6.77 so that the exterior of the hull can radiate heat away to unobstructed space. The changes described above establish the final colony configuration. The next sections detail the designs of the parabolic mirror and electrical generating system, the docking system, the flat mirror, and the shadow reflector. VI.9.3: Parabolic Mirror and Electrical Generating System: The candidate electrical generating systems considered by the ES group were thermal engines and solar cells. Thermal engines, as described by Woodcock and Gregory (6.7), use mirrors to focus sunlight, warming a collector. A working fluid is heated inside the collector and passes through turbines to produce shaft power. This shaft power runs generators to produce electrical power. The working fluid then passes through a radiator, providing the low-temperature end of the cycle. Figure 6.32 depicts two possible thermal engine configurations for the colony. In Figure 6.32(a), the collector and the second mirror for agricultural sunlight are in the same place and the parabolic mirror collects sunlight for both functions. Should engineering problems preclude a combined collector and second mirror, the collector could be further out on the boom (Figure 6.32(b)). This calls for an outer section of the parabolic mirror with a different curvature to focus sunlight on the collector. In either configuration, there would be several turbines and generators on the boom so that failure and maintenance of components would not deprive the colony of power. The ES group worried about placing this large machinery and radiator on the boom because of the anticipated gyroscopic stresses (described in Section VI.8.4) and because massive rotating components create safety hazards and possible torques on their mountings. For these reasons, the ES group decided to use the simpler solar cell concept. Figure 6.33 shows the final configuration of the parabolic mirror and solar cell array. The design allows a 25-meter radius hole at the pericenter of the parabolic mirror for the hull's window and the trusswork around it. Thus, the actual projected area of the mirror is:
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