Figure 3.3-20 and 3.3-21 summarize the results of the study and show clearly the thermally superior performance of the triangular geometry, both from the standpoint of lower maximum temperature and smaller temperature difference. 3.3.2.5 Temperature Profile Temperature profiles along the beam cap elements were calculated for the MPTS in various orbital positions during the equinoxes and solstices. Orbital variations in the temperature profiles are caused by the varying angle, normal to the structure made with the sun vector, as a result of the antenna being earth oriented. The "North-South" and "East-West" beam caps have been assumed to be oriented at 45° to the sun line to minimize differential solar inputs. The temperature profiles change during the year as a result of the solar load on the structure varying with the changes in Earth-Sun distance and the inclination of the solar vector to the orbital plane. Figure 3.3-22 shows three temperature profiles for a beam cap member located one meter above the antenna surface. Two profiles are for the extremes of the full sun condition, that is, when the structure is located at the sub-solar point of the orbit during the equinoxes and the summer solstice. It is observed that the yearly variation in the temperature profile of a beam cap element is only a few degrees. This is contrasted with the orbital variation which is much greater. Figure 3. 3-22 shows that as the structure near the center of the antenna goes around an orbit its temperature will change approximately 50°K. Near the perimeter of the structure, where the sun's load represents a greater fraction of the total heat load on the structure, the temperature of the beam elements shifts roughly 150°K as the MPTS moves around an orbit. This swing in temperature may be significant when one considers that there will be over 10,000 such cycles during the 30-year life of the MPTS. The temperature swing near the structure perimeter is reduced when a more nearly uniform microwave converter spacing is used. The temperature profiles shown in Fig. 3.3-22 are those associated with a scale factor p = 466 meters* which produces a 10 to 1 power ratio from the center to the antenna tip. With a scale factor of p= 557 meters the power ratio is reduced to 5 to 1 and, as Fig. 3. 3-23 shows, the temperature shift near the perimeter is reduced from its previous value of 150° to 120°K. The swing near the center, however, has increased from 50° to 60° K.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==