6.63 of such failures would be difficult since the radiator would spin at 2 to 3 RPM. Furthermore, meteorite impacts would cause leaks and these would require localized leakage control systems such as a set of solenoid valves to cut off airflow to pipes · showing pressure losses. Since the radiator structure would be under centrifugal stress, the ES group worried about the effect of a major impact on it. A large hole could propagate and tear the structure apart. Specifically, the ES group worried about the consequence of a docking accident if the external radiator was on the docking port end of the colony. The ES group felt that designing the radiator to withstand an accidental ramming by a cargo container or transfer pod would complicate the structural design and fabrication too much and therefore decided to put the external radiator at the window end of the colony. This design made the disc configuration unacceptable since such a disc would hide the parabolic mirror from the Sun. The remaining configuration, a sunward-side cylinder, is shown in Figure 6.26. Both the second mirror, the external radiator, and the hull orientation thrusters (shown in Figure 6.21 but not in Figure 6.26) were supported on a hollow trusswork extending from around the hull's window. The ES group anticipated that the trusswork might also support a solar collector and power plant if it were decided to generate electricity with a thermal engine. This trusswork structure was named the boom. The ES and SDA groups worried about the masses at the end of this boom because of the two-gyroscope affect described above. However, to minimize these masses, the only significant reduction possible was in the size of the radiator. In light of all the disadvantages listed above, the ES group concluded that the external radiator concept was unacceptable unless the radiator mass could be greatly reduced. However, to dissipate 2.49xl0 7 watts with a radiator area smaller than 7.73xl0 4 m 2 and a reasonable mass flow through the radiator, an inlet temperature higher than 295°K was required. Therefore, power had to flow from a 295°K hull to a higher-temperature radiator necessitating a heat pump (also called a refrigerator circuit), thus making the system ''active''.
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