As the development of a space transportation system proceeds over the next decades, the cost of orbiting payloads will drop from thousands of dollars per kg for the Saturn-type vehicles used for the manned lunar landing, to hundreds of dollars per kg for the space shuttle, to tens of dollars per kg for the space freighters. 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 subsystems rather than support loads. The latter, under normal operating conditions, are orders of magnitude less than those experienced by structures on the surface of the Earth. But 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. Depending upon the dimensions of the structure, if an aluminum alloy is used, these gradients would cause dimensional changes of 50 to 100 m. Graphite composites also show promise for use as the structural materials. They 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 has never been fabricated on Earth. Therefore, unique construction methods will be required for the structures which will be needed 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. 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 which would provide launch-vehicle docking stations and habitats for 500 crew members. The construction facility will be designed to assemble the solar energy conversion system and the microwave transmission antenna. At first, the crews constructing the SPS and participating in industrial activities would live in habitats designed to provide the types of comforts now found on off-shore oil-drilling platforms. Partially closed ecological life-support systems will reduce the requirements for food supplies from Earth. The crew would return to Earth at intervals, which may extend over weeks and months, just as is the custom on Earth at remote construction projects. But there may come a time when the space habitats will be not only utilitarian, but places where people may wish, for various other reasons, to stay for extended periods. Whether people will work and live in space habitats and stay for years, decades, or generations is not possible to project with certainty. What is certain is that mankind will increasingly utilize Space to take
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