cooler than the triangle proper. Increasing the emissivity of the tabs on the side facing the antenna surface results in higher tab temperatures. Therefore, it is possible to reduce the temperature gradient within the member by proper selection of the emissivity of the tabs. Although applying a coating to the tabs will increase manufacturing costs, the smaller temperature gradient will permit a thinner, and hence lighter, structural member to be used. The net effect may be a reduction in the total system cost. Figure 3.3-19 presents the maximum temperature and temperature difference that Figure 3.3-19 exists in triangular members. The results are shown with and without a side solar load for two different size tabs with various emissivity values on the tabs. The emissivity value that minimizes the temperature difference is in the neighborhood of 0.2, which can be achieved by applying an anodized coating 0.1 microns in thickness (Ref 23). The maximum temperature difference is reduced to 30° K with solar load and 12° K without solar load. Figure 3. 3-19 also shows that there is no thermal advantage to the larger tabs. However, opening the triangle beyond the 60° angle considered in the study will reduce both the maximum temperature and the temperature difference within the member. In the limit, opening the triangle completely to a flat plate produces the lowest possible temperature, 337°K (147°F), for white paint on the top ( ) and unfinished aluminum( ) on the bottom. This is, of course, at the complete expense of the member strength. No doubt there is an optimum angle. The following conclusions can be drawn from this study of candidate beam geometries: • Tube geometry is the worst from a thermal standpoint. The highest temperatures and largest temperature differences are achieved with this geometry. The use of aluminum or graphite/epoxy tubing near the center of the antenna will not be possible without the use of some insulation between the tubing and the antenna surface. (The insulation should not encapsulate the tubing.) • The high-hat section is not an attractive geometry although the temperature picture is somewhat better than the tubular geometry. The tube with its greater rigidity is preferred over the high-hat. • The triangular section is the best geometry of those studied. It has the lowest temperature and the smallest temperature differences. It can be easily manufactured and made of aluminum. Whether it is economically justifiable to anodize the bottom of the side tabs remains to be investigated.
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