**4.7 Multihundred Watt (MHW) RTG**

In anticipation that NASA would require higher power RTGs for increasingly ambitious robotic science missions in the future, the AEC contracted with GE to conduct a technology readiness effort for an RTG with a power capability in the range of several hundred We. Development of this unit, which later became known as the MHW-RTG, was initiated in anticipation that NASA would conduct a Grand Tour mission of the planets. This was realized with the Voyager missions launched in 1977. At the same time, the DOD also had a requirement for a hundred watt-class RTG, and requested the AEC to develop such a unit for two communication satellite technology demonstrators built by MIT's Lincoln Laboratory. These Lincoln Experimental Satellites (LES) 8 and 9 were launched together in 1976.

The MHW-RTG represented a dramatic advancement in RTG technology with its use of Silicon-Germanium (Si-Ge) thermoelectric materials and a much higher temperature heat source. The higher hot-side temperature translated to greater power conversion efficiency, and, most importantly, enabled radiation of waste heat at higher temperatures. This allowed a substantial reduction in radiator size and a significant increase in specific power over its Pb-Te/TAGS predecessors. Thermocouples made of Si-Ge can operate over a broad temperature range, up to 1,000 C, much higher than telluride-based thermocouples. Plus with a Silicon Nitride coating, Si-Ge does not sublimate significantly, and allows operation without a cover gas in the vacuum of space.

The MHW-RTG had a length of 58.3 cm and fin span of 39.7 cm (Fig. 19). The converter housing consisted of a beryllium outer shell and pressure domes, with unicouples attached directly to the outer shell. Like SNAP-19, the heat source was designed to immobilize and contain the fuel in the event of a launch abort. It was shaped as a right circular cylinder, and contained twenty-four 3.7-cm diameter fuel containers of PuO2 (Fig. 20). Each fuel container produced 100 Wt, and had a metallic iridium shell containing the PuO2 fuel and a graphite impact shell, which provided the primary resistance to mechanical impact loads.

Fig. 19. MHW-RTG. Cutaway view on left. Installation in test fixture on right.

Fig. 20. MWH-RTG heat source.

444 Radioisotopes – Applications in Physical Sciences

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

In anticipation that NASA would require higher power RTGs for increasingly ambitious robotic science missions in the future, the AEC contracted with GE to conduct a technology readiness effort for an RTG with a power capability in the range of several hundred We. Development of this unit, which later became known as the MHW-RTG, was initiated in anticipation that NASA would conduct a Grand Tour mission of the planets. This was realized with the Voyager missions launched in 1977. At the same time, the DOD also had a requirement for a hundred watt-class RTG, and requested the AEC to develop such a unit for two communication satellite technology demonstrators built by MIT's Lincoln Laboratory. These Lincoln Experimental Satellites (LES) 8 and 9 were launched together in

The MHW-RTG represented a dramatic advancement in RTG technology with its use of Silicon-Germanium (Si-Ge) thermoelectric materials and a much higher temperature heat source. The higher hot-side temperature translated to greater power conversion efficiency, and, most importantly, enabled radiation of waste heat at higher temperatures. This allowed a substantial reduction in radiator size and a significant increase in specific power over its Pb-Te/TAGS predecessors. Thermocouples made of Si-Ge can operate over a broad temperature range, up to 1,000 C, much higher than telluride-based thermocouples. Plus with a Silicon Nitride coating, Si-Ge does not sublimate significantly, and allows operation

The MHW-RTG had a length of 58.3 cm and fin span of 39.7 cm (Fig. 19). The converter housing consisted of a beryllium outer shell and pressure domes, with unicouples attached directly to the outer shell. Like SNAP-19, the heat source was designed to immobilize and contain the fuel in the event of a launch abort. It was shaped as a right circular cylinder, and contained twenty-four 3.7-cm diameter fuel containers of PuO2 (Fig. 20). Each fuel container produced 100 Wt, and had a metallic iridium shell containing the PuO2 fuel and a graphite impact shell, which provided the primary resistance to mechanical impact loads.

Fig. 19. MHW-RTG. Cutaway view on left. Installation in test fixture on right.

mission in a lifeboat mode situation.

**4.7 Multihundred Watt (MHW) RTG** 

without a cover gas in the vacuum of space.

1976.

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

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. 21. LES-8 and 9 in orbit.

Radioisotope Power: A Key Technology for Deep Space Exploration 447

The GPHS-RTG used the same Si-Ge alloy unicouples used in the MHW-RTG. Because production of the unicouples had been stopped after the Voyager program there was a need to restart production. However, the rest of the design was very different. For one, the converter housing was made of a less expensive and more manufacturable Aluminum 2219- T6 alloy, instead of the beryllium used in the MHW-RTG. Another big difference was the heat source, which employed an assembly of newly developed General Purpose Heat Source (GPHS) modules. This modular approach to heat source design opened the door for developing RTGs of different sizes and powers in the future, but it required an extensive development and qualification program to replace the fuel sphere assemblies used in the MHW-RTG. Finally, DOE had decided to move the RTG assembly and testing work from its RTG contractors to DOE's Mound Laboratory, which necessitated a rapid buildup of the

The GPHS-RTG, shown in Fig. 2, was composed of two main elements: a linear stack of 18 GPHS modules and the converter. The converter surrounds the heat source stack, and consists of 572 radiatively-coupled Si-Ge unicouples, which operate at a hot side temperature of 1,275 K and a cold side/heat rejection temperature of 575 K. The outer case of the RTG provides the main support for the converter and heat source assembly, which is axially preloaded to withstand the mechanical stress environments of launch and to avoid separation of GPHS modules. The converter also provides axial and mid-span heat source supports, a multifoil insulation packet and a gas management system. The latter provides an inert gas environment for partial power operation on the launch pad, and also protects the multifoil and refractory materials during storage and ground

The complete GPHS-RTG has an overall length of 114 cm and a fin span of 42.2 cm. Its mass of 55.9 kg and BOM power level of up to 300 We provides a specific power of 5.1 to 5.3

The Galileo spacecraft (Fig. 23) was launched on 18 October 1989 on the Space Shuttle, after a 3.5-year delay caused by the Challenger accident. Forced to take a long, circuitous trajectory involving Earth and Venus gravity assists, Galileo arrived at Jupiter in December 1995, The Orbiter spacecraft investigated the Jupiter and its Galilean satellites from space, while the Galileo Probe, which was battery-powered but kept warm via a number of small radioisotope heater units, entered Jupiter's atmosphere on 7 December 1995. Both GPHS-RTGs met their end of mission (EOM) power requirements, thus allowing NASA to extend the Galileo mission three times. However on 21 September 2003, after eight years of service in orbit about Jupiter, the mission was terminated by intentionally forcing the orbiter to burn up in Jupiter's atmosphere. This was done to avoid any chance of contaminating local moons, especially Europa, with micro-organisms

The Ulysses (Fig. 24) was launched nearly a year later by the Space Shuttle on 6 October 1990. The mission included a Jupiter gravity assist performed on 8 February 1992 in order to place the spacecraft in a trajectory over the polar regions of the Sun. The single GPHS-RTG performed flawlessly and exceeded its design requirement. As a result, the Ulysses mission was extended beyond its original planned lifetime goal, thus allowing it to take measurements over the Sun's poles for the third time in 2007 and 2008. However after it became clear that the power output from the RTG would be insufficient to operate science

infrastructure at a new location.

We/kg, far greater than any of its predecessors.

operations.

from Earth.

The Voyager 2 spacecraft launched on 20 August 1977 aboard a Titan-Centaur launch vehicle (Fig. 22). Each Voyager probe carried three MHW generators. Voyager 1 followed on 5 September 5, also aboard a Titan-Centaur rocket.

Fig. 22. Voyager spacecraft.

The Voyager spacecraft explored the most territory of any mission in history, including all the giant planets of the outer solar system, 48 of their moons, and the unique system of rings and magnetic fields those planets possess. The final planetary encounter was conducted by Voyager 2, which had its closest approach with Neptune on 25 August 1989. Although Pioneers 10 and 11 were the first spacecraft to fly beyond all the planets, Voyager 1 passed Pioneer 10 to become the most distant human-made object in space. As of 11 August 2007, the power generated by the spacecraft had dropped to about 60% of the power at launch. This is better than the pre-launch predictions based on a conservative thermocouple degradation model. As the electrical power decreases, spacecraft loads must be turned off, eliminating some spacecraft capabilities.

#### **4.8 General Purpose Heat Source (GPHS) RTG**

Following the successful launches of the Voyager spacecraft, DOE turned its focus on developing a new selenide-based RTG for NASA's planned International Solar Polar Mission (ISPM) and the Jupiter Orbiter Probe, which later became the Ulysses and Galileo missions, respectively. Nuclear power was required for these missions, since they would both operate in the vicinity of Jupiter with its low solar energy flux, cold temperatures and intense radiation environment. Both missions were to be launched in the mid-1980s aboard the then under development U.S. Space Shuttle.

Upon determining that selenide thermoelectrics would not be suitable for long-duration missions, DOE went back to Si-Ge technology and considered modifying flight spares of the MHW-RTG for use on Galileo. However, the joint NASA-ESA ISPM team requested a new, larger, more powerful RTG for their spacecraft. When the Galileo project saw the benefits of the planned ISPM RTG they requested two for the Galileo spacecraft. As a result the ISPM RTG was renamed the GPHS-RTG.

The Voyager 2 spacecraft launched on 20 August 1977 aboard a Titan-Centaur launch vehicle (Fig. 22). Each Voyager probe carried three MHW generators. Voyager 1 followed

The Voyager spacecraft explored the most territory of any mission in history, including all the giant planets of the outer solar system, 48 of their moons, and the unique system of rings and magnetic fields those planets possess. The final planetary encounter was conducted by Voyager 2, which had its closest approach with Neptune on 25 August 1989. Although Pioneers 10 and 11 were the first spacecraft to fly beyond all the planets, Voyager 1 passed Pioneer 10 to become the most distant human-made object in space. As of 11 August 2007, the power generated by the spacecraft had dropped to about 60% of the power at launch. This is better than the pre-launch predictions based on a conservative thermocouple degradation model. As the electrical power decreases, spacecraft loads must be turned off,

Following the successful launches of the Voyager spacecraft, DOE turned its focus on developing a new selenide-based RTG for NASA's planned International Solar Polar Mission (ISPM) and the Jupiter Orbiter Probe, which later became the Ulysses and Galileo missions, respectively. Nuclear power was required for these missions, since they would both operate in the vicinity of Jupiter with its low solar energy flux, cold temperatures and intense radiation environment. Both missions were to be launched in the mid-1980s aboard

Upon determining that selenide thermoelectrics would not be suitable for long-duration missions, DOE went back to Si-Ge technology and considered modifying flight spares of the MHW-RTG for use on Galileo. However, the joint NASA-ESA ISPM team requested a new, larger, more powerful RTG for their spacecraft. When the Galileo project saw the benefits of the planned ISPM RTG they requested two for the Galileo spacecraft. As a result the ISPM

on 5 September 5, also aboard a Titan-Centaur rocket.

Fig. 22. Voyager spacecraft.

eliminating some spacecraft capabilities.

**4.8 General Purpose Heat Source (GPHS) RTG** 

the then under development U.S. Space Shuttle.

RTG was renamed the GPHS-RTG.

The GPHS-RTG used the same Si-Ge alloy unicouples used in the MHW-RTG. Because production of the unicouples had been stopped after the Voyager program there was a need to restart production. However, the rest of the design was very different. For one, the converter housing was made of a less expensive and more manufacturable Aluminum 2219- T6 alloy, instead of the beryllium used in the MHW-RTG. Another big difference was the heat source, which employed an assembly of newly developed General Purpose Heat Source (GPHS) modules. This modular approach to heat source design opened the door for developing RTGs of different sizes and powers in the future, but it required an extensive development and qualification program to replace the fuel sphere assemblies used in the MHW-RTG. Finally, DOE had decided to move the RTG assembly and testing work from its RTG contractors to DOE's Mound Laboratory, which necessitated a rapid buildup of the infrastructure at a new location.

The GPHS-RTG, shown in Fig. 2, was composed of two main elements: a linear stack of 18 GPHS modules and the converter. The converter surrounds the heat source stack, and consists of 572 radiatively-coupled Si-Ge unicouples, which operate at a hot side temperature of 1,275 K and a cold side/heat rejection temperature of 575 K. The outer case of the RTG provides the main support for the converter and heat source assembly, which is axially preloaded to withstand the mechanical stress environments of launch and to avoid separation of GPHS modules. The converter also provides axial and mid-span heat source supports, a multifoil insulation packet and a gas management system. The latter provides an inert gas environment for partial power operation on the launch pad, and also protects the multifoil and refractory materials during storage and ground operations.

The complete GPHS-RTG has an overall length of 114 cm and a fin span of 42.2 cm. Its mass of 55.9 kg and BOM power level of up to 300 We provides a specific power of 5.1 to 5.3 We/kg, far greater than any of its predecessors.

The Galileo spacecraft (Fig. 23) was launched on 18 October 1989 on the Space Shuttle, after a 3.5-year delay caused by the Challenger accident. Forced to take a long, circuitous trajectory involving Earth and Venus gravity assists, Galileo arrived at Jupiter in December 1995, The Orbiter spacecraft investigated the Jupiter and its Galilean satellites from space, while the Galileo Probe, which was battery-powered but kept warm via a number of small radioisotope heater units, entered Jupiter's atmosphere on 7 December 1995. Both GPHS-RTGs met their end of mission (EOM) power requirements, thus allowing NASA to extend the Galileo mission three times. However on 21 September 2003, after eight years of service in orbit about Jupiter, the mission was terminated by intentionally forcing the orbiter to burn up in Jupiter's atmosphere. This was done to avoid any chance of contaminating local moons, especially Europa, with micro-organisms from Earth.

The Ulysses (Fig. 24) was launched nearly a year later by the Space Shuttle on 6 October 1990. The mission included a Jupiter gravity assist performed on 8 February 1992 in order to place the spacecraft in a trajectory over the polar regions of the Sun. The single GPHS-RTG performed flawlessly and exceeded its design requirement. As a result, the Ulysses mission was extended beyond its original planned lifetime goal, thus allowing it to take measurements over the Sun's poles for the third time in 2007 and 2008. However after it became clear that the power output from the RTG would be insufficient to operate science

Radioisotope Power: A Key Technology for Deep Space Exploration 449

Fig. 25. Cassini spacecraft. Pre-launch checkout of RTG on left. Artist concept of vehicle on

The most recent mission to use a GPHS-RTG is the New Horizons mission to Pluto (Fig. 26), which was launched on 19 January 2006 aboard an Atlas V 551. The spacecraft is currently on a 9.5-year transit to Pluto and Charon. At encounter, which is expected in July 2015, New Horizons will characterize and map the surfaces of Pluto and Charon and their atmospheres. From 2016 to 2020, the spacecraft will continue to conduct encounters with one or two Kuiper Belt Objects. So far, it is anticipated that the RTG will exceed its power

Fig. 26. New Horizons spacecraft. Pre-launch integration with spacecraft on left. Artist

limit its modularity and ease of integration on future small to mid-size spacecraft.

Although the GPHS-RTG served well on Ulysses and Galileo and continues to meet requirements for Cassini and New Horisons, it is not suitable for future missions on Mars and other planetary bodies with atmospheres. The GPHS-RTG was only designed to function effectively in a vacuum environment. Furthermore, its relatively large size and power level

DOE and NASA are currently developing a new generation of RPS generators that could be used for a variety of space missions. One is the Multi-Mission RTG (MMRTG), which has

concept of New Horizons flyby of Pluto and Charon on right.

right.

and lifetime requirements.

**4.9 Multi-mission RTG (MMRTG)** 

instruments and keep onboard hydrazine propellant from freezing, the decision was made to end the mission on 1 July 2008.

Fig. 23. Galileo spacecraft. Pre-launch assembly on left. Artist concept of spacecraft in orbit around Jupiter on right.

Fig. 24. Ulysses spacecraft. Installation and checkout of RTG on left. Artist concept of vehicle on right.

The third mission to use the GPHS-RTG was Cassini (Fig. 25), which was launched, along with the ESA-built Huygens Titan Probe, on 15 October 1997 aboard a Titan IV/Centaur launch vehicle. Cassini achieved Saturn orbit insertion on 1 July 2004 after a 6.7-year transit involving gravity assists about Venus and Earth. The Huygens probe, which carried the same radioisotope heater units as Galileo, successfully landed on Titan and provided the first close-up views of that enigmatic world. Because of mission complexity, Cassini needed more power than used on previous flagship-class missions. The three GPHS-RTGs that were used have so far operated flawlessly and have exceeded their expected power output. The mission has now been approved for an extension to 2017.

instruments and keep onboard hydrazine propellant from freezing, the decision was made

Fig. 23. Galileo spacecraft. Pre-launch assembly on left. Artist concept of spacecraft in orbit

Fig. 24. Ulysses spacecraft. Installation and checkout of RTG on left. Artist concept of vehicle

The third mission to use the GPHS-RTG was Cassini (Fig. 25), which was launched, along with the ESA-built Huygens Titan Probe, on 15 October 1997 aboard a Titan IV/Centaur launch vehicle. Cassini achieved Saturn orbit insertion on 1 July 2004 after a 6.7-year transit involving gravity assists about Venus and Earth. The Huygens probe, which carried the same radioisotope heater units as Galileo, successfully landed on Titan and provided the first close-up views of that enigmatic world. Because of mission complexity, Cassini needed more power than used on previous flagship-class missions. The three GPHS-RTGs that were used have so far operated flawlessly and have exceeded their expected power output.

The mission has now been approved for an extension to 2017.

to end the mission on 1 July 2008.

on right.

around Jupiter on right.

Fig. 25. Cassini spacecraft. Pre-launch checkout of RTG on left. Artist concept of vehicle on right.

The most recent mission to use a GPHS-RTG is the New Horizons mission to Pluto (Fig. 26), which was launched on 19 January 2006 aboard an Atlas V 551. The spacecraft is currently on a 9.5-year transit to Pluto and Charon. At encounter, which is expected in July 2015, New Horizons will characterize and map the surfaces of Pluto and Charon and their atmospheres. From 2016 to 2020, the spacecraft will continue to conduct encounters with one or two Kuiper Belt Objects. So far, it is anticipated that the RTG will exceed its power and lifetime requirements.

Fig. 26. New Horizons spacecraft. Pre-launch integration with spacecraft on left. Artist concept of New Horizons flyby of Pluto and Charon on right.

#### **4.9 Multi-mission RTG (MMRTG)**

Although the GPHS-RTG served well on Ulysses and Galileo and continues to meet requirements for Cassini and New Horisons, it is not suitable for future missions on Mars and other planetary bodies with atmospheres. The GPHS-RTG was only designed to function effectively in a vacuum environment. Furthermore, its relatively large size and power level limit its modularity and ease of integration on future small to mid-size spacecraft.

DOE and NASA are currently developing a new generation of RPS generators that could be used for a variety of space missions. One is the Multi-Mission RTG (MMRTG), which has

Radioisotope Power: A Key Technology for Deep Space Exploration 451

MSL is considerably larger than the Mars Exploration Rovers that landed on the planet in 2004. It will carry more advanced scientific instruments than any other Mars mission to date, including analysis of samples scooped up from the soil and drilled powders from rocks. It will also investigate the past and present ability of Mars to support life. The MSL rover will use power from an MMRTG to supply heat and electricity for its components and science instruments. A coolant loop and heat exchanger coupled with the MMRTG radiators will transport waste heat to the electronics, thus extending operation of the rover into the Martian night and winter season. The goal is to operate for at least one Martian

The MMRTG could be used on a number of other potential missions in the future. One exciting prospect is to use the MMRTG as the principal electrical power and heat source for a Titan aerobot/balloon mission (Fig. 29). In this scenario, the considerable waste heat produced by the MMRTG would be used to heat a gas and generate buoyancy for a balloon carrying a long-lived payload, in addition to providing electrical power to onboard

When the potential of radioisotope power became apparent in the 1950s, the original focus was on development of dynamic power conversion systems. Most of these activities concentrated on applying the high efficiencies achievable with Brayton and Rankine cycles,

Although thermoelectric technology supplanted this approach and became the dominant power conversion option for every RPS flown in space, work on Dynamic Isotope Power Systems (DIPS) continued at various times throughout the intervening decades. The principal focus of these efforts was on eventual development of power systems capable of producing up to tens of kilowatts of power. These higher power technologies would be used in conjunction with the ambitious crewed missions anticipated in the future. The studies of DIPS pointed to its excellent suitability for lunar and planetary surface

in expectation that systems would evolve to larger power levels in the future.

year (i.e., two Earth years) over a wide range of possible landing sites.

instruments.

Fig. 29. Titan Aerobot.

**4.10 Advanced Stirling Radioisotope Generator (ASRG)** 

been designed to operate on planetary bodies with atmospheres, such as Mars, as well as in the vacuum of space. The MMRTG's smaller size of about 110 We is more modular in design and flexible in meeting the needs of a broader range of different missions as it generates electrical power in smaller increments. The design goals for the MMRTG include ensuring a high degree of safety and reliability, optimizing power levels over a minimum lifetime of 14 years, and minimizing mass.

The MMRTG (Fig. 27) is designed to use a heat source consisting of eight Step 2 GPHS modules. These Step 2 modules have additional material in the GPHS aeroshell that improves structural integrity and performance. Although the Pb-Te/TAGS thermoelectric materials are the same as those used on SNAP-19, and represent a thoroughly flight proven technology, the physical dimensions and material changes to improve performance have resulted in different degradation compared to the SNAP-19. The MMRTG generator has a fin span of 64 cm, a length of 66 cm, and a mass of about 45 kg. Its BOM power level of approximately 110 We yields a specific power that is less than the SNAP-19. However, the purpose in pursuing this unit is not to advance state-of-the-art in specific power, but to minimize development risk, while providing an RPS capable of operating in different mission environments.

Fig. 27. Multi-Mission RTG (MMRTG). Cutaway schematic of power unit on left. MMRTG Qualification Unit undergoing tests on right.

The MMRTG is being developed to serve as the primary power source on the Mars Science Laboratory (MSL), a concept of which is shown in Fig. 28. This mission is currently planned for launch in 2011, and is anticipated to land on Mars in 2012.

Fig. 28. Mars Science Laboratory.

been designed to operate on planetary bodies with atmospheres, such as Mars, as well as in the vacuum of space. The MMRTG's smaller size of about 110 We is more modular in design and flexible in meeting the needs of a broader range of different missions as it generates electrical power in smaller increments. The design goals for the MMRTG include ensuring a high degree of safety and reliability, optimizing power levels over a minimum

The MMRTG (Fig. 27) is designed to use a heat source consisting of eight Step 2 GPHS modules. These Step 2 modules have additional material in the GPHS aeroshell that improves structural integrity and performance. Although the Pb-Te/TAGS thermoelectric materials are the same as those used on SNAP-19, and represent a thoroughly flight proven technology, the physical dimensions and material changes to improve performance have resulted in different degradation compared to the SNAP-19. The MMRTG generator has a fin span of 64 cm, a length of 66 cm, and a mass of about 45 kg. Its BOM power level of approximately 110 We yields a specific power that is less than the SNAP-19. However, the purpose in pursuing this unit is not to advance state-of-the-art in specific power, but to minimize development risk, while providing an RPS capable of operating in different

Fig. 27. Multi-Mission RTG (MMRTG). Cutaway schematic of power unit on left. MMRTG

The MMRTG is being developed to serve as the primary power source on the Mars Science Laboratory (MSL), a concept of which is shown in Fig. 28. This mission is currently planned

lifetime of 14 years, and minimizing mass.

Qualification Unit undergoing tests on right.

Fig. 28. Mars Science Laboratory.

for launch in 2011, and is anticipated to land on Mars in 2012.

mission environments.

MSL is considerably larger than the Mars Exploration Rovers that landed on the planet in 2004. It will carry more advanced scientific instruments than any other Mars mission to date, including analysis of samples scooped up from the soil and drilled powders from rocks. It will also investigate the past and present ability of Mars to support life. The MSL rover will use power from an MMRTG to supply heat and electricity for its components and science instruments. A coolant loop and heat exchanger coupled with the MMRTG radiators will transport waste heat to the electronics, thus extending operation of the rover into the Martian night and winter season. The goal is to operate for at least one Martian year (i.e., two Earth years) over a wide range of possible landing sites.

The MMRTG could be used on a number of other potential missions in the future. One exciting prospect is to use the MMRTG as the principal electrical power and heat source for a Titan aerobot/balloon mission (Fig. 29). In this scenario, the considerable waste heat produced by the MMRTG would be used to heat a gas and generate buoyancy for a balloon carrying a long-lived payload, in addition to providing electrical power to onboard instruments.

Fig. 29. Titan Aerobot.

#### **4.10 Advanced Stirling Radioisotope Generator (ASRG)**

When the potential of radioisotope power became apparent in the 1950s, the original focus was on development of dynamic power conversion systems. Most of these activities concentrated on applying the high efficiencies achievable with Brayton and Rankine cycles, in expectation that systems would evolve to larger power levels in the future.

Although thermoelectric technology supplanted this approach and became the dominant power conversion option for every RPS flown in space, work on Dynamic Isotope Power Systems (DIPS) continued at various times throughout the intervening decades. The principal focus of these efforts was on eventual development of power systems capable of producing up to tens of kilowatts of power. These higher power technologies would be used in conjunction with the ambitious crewed missions anticipated in the future. The studies of DIPS pointed to its excellent suitability for lunar and planetary surface

Radioisotope Power: A Key Technology for Deep Space Exploration 453

Activities are focused on developing and testing the ASRG-EU in thermal and vibrational

The ASRG-EU uses two axially-opposed Advanced Stirling Convertors (ASCs), operating at a hot-end temperature of 650 deg C, producing about 140 We. Sunpower is developing the ASC under a 2002 NASA Research Announcement (NRA) with GRC. The low mass of the

The ASRG has achieved a TRL 6 (system demonstration in a relevant environment) with operation at qualification level thermal and dynamic environments. Tests on the ASRG-EU were completed in June 2008 at the Lockheed-Martin Space System Company in King of Prussia, PA. These evaluations included thermal balance, thermal performance, mechanical disturbance, sine transient, random vibration, simulated pyrotechnic shock and electromagnetic interference and magnetic field emission tests. Over 1,000 hours of successful EU operating time with numerous startup and shutdown cycles were accumulated during the testing at Lockheed-Martin. The ASRG-EU is now undergoing extended/multi-year duration testing at NASA GRC. It has achieved over 11,000 hours of successful operation as of April

Ongoing ASRG-EU tests use electrical resistance heaters that simulate the heating characteristics of the actual GPHS module. Avoiding use of nuclear materials during early phases of development greatly facilitates testing and evaluation of the ASRG

Fig. 30. Advanced Stirling Radioisotope Generator (ASRG).

ASC is key to the ASRG's high overall system specific power.

subsystems.

environments that closely approximate qualification-level tests (Fig. 31).

2011, and is expected to exceed 14,000 hours of operation by the end of 2011.

exploration, particularly surface rovers, remote science stations and backup power supplies to central base power.

Interest in DIPS was particularly high during the Space Exploration Intitiative (SEI) of the early-1990s. However with the demise of that effort in 1992, the focus shifted to determine how dynamic power conversion could benefit radioiosotope power systems in the multi-hundred watt range. During the 1990s, several advanced dynamic and static conversion technologies were researched and evaluated. Several technologies that had appeared promising initially proved to be ill-suited for the unique demands of deep space missions. In the end, it became apparent that the free-piston Stirling engine offered the best hope of advancing the efficiency of future generators, while offering lifetimes up to a decade or two. Unlike previous DIPS designs, which featured turbomachinery-based conversion technologies (e.g. Brayton), small Stirling DIPS could be advantageously scaled down to multihundred-watt unit size while preserving size and mass competitiveness with RTGs.

In 2002, NASA and DOE began a Stirling Radioisotope Generator (SRG) project focused on evaluating and demonstrating a unit for flight development. The work was initiated to provide a back-up RPS for the MSL mission. The unit used Stirling convertors built and tested under a technology development effort funded by DOE. Although the SRG could achieve a four-fold reduction in fuel requirements for the same power, the final system specific power of the unit was only slightly better than the MMRTG.

In less than two years, it became apparent that the MMRTG would be selected by NASA's Mars program, so that the rover could make use of the significant waste heat produced by that unit. Finally, a small business technology project initiated in the early 2000s with Sunpower Technologies in Athens, Ohio, indicated that convertors with much better mass performance could be developed and substituted into an SRG-based design. Such a unit could potentially achieve specific powers of about 7 We/kg. With the advancement in Stirling generator heater head materials and with improved temperature margin and higher temperature operation, units with specific powers greater than 8 We/kg may be possible..

In 2005, the decision was made to redirect efforts toward development of an Advanced SRG (ASRG) technology demonstration Engineering Unit (EU). The effort drew upon the work that had gone on previously with the controller, housing and insulation systems for the SRG, but incorporated use of the higher specific power Sunpower generators. In addition to high specific power, the ASRG would likely achieve an efficiency over 30%. This is four to five times higher than that from a GPHS-RTG, and is particularly important for conserving the very limited worldwide supply of Pu-238 fuel.

The ASRG, which is shown in Fig. 30, is being developed under the joint sponsorship of the U.S. Department of Energy (DOE) And NASA. The eventual flight units are expected to produce over 130 We in a space environment and to have a mass of 32 kg or less. The prime contractor is Lockheed-Martin Corporation of Valley Forge, PA, with Sunpower, Inc. of Athens, Ohio as the main subcontractor. NASA Glenn Research Center (GRC) is supporting the technology development, along with evaluation and testing of the Stirling convertors used in the device. In addition to improving fuel utilization efficiency over previous RPS, the ASRG is being designed for multi-mission use in deep space, and within the atmosphere of Mars and possibly Titan.

exploration, particularly surface rovers, remote science stations and backup power supplies

Interest in DIPS was particularly high during the Space Exploration Intitiative (SEI) of the early-1990s. However with the demise of that effort in 1992, the focus shifted to determine how dynamic power conversion could benefit radioiosotope power systems in the multi-hundred watt range. During the 1990s, several advanced dynamic and static conversion technologies were researched and evaluated. Several technologies that had appeared promising initially proved to be ill-suited for the unique demands of deep space missions. In the end, it became apparent that the free-piston Stirling engine offered the best hope of advancing the efficiency of future generators, while offering lifetimes up to a decade or two. Unlike previous DIPS designs, which featured turbomachinery-based conversion technologies (e.g. Brayton), small Stirling DIPS could be advantageously scaled down to multihundred-watt unit size while preserving size and mass

In 2002, NASA and DOE began a Stirling Radioisotope Generator (SRG) project focused on evaluating and demonstrating a unit for flight development. The work was initiated to provide a back-up RPS for the MSL mission. The unit used Stirling convertors built and tested under a technology development effort funded by DOE. Although the SRG could achieve a four-fold reduction in fuel requirements for the same power, the final system

In less than two years, it became apparent that the MMRTG would be selected by NASA's Mars program, so that the rover could make use of the significant waste heat produced by that unit. Finally, a small business technology project initiated in the early 2000s with Sunpower Technologies in Athens, Ohio, indicated that convertors with much better mass performance could be developed and substituted into an SRG-based design. Such a unit could potentially achieve specific powers of about 7 We/kg. With the advancement in Stirling generator heater head materials and with improved temperature margin and higher temperature operation, units with specific powers greater than 8 We/kg may be

In 2005, the decision was made to redirect efforts toward development of an Advanced SRG (ASRG) technology demonstration Engineering Unit (EU). The effort drew upon the work that had gone on previously with the controller, housing and insulation systems for the SRG, but incorporated use of the higher specific power Sunpower generators. In addition to high specific power, the ASRG would likely achieve an efficiency over 30%. This is four to five times higher than that from a GPHS-RTG, and is particularly important for conserving

The ASRG, which is shown in Fig. 30, is being developed under the joint sponsorship of the U.S. Department of Energy (DOE) And NASA. The eventual flight units are expected to produce over 130 We in a space environment and to have a mass of 32 kg or less. The prime contractor is Lockheed-Martin Corporation of Valley Forge, PA, with Sunpower, Inc. of Athens, Ohio as the main subcontractor. NASA Glenn Research Center (GRC) is supporting the technology development, along with evaluation and testing of the Stirling convertors used in the device. In addition to improving fuel utilization efficiency over previous RPS, the ASRG is being designed for multi-mission use in deep space, and

specific power of the unit was only slightly better than the MMRTG.

the very limited worldwide supply of Pu-238 fuel.

within the atmosphere of Mars and possibly Titan.

to central base power.

competitiveness with RTGs.

possible..

Fig. 30. Advanced Stirling Radioisotope Generator (ASRG).

Activities are focused on developing and testing the ASRG-EU in thermal and vibrational environments that closely approximate qualification-level tests (Fig. 31).

The ASRG-EU uses two axially-opposed Advanced Stirling Convertors (ASCs), operating at a hot-end temperature of 650 deg C, producing about 140 We. Sunpower is developing the ASC under a 2002 NASA Research Announcement (NRA) with GRC. The low mass of the ASC is key to the ASRG's high overall system specific power.

The ASRG has achieved a TRL 6 (system demonstration in a relevant environment) with operation at qualification level thermal and dynamic environments. Tests on the ASRG-EU were completed in June 2008 at the Lockheed-Martin Space System Company in King of Prussia, PA. These evaluations included thermal balance, thermal performance, mechanical disturbance, sine transient, random vibration, simulated pyrotechnic shock and electromagnetic interference and magnetic field emission tests. Over 1,000 hours of successful EU operating time with numerous startup and shutdown cycles were accumulated during the testing at Lockheed-Martin. The ASRG-EU is now undergoing extended/multi-year duration testing at NASA GRC. It has achieved over 11,000 hours of successful operation as of April 2011, and is expected to exceed 14,000 hours of operation by the end of 2011.

Ongoing ASRG-EU tests use electrical resistance heaters that simulate the heating characteristics of the actual GPHS module. Avoiding use of nuclear materials during early phases of development greatly facilitates testing and evaluation of the ASRG subsystems.

Radioisotope Power: A Key Technology for Deep Space Exploration 455

Nuclear Electric Propulsion (NEP) has been studied since the early 1960's because of its potential for future high-energy space missions. Almost all NEP assessments to date have assumed fission as the nuclear energy source. Unlike solar-powered electric propulsion (SEP) systems, NEP operation is generally independent of distance and orientation with respect to the Sun. Over the last decade, several studies have pointed to Radioisotope Power Systems (RPS), instead of reactor power sources, as the best way of implementing NEP. Radioisotope-based NEP, also known as Radioisotope Electric Propulsion (REP), has been evaluated before, but has not been seriously considered for flight due to the low specific power range of traditional RPS (e.g., 3 to 5 We/kg). However, the prospects for REP have improved substantially with the advent of the ASRG and its likely improvement in specific

In this capacity, REP would principally be used as an interplanetary stage for long-duration deceleration and acceleration in deep space. At remote destinations, REP would perform deceleration, orbit insertion and maneuvers around outer planets and other planetary bodies. REP-based spacecraft could also provide ample power at destination for sophisticated science instruments and communications, but it would fit better within the relatively modest kilowatt-scale power requirements of the space science community.

Radioisotope power systems will continue to play an important role in NASA's exploration efforts. These systems also have the potential for use in a variety of new applications, which would benefit from the technology's versatility in a broad range of space and planetary environments. In the near-term, the MMRTG will expand the capability for conducting science on the surface of Mars. The ASRG will enable even higher performance missions. These units will also enable more ambitious exploration of other planetary surfaces and provide a reliable means of powering spacecraft in deep space. Current activities would also allow the potential development of new systems that could expand application of RPS to smaller science missions. The key to successful implementation of RPS is to maintain close ties with potential users and the science community at large. With these advancements, radioisotope power systems and technology will offer tremendous benefits

Angelo, J.A. and Buden, D., *Space Nuclear Power*, Orbit Book Co., Malabar, FL, 1985, pp. 133-

Bennett, G.L., "Space Nuclear Power: Opening the Final Frontier," AIAA-2006-4191, 4th International Energy Conversion Engineering Conference, June 2006. Bennett, G.L., and Skrabek, E.A., "Power Performance of U.S. Space Radioisotope

Chan, J., Hill, D., Hoye, T., and Leland, D., "Development of Advanced Stirling

Thermoelectric Generators," in proceedings of *15th International Conference on* 

Radioisotope Generator for Planetary Surface and Deep Space Missions," AIAA-2009-5768, 6th International Energy Conversion Engineering Conference, July 28-30,

power.

**6. Conclusion** 

**7. References** 

157.

2009.

for future exploration endeavors.

*Thermoelectrics*, June 1996, pp. 357-372.

Fig. 31. ASRG Engineering Unit.
