manufacture spare parts for the base. Thus, only the information of how to make spare parts for all of the relevant equipment (at least those which are non-critical) would need to be provided, and not the parts themselves. Statistically, not all of the items for which you might want spares are going to break. Also, the volume constraints and warehousing of these parts would be relaxed. Eventually, the stock could be supplied by the ISRU facilities, at least for low-tech needs where low grade alloys would serve. Eventually, the manufacturing plant could develop solar power generating capabilities for the outpost, even bootstrapping the productivity of the base. This near- term benefit would perhaps justify the effort in itself, and would act as the technology demonstration for larger scale manufacture for a space solar power program. 9.5.2 In-Space Manufacturing The potential for in-space manufacturing is enormous. However, a number of scientific, economic, political, and structural problems need to be solved before this potential can be realized in commercial projects. Manufacturing operations in the micro-gravity conditions of space, rather than in the low gravity of the Moon, will provide their own set of benefits and limitations. Containerless processing of samples in space for the production of unique glasses from materials that are reluctant glass formers, fabrication of unique shapes and configurations without sagging or physical contact, such as concentric glass shells, production of ultrapure glass for use in optical wave guides, etc are good examples or advantages of micro-gravity manufacturing. Perhaps only physical processing of materials should be planned for the in-space segment, leaving the chemical processing to a lunar base, asteroid base, or even Earth. The wake shield facility, to be launched later this year or early in 1993, is an example of an experiment which will utilize the near-perfect vacuum of space to advantage. It will investigate the use of molecular beam epitaxy to produce semi-conductor devices such as solar cells. GaAs is one of the most important III-V semiconductors, with uses ranging from microwave devices to solid state lasers. In microgravity the role of thermocapillary convection becomes appreciable therefore the deposition method considerably improves the performance of the materials. Superconducting compounds represent another class of materials with a potential for space manufacturing. These may open up new technological possibilities and would find numerous applications in various types of space power systems. Other products which might be enabled by the conditions found in space might include large, thin film structures and foamed metal beams. By manufacturing satellite structural components in orbit, it is possible that less mass will be required because launch loads and the special mechanisms required for deployment will be obviated. However, manned deployement of these structures could be a problem. On-orbit microgravity manufacturing tests which aim at developing technologies useful in the manufacturing of useful materials or devices and assembly studies should continue on Shuttle, Mir, and unmanned flights. Of course, all of the processes described above need to be traded against the mass required in-space and on the lunar surface to produce these products. 9.5.3 Schedule Issues for Space Manufacturing Technology Certainly, our ability to manufacture items in space is very low. This program is not the only one in which manufacturing methods will be required, though. The maintenance and expansion of a lunar outpost will be enabled by the use of these technologies. The possibility of producing specialty electronics, nanotechnology products, or biotechnology products is another avenue for expansion of these technologies into space. One can see, then, that the development of technologies related to space manufacturing has a schedule of its own which interacts with many programs. Figures 9.15 portrays the tasks for in-space manufacturing programs related to space solar power program and other projects. Milestones in any one of these programs can be considered advances for all of them. Programs and milestones which are important for manufacturing in space include: • First flight of the Wake Shield Facility (WSF) to explore semiconductor growth by molecular beam epitaxial growth. Aim at specialty electronic market, but demonstrate photovoltaic device manufacture. • Perform trade studies to explore the optimum location to perform each manufacturing step for SPS construction, and begin paper studies related to these manufacturing techniques.
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