The maximum waste heat flux from the amplitron subarrays is established by the maximum operating temperature of the amplitron and the size of the fin radiator required to dissipate the waste heat. For an amplitron rated at 5 kW of radiated microwave energy and 85 percent efficiency, 0. 882 kW of waste heat must be dissipated, which requires a 48 cm diameter fin radiator. At minimum spacing between radiators, the maximum package density of the amplitrons is 4. 3 per m2. Thermal analysis studies indicate that 87. 5 percent of the waste heat is directed toward the primary structure. An assumed additional 200 W of amplifier waste heat produces a total waste heat flux of 3. 5 kW/m2. This value determines the maximum anticipated structural element temperatures. 7. 3. 3. 2 STRUCTURAL TEMPERATURES Structural temperatures of the microwave antenna are shown as a function of radiated waste heat for flat structural elements both parallel and perpendicular to the antenna plane. For parallel structural elements, one side absorbs heat from the antenna while the other side radiates heat to space, so that its temperature can be lowered by lowering the emissivity value of the absorbing side. For a perpendicular member, however, the sides of the elements are both absorbing and radiating, so that its temperature is independent of emissivity and is given by the curve e = 1 (Fig. 7-42). Although several factors not considered here can increase structural temperatures (solar heating, structural shapes curved for efficiency, and degraded emissivity values), maximum structural temperatures are not expected to preclude the use of aluminum or graphite epoxy at the maximum waste heat flux of 3. 5 kW/m2. 7. 3. 3. 3 STRUCTURAL TEMPERATURE GRADIENTS The in-plane structural temperature gradients from the center to the edge of the antenna are extreme and can be associated directly with the waste heat profile. Transverse structural temperature gradients are minimal over much of the antenna. Average flux to a point on the structure does not change very much with distance, as can be seen by the view angle depicted in Figure 7-43. Near the edge of the antenna, transverse gradients become more pronounced because of the increased view of space with increased distance from the antenna. 7. 3. 3.4 STRUCTURAL COOL-DOWN RATE IN THE EARTH'S SHADOW The cool-down rate for a 0. 05 cm aluminum plate parallel to and near the center of the antenna is approximately 9°C/min (Fig. 7-44). The combined structural/thermal design impact to the antenna will require much more detailed study.
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