Fig. 4-68. Radiator T Fig. 4-70. Effect of Power Level on Radiator System Specific Mass (Optimum LT) level of a radiator is to be increased, additional panel area must be provided around the periphery (the radiator is a single-plane structure to minimize view factor and meteoroid effects). The headers associated with this added area are obviously longer (and, consequently, more massive) than those associated with an equal area near the center. Thus, radiator specific mass is a function of the power level of the system, and becomes an important factor in the selection of ideal power satellite module size, particularly if the radiator operates at a relatively lower temperature range (and is, consequently, more area-intensive). 4.10.5 Occultation Effects Solar occultation will occur for varying periods of up to 70 minutes (1.167 hours) duration. During these periods, the NaK radiator is subject to cool down from its normal operating temperature. A transient thermal analysis was conducted to determine whether the NaK in the radiator panel tubing will freeze. The cooling rates with and without circulation were determined. The results (Figure 4-71) indicate a high probability that freezing will occur during longer occultation periods (>38 minutes). (The NaK helium heat exchanger mass was not included which would delay the freezing time somewhat.) At the end of occultation, to thaw the radiator, it is anticipated that the collector facets can be oriented to direct reflected solar energy to the radiator surface. When the NaK has melted the facets would be redirected to the cavity aperture to start up the cycle. However, thawing the radiator is considered as potentially F Fig. 4-69. Effect of Significant Radiator Parameters on Total Mass model includes material strength allowables. Consequently, the wall thickness of the panel tubes and headers must increase as temperature increases to yield the 30 year creep rupture strength. In Figure 4-70, minimum radiator specific mass is plotted versus thermal power dissipated for five inlet temperatures. The variation with power level may be explained as follows: The single source of the power to be dissipated is located at the approximate center of the radiator. If the power
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