SNAP-27 The SNAP-27 RTGs were developed to power the experiments of NASA's Apollo Lunar Surface Experiments Package (ALSEP). The RTG design requirement was to provide at least 63.5 We at 16 V DC 1 year after lunar emplacement. (In the case of Apollo 17, the requirement was 69 We 2 years after emplacement.) The use of RTGs to power the ALSEPs was a natural choice because of their light weight, reliability, and ability to produce full electrical power during the long lunar night-day cycle. Since the ALSEPs were to be manually positioned by the astronauts, the RTG designers took advantage of this assembly capability. The converter and the sealed-fuel-capsule assembly were kept separately in the Lunar Module and integrated on the Moon as shown in Fig. 12. This approach allowed optimization of the electrical, mechanical, and thermal interfaces of the two major hardware subsystems of the RTG [16], Figure 13 is a schematic of the SNAP-27 RTG. A total of five RTG-powered ALSEPs were placed on the Moon. In each case the RTGs exceeded their mission requirements in both power and lifetime (all were still operating when NASA shut down the stations on 30 September 1977). Through this performance beyond mission requirements, the SNAP-27 RTGs enabled the ALSEP stations to gather long-term scientific data on the internal structure and composition of the Moon, the composition of the lunar atmosphere, the state of the lunar interior, and the genesis of lunar features [17].
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