6.67 power plant or electrical generating system. The heat pwnp itself would require maintenance and repair and its operation could pose threats (vibration, failure of high-speed components 1 production of toxic substances, severe temperature gradients) to the colony. The increased temperature of the radiator might require heatresistant materials and more complicated maintenance and repair techniques. Therefore, decisions between passive and active external radiators and between types of active heat pumps would involve qualitative as well as quantitative tradeoffs. Appendix VI.H presents a description and analysis of the Brayton heat pump system. This cycle would allow a significant reduction in radiator size at the cost of a large mass flow through the pump and radiator (therefore requiring bulky machinery) and a substantial increase in electrical power requirement. Appendix VI.I describes and investigates the Rankine heat pump system using three different refrigerants. This cycle is more efficient than the Brayton cycle because it stores the power it carries as a liquid-vapor phase change rather than an increase in gaseous temperature. This allows the Rankine system a smaller mass flow and thus smaller equipment and a lesser electrical power consumption. Also, the equivalent radiator temperature would be higher since the condensation process within would not lower the temperature of the refrigerant. In spite of the reduction in external radiator size made possible by the active systems, the ES group was still dissatisfied with the concept. The water-filled Rankine heat pump system described in Appendix VI.I.6 would have an external radiator massing l.64xlo 5 kg which was felt to be excessive. Together with the active radiator concept's other disadvantages (described above), the size problem led the ES group to look for alternative solutions to the thermal problem. VI.8.6: Heat Pipes through the Shield: As stated in Section VI.8.3, the insufficient power output could also be remedied by an increase in the thermal conductivity of the shield. To bring this conductivity up to the needed 40 watts/m-°K (see Section VI.8.3 and Appendix VI.F),
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