3.3.2.3 High-Hat Section The high temperature and gradients within the tube geometry are caused basically by the bottom segment of the tube not being able to radiate directly to deep space and the top segment not receiving radiant energy directly from the antenna surface. A "high-hat" geometry does not have this disadvantage. Instead each segment has some view of both the antenna surface and deep space; albeit the fractional view of each varies from segment to segment. Figure 3. 3-17 shows the temperature distribution in a high-hat section that is dimensioned L x 6L x 4L. Two cases are shown: One where both sides of the member are painted white, and the other case where the side towards the antenna surface has a low emissivity coating () or, if aluminum were used, the side towards the antenna surface is left untreated. 2 A solar load of 1356 watts/meter was applied to the right side of the high-hat to obtain the maximum temperature gradient. The following conclusions can be drawn from Fig. 3.3-17: • The high-hat section runs cooler than the tubular cross-section • Temperature gradients are smaller in the high-hat than in the tube • Both sides of the high-hat should be painted white as this reduces the maximum temperature difference from 196°K to 76°K • Aluminum or graphite/epoxy cannot be used without some insulation between the member and the antenna surface. 3.3.2.4 Triangular Cross-Section The next geometry considered was that of a triangle. In this geometry each segment of the triangle has a good view of cold space. Figure 3.3-18 shows the temperature distribution within such a member with and without a solar load. Of great significance here is the low maximum temperature of 439° K (330° F) which permits aluminum or graphite/epoxy to be used. This geometry can be easily manufactured with one side of the aluminum sheet left untreated and the other side painted white or given an alzac finish. Graphite/epoxy would have the top painted white and the side facing the antenna would be aluminum foil bonded into the epoxy. The maximum temperature differences in the triangular shaped member are seen to be 73°K with a side solar load and 56°K without a solar load. The two side tabs are running
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