Fig. 4-64. Liquid Metal (NaK) Loop radiator panels arranged as in Figure 4-63. The cooled liquid is passed through output headers and feeders and over the diode collectors, completing the cycle. An accumulator is used to provide a positive pressure at the pump inlet. Isolation valves are provided at the inlet and outlet of each panel to enable any panel(s) to be cut out of the cooling loop to prevent loss of coolant in the event of leaks due to meteoroid puncture or other causes. It has been shown that the motion of parts of the radiator such as headers and panels relative to each other has been considered in the design. The "halo" configuration of the radiator minimizes relative motion of its parts due to temperature differentials. Another factor for consideration in the radiator design is metal creep due to stress. Figure 4-65 shows the creep (or strain) of Haynes 188 material in 30 years as a percentage of original length, plotted against the constant stress level required to produce the creep, for three different temperatures. Fig. 4-65. Stress Versus Creep — Haynes 188 Note that a decrease in stress level causes a disproportionate decrease in creeo; e.g., a decrease in stress from 7 to 5 x 107 N/M^ at 1033K causes a decrease in the 30 year creep from 7.5 to 0.75 percent. If the stress level for a creep of 10% in 30 years is reduced by 50% the creep becomes very small— approximately 0.1%. The radiator headers and feeders are designed for relatively high stress such that significant creep (approximately 10%) occurs over the design life (30 years). Figure 4-66 shows a circular section of feeder or header tube. It should be noted that the circumferential stress is twice the axial stress. The wall thickness of the header or feeder is thus determined by the circumferential stress. However, Figure 4-65 showed that creep decreases at a much higher rate than stress. Thus, the axial creep will be very small compared to the circumferential creep. The graph in Figure 4-66 shows that for a 10% creep the volume of the feeder or header increases approximately 22% in 30 years. Fig. 4-66. Header or Feeder Volume Versus Creep This large volume increase is too great for the NaK accumulator to handle. A yearly, or two-yearly, "topping-up" of the system will be required. 4.10.4 Radiator System Optimization Figure 4-67 is a portion of the interactions diagram of a liquid metal cooled generation system. It represents a math model which is computerized to determine minimum radiator system mass. It is a portion of a large math model of the complete powersat module.
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