraviolet degradation of exposed materials and coatings. Fortuitously, the solar cells and reflectors have about the same characteristic decay time (about 12 seconds) to a step function occultation. Although the structural decay times are quite variable, a typical characteristic decay time is on the order of five minutes. Therefore, the solar cells and reflectors will virtually track the sun set and sun rise; however, the structure will lag somewhat in response. Significant differences between thermal strains must be accommodated either through detailed design features or preferably through strain allowances afforded by the structural configuration. Thermal strain between the nominal operating condition and the assembly environment must be accommodated by the assembly process. The solar wind pressure is three orders of magnitude below the light pressure, but this hard radiation energy flux is seven orders of magnitude below the thermal radiation energy flux. The solar wind influences the earth's magnetic field causing daily fluctuations relative to the SPS and sporadic fluctuations which are not well understood. Also, the charged particle flux can degrade material characteristics such as surface reflectivity. The structural requirement of maintaining configuration integrity within potential dynamic motion is more restrictive than the requirements for either the static loads or the environment. Proximity of the system frequency to any excitation frequency establishes the ratio of the system kinetic energy relative to the potential energy characterized by the stress and displacement of the structure. The most significant dynamic loading frequencies to the large array are the twice daily (2.3 x 10"5 Hz) and daily gravity gradient loading cycles. The orbit fluctuations of 8000 km (due to the solar pressure) and the magnetic field fluctuations occur on a daily cycle. To keep the dynamic motions of the overall station to a reasonable level, a minimum natural frequency criterion of 2.3 x 10_1+ Hz was selected. This establishes a membrane tension level in the cable/column array of about .25 N/m. The minimum structural thickness of the "truss" configuration is on the order of 600 meters. The natural frequency of any configuration is directly proportional to the ratio of its shortest dimension to its longest dimension. In absolute terms, the natural frequency is inversely proportional to a characteristic dimension. It is estimated that the control system will provide loads at frequencies which will not exceed .2 Hz. This is orders of magnitude above the fundamental or low order array frequencies; however, potential interactions between the control system and individual or small groups of structural elements exist. c. Structural Configurations General: The design of a solar power station must primarily address the financial investment required to obtain a flow of electrical energy into the existing ground network. The selection of an SPS configuration should be based on a cost optimization obtained through a proper balance of total system weight, transportation requirements, maintenance characteristics, and the total time from investment to electrical power production. Total time includes the time from the commitment of resources through processing, transport, assembly, checkout, and operation.
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