The effect of on the maximum structural temperatures is shown in Fig. 3. 3-29 for two efficiencies, 70 and 85%, and two types of members, vertical and horizontal. Figure 3.3-29 reinforces what was mentioned early that a vertical member or column will run hotter than a horizontal beam at the center of the antenna. But more to the relevancy of is the trend shown of decreasing temperature with increasing . It is obvious that the choice of may well impose a constraint on material selection. It is recognized that will influence the microwave transmission efficiency and the total power that can be handled by the MPTS. It should now be obvious that has a strong effect on structural temperatures and material selection which must be accounted for in any studies to optimize the power received on Earth. Before closing this section it should be noted that there are other ways of reducing the maximum structural temperature than by increasing p; some of the methods are: • Alter the microwave converter radiator design so that more of the waste heat . is rejected in the transmission direction of the microwaves (toward Earth) and less is radiated towards the antenna support structure. In the present study, 87.5% of the microwave converter waste heat is being radiated towards the structure and 12.5% toward Earth. • Employ heat pipes to smooth out the heat rejection profile so as to produce a nearly uniform profile across the surface of the antenna (see Fig. 3. 3-30). • Design the microwave converter radiator surfaces to be geometrically and spectrally selective so as to reduce the amount of solar energy that is absorbed and to alter the distribution of radiant flux emanating from the antenna to a more nearly uniform one. • Use special coatings on the structural members. For the present study white paint () was used on the side of members facing space and an aluminum finish ( lightly anodized) for the side facing the antenna surface. Coatings such as silver teflon ( ) and gold ( ), respectively could be substituted for the white paint and aluminum. It must be noted that most coatings degrade as a result of exposure to ultraviolet radiation and particulate radiation emanating from galactic sources and the Van Allen belt. The 30-year MPTS design life demands that serious consideration be given to establishing the extent of degradation. At the present time there is a dearth of data for ultraviolet exposures greater than 10 sun-hours; the MPTS will have an exposure of approximately sun-hours.
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