**6. SNAP-27 on Apollo**

For the Apollo missions to the Moon, RTGs were a natural choice to power scientific instruments during the long (14-Earth-day) lunar night. To provide this power, the U.S. Atomic Energy Commission (AEC) provided NASA with SNAP-27 RTGs built by General Electric (GE, now part of Lockheed-Martin). The SNAP-27 RTGs were designed to provide at least 63.5 We at 16 V one year after lunar emplacement. (In the case of Apollo 17, the requirement was 69 We two years after emplacement). Figure 7 shows Apollo 12 astronaut Alan L. Bean removing the SNAP-27 fuel-cask assembly from the Lunar Module on 19 November 1969. This was the first use of electricity-producing nuclear power on the Moon.

All five SNAP-27 RTGs (Apollo 12, 14, 15, 16, 17) exceeded their mission requirements in both power and lifetime thereby enabling the Apollo Lunar Surface Experiment Packages (ALSEPs) 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 surface features (Pitrolo, et al., 1969, Bates, et al., 1979). On Apollo 11 the experiment package deployed on the lunar surface was named Early Apollo Scientific Experiments Package (EASAP) and consisted of the laser ranging retro-reflector (LRRR, also deployed on each following Apollo mission, and are still in use today) and the passive seismic experiments package (PSEP). The PSEP utilized 2 RHUs called the Apollo Lunar Radioisotopic Heater (ALRH) for thermal control (Apollo 11 Lunar Landing Mission Press Kit, 1969*)* and also had a solar array power system that lasted three weeks. The ALRHs contained ~34 gm of 238Pu producing 15 Wth each. The subsequent PSEP stations utilized power from the SNAP-27 RTGs.

U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future 483

With NASA developing the higher-powered Voyager 1 and Voyager 2 spacecraft (see Figure 8) as the next generation of outer planet explorers the bar was raised for RTG performance. To meet this demand, the AEC funded GE (now part of Lockheed-Martin) to develop the Multi-Hundred Watt Radioisotope Thermoelectric Generator (MHW-RTG), which was based on the use of a silicon-germanium alloy. Silicon-germanium, as noted earlier, can be operated at higher temperatures (~1300 K) than the telluride-based thermoelectrics (~800-900 K). Higher temperatures mean higher heat rejection temperatures, which mean smaller radiators hence lower unit masses. Combining the higher temperature with multifoil insulation (instead of bulk insulation) and vacuum operation (instead of using a cover gas) can yield a specific power that is 40% to over 70% higher than that of a telluride-based RTG (Bennett, et al., 1984). The basic layout of a

Fig. 9. Cutaway of the General-Purpose Heat Source Radioisotope Thermoelectric Generator

The GPHS-RTG can produce over 300 We at initial fueling. The overall diameter is 42.2 cm

and the length is 114 cm. The mass is 55.9 kg (Image credit: DOE).

**7. Silicon-germanium RTGs** 

silicon-germanium RTG is shown in Figure 9.

(GPHS-RTG).

Fig. 7. Apollo 12 astronaut Alan L. Bean removing the SNAP-27 fuel-cask assembly from the Lunar Module. The SNAP-27 converter is shown in front of Bean ready to receive the fuelcask assembly. (NASA)

Fig. 8. Artist's concept of a Voyager spacecraft flying by Jupiter and Saturn. The three MHW-RTGs are shown on the boom above the spacecraft. The average power of each MHW-RTG was 158 We. The overall diameter was 39.73 cm and the length was 58.31 cm. The average flight mass for a Voyager MHW-RTG was 37.69 kg. (Image credit: NASA/JPL/Caltech)
