6.68 the ES group considered inserting heat pipes in the shield to channel the power to its outer surface. A heat pipe (6.8), as shown in Figure 6.28, is a closed chamber whose inside walls are lined with a capillary structure (or wick) that is saturated with a volatile fluid. Vapor heat transfer is the principal power-transport mechanism between the evaporator section at one end and the condensor section at the other. The condensed fluid returns to the evaporator end by capillary action. Several properties of the heat pipe make it particularly suitable for increasing the ther~al conductivity of the shield. Vaporheat-transfer devices can have several thousand times the heattransfer capacity of the best metallic conductors. The temperature loss through the pipe is very small so that it is essentially an isothermal device. Also, the evaporation and condensation functions of a heat pipe are effectively independent operations connected only by the streams of vapor and liquid in the pipe. This makes it possible to separate the heat sink from the heat source by a large distance and allows the heat sink temperature to remain constant in spite of large variations in the power input to the heat pipe. Since the power through-put of a heat pipe is limited by its size and its fluid's properties, the ES group considered locating a number of heat pipes through the shield, spread out evenly over the shield surface. However, the Fabrication and Test Group worried that these heat pipes would increase the complexity of fabrication of the shield, require frequent inspection, and pose serious repair problems if they failed. In spite of these objections, the ES group felt that heat pipes were preferable to the active external radiator concept. VI.9: CONFIGURATION CHANGE VI.9.1: Reasons for Changes: Up to this point in the design process, the study group had become increasingly dissatisfied with the proposed overall configuration. While there were no objections to
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