**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along with the development of the higher efficiency Advanced Stirling Radioisotope Generator (ASRG) (see Section 10) (unpublished Report of the RPS Provisioning Strategy Team, 2001).

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

U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future 487

Fig. 12. Cutaway of the Multi-Mission Radioisotope Thermoelectric Generator (Hammel, et al., 2009) The MMRTG is designed to produce ~110 We at BOM with a mass of ~45 kg. The MMRTG is about 64 cm in diameter (fin-tip to fin-tip) by 66 cm long. (Image credit:

Fig. 13. The MMRTG Thermoelectric (TE) Couple in illustration and in a Test Fixture (Image

Like the SNAP-19 RTGs, the MMRTG is a sealed RTG with a cover gas. (The MHW-RTGs, GPHS-RTGs and Transit RTG were operated in a vacuum.) The heat source is sealed from the converter by a thin metal liner. Helium buildup from the natural decay of the Pu-238 fuel is prevented by venting it directly to the exterior of the MMRTG. The hermetically sealed converter contains an argon cover gas that reduces parasitic heat losses and protects the thermoelectric elements. With this venting and cover gas arrangement the MMRTG can operate in space or in an atmosphere (e.g., the surface of Mars or Titan) (Hammel, et al., 2009).

DOE).

credit: NASA/JPL/Caltech)

Fig. 11. Artist's concept of the Mars Science Laboratory (MSL) Curiosity rover with the MMRTG shown attached to the back end (right side in the picture). MSL is ~3 m long (not including the arm), 2.7 m wide, 2.1 m tall with a mass of 900 kg. The arm can reach about 2.1 m. (Image credit: NASA/JPL/Caltech).

The overarching science goals of the MSL mission are to search for clues about whether environmental conditions (such as the existence of water for significant periods) could support microbial life today or in the past, and to assess whether the environment has favored the preservation of this evidence. MSL will be the first interplanetary mission to use a sky crane to land and the first to use guided entry to land in a precise location. MSL is designed to last for one Mars year (~687 Earth days) and to travel 20 km during its prime mission.

The MMRTG is designed to provide about 110 We on the surface of Mars at 28 to 32 V. The conversion is achieved using 16 thermoelectric modules of 48 telluride-based thermoelectric elements (Hammel, et al., 2009). The MMRTG is designed to have a minimum lifetime of 14 years. The MMRTG employs a flexible modular design approach that would allow the MMRTG concept to meet the power requirements of a wide range of missions.

Figure 12 shows a cutaway of the MMRTG. The MMRTG gets its ~2-kWt of thermal power from eight GPHS modules, the same heat source technology that was successfully used in the GPHS-RTGs and is planned for use in the ASRG. Like the GPHS-RTG, the converter housing and the eight heat rejection (radiator) fins are made of aluminum. The core assembly with 16 thermoelectric modules, each containing 48 couples (see Figure 13), are located under the eight fins with eight pairs of two modules aligned axially (Hammel, et al., 2009).

The thermoelectric modules are spring loaded to enhance conduction of heat from the GPHS modules and to enhance conduction of heat from the cold junction of the thermoelectric elements into the module bar and then into the converter housing. A bulk insulation system composed of the material Min-K reduces heat losses, in effect forcing the heat to travel through the thermoelectric elements. To enhance reliability the thermoelectric couples are electrically arranged in series and parallel. This redundant arrangement prevents loss of power should one or even several thermoelectric elements fail (Hammel, et al., 2009).

Fig. 11. Artist's concept of the Mars Science Laboratory (MSL) Curiosity rover with the MMRTG shown attached to the back end (right side in the picture). MSL is ~3 m long (not including the arm), 2.7 m wide, 2.1 m tall with a mass of 900 kg. The arm can reach about 2.1

one Mars year (~687 Earth days) and to travel 20 km during its prime mission.

MMRTG concept to meet the power requirements of a wide range of missions.

fins with eight pairs of two modules aligned axially (Hammel, et al., 2009).

The overarching science goals of the MSL mission are to search for clues about whether environmental conditions (such as the existence of water for significant periods) could support microbial life today or in the past, and to assess whether the environment has favored the preservation of this evidence. MSL will be the first interplanetary mission to use a sky crane to land and the first to use guided entry to land in a precise location. MSL is designed to last for

The MMRTG is designed to provide about 110 We on the surface of Mars at 28 to 32 V. The conversion is achieved using 16 thermoelectric modules of 48 telluride-based thermoelectric elements (Hammel, et al., 2009). The MMRTG is designed to have a minimum lifetime of 14 years. The MMRTG employs a flexible modular design approach that would allow the

Figure 12 shows a cutaway of the MMRTG. The MMRTG gets its ~2-kWt of thermal power from eight GPHS modules, the same heat source technology that was successfully used in the GPHS-RTGs and is planned for use in the ASRG. Like the GPHS-RTG, the converter housing and the eight heat rejection (radiator) fins are made of aluminum. The core assembly with 16 thermoelectric modules, each containing 48 couples (see Figure 13), are located under the eight

The thermoelectric modules are spring loaded to enhance conduction of heat from the GPHS modules and to enhance conduction of heat from the cold junction of the thermoelectric elements into the module bar and then into the converter housing. A bulk insulation system composed of the material Min-K reduces heat losses, in effect forcing the heat to travel through the thermoelectric elements. To enhance reliability the thermoelectric couples are electrically arranged in series and parallel. This redundant arrangement prevents loss of

power should one or even several thermoelectric elements fail (Hammel, et al., 2009).

m. (Image credit: NASA/JPL/Caltech).

Fig. 12. Cutaway of the Multi-Mission Radioisotope Thermoelectric Generator (Hammel, et al., 2009) The MMRTG is designed to produce ~110 We at BOM with a mass of ~45 kg. The MMRTG is about 64 cm in diameter (fin-tip to fin-tip) by 66 cm long. (Image credit: DOE).

Fig. 13. The MMRTG Thermoelectric (TE) Couple in illustration and in a Test Fixture (Image credit: NASA/JPL/Caltech)

Like the SNAP-19 RTGs, the MMRTG is a sealed RTG with a cover gas. (The MHW-RTGs, GPHS-RTGs and Transit RTG were operated in a vacuum.) The heat source is sealed from the converter by a thin metal liner. Helium buildup from the natural decay of the Pu-238 fuel is prevented by venting it directly to the exterior of the MMRTG. The hermetically sealed converter contains an argon cover gas that reduces parasitic heat losses and protects the thermoelectric elements. With this venting and cover gas arrangement the MMRTG can operate in space or in an atmosphere (e.g., the surface of Mars or Titan) (Hammel, et al., 2009).

U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future 489

This time the CBC was chosen and Rocketdyne developed a basic 2.5-kWe DIPS module that could be used in space or on planetary surfaces. Figure 15 illustrates the basic components of a 2.5-kWe modular DIPS power conversion unit (PCU). While again changing national priorities did not allow DIPS to be developed into a flight system, the basic technology exists to provide an RPS with powers spanning the range from 2 to ≥10 kWe (Rockwell, 1992). Section 10 describes a lower power successor to DIPS, the Advanced Stirling Radioisotope Generator (ASRG) that will provide increased efficiency and use less Pu-238 fuel than existing RTGs

Fig. 15. Components of the proposed 2.5-kWe modular DIPS Power Conversion Unit (PCU). Overall dimensions for the 2.5-kWe module are 2.44 m x 3.55 m x 0.5 m. (Image credit:

The ASRG employs an advanced, high efficiency, dynamic Stirling engine for heat-toelectric power conversion. This process is roughly four times more efficient than presently utilized thermoelectric devices. As a result, the ASRG produces comparable power to the MMRTG with only one quarter of the Pu-238, extending the supply of radioisotope fuel available for future space science missions. The higher efficiency provided by dynamic systems, such as the ASRG, could become an enabling power system option for higher power kilowatt class power systems envisioned for flagship class science spacecraft, large

The ASRG utilizes an advanced Stirling free-piston heat engine consisting of two major assemblies, the displacer and piston, that reciprocate to convert heat to electrical power as shown in Figure 16. Heat from the GPHS module is conductively coupled to the heater head (not shown). Helium is used as the working fluid and is hermetically contained within the convertor enclosure. The displacer shuttles helium between the expansion

**10. Advanced Stirling Radioisotope Generator (ASRG)** 

planetary rovers, and systems in support of human exploration activities.

Rocketdyne).

Modeling of the MMRTG performance indicates that the MMRTG will be able to provide the necessary power to enable MSL to achieve its objectives (Hammel, et al., 2009).
