temperatures.) It is seen that there is well over a 200°K temperature difference between a column at the perimeter of the structure and the one at the center. This large temperature difference does not pose a deflection problem provided the sizing of the vertical members reflects the different operating temperatures. However, careful design consideration will have to be given to the vertical members to minimize deflections induced by the different temperature swings (viz., 18°K for a member located near the center, 61°K for a member near the perimeter) caused by the varying sun load. Use of a low solar absorptance coating on vertical members will diminish the Sun's influence so far as a direct solar load on the members is concerned. ( corresponding to white paint was used in the present study.) However, since the sun's energy is absorbed by the antenna surface and then reradiated as energy in the infrared region, it is seen that the low coating will not eliminate entirely the difference in temperature swings between columns. Organic matrices with their low thermal expansion coefficients may well be the answer. The columns near the perimeter are prone to having large temperature gradients along their length and through their cross-section and therefore will probably not be tubular in cross-section. The gradients tend to exist because of the different views elements on the column have of the antenna surface and space. This contrasted with the columns near the center. Every element along these columns, regardless of orientation or position, has the same view of the antenna surface and space, 0.5 each. Consequently, columns near the center will be essentially uniform in temperature but rather hot (viz. 482 - 500°K). Coatings, insulation or geometry selections will not yield any significant reduction in these column temperatures. Near the center of the antenna, the columns will have to be made from a material such as polyimide that can sustain the temperature level of 500° K. If aluminum or graphite/epoxy is to be used for these vertical members, waste heat flux at the center of the antenna must be reduced. One way of accomplishing this reduction while maintaining the total power level of the MPTS is to space the microwave converters differently which can be achieved by increasing the scale factor . Increasing p has the effect of reducing the power transmitted from the center of the antenna and increasing it at the perimeter. A discussion of the effects of varying p are given next. 3. 3.2.8 Effect of Microwave Converter Spacing Maximum structural temperatures are dictated by the maximum waste heat flux. These maximum values occur at the center of the antenna where the microwave converters are most densely packed. The microwave converter spacing is given by meters where = the converter spacing at the center of the antenna in meters.
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