10.3 colony and separate shield, and permits dissipating heat directly to space from the outer hull. The study group concludes that any colony design should investigate the shield-included option. Minimum structural mass, though desirable, must take second place to safety considerations. In the case of the MIT Prototype Space Colony, an increase in required mass does not push up the overall cost significantly unless it either requires the use of two Transport Linear Accelerators on the Moon instead of one, or extends the time required for construction, thus adding to the discounted program costs. However, different transportation scenarios and construction schedules would exhibit different sensitivities. The minimum mass criterion does strongly favor steel as a hull material, since the design would require a mass of aluminum 5 times (a thickness 15 times) as large as that of steel to meet the Leak Before Break criterion. X.1.4: Construction: The study group concludes that electron beam welding, while useful in joining small (1 meter x 1 meter) pieces together, is not desirable for joining larger sections, because of the extreme difficulty in maintaining close alignment of the sections during welding. The construction of the MIT Prototype Space Colony relies on a subassembly technique: electron-beam welding stock parts into larger (20m x 20m) hull sections, and assembling these with fasteners fitted through drilled holes. Its construction requires a peak work force of 140-160 people, a 60-megawatt power plant, and 5 years. The study group also concludes that the masses of manufacturing machines designed for space operations should be roughly the same as their Earth counterparts, because gravitational loads are not the significant effects in sizing Earth-based machines. X.1. 5: Inspection and Repair: Given the proposed colony's plate/ stiffener construction and double-hull, which meet the Fail-Safe and Leak Before Break criteria, quality assurance during manufacture and inspection after final assembly are simple procedures. Existing inspection methods can guarantee a high degree of safety and long lifetime (greater than 30 years). Because of the double-hull
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