Fig. 4-84. Radiator Mass Comparison: Heat Pipe (Constant Section Manifolds) and Tube/ Fin (Tapered Manifolds) Since, intuitively, it does not seem possible that heat pipes replacing tubes filled with NaK could account for such a large increase in mass, the two systems were broken down into their constituent masses. Figure 4-85 compares the heat pipe/fin radiator (constant section manifolds) with tube/fin radiator for inlet temperatures of 644K (700°F) and 900K (1160°F), and radiator power of 32 x 106 Kw/SPS. Each is broken down into three major mass elements: (a) manifolds and NaK and pump mass, (b) panels, and (c) pump penalty mass. It is obvious that the widest difference is in (a). Since the pump mass is comparatively small, the difference must be in the manifolds and NaK. Observation of the design of the two systems shows that the tapered headers (manifolds) of the tube/fin radiator must contribute largely to the lower mass compared to the constant section manifolds of the heat pipe fin radiator. Analyses of radiators with tapered headers as in the tube/fin radiator concept but with the tube/fin panels replaced by heat pipe/fin panels was performed. The radiator design is shown in Figure 4-86. Fig. 4-86. Heat Pipe Radiator With Tapered Manifolds Results of the analyses are shown in Table 4-33. It can be clearly seen that using tapered manifolds instead of constant section manifolds reduces the mass by more than 50%. Figure 4-87 is a graphical comparison of masses of heat pipe/fin radiators and tube/fin radiators, both with tapered manifolds. Figure 4-87 shows the heat pipe/fin radiator mass estimate as a discontinuous curve. The reason for this is that the various heat pipe transport fluid has a fairly sharply defined operating temperature range. The radiator has been optimized for the appropriate transport fluid for a given inlet temperature. Fig. 4-85. Effect of Manifold Taper on Radiator Mass
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