8.27 The rate limiting step of the refining, manufacturing and fabrication process is this lay-up and welding procedure. In reality, it may be necessary to have two such fabrication operations going on side by side. In the construction of a larger colony or if the construction time of this colony were to be reduced, one would certainly need many lay-up and welding shacks. VIII.6: WORK ENVIRONMENT AND SUMMARY VIII.6.1: Work Shacks VIII.6.1.1: General Description: The construction operations are all housed in construction shacks or cylinders. These are at least semipermanent structures which house all the refining, manufacturing, and fabrication operations required to build the first colony. Since these shacks possess a fairly sophisticated and versatile industrial capability, they will probably form the beginning of the space manufacturing complex and indeed they are so intended. The cylinders serve several purposes. They are as pressure hulls to provide the same atmosphere inside the shack that will event ually be us ed in the colony, a 50% oxygen-SO% nitrogen mixture. This means that the humans who must run and maintain the equipment work in a shirt-sleeve environment, but in zero-g. It also means that the machines are working in an atmosphere so that lubrication is not a major problem. The cylinder also provides a base to which the machinery can be attached and against which it can apply loads. VIII.6.1.2: Design: In this preliminary estimate, the only loads used in the design of the shacks are the atmospheric pressure loads. Throughout their use, however, the shacks are subjected to significant periodic dynamic loads. These loads are due to the inertia forces of accelerating slabs of metal and rotating machinery, and the interaction of the work pieces and machine tools. In order to keep the stress levels low and thus avoid fatigue problems in the shack structure, a low design allowable stress of 140 MN/m 2 (20 , 000 psi) was used. Before the design can proceed, one must know the size of the shacks to be built. The largest single component is the blooming
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