6.57 are set equal for simplicity. The shield is modeled as a continuously solid flat wall so that power flows through it entirely by convection. The analysis neglects any energy radiated onto the outside of the shield by the back of the parabolic mirror. Figure 6.24 (a copy of Figure 6.Fl) shows the effect of the thermal conductivity of the shield on the power flow from the hull out to space. A solid aluminum shield would allow 3.6xl0 7 watts to radiate away. A solid glass shield would limit the output to 8.6xlo 6 watts. To match the power output to the power input of 3.35xl0 7 watts, the thermal conductivity of the shield would have to be 40 watts/m-°K,roughly the thermal conductivity of lead. However, a shield of lunar dirt suspended in vacuum would have a thermal conductivity worse than that of glass. Much of the lunar soil is silica, but the vacuum between the glass particles would insulate them from each other. If the shield was made from fused silica, the shield's thermal conductivity could be modeled as that of solid glass (1 watt/m-°K). This is an approximation since the fused silica would have gaps and impurities in it; furthermore, a solid glass shield would have a thickness other than 2.5 meters. For simplicity, however, the shield was assumed to be 2.5 meters thick and to have 1 watt/m-°K thermal conductivity. Then the expected power output to space was: Poutputhull + shield 8.6xl0 6 watts which was 2.49xl0 7 watts short of the needed output. The ES group saw two possibilities to supplement this insufficient power output. The radiative area could be increased, or the thermal conductivity of the shield could be improved. The Fabrication and Test Group anticipated increased complexity in the manufacture and fabrication of a higher-thermal-conductivity shield. The ES group therefore first investigated an increase in radiative area . VI.8.4: Output by Passive External Radiator: The ES group considered a passive external radiator. The colony's air could be
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