Unfortunately there were also several problems with this design. The very complex temperature gradient in the radiator was felt to present severe problems to the design of the primary reflector. The resulting complex thermal expansion pattern and fatigue resulting from thermal cycling seem likely to degrade the optics. In addition, as the radiator design (radial heat pipes on a one sided disk) is inefficient, it was necessary to make the dish relatively thick (and therefore massive) to minimize the temperature drop. Finally, it appeared that mass efficiency increased significantly as the number of heat pipes increased and as concentrator size decreased. Optimizing in this way alleviated the first two problems to some degree but also decreased the minimum heat pipe radius. For a unit cell radius of 10 - 100 cm the heat pipe dimensions became so small as to be impractical. Larger heat pipes could be used but that would incur a severe mass penalty. For these reasons focus was placed on the second option, a single finned heat pipe projecting axially back from the primary reflector. The major benefits of this approach are simplicity and economy of scale (using a larger heat pipe). At the same time, however, analysis of the reradiation became difficult. Reradiation between fins (taken here to be coaxial disks) on a single pipe was easily evaluated (6,p635) but for radiation between concentrators no simple approximation was found. As a standard of comparison it was estimated that 25 percent of the heat emitted from a heat pipe/fin assembly would be reabsorbed by other units. Using this assumption and foregoing the requirement that the fins be thick relative to any high emissivity coating gave mass efficiency results that were approximately equivalent to that of the disk radiator. In addition to the problems mentioned above there were some difficulties encountered that were common to both applications of heat pipes. The first was transferral of heat from the comparatively large solar cell to the much smaller radius heat pipe. The most promising method seemed to be wrapping the evaporator portion of the pipe into a spiral disk which would cover the back of the solar cell. There is not much information, however, on complex geometries for heat pipes and this application remains to be proven acceptable. A second problem lies in the choice of a proper structural material I working fluid combination for this temperature (assumed to be approximately 470 K). Water seems to be the most likely candidate for a working fluid, particularly when striving for a low non-lunar mass. Unfortunately, few lunar materials are compatible with water at temperatures above 100 C (1,ppl03-106) because generation of even small amounts of oxides or non-condensable gas can seriously impair the performance of a heat pipe(l,pl03). There is evidence to suggest that some steels are workable but this is not yet sufficiently documented(1,ppll0-lll). This leads to a final problem: the lifetime of the heat pipe. Currently most space applications require lifetimes of 5 to 10 years and even that has proved to be a real challenge for heat pipe design(1,pp!08-110). The 30 year lifetime required by the SPS will place a much greater strain on the heat pipes as well as on the verification process. 2.2.4.4 Construction For analysis, the primary reflector is approximated as a circular dish. In practice, however, it is more likely to be constructed as a square or hexagonal section of a panel to facilitate assembly. As these panels will
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