8 . 9 VIII.4: MANUFACTURE VIII.4.1: General Comments on Space Manufacture: Since little is known about large-scale space manufacturing, we must base our estimates on procedures, masses of equipment, and power required on present-day Earth manufacturing techniques. This is not a bad estimate, however, because the primary design criteria and design loads in present-day machine tools are the inertial loads of accelerating a work piece through the tool, and the loads due to interaction between the work piece and tool. The gravitational loads are less significant. Thus, a machine tool designed to operate in zero-g would be very similar to one operating on the Earth. There are, in fact, several disadvantages to manufacturing in zero-g. Gravity is now used to keep work pieces down on work beds and rollers and to move pieces from one tool to another. The more elaborate handling systems required in zero-g would roughly compensate for the decrease in system weight due to absence of gravitational load. Present-day Earth manufacturing techniques have, therefore, been used as a baseline throughout and, where practical, items in this report are based on actual existing machine tools. It is possible, however, that a process known as "ultrasonic rolling and drawing" (8.51 could significantly reduce the mass and energy required, but not enough information is available at this time to use this process as the baseline space manufacturing technique. In calculating productivities, an 80 % duty cycle has been assumed. For a machine, this represents 19.2 hours of up-time and 4.8 hours of down-time per day. For workers, this is equivalent to three 8-hour shifts, each with a 1-hour lunch break and two 15-minute breaks. VIII.4.2: Material Flow: Figure 8.3 shows the flow of material through the manufacturing process for the plates, fasteners, major and minor stiffeners needed for the assembly of the colony. The process begins by taking the 1-ton ingots produced by the refinery and rolling them in a blooming mill down to blooms and slabs.
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