effort will involve considerable system engineering and technology development which will greatly benefit from the activities currently under way in support of the present space shuttle development. Orbital Assembly. — Operations in space involving assembly have been limited to docking two actively maneuvering vehicles together. However, studies of modular space stations which dock a number of similar masses to form a large complex in orbit have been performed. In general, to effect a mating, attitude control is required on both target and docking vehicles. After a docking vehicle contacts a space assembly, several things happen: (l)the assembly experiences loads; (2) modular elements deflect relative to each other, and (3) possibly disruptive control forces and G loads are transmitted throughout the space assembly. In general, the weak link in a space assembly is the docking interface, since it has a smaller cross-sectional area than the prime construction element. Relatively large space assemblies have been analysed in modular space station studies and found to be controllable during assembly (34), providing that a prescribed build-up sequence is followed and that grossly asymmetric configurations are avoided. In addition, there is a significant interplay between docking contact velocities, target and docking vehicle flight control, and docking mechanism characteristics. In general, direct vehicle docking contact velocities of about 0.5 fps and manipulator docking contact velocities of 0.1 fps are compatible with currently planned spacecraft systems. The large, more flexible SSPS-type spacecraft will require significantly lower contact velocities because of the size and flexibility of the system and its potential damping characteristics. Zero or near-zero contact velocities may be required, together with appropriate dooking or joining mechanisms and control techniques during assembly. Because a considerable portion of the SSPS structure may be part of the power distribution system, new joining and assembly techniques may be required. Assembly sequences and modes and desirable assembly altitudes have to be identified to define the assembly requirements for the SSPS's large area and light-weight structure. Once the framework for the basic SSPS structure has been developed, including the potential sizes and sub-element characteristics of major components, such as the solar collector arrays, the transmitting antenna and structural members, alternative assembly sequences, modes, and altitudes can be evaluated. These could include automatic or operator assembly options, and assessments as to the portions of the assembly process to be carried out at low-Earth orbit altitudes, at intermediate or at synchronous altitudes. With this knowledge, the appropriate control techniques during the assembly process can then be identified and developed. Key Environmental Issues Resource Use. — The SSPS represents an approach to power generation which does not use a terrestrial energy source. Thus, the environmental degradation associated with mining, transportation, or refining of natural energy sources is absent. Natural resources will have to be used to produce the components for the SSPS and the propellants for transportation to orbit. Nearly all the materials to be used for the components are abundant. For each SSPS, the rare materials required, such as platinum or gallium, would be less than 2% of the supply available to the United States per year as Table 23 shows.
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