**7. Silicon-germanium RTGs**

482 Radioisotopes – Applications in Physical Sciences

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 fuel-

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)

cask assembly. (NASA)

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 silicon-germanium RTG is shown in Figure 9.

Fig. 9. Cutaway of the General-Purpose Heat Source Radioisotope Thermoelectric Generator (GPHS-RTG).

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).

U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future 485

Fig. 10. Progress in RTG development. (Rockwell, 1992)

Provisioning Strategy Team, 2001).

late fall of 2011 to arrive at Mars in August 2012.

**8. Multi-mission Radioisotope Thermoelectric Generator (MMRTG)** 

Following the successes of such flagship missions as Galileo and Cassini, NASA turned its attention to providing smaller "faster, better, cheaper" science spacecraft. In looking for an RPS which would satisfy that mandate along with being able to operate both in space and on the surface of a planetary body (e.g., Mars), a joint NASA/DOE team recommended development of the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) along with the development of the higher efficiency Advanced Stirling Radioisotope Generator (ASRG) (see Section 10) (unpublished Report of the RPS

The MMRTG, built by Rocketdyne and Teledyne, is based on the telluride thermoelectric technology used in the SNAP-19 RTG program which had shown that it could work in space (Nimbus III, Pioneers 10/11) and on a planetary surface (Viking Landers 1 and 2). The first mission to employ the MMRTG will be the Mars Science Laboratory (MSL), whose rover has been named "Curiosity" (see Figure 11). The 900-kg MSL is scheduled to be launched in the

#### **7.1 MHW-RTG**

The MHW-RTG objective was to provide at least 125 We after five years in space. It was designed to produce at least 150 We at BOM, making it the highest-powered RTG at the time (1970s). Once the program was under way, the U.S. Air Force requested four MHW-RTGs for its communications satellites Lincoln Experimental Satellites 8 and 9 (LES-8/9) (Bennett, et al., 1984). As it turned out, LES-8/9 were launched prior to the Voyager launches (1976 versus 1977). Each LES carried two MHW-RTGs. The MHW-RTGs performed so well that the two communications satellites were used for years, including in the first Gulf War and to relay e-mail messages from stations in Antarctica.

Each Voyager spacecraft carried three MHW-RTGs (see Figure 8). The MHW-RTGs performed so well that Voyager 2 was retargeted after its flyby of Saturn (1981) to fly by Uranus and Neptune giving the human race its first close-up views of those distant worlds. Both Voyagers are still operating, almost 34 years after launch.

#### **7.2 General-purpose Heat Source Radioisotope Thermoelectric Generator (GPHS-RTG)**

For the Galileo and Ulysses missions the U.S. Department of Energy funded GE (now Lockheed Martin) to develop the General-Purpose Heat Source Radioisotope Thermoelectric Generator (GPHS-RTG), a power source essentially equivalent to two MHW-RTGs (see Figure 9). Where the MHW-RTG produced at least 150 We at BOM, the GPHS-RTG was capable of producing 300 We at BOM. Where each MHW-RTG had 312 silicon-germanium thermoelectric elements (called "unicouples"), each GPHS-RTG had 572 unicouples (Bennett, et al., 1984, Bennett, et al., 2006).

NASA's Galileo Orbiter carried two GPHS-RTGs to power its successful exploration of the Jovian system. The Ulysses spacecraft, which was built by the European Space Agency (ESA), carried one GPHS-RTG for its exploration of the polar regions of the Sun (Bennett, et al., 2006).

In 1997, NASA again used the GPHS-RTG, this time three of them to power the Cassini spacecraft that is still in orbit around Saturn. The GPHS-RTGs have performed so well that the mission has been extended several times (Bennett, et al., 2006). Figure 10 illustrates the progress that has been made in RTG performance – *in the span of a little over 30 years the power produced by a space RTG has increased over one-hundredfold!* 

The most recent launch of the GPHS-RTG was in 2006 on the New Horizons spacecraft, which is traveling to Pluto. Because of the unavailability of a full complement of fresh Pu-238 fuel, the GPHS-RTG for New Horizons utilized some existing fuel that had decayed for 21 years since its production, yielding 245.7 We of power at BOM instead of the possible 300 We. Still, it is expected that the GPHS-RTG will provide sufficient power (~200 We) at the time of Pluto encounter to meet all of the mission's scientific and operational requirements. Once Pluto and its principal satellite Charon have been visited, New Horizons is designed to continue beyond to explore Kuiper Belt Objects (KBOs) (Bennett, et al., 2006).

Changes have been made in the general-purpose heat source (GPHS) that is the heart of the GPHS-RTG. For New Horizons, additional aeroshell material was added which increased the mass of the RTG. Additional material increases are planned for the GPHS modules to be used to power the MMRTG for MSL. While these changes have the effect of increasing the mass of the GPHS-RTG over the Galileo/Ulysses GPHS-RTGs there are design improvements, which could recreate the high specific power of the GPHS-RTG (Vining and Bennett, 2010).

The MHW-RTG objective was to provide at least 125 We after five years in space. It was designed to produce at least 150 We at BOM, making it the highest-powered RTG at the time (1970s). Once the program was under way, the U.S. Air Force requested four MHW-RTGs for its communications satellites Lincoln Experimental Satellites 8 and 9 (LES-8/9) (Bennett, et al., 1984). As it turned out, LES-8/9 were launched prior to the Voyager launches (1976 versus 1977). Each LES carried two MHW-RTGs. The MHW-RTGs performed so well that the two communications satellites were used for years, including in

Each Voyager spacecraft carried three MHW-RTGs (see Figure 8). The MHW-RTGs performed so well that Voyager 2 was retargeted after its flyby of Saturn (1981) to fly by Uranus and Neptune giving the human race its first close-up views of those distant worlds.

**7.2 General-purpose Heat Source Radioisotope Thermoelectric Generator (GPHS-**

For the Galileo and Ulysses missions the U.S. Department of Energy funded GE (now Lockheed Martin) to develop the General-Purpose Heat Source Radioisotope Thermoelectric Generator (GPHS-RTG), a power source essentially equivalent to two MHW-RTGs (see Figure 9). Where the MHW-RTG produced at least 150 We at BOM, the GPHS-RTG was capable of producing 300 We at BOM. Where each MHW-RTG had 312 silicon-germanium thermoelectric elements (called "unicouples"), each GPHS-RTG had 572 unicouples

NASA's Galileo Orbiter carried two GPHS-RTGs to power its successful exploration of the Jovian system. The Ulysses spacecraft, which was built by the European Space Agency (ESA), carried one GPHS-RTG for its exploration of the polar regions of the Sun (Bennett, et al., 2006). In 1997, NASA again used the GPHS-RTG, this time three of them to power the Cassini spacecraft that is still in orbit around Saturn. The GPHS-RTGs have performed so well that the mission has been extended several times (Bennett, et al., 2006). Figure 10 illustrates the progress that has been made in RTG performance – *in the span of a little over 30 years the power* 

The most recent launch of the GPHS-RTG was in 2006 on the New Horizons spacecraft, which is traveling to Pluto. Because of the unavailability of a full complement of fresh Pu-238 fuel, the GPHS-RTG for New Horizons utilized some existing fuel that had decayed for 21 years since its production, yielding 245.7 We of power at BOM instead of the possible 300 We. Still, it is expected that the GPHS-RTG will provide sufficient power (~200 We) at the time of Pluto encounter to meet all of the mission's scientific and operational requirements. Once Pluto and its principal satellite Charon have been visited, New Horizons is designed to continue beyond to explore

Changes have been made in the general-purpose heat source (GPHS) that is the heart of the GPHS-RTG. For New Horizons, additional aeroshell material was added which increased the mass of the RTG. Additional material increases are planned for the GPHS modules to be used to power the MMRTG for MSL. While these changes have the effect of increasing the mass of the GPHS-RTG over the Galileo/Ulysses GPHS-RTGs there are design improvements, which

could recreate the high specific power of the GPHS-RTG (Vining and Bennett, 2010).

the first Gulf War and to relay e-mail messages from stations in Antarctica.

Both Voyagers are still operating, almost 34 years after launch.

(Bennett, et al., 1984, Bennett, et al., 2006).

*produced by a space RTG has increased over one-hundredfold!* 

Kuiper Belt Objects (KBOs) (Bennett, et al., 2006).

**7.1 MHW-RTG** 

**RTG)** 

Fig. 10. Progress in RTG development. (Rockwell, 1992)
