Multi-Mission RTG (MMRTG)

The Multi-Mission RTG is NASA and the Department of Energy’s current flagship nuclear power source for space missions. It is the power source for not only NASA’s current flagship planetary mission, but the next two New Frontiers missions as well. These are Curiosity (Mars Science Laboratory), Mars 2020 (soon to be renamed), and now Dragonfly (just recently announced).

The Multi-Mission RTG was the second design to use the GPHS fuel architecture (for more information on that, check out our GPHS page), after the GPHS-RTG used on Galileo and Cassini (more on that here). It also revived and improved a technology we’ve seen before, on the SNAP-19 for Pioneer and Viking (more information here).

This is also the first RTG to be built for NASA in decades that was designed to operate in an atmosphere, a major thermal management change from the typical spaceborne MHW-RTG and GPHS-RTG systems. RTGs have of course been designed to reject heat into both atmosphere and into liquid during the SNAP RTG program, which included naval and air force programs (more on those systems here), but this application hadn’t been used by NASA since the SNAP-19 (the Viking landers used two of the generators, details available here). This complicates the thermal environment for the RTG, something we’ll discuss in the heat rejection section below (or you can skip ahead).

The MMRTG combines the modularity and environmental safety of the GPHS module with another upgraded legacy system: the PbTe/TAGS thermoelectric generators from SNAP-3 and SNAP-19. By doping the thermocouples even further, greater efficiency was possible. We’ll discuss this more below as well.

All of the non-GPHS elements of the MMRTG are manufactured by Teledyne (the company that the thermocouple inventors worked for in the 1960s and 1970s), and Aerojet Rocketdyne. Lockheed Martin and the Department of Energy provide the systems and materials for the GPHS modules that fuel the MMRTG.

As of July 2019, the MMRTG has been flight proven by NASA on the Mars Science Laboratory (Curiosity, which has used the MMRTG since 2004), and has been selected for two currently-future missions: Mars 2020 (RTG delivered to Idaho National Laboratory in August of 2018) on Mars, and Dragonfly, which is planned to explore Titan.

Let’s look at the subsystems themselves. You can jump to each section here:

Radioisotope Fuel Elements and Assembly

The MMRTG, as all modern NASA RTGs are, is based on the General Purpose Heat Source. This is a modular radioisotope heating unit, using small pellets of 238Pu encased in a multilayered radiation and re-entry thermal protection system.

More on the GPHS is available here.

Curiosity GPHS stack, image INL/DOE

The radioisotope power module of the MMRTG is composed of eight GPHS modules (providing a maximum total of 1,960 Wt at the beginning of module life, although a margin is often applied in the mission planning phase to ensure a healthy margin for fuel age) with mounting brackets for the rectangular assemblies to be integrated into the cylindrical fuel element housing.

This housing is called the isolation liner, is made out Haynes 25 alloy, and is surrounded by a graphite sleeve, called the thermal distribution block. This ensures that the thermal load from the isolation liner is distributed as evenly as possible across the hot shoe of the thermoelectric generator, which it interfaces with directly via a spring loading system integrated to the hot shoe of the TEG. No other mechanical attachments occur between the TEG and the heat source assembly. Both the isolation liner and thermal distrobution block have provision to release helium gas that builds up as the 238Pu fuel decays.

At the base of the fuel stack is a bracket that connects to the mounting interface in the housing to provide mechanical support for the fuel stack.

A degredation of 0.5% in power available will occur from time of fuel element refinement, including production time, integration time for the GPHS, the MMRTG, not including the transit time to the mission location.

Thermoelectric Generator and other Power Conversion Systems

Thermocouple diagram, image NASA

The thermoelectric generator (TEG) assembly of the MMRTG is based on a legacy design: the lead telluride/telluride arsenic germanium selenite (PbTe/TAGS85) thermoelectric thermocouple system. This was first used in the SNAP-19 RTG (more information available here). The MMRTG uses an updated configuration, using 768 thermocouples configured as two series-parallel chains for fault tolerance: any thermocouple that fails will do so individually, resulting in a negligible and isolated loss of power that is easy and reliable to integrate into mission planning.

The two legs of the thermocouple are made from lead telluride (PbTe) and TAGS, or the alloy of tellurium, antimony, and germanium. The ratio of these materials is reflected in the name of the particular alloy: TAGS85, where 85% of the material was PbTe, and 15% was the TeSbGe2 alloy. This material was first flown on the SNAP-19 RTG, invented by two engineers at Teledyne in the 1960s. However, the material was modified over the years, incorporating greater understanding of the impacts of quantum mechanics on the Seebeck effect, crystalline structure, and most especially band theory, which looks at valence states of electron orbitals as they apply to

This was improved with a number of methods: using improved doping of both the n and p legs of the thermocouples, as well as dividing the thermocouple p-leg into two parts: one optimized better for the hot shoe end, made out of TE2003 (a PbSnTe alloy), and one (longer) for the cold shoe end made of TAGS-85, with the N leg made out of a PbTe alloy called TE1010.

Spring mounting system exploded diagram, image NASA/JPL

The cold foot of the thermocouple is mechanically integrated to a spring loading system, which presses the hot shoe against the graphite thermal distribution block. The hot foot is made out of nickel, but I have been unable to determine the cold foot composition.

The thermocouples are surrounded by thermal insulation to minimize heat transfer outside the thermocouples themselves.

Unfortunately, one significant barrier for the use of these materials is differential sublimation of the thermoelectric materials themselves. This is unfortunate, but there are a number of ways to manage this effect. In the current incarnation, a mix of argon and helium are used as a cover gas, but other cover gas compositions are also possible. Additionally, silica insulation is used immediately surrounding the thermocouples to reduce sublimation rates.

Coatings on the thermocouples themselves have also been proposed over the years to minimize this effect. Dr Skrabek tested ceramics, phosphate glasses, lead oxide enamels, and high temperature engine paints in the early 1970s, but all failed as the materials vaporized and ruptured the coating layer.

Research in 2012-2013 at University of Dayton looked at Sol-Gel coatings of Al2O3, TiO2, and SiO2, however initial testing showed that the coatings were non-uniform, possibly weren’t densified correctly, and failed to stem sublimation of the thermocouples. This led to a decision to abandon the Sol-Gel process.

On the plus side, it was determined that the Ge component of the thermocouples was the main cause of sublimation failures in the TEG, allowing for a more targeted coating strategy. It also demonstrated that Al2O3 offered the most promise of all the coatings despite the flaws in the application process.

More recent work at U of D from 2015 studied the use of atomic layer deposition (ALD), a highly precise gas phase coating process using a precursor chemical followed by the external coating. This allows for angstrom-level coating thickness precision, ensuring a consistent thickness and density of coating, addressing the problems of the Sol-Gel process. This process was repeated 200-400 times, to produce a thickness of 30-40 nm.

Augur Electron Microscopy (AES) was used to determine composition as a function of depth both before and after testing in a vacuum at 350 C for up to 3000 hours. The profiles for each TAGS-85 element are available below.

In addition to these processes, it was determined that there may be promise in using a very thin ALD deposited layer of HfO2 between the TAGS-85 and the Al2O3 may provide even greater protection. This is a reasonably common coating for many nuclear materials (it was used in the SNAP U-ZrH fuel element hydrogen barrier enamels, for instance), and so has been studied well enough for this to be implemented with little in the way of material behavior uncertainty, but I haven’t found any results.

This work provides promise for the long-term life of the TAGS-85 system of thermocouples, although it’s unclear if this will be used in the MMRTG. In addition, the future of the MMRTG is not based on the TAGS-85, but skudderite thermocouples. This is the enhanced MMRTG, or eMMRTG, which is currently under development. This will be covered more in depth in the future.

Whether the MMRTG will continue to use TAGS-85, the future of the thermocouple is bright, and the MMRTG will continue to serve as NASA’s most reliable current nuclear power source for decades to come.

Thermal Management and Integration Hardware

Fueled MMRTG in hot box at INL immediately after fueling completion, image INL/DOE

The standard MMRTG complete housing measures 0.65 m in diameter and 0.69 m in height, and consists mainly of a cylindrical housing surrounding the thermoelectric modules made out of Al 2219. It uses eight fins made out of an aluminum alloy (Al6063) to reject heat by either radiation or by convection in an atmosphere. The base of the unit contains the mounting interface and electrical supply connectors used to integrate the power supply into the spacecraft, as well as manage the launch stresses on the RTG.

Heat is transferred via radiation as well as convection from helium gas, both pre-inventoried into the GPHS assembly and generated through alpha emission.

The MMRTG is fascinating for a number of reasons in the context of thermal management. The two most prominent (upon investigation) differences between the MMRTG and almost every other off-Earth RTG design are:

  • The radiators are designed to work in both vacuum and atmospheric conditions, and
  • The MMRTG is able to use a pumped coolant thermal management system during operation, the first to do so during a mission with the Mars Science Laboratory cruise stage. This was not only critical for the cruise stage thermal management, but also had an impact on the Entry, Descent, and Landing Skycrane.

As a note, most if not all RTGs use pumped thermal management systems while the RTG is on the launchpad, something that is absolutely necessary in most cases thanks to the large amount of thermal energy that is produced by the 238Pu fuel (up to 1893 Wt at beginning of life). In space as part of a composite spacecraft, such as the Mars Science Laboratory and Mars 2020 rovers, thermal transfer points are mechanically and thermally attached to the radiator fins of the RTG. The cruise stage for Curiosity had a total of 23 m of aluminum tubes in a two-split flow configuration integrated to the cruise stage for this purpose, which when welded to a 1.5 mm aluminum sheet creates a radiator about 6 m^2 in area.

This is integrated into the base of the fins on the radiator, which were then jettisoned for surface operations (although exactly when and how this occurs is unclear).

The MMRTG was designed for both atmospheric and deep space missions, which complicates the thermal management side of things.

The vacuum variation, which has never been flown, but is considered to be TRL 9 (or flight-proven) due to the similarity in vacuum environments with Mars (5-10 mbar is a reasonable vacuum as far as this system is concerned). The fin temperature must be maintained above -269 C (4 K), if temperatures colder than this are required insolation must be applied to the heat sink in order to prevent material damage due to the extreme thermal gradient across the materials.

For Mars, the thermal variation of -123 C to +5 C was built into the design of the thermal management system. As you can see in the diagram of the Curiosity RTG mounting, the surface applications use heat exchanger “shields” on the sides of the unit. This allows for rover thermal management systems to use the waste heat from the RTG to manage the temperature of various instruments on the MSL-type rovers. This also controls how much heat is rejected into the environment, and doesn’t require integrated RTG-to-spacecraft heat exchangers to be built into the design of the system, increasing the flexibility of the basic design.

Titan, the latest location that the MMRTG has been announced to be used in, is an unusual environment in a number of ways. Titan is actually one of the coldest – if not THE coldest – location in the Solar System, and the atmospheric temperature is below the minimum fin root temperature. This requires a different configuration than the Martian configuration, which can be seen in the cylindrical housing the MMRTG is placed in on the initial design concept of Dragonfly. This additional insulating layer also makes it possible to use more of the waste heat from the RTG to maintain the thermal environment within the rover, a major concern at temperatures that low. This is similar to the way the Mars surface configuration is designed, but in a more complete way to address both the increased thermal control needs of the spacecraft as well as the insulation requirements of the RTG. Final thermal designs for the Dragonfly mission are still a long way away, but these considerations and more will play crucial roles in determining the final design of that RTG system.

Finally, the color of the outer casing and fins of the MMRTG plays a key role in thermal management. While Curiosity and Mars 2020 use white painted MMRTG fins and casings, a black paint for colder temperatures has been invented as well for deep space missions.

The spacecraft interface consists of a mounting bracket which connects to the spacecraft for mechanical attachment, as well as an electrical and telemetry connection to the spacecraft through a single connector. Mechanical integration uses a four bolt mounting interface. The only telemetry provided is from platinum resistance thermometers within the RTG.

Assembly, Testing, Integration and Launch

Here’s a good video of the fueling, testing, and integration process for the Curiosity Rover at Idaho National Laboratory, with an interview of the personnel involved in the program:

All handling of the fueled portion of the MMRTG is handled in a hot cell until integration of the outer casing is completed. This minimizes radiological hazards to the workers.

After fueling, the RTG undergoes a series of tests for radiological contamination during the fueling and integration process, vibration and shock testing, as well as electromagnetic interference from the RTG. Because the use of a stand-off boom, such as the Pioneer 10 & 11 and Voyagers 1 & 2 spacecraft used, is not an option for minimizing EM interference, the MMRTG is held to an exacting standard of having a field of less than 25 nT.

EM testing and characterization, image INL/DOE

Once this process is complete, the MMRTG is ready for delivery. This will only happen once the rover has been integrated with both the cruise stage and the launch vehicle.

Moving MMRTG for Curiosity within INL

Integration with Spacecraft

The RTG is only ever integrated onto the spacecraft at the launch pad, due to nuclear material security concerns, waste heat management simplification, and radiological safety.

MMRTG mounted to spacecraft. Note access hatch visible around RTG. Image JPL

Interestingly, the entire RTG was integrated into Curiosity on the launch pad, after the rest of the rover and cruise stage was already integrated into the launch vehicle (a ULA Atlas 541). This will be done with Mars 2020 as well, with a mass simulator being used in its place.

This places additional complications on launching a mission using an MMRTG. Often, the clean rooms that the rovers are built in are far more stringent than those used for vehicle integration, but because the spacecraft isn’t entirely complete until after power source integration, access to the spacecraft in the fairing after it’s been integrated to the rest of the launch stack must be maintained at the much higher level of stringency usually seen in the assembly facilities at JPL.

This also complicates radiological materials handling requirements from the DOE and NASA, due to the more constricted space at the top of the launch tower, the necessity for NNSA security personnel to ensure proper fissile fuel security (the RTG uses alpha decay, but due to the fact that 238Pu CAN undergo fission, and is highly refined, those rules apply to 238Pu fueled RTGs in the US), and even the swaying of the launch vehicle and tower due to weather.

Theoretically there’s also an advantage here, although this isn’t the way things are done presently and the gains will be marginal due to the long half-life of 238Pu. The fuel can be produced far closer to the time that it will be integrated into the spacecraft, maximizing the specific power of the radioisotope itself if the fuel, and therefore mission life and power output.

The 89.9 year half-life of 238Pu makes this only a marginal gain, however, and fueling of each GPHS takes well over two months. Additionally, verification and validation testing takes more time before the integrated MMRTG can be delivered for integration. Finally, most mission design references that cover the MMRTG assume older fuel with a lower specific power density

Missions

The MMRTG is designed for a wide range of missions, and as the standard RTG for NASA design missions at the moment has a wide range of proposals that have used it as a power source. However, only one flown, and two approved, missions, have incorporated this power supply: Curiosity, currently on Mars, Mars 2020, which is undergoing final construction, and the newly selected Dragonfly mission to Titan.

Mars Science Laboratory (Curiosity)

Coriosity MMRTG at Gale Crater, image NASA

NASA’s current nuclear powered planetary flagship, the Mars Science Laboratory – better known as the Curiosity rover – is currently active in Gale Crater in Mars’ northern hemisphere. It launched on a United Launch Alliance Atlas 5 in the 541 configuration on

Curiosity is covered incredibly exhaustively elsewhere, so I won’t cover it in depth here. However, the particular application of the MMRTG is worth examining, since this is the only real-world experience with this system available.

Mars 2020

The Mars 2020 rover from the beginning has been meant to integrate as much of the engineering design of MSL as possible into a rover focused on different scientific and engineering goals once on Mars. The two rovers are nearly identical when it comes to their power supplies, however, so from the perspective of the RTG itself there’s little difference.

If Curiosity is, at its core, an organic chemistry laboratory, Mars 2020 is a mobile geology laboratory. Additionally it is designed to test the ability to collect, seal, and cache samples taken by the rover for a future sample return mission (although no mission to collect or return the samples has received any significant funding). Additionally, it will carry a small, solar powered flying drone with counter-rotating blades to provide lift to increase the science return of the mission and demonstrate a modular, lightweight instrument delivery system for future Martian mission.

Again, this has been covered elsewhere, so I won’t cover it in depth here. Check out the References section for more in-depth design documentation for Mars 2020 and how it differs from Curiosity.

Dragonfly

The latest in the MMRTG family, Dragonfly is an eight-bladed quadcopter design that will explore Saturn’s moon Titan. It in many ways is similar to the Curiosity and Mars 2020 rovers, but adapted to flight in Titan’s incredibly dense atmosphere rather than drving across the surface of Mars. It was approved as a New Frontiers-class mission in June of 2019, set to land in 2034.

This is an incredible mission on a lot of levels. I’ve covered this in a blog post, here:

While the MMRTG has been performing within the design envelope for the needs of the Curiosity mission on Mars for years, it has shown some moderately concerning degradation behaviors that the Dragonfly team are taking into account when designing the power profile for Dragonfly.

The JPL “Radioisotope Power Systems Reference Book for Mission Designers and Planners,” by Young Lee and Brian Barstow at JPL, covers many of the details of the MMRTG, as well as recommendations on the design margins that should be adhered to when applying this power supply to a mission proposal.

The first concern is fuel age. While the MMRTG on Curiosity supplied 114 We of power on landing on Mars (and this includes three years in storage, and four years on vehicle integration, launch, and cruise), the Reference Book lists the conservative power output for a brand new MMRTG to be only 107 We on the surface of Mars. This is also a longer time period than the Mars 2020 rover’s MMRTG, which was delivered in August of 2018 for a 2020 launch, although when the fuel was produced is something I have yet to determine. This bodes well for having sufficient power from the 238Pu fuel for the Dragonfly at the beginning of its surface mission, nine years after launch.

The second concern, as discussed in the Thermoelectric Generators section (you can jump to that location here), is sublimation of the TEG’s materials. This is a bigger unknown on Dragonfly than on Curiosity, due to the longer cruise stage and different thermal environment on Titan. Significant margin for power supply has been provided in the mission planning, though, so this shouldn’t be a significant impact.

Because of the long cruise phase of the mission, the Dragonfly team assume that the MMRTG will only be able to provide about 70 We of power once the mission arrives at Titan. This is sufficient to power the Li ion batteries onboard the lander (held in a thermal insulative box and kept at temperature with waste heat from the RTG) for both flight and energy-intensive testing, as well as provide power for the scientific instruments that will be running during grounded science operations.

According to Lorentz et al at JHUAPL in 2018:

“Although sample acquisition and chemical analysis are somewhat power-hungry activities, they require only a few hours of activity. Science activities that require continuous monitoring, namely meteorological and seismological measurements, although of low power, actually dominate the payload energy budget. Indeed, for these extended periods, the lander avionics are powered down and data acquisition is performed only by the instrument, to maximize the rate of recharge of the battery.”

This means that there is more than enough margin for a years-long successful mission from even an MMRTG degrading at the high end of the degradation curve, ensuring that the power supply will likely not be a cause for significant concern of mission failure.

More details will be available as the mission is further refined. For a more detailed look into Dragonfly, check out the Dragonfly blog post here: [insert link]

References and Further Reading

Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) program overview, Ritz et al 2004 https://trs.jpl.nasa.gov/handle/2014/38246

A methodology of MSL breakup analysis for Earth accidental reentry and its application to breakup analysis for Mars off-nominal entry Salama, Ahmed; Ling, Lisa 2005 https://trs.jpl.nasa.gov/handle/2014/39653

Usage of Multi-Mission Radioisotope Thermoelectric Generators (MMRTGs) for Future Potential Missions (slides) 2017 https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170000017.pdf

Mass Properties Testing and Evaluation for the Multi-Mission Radioisotope Thermoelectric Generator, Felicione 2009 https://inldigitallibrary.inl.gov/sites/sti/sti/4502646.pdf

Sublimation Suppression Coatings for Thermoelectric Materials, Barklay et al 2015 http://anstd.ans.org/wp-content/uploads/2015/07/5048_Barklay-et-al.pdf

Mars Science Laboratory

The Mars Science Laboratory’s MMRTG : a missions perspective, Woerner, David 2012 https://trs.jpl.nasa.gov/handle/2014/43174

The F1 Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) : a power subsystem enabler for the Mars Science Laboratory (MSL) Mission 2013 https://trs.jpl.nasa.gov/handle/2014/43261

Multi-mission radioisotope thermoelectric generator experience on Mars, Wood 2016 https://trs.jpl.nasa.gov/handle/2014/46057

Multi-Mission Radioisotope Thermoelectric Generator Heat Exchangers for the Mars Science Laboratory Rover (internal magazine article?) 2012 https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120006581.pdf

CFD analysis for assessing the effect of wind on the thermal control of the Mars Science Laboratory Curiosity Rover, Bhandari, Pradeep; Anderson, Kevin 2013 https://trs.jpl.nasa.gov/handle/2014/44910

Mars Science Laboratory thermal control architecture, Bhandari et al 2005 https://trs.jpl.nasa.gov/handle/2014/38066

Pumped fluid look heat rejection & recovery systems for thermal control of the Mars Science Laboratory, Bhandari 2006 https://trs.jpl.nasa.gov/handle/2014/40684

Mars Science Laboratory launch pad thermal control Bhandari et al 2011 https://trs.jpl.nasa.gov/handle/2014/42169

Launch pad closeout operations for the Mars Science Laboratory’s Heat Rejection System, Mastropietro et al 2012 https://trs.jpl.nasa.gov/handle/2014/44989

Mars Science Laboratory Rover integrated pump assembly bellows jamming failure, Johnson 2012 https://trs.jpl.nasa.gov/handle/2014/42426

Summary of Mars Science Laboratory Rover System Thermal Test, Novak 2012 https://trs.jpl.nasa.gov/handle/2014/42414

Mars 2020

Mars 2020 mission design and navigation overview, Abilleira et al 2019 https://trs.jpl.nasa.gov/handle/2014/45974

Leveraging heritage on the Mars 2020 project, Wallace et al 2017 https://trs.jpl.nasa.gov/handle/2014/46262

Nuclear Risk Assessment for the Mars 2020 Mission Environmental Impact Statement, Clayton et al 2014 https://prod-ng.sandia.gov/techlib-noauth/access-control.cgi/2013/1310589.pdf

Aerojet Rocketdyne press release on Mars 2020 MMRTG delivery to INL, 7 August 2018 https://www.rocket.com/article/aerojet-rocketdyne-delivers-power-generator-mars-2020-rover

Preliminary surface thermal design of the Mars 2020 Rover, Novak 2015 https://trs.jpl.nasa.gov/handle/2014/45738

Dragonfly

Note: Dragonfly’s mission design is in a relatively early stage, and as such specific power supply and thermal management reporting as an independent spacecraft design consideration haven’t been published. The following links are of the mission in general, with RTG-specific information distributed through several papers

Dragonfly mission homepage, Johns Hopkins Applied Physics Laboratory http://dragonfly.jhuapl.edu/index.php

Dragonfly: A Rotorcraft Lander Concept for Scientific Exploration at Titan, Lorenz et al 2018 http://dragonfly.jhuapl.edu/News-and-Resources/docs/34_03-Lorenz.pdf

Dragonfly: New Frontiers mission concept study in situ exploration of Titan’s prebiotic organic chemistry and habitability, (presentation slides) Turtle et al 2018 https://www.lpi.usra.edu/opag/meetings/feb2018/presentations/Turtle.pdf

Preliminary Interplanetary Mission Design and Navigation for the Dragonfly New Frontiers Mission Concept, Scott et al 2018 https://www.researchgate.net/publication/327110307_Preliminary_Interplanetary_Mission_Design_and_Navigation_for_the_Dragonfly_New_Frontiers_Mission_Concept