with reduced heat resistance," similar to the upper set of curves of Fig. 6. It was therefore sufficient to draw only the Troc-curves for the "basic HPSE" in Fig. 7 as representative for all of the three cases. Trec varies from 847 to 908°C and is thus only from 26 to 60 K above the relevant LiF Phase change temperature TPC = 848°C. The mean gas temperature THe (in the heater tube of the Stirling engine over its length within the heat pipe of the HPSE) plays an important role. Subject to the details of the next two paragraphs, it is a measure of the maximum working gas temperature in the Stirling engine which determines the thermal-to-mechanical effeciency 7/th of this engine. 7jth increases with THe which should therefore be as large as possible. The calculations yielded for the HPSEs of cases (1) through (3), respectively, as described in Figs. 1, 2, 6, and 7, both for the insolation and the eclipse periods. Therefore, while the addition of the intermediate heat pipe has little influence on the relevant heat flows (Figs. 3 and 4) and the heat pipe and receiver temperatures (Figs. 6 and 7), it reduces THe by 50.8 K and, according to a NASA-published diagram1, 7?th by ca. 5% (abs.). For this reason - and also because of its lack of redundancy as to possible damage by meteorite impact etc. - an intermediate commmon heat pipe should not be used unless it were unavoidably necessary for an equidistribution of the heat load between the heat pipes and storage units of the HPSEs.
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