**4.6 SNAP-27**

During the 1960's, scientists involved with the Apollo program envisioned placing scientific stations on the lunar surface that could transmit data long after the astronauts returned to Earth. They were interested in many measurements, including fluctuations in solar and terrestrial magnetic fields, changes in the low concentrations of gas in the lunar atmosphere, and internal structure and composition of the Moon. These ideas culminated in the Apollo Lunar Surface Experiment Package (ALSEP), led by Bendix Aerospace Systems Division. The requirement for multi-year operation and survival over many 14-day lunar day/night cycles favored use of RPS as the primary power source for ALSEP. Although NASA looked at using the new SNAP-19 for this application, ALSEP power requirements would have necessitated multiple SNAP-19s per mission and considerable effort in deployment by the Apollo crew. Instead, the AEC was requested to develop a new RTG, called the SNAP-27 (Fig. 17).

Special features were added to the SNAP-27 to ensure safety and facilitate its deployment by the astronauts on the lunar surface (Fig. 18). Chief of these was the separate storage of the heat source in a graphite lunar module fuel cask (GLFC) carried on the Lunar Excursion Module (LEM). The GLFC enclosed the fuel module during the trip to the Moon, and provided thermal and blast protection in the event of a launch pad explosion, launch abort, or reentry into the Earth's atmosphere and ground impact.

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Fig. 17. SNAP-27.
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The 12-sided converter used Pb-Te thermoelectric "Isotec" panels operated at a low hot-side temperature of 673 K in a vacuum, thus eliminating the need for hermetic sealing and a cover gas to inhibit thermoelectric material sublimation. Each of the 12 Isotec panels contained 36 Pb-Te thermocouples arranged in a series-parallel matrix with four couples in a

The TRIAD satellite (Fig. 16) was launched on September 2, 1972 from Vandenburg Air Force Base into a 700 to 800 km orbit. The short-term objectives of the TRIAD satellite were successfully demonstrated, including a checkout of RTG performance. However, a telemetry-converter failure onboard the spacecraft caused a loss of telemetry data about a month into the mission. This, in turn, precluded measuring the Transit-RTG power level versus time. However, the TRIAD satellite continued to operate normally for some time and

During the 1960's, scientists involved with the Apollo program envisioned placing scientific stations on the lunar surface that could transmit data long after the astronauts returned to Earth. They were interested in many measurements, including fluctuations in solar and terrestrial magnetic fields, changes in the low concentrations of gas in the lunar atmosphere, and internal structure and composition of the Moon. These ideas culminated in the Apollo Lunar Surface Experiment Package (ALSEP), led by Bendix Aerospace Systems Division. The requirement for multi-year operation and survival over many 14-day lunar day/night cycles favored use of RPS as the primary power source for ALSEP. Although NASA looked at using the new SNAP-19 for this application, ALSEP power requirements would have necessitated multiple SNAP-19s per mission and considerable effort in deployment by the Apollo crew. Instead, the AEC was requested to develop a new RTG, called the SNAP-27

Special features were added to the SNAP-27 to ensure safety and facilitate its deployment by the astronauts on the lunar surface (Fig. 18). Chief of these was the separate storage of the heat source in a graphite lunar module fuel cask (GLFC) carried on the Lunar Excursion Module (LEM). The GLFC enclosed the fuel module during the trip to the Moon, and provided thermal and blast protection in the event of a launch pad explosion, launch abort,

or reentry into the Earth's atmosphere and ground impact.

row in webbed, magnesium-thorium corner posts with Teflon insulators.

provided magnetometer data using power from the RTG.

Fig. 16. Transit TRIAD Satellite.

**4.6 SNAP-27** 

(Fig. 17).

Fig. 18. Use of SNAP-27 on the Moon. Alan Bean deploying SNAP-27 on Apollo 12.

Thermal energy from the fuel capsule was transferred to the generator hot frame by radiative coupling. When deployed on the lunar surface, the fuel capsule operated at 1005 K, while the Inconel 102 alloy hot frame was 880 K. The hot junction temperature ranged between 855 K and 865 K, reflecting an overall temperature drop of 15 to 25 K. On the Moon's surface, where temperatures can vary from 350 K during the lunar day to a frigid 100 K during the lunar night, the generator's cold side temperature operated at 545 K. Pb-Te served as the TE material and the couples were assembled in a series-parallel electrical arrangement to prevent string loss. The power capability for the 19.6 kg RTG was at least 63.5 We at 16 Vdc for one year after lunar emplacement. The converter was 46 cm tall and 40 cm wide across the fins. The specific power was greater than 3.2 We/kg, which represented a 10% increase over the Pioneer SNAP-19.

The five units deployed on the lunar surface from 1969 to 1972 operated flawlessly. Telemetry data from their operation stopped in 1977 when the ALSEPs were intentionally shutdown. Until then, their degradation in performance matched all predictions.

The only potential problem with SNAP-27 occurred with the Apollo-13 mission, when there was concern over the SNAP-27 onboard the LEM reentering the Earth's atmosphere. Normal reentry trajectory and velocity were achieved as had been assumed in the prelaunch review accounting for this type of event. The detached LEM broke up on reentry, as

Radioisotope Power: A Key Technology for Deep Space Exploration 445

The power converter contained 312 Si-Ge unicouples arranged in 24 circumferential rows with each row containing 13 couples. The MHW-RTGs flown on LES 8 and 9 had an average mass of 39.7 kg, BOM power of 154 We, and specific power of 3.9 We/kg. The six RTGs for Voyager were modified to yield a higher specific power of 4.2 We/kg, based on an

LES 8 and 9 were launched together aboard a Titan IIIC launch vehicle on 15 March 1976, and were deployed to a geosynchronous orbit altitude of approximately 36,000 km (Fig. 21). Each LES used two MHW generators (Fig. 19), which provided primary power for all spacecraft systems. The MHW-RTGs more than met the mission goals for lifetime. They also enabled the demonstration of improved methods for maintaining voice or digital data circuits among widely separated mobile communications terminals. Although its RTGs were still providing usable electric power, LES-8 was turned off on 2 June 2004 due to

control difficulties. LES-9, however, continues to operate over 30 years after launch.

Fig. 20. MWH-RTG heat source.

Fig. 21. LES-8 and 9 in orbit.

average mass of 37.7 kg and BOM power of 158 We.

anticipated, while the graphite-encased Pu-238 fuel cask survived the breakup and went down intact in the 20,000 foot deep Tonga Trench, as had been projected for an aborted mission in a lifeboat mode situation.
