6. ORBITAL ASSEMBLY AND MAINTENANCE The absence of gravity and of the influence of forces shaping the terrestrial environment presents a unique freedom for the design of Earth-orbiting structures and provides a new dimension for the design of the structure required for the SPS, its fabrication, its assembly, and its maintenance in LEO and GEO. In GEO, the function of the structure is to define the position of sub-systems rather than support loads which under normal operating conditions are orders of magnitude less than those experienced by structures on the surface of the Earth. The structure will have to be designed to withstand loads imposed during assembly of discrete sections which may be fabricated in orbit and then joined to form continuous structural elements. The structure will therefore have to be designed to withstand both tension and compression forces which may be imposed during assembly and during operation when attitude control is required to maintain the desired relationship of the solar collectors with respect to the Sun and of the transmitting antenna with respect to the receiving antenna on Earth. The immensity of the structure alone ensures that it would undergo large dimensional changes as a result of the significant variations in temperatures that will be imposed on it during periodic eclipses. During such eclipses, temperature variations as large as 200 K could be imposed, leading to substantial temperature gradients, which, depending upon the dimensions of the structure, would cause dimensional changes of 50 -100 m if an aluminum alloy is used. Both aluminum alloys and graphite composites show promise for use as the structural materials. Graphite composites have a very small coefficient of thermal expansion compared to the aluminum alloys, but the aluminum structure could be insulated to reduce undesirable thermal effects. The contiguous structure of the SPS is of a size which does not yet exist on Earth or in space. Therefore, unique construction methods will be required to erect the structures which are used to position and support the major components such as the solar arrays to form the solar collectors and the microwave subarrays to form the transmitting antenna. The basic approaches to constructing the required large space structure are as follows (18): • Deployable systems, using elements fabricated on Earth; • Erectable systems, using elements fabricated on Earth; and • Erectable systems, using elements fabricated in space An automated machine capable of producing triangular truss shapes of desired sizes and material thicknesses in a modular configuration (19) has been constructed. This automated beam builder consists of roll-forming units which are fed with coiled strip material and automatically impart the proper shape to the individual strip, weld and fasten the individual elements, control dimensions and produce the complete structural members. The machine could produce the structural member in increments of 1.5 m at a rate of 0.5 m/min for continuous production of structural beams in space. Warehousing logistics and inventory control will be required to effectively manage the flow of material to the SPS construction facility, which will be designed to handle about 100,000 tons per year. The construction facility could be a large lightweight rectangular structure with dimensions of about 1.4x2.8 km. It would provide for launch-vehicle docking stations and 100-person crew cylindrical modules with dimensions of about 17 m diameter by 23 m long. The construction facility will be
RkJQdWJsaXNoZXIy MTU5NjU0Mg==