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RHU RTGs Spacecraft Concepts

Dragonfly: NASA’s Newest Nuclear Powered Spacecraft

Hello, and welcome back to Beyond NERVA! The blog itself has been quiet for a while for a number of reasons, but the website continues to grow! I’ve added extensive sections on radioisotope power sources and many details of their operation. There are now pages for radioisotope power sources in general (which you can find here), the fuel elements and heating units used in these systems (here), the considerations for fuel selection (which can be found here), the various options for the fuel itself (each of these has its own page: 238Pu, 241Am, and 210Po). I also covered the RTGs of the SNAP era (available here), which evolved concurrently with the SNAP-2, SNAP-10, and SNAP-8 reactors that we’ve already looked at in depth (each of those now has its own page, as well as the experimental and development reactors associated with the systems, just click on their names to find them!), as well as the Multi-Hundred Watt RTG that powered the Voyager spacecraft, the General Purpose Heat Source RTG which powered Galileo, Ulysses, and Cassini, and still powers New Horizons, and as this is released a page will be coming online about the Multi-Mission RTG, NASA’s current workhorse RTG! Make sure to check out those pages for in depth information on an ecosystem of power supplies which are fascinating, but often overlooked (including by me) for the flashier, higher-powered fission reactor proposals.

Today, we’re going to look at a particular application of the Multi-Mission RTG, or MMRTG.

Concept model of Dragonfly lander in flight configuration, image JHUAPL

This is the newly announced Dragonfly mission to Titan. This mission will only be the second to touch down on the surface of that Saturnian moon, and promises to transform our understanding of the complex hydrocarbon cycles, liquid water (which is under the surface of Titan), and complex organic chemistry that may hold clues to the early atmospheric conditions of Earth.

Congratulations are very much in order for the Johns Hopkins University Applied Physics Laboratory team for their successful mission concept!

Lander in surface, image JHUAPL

This mission will be primarily based around the quadcopter which has gained the most attention in the recent announcements. Using eight rotors mounted to four motor mounts, this system will charge a set of lithium ion batteries with the output of the MMRTG, and then use this higher-power-density supply to fly in short hops, first scouting out the landing point where it will settle for the science gathering portion after the one it will be landing on for that flight, and then land at an already-scouted position on the Titanic surface. There a suite of instruments, including a mass spectrometer, a neutron and gamma spectrometer, a meteorological and seismic sensor suite, and a camera suite. There may be others that are yet to be fully determined but will be refined over the course of the final mission planning (which can begin now that the mission has been selected for funding.

Flying on Titan is far easier than on Earth due to the high atmospheric density (over 4 times higher than Earth’s at sea level) and low gravity (1/7 g) of the moon, and as such a host of proposals for flying probes have been proposed over the years, from balloons to helicopters to airships to airplanes – and even a radioisotope thermal rocket proposal! Furthermore, both surface and lake landers have been proposed.

As the team pointed out in their proposal, the Dragonfly concept combines the benefits of a Curiosity-class lander’s equipment suite and surface science capability with the mobility of an aerial movement platform. This provides the scientific advantages of surface deployment with the ability to relocate the lander at a far higher rate of travel than a wheeled rover such as Curiosity. This provides a good balance of the advantages of each design type.

Let’s first look at the mission itself, from launch to landing on Titan, followed by the science goals of the lander, and finally we’ll look at the power supply relatively briefly, focusing on the thermal management strategies needed for both the lander and the RTG itself. All of the information here is as of July 2019, so much is still subject to change in the next few years before the launch window.

For a more in-depth look at the MMRTG itself, check out the MMRTG page here!

If you’re looking for a particular aspect of the mission, the links below will jump you to the appropriate section, but as with all missions, each phase or component informs every other one, so by skipping ahead you may miss some interesting tidbits about this incredible mission!

Mission Profile Pre-Titan: Launch, Cruise, and EDL

Launch of Atlas V-541 configuration for Curiosity, November 26, 2010 from Cape Canaveral AFS, image ULA

Dragonfly will launch on either an Atlas 541 or equivalent launcher on April 12th, 2025, and conduct a series of flybys of various planets to get out to the Saturnian system. This means that a smaller launcher can be used, and therefore a less expensive one to save more funding for the spacecraft and science teams. However, since this launch vehicle (to my knowledge) hasn’t firmly been pinned down, differences in the actual launcher – as well as any possible onboard non-optimal launch conditions, could change the exact timing of the gravitational assists listed below.

The first gravitational assist will be on April 11th, 2026 from Earth, followed by a Venusian gravitational assist on 4/16/2017, another Earth gravitational assist on 5/27/2028, and a final Earth gravitational assist on 9/3/2031. While there are options to rearrange these gravitational assists, this one was selected due to a number of orbital mechanical factors. Sadly, Jupiter will be out of phase with Saturn at the appropriate time, so it’s impossible to use the large planet’s convenient gravity well to shorten the trip to the Saturnian system.

Image JHUAPL

After this series of four inner-system gravitational assists, Dragonfly will have the necessary dV to get to Titan. A mid-course correction around December 2, 2031 (while the spacecraft is between Mars and Juptier) will ensure that the spacecraft is oriented correctly for Titan capture.

Fueled MMRTG for Curiosity. External radiator piping seen as silver “U” between blades of radiator. Image DOE/INL

During this time, the MMRTG will use its secondary coolant loop (visible as the silver tubes at the base of the RTG’s fins in the above image) combined with a pumped coolant loop similar to what was used on Curiosity’s cruise stage, in order to reject the waste heat from the RTG during the time that it’s enclosed in the heat shield and cruise stage. It’s unclear in the mission documentation whether the RTG will be used to power any instruments during cruise, as Curiosity’s particle detection system was. The instrumentation on Dragonfly is significantly different from Curiosity, and it’s not apparent whether any of the instruments would either be useful in cruise, or perhaps whether operation of the cruise stage would interfere with those experiments that could be useful to the point that the data would simply be too messy or corrupted to bother.

Mars Science Laboratory cruise stage thermal management diagram, image NASA/JPL

The cruise stage design is also something that I haven’t been able to find, so it’s possible (although unlikely due to increased electrical bus complexity) that the MMRTG could be used to power scientific instruments on the cruise stage itself. While the majority of the time the spacecraft will be doing the series of gravitational assists in the inner Solar System, allowing for the use of solar panels, by the time it is heading out of the inner Solar System the power available from solar radiation will be dropping off exponentially. This could likely be handled by battery power on board the cruise stage, or it could possibly be handled by the MMRTG. However, the increased complexity of the electrical system should it be connected to both the cruise stage as well as the lander may be sufficient to have the mission planners decide to not go this route.

Entry, Descent, and Landing for Dragonfly

Finally, at the close of 2034 (12/30), the spacecraft will reach Titan, and perform its entry, descent and landing procedures.

For those that remember Curiosity’s landing, this was (rightly) touted as the “Seven Minutes of Terror,” involving a huge number of complex and risky maneuvers with a collection of sometimes exotic craft to land on Gale Crater. These included: a set of hypersonic parachutes designed for the thin Martian atmosphere, followed by the use of an eight-engined “sky crane,” which hovered meters off the surface of Mars to lower the rover on a winch and cable system (during which the rover’s wheel and bogey driving system deployed for the first time), followed by placing the rover on the surface of Mars, disconnecting the cables, and then flying away to crash at some distance. This was due to the combination of thin atmosphere, reasonably high gravity, and the need to minimize dust and debris from collecting on the surface of the rover (to protect the wet and dry lab sample collection system on the surface of the rover from being contaminated). It was a stunning series of firsts in an EDL system, and one which rightly received worldwide attention – and garnered NASA to repeat the process for Mars 2020.

Titan, on the other hand, is a whole different ballgame. Its thick atmosphere and low gravity make it a very different EDL environment, one that is both easier to deal with but also requires a different set of conditions to be followed. Due to the fact that the gravity is so low (1/7 of Earth’s), Titan’s sensible atmosphere extends far higher than even Earth’s does. In fact, even though the atmospheric density is 4 times that of Earth’s at the surface, the atmospheric pressure is only slightly higher (1.4x). Combining a far lighter-weight rover (weight=mass x gravity), with far better lift, and onboard flight capability, the sky-crane maneuver isn’t necessary on Titan.

The beginning will look similar: the cruise stage is outwardly very similar to the MSL cruise stage, and will also be ejected as the craft begins to enter the atmosphere. A set of parachutes will slow the craft further, until an optimal velocity is achieved above the surface of Titan. During this time, it’s likely that the radar designed to map the Titanic surface in flight will be used to verify the lander’s location over the surface and find an acceptable landing location for its first touchdown. Based on the typical flight profile of Dragonfly, and depending on the power level in the batteries (which would likely be fully charged) compared to how much power it will take to get to a safe landing location, the next flight’s landing location may be scouted as well.

This video from JHUAPL shows the EDL process for Dragonfly:

Video JHUAPL

The location of the first landing location (as well as all other subsequent locations) is dependent on communication with Earth. Dragonfly is designed to communicate directly with Earth, something which is standard for outer solar system missions due to the lack of a communications architecture in a useful location (the one exception is the Huygens probe, also deployed to Titan, which used the Cassini spacecraft as a communications relay). This places certain limits on the location that Dragonfly can deploy to on the Titanic surface, and also where the lander will move across the surface during its ground science mission – but more on that in the surface mission section.

Image JHUAPL

The mission planners for Dragonfly have selected a similar landing latitude and season as the Huygens mission’s landing in order to maximize the knowledge of the atmospheric conditions for the initial, riskiest portion of the lander’s atmospheric operations. This also maximizes communications availability with Earth and works well with orbital mechanical constraints upon entering the Saturnian system and aerocapture by Titan itself.

After the lander is safely on the surface of Titan, it will deploy its communications dish, send data back to Earth, and begin surface science experiments. This will be the start of the Dragonfly surface mission, which will last a number of years – until either a failure occurs on the lander which prevents its further operation, or the MMRTG degrades to the point that communication with Earth is no longer possible (the science equipment takes less power than the communications do, so even if Dragonfly isn’t able to fly it can still provide valuable scientific data – if it can phone home).

This begins the purpose of the whole mission, which will be explored in the next section

Titan Surface Mission and Flight Profile

Titan is a fascinating place. The coldest location in the Solar System thanks to its location in the outer solar system, complex hydrocarbon cycles which in may ways mimic the hydrological cycles of Earth (but with a complex set of liquid hydrocarbons rather than water), and a chemical profile that may be similar to Earth’s at the beginning of the evolution of life on Earth provide a fascinating place to conduct science missions, with implications reaching not only far back into Earth’s past, but also have a major impact on the future of humanity in the Solar System.

For an in-depth look at the scientific appeal of Titan, I highly recommend reading Ralph Lorentz’s “The Exploration of Titan,” available here: http://dragonfly.jhuapl.edu/News-and-Resources/docs/apltechdigest.pdf

The surface mission of Dragonfly largely comes in two major phases: landed science instruments and communications, and movement. While some data is collected in flight (mostly imaging), the in depth data collection and transmission are done on the surface of the moon. This is important, because while the MMRTG is the best power supply available for this mission, it isn’t able to provide the power needed for flight as quickly as needed. This means that a set of lithium ion batteries, stored in an insulated box that’s heated by waste heat from the MMRTG, are charged while on the surface, and then once the desired power level is reached, a new flight can occur.

Battery charge vs time, Image JHUAPL

Let’s begin by looking at the science instruments which will be included on Dragonfly. This list is quoted from “Dragonfly: A Rotorcraft Lander Concept for Scientific Exploration of Titan,” a white paper from Lorentz et al at JHUAPL on January 9th of 2019:

DraMS—Dragonfly Mass Spectrometer (Goddard Space Flight Center). A central element of the payload is a highly capable mass spectrometer instrument, with front-end sample processing able to handle highmolecular-weight materials and samples of prebiotic interest. The system has elements from the highly successful SAM (Sample Analysis at Mars) instrument on Curiosity, which has pyrolysis and gas chromatographic analysis capabilities, and also draws on developments for the ExoMars/MOMA (Mars Organic Material Analyser).

DraGNS—Dragonfly Gamma-Ray and Neutron Spectrometer (APL/Goddard Space Flight Center). This instrument allows the elemental composition of the ground immediately under the lander to be determined without requiring any sampling operations. Note that because Titan’s thick and extended atmosphere shields the surface from cosmic rays that excite gammarays on Mars and airless bodies, the instrument includes a pulsed neutron generator to excite the gamma-ray signature, as also advocated for Venus missions. The abundances of carbon, nitrogen, hydrogen, and oxygen allow a rapid classification of the surface material (for example, ammonia-rich water ice, pure ice, and carbonrich dune sands). This instrument also permits the detection of minor inorganic elements such as sodium or sulfur. This quick chemical reconnaissance at each new site can inform the science team as to which types of sampling (if any) and detailed chemical analysis should be performed.

DraGMet—Dragonfly Geophysics and Meteorology Package (APL). This instrument is a suite of simple sensors with low-power data handling electronics. Atmospheric pressure and temperature are sensed with COTS sensors. Wind speed and direction are determined with thermal anemometers (similar to those flown on several Mars missions) placed outboard of each rotor hub, so that at least one senses wind upstream of the lander body, minimizing flow perturbations due to obstruction and by the thermal plume from the MMRTG. Methane abundance (humidity) is sensed by differential near-IR absorption, using components identified in the TiME Phase A study. Electrodes on the landing skids are used to sense electric fields (and in particular the AC field associated with the Schumann resonance, which probes the depth to Titan’s interior liquid water ocean) as well as to measure the dielectric constant of the ground. The thermal properties of the ground are sensed with a heated temperature sensor to assess porosity and dampness. Finally, seismic instrumentation assesses regolith properties (e.g., via sensing drill noise) and searches for tectonic activity and possibly infers Titan’s interior structure.

DragonCam—Dragonfly Camera Suite (Malin Space Science Systems). A set of cameras, driven by a common electronics unit, provides for forward and downward imaging (landed and in flight), and a microscopic imager can examine surface material down to sand-grain scale. Panoramic cameras can survey sites in detail after landing: in many respects, the imaging system is similar to that on Mars landers, although the optical design takes the weaker illumination at Titan (known from Huygens data) into account. LED illuminators permit color imaging at night, and a UV source permits the detection of certain organics (notably polycyclic aromatic hydrocarbons) via fluorescence.

Engineering systems. Data from the inertial measurement unit (IMU) may be used to recover an atmospheric density profile via the deceleration history during entry. IMU and other navigation data may provide constraints on winds during rotorcraft flight. Additionally, the radio link via Doppler and/or ranging measurements may shed light on Titan’s rotation state, which, in turn, is influenced by its internal structure.”

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

This is a diverse set of instruments, and offer a wonderful range of science data return in a compact and highly mobile lander platform.

For more information on how these instruments will be applied in studying the surface composition of Titan, check out this paper from Trainer (NASA GSFC) et al: “DRAGONFLY: INVESTIGATING THE SURFACE COMPOSITION OF TITAN” https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20180003047.pdf

It’s unclear whether all electrical power will be routed through these batteries or not. While there are advantages from a power conditioning point of view (ensuring the correct voltage and wattage, preventing power dropouts or spikes to sensitive instruments, etc), it can also cause battery life complications due to the constant discharging of the batteries themselves – and degradation of especially the anode in the batteries. It’s unclear which power conditioning scheme will be used for the always-on systems, such as the meteorological system, but the high powered systems will likely draw power from the batteries exclusively.

Flight profile representation, image JHUAPL

In order to deploy these instruments, the lander will fly from one location to another, scouting out the location of the landing after the target that it will be landing at on that mission. A good schematic of the flight profile can be seen below:

Flight profile as function of distance and altitude, image JHUAPL
Image JHUAPL

This ensures that the flight time for the next flight can be maximized, since a known safe landing location is already known before the lander takes off for its next flight, as well as providing margin in flight time for all legs of the mission after the initial one. It’s unclear if the first flight will also use this profile, since that’s part of the entry, descent, and landing sequence, but there’s no apparent reason why it couldn’t if the lander is ejected from the backshell at moderate altitude and velocity.

Direct-to-Earth Communications: Mission Profile Implications

Due to the remote nature of this mission, there aren’t many (if any) options available to use communications hubs off Earth as relays for Dragonfly, a major contrast to Martian operations where the many orbiters around the planet also serve as communications satellites for the various landers and rovers operating on the surface of Mars. This means that in order for the scientific and engineering data from Dragonfly to be returned to Earth, and additional commands to be transmitted, the rover needs line of sight to Earth. This is done via a deployable high gain antenna, which will be stowed for flight operations to reduce drag and stress on the antenna itself.

This complicates matters in two ways: if the lander is at too high or too low a latitude, there’s no line-of-sight available to Earth from the Titanic surface, meaning that communications is impossible; second, although the length of the sol (the extraterrestrial equivalent of a day, in the case of Titan this is almost 16 Earth days, the same as Titan’s orbital period around Saturn).

While it may be possible to do a hop outside the communications window, collect scientific data, then return to the communications window to transmit the data, it also increases the chance of an unrecoverable failure due to the lack of ability for the engineering team on Earth to troubleshoot and resolve any potential spacecraft failure.

Available landing locations in polar view, image JHUAPL

The lander isn’t tied to the ability to gather sunlight like a solar-powered spacecraft is, but at the same time the ability to communicate with Earth and having the Sun visible are conditions that pretty closely overlap, so night-time scientific data gathering on this mission essentially means that all data would need to be stored on board the spacecraft, and the rate of data collection and data storage capabilities of Dragonfly aren’t clear from current documentation. This means that, should the lander need to overnight (a very real possibility) on Titan, some of the data may need to be written over in order to make room for more immediately interesting scientific data.

Whether this is an avoidable circumstance or not is something that I’ve been unable to determine, but the mission design team have made provisions for overnighting Dragonfly on Titan, and this may in fact be required to allow for the time necessary to recharge the batteries to flight condition. If this is the case, it’s likely that the mission’s data storage capabilities will be sufficient to collect all desired data through the Titanic night and transmit them during daytime surface operations, when line-of-sight with Earth is possible.

Now that we’ve looked at what Dragonfly is going to be doing on the surface of Titan, let’s look at how it will do it from a power point of view: the Multi-Mission RTG, or MMRTG.

Multi-Mission RTG: NASA and the DOE’s Flagship RTG

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).

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.

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.

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.

This has significant implications for Dragonfly, as we’ll see below.

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 profile for a lander or rover.

In space as part of a composite spacecraft, such Dragonfly in cruise, 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.

Cruise stage configuration. The silver ring on the cruise stage is likely the radiator for the MMRTG. Image JHUAPL

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).

Coolant tube configuration, image NASA/JPL

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.

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. 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. I presume the same will be done for Dragonfly.

For more information on the MMRTG in general, check out the MMRTG page here!

Multi-Mission RTG Use for Dragonfly

While the MMRTG (more information available here) was designed to handle the Titan environment from the beginning, there are many difference between this version of the MMRTG and those that are (and will be) used on Mars. Additionally, the fact that aerodynamics and mass distribution are a major design criterion for Dragonfly places additional requirements on the use of the system.

Martian MMRTG configuration. Note the two shields on either side; this is extended into a full cylinder on Dragonfly. Image JPL

A standard MMRTG uses an eight bladed, cylindrical radiator to reject heat. This is present on both the Mars and deep space version of the MMRTG, but the Martian version uses a pair of shields on either side of the rover to both control air flow past the radiator (limiting the amount of convective heat loss) and to capture waste heat from the RTG to heat certain temperature-sensitive system components. These shields don’t touch the radiator, and the configuration of the radiator is the same as the outer space version of the system (although the deep space version may be painted black instead of white for thermal management).

All systems that use radiators have a minimum and maximum operating temperature for the radiator itself. For the MMRTG it’s based on the temperature at the root of each fin on the radiator. While the MMRTG was designed with a maximum allowable temperature of 200 C (!) (with a corresponding loss in conversion efficiency due to a lower thermal gradient) for Martian or orbital operations in an absolute worst-case scenario, the system faces the opposite problem on Titan: the incredibly low surface temperature is well below the minimum operating temperature at the fin root temperature of -269 C. This requires the radiator to be insolated from the exterior environment to a certain degree, meaning that the heat rejection system needs to be changed.

Of course, the fins on the radiator aren’t the most aerodynamic things around: not only would they cause changes in yaw and roll to be more difficult, but the upward angle of the RTG’s mounting causes problems with drag as well.

Rear view of Dragonfly in flight. Note the angled cylinder on the back of the lander: this is the MMRTG housing. Image JHUAPL

Fortunately, both of these problems can be addressed together, by taking an idea that’s already in use on the Curiosity rover, extending and adapting it for the Titanic environment. On Mars, as we already mentioned, a pair of shields are used on either side of the rover. For Dragonfly, those shields are extended to make a cylindrical housing for the MMRTG, as seen at the back of the lander. Not only does this ensure that the RTG components are kept at the appropriate temperature, but also that the lander has improved aerodynamic conditions. This is especially important because while lift is easy to gain on Titan, drag is also far more powerful.

Caution and the RTG: Unknown Unknowns in Mission Design and Power Supply Fidelity

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.

Sadly, radioactive decay isn’t the only cause of regular degradation for the MMRTG. The thermoelectric generator itself, the GeTe/TAGS-85 thermocouples that are the children of those used by the Pioneer 10 and 11 probes, lose their thermoelectric materials (mainly germanium) over time through sublimation and migration out of the thermocouples themselves. This is currently reduced by using a cover gas of argon and helium in a carefully controlled ratio, but sadly the utility of this seems to be less than ideal on the Martian surface. Additionally, silicon insulation immediately surrounding the thermocouples can assist in reducing sublimation, but whether that’s currently in use or not is unclear.

Ideal power output as a function of time, image JPL

The MMRTG’s output was assumed to degrade between 3.5% and 4.8% per year, between the decay energy decline in the 238Pu fuel elements and the sublimation of the materials in the thermocouples themselves. Sadly, real-world data from Curiosity shows that the MMRTG on board the rover is degrading at the top end of that scale: 4.8%. This is a fact to cause worry for a mission planner, and one that the Dragonfly team at JHUAPL have taken into account in the mission design.

While there are advanced mitigation techniques (such as cladding them in Al2O3 with an atomic layer deposition process for a highly regular clad similar to nuclear fuel elements, but far more exacting than average) have been proposed and demonstrated by the University of Dayton, it’s unclear whether these mitigation techniques will be used on Dragonfly due to the unknowns that they can introduce into the system.

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.

Titan, Here We Come!

The exploration of Titan has long been a goal, ever since the Voyager spacecraft first sent data back from this fascinating moon. Cassini and Huygens sent back reams of data, but sadly only served to whet our appetite for more.

Now, Dragonfly will provide us far more data, in an incredibly mobile platform, on the composition, chemical processes, and weather on Titan. This will not only increase our knowledge of the moon itself, but early chemistry on Earth and how it could have led to the rise of life in the Solar System.

Sadly, it will be 2034 before we receive data back from Dragonfly on the surface of Titan, but this is not unusual for so distant a location. Until then, we can only follow the mission development, cheer on the team at Johns Hopkins University APL, and wait with bated breath for the first data to be transmitted from this distant, intriguing moon.

Dragonfly team with Earth analogue test article, Image credit JHUAPL

For more information on the MMRTG, make sure to check out the new MMRTG page, available here: MMRTG page.

More coming soon!

References and Additional Resources

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

The Exploration of Titan, Lorentz 2018 http://dragonfly.jhuapl.edu/News-and-Resources/docs/apltechdigest.pdf

DRAGONFLY: INVESTIGATING THE SURFACE COMPOSITION OF TITAN, Trainer et al 2018 https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20180003047.pdf

Categories
Development and Testing RHU RTGs Test Stands

ESA’s RTG Program: Breaking New Ground

Hello, and welcome back to Beyond NERVA! I would like to start by apologizing for the infrequency in posting recently. My wife is currently finishing her thesis in wildlife biology (multispecies occupancy, or the combination of statistical normalization of detection likelihood with the interactions between various species in an ecosystem to determine inter-species interactions and sensitivities), which has definitely made our household more hectic in the last few months. She should defend her thesis soon, and things will return to somewhat normal afterward, including a return to more frequent posting.

Today, we return to radioisotope thermoelectric generators (RTGs), which have once again been in the news. This time, the new is on a happier note than the passing of one of the pioneers in the field: the refining of a different type of fuel for RTGs: Americium 241. This work was done at the United Kingdom’s National Nuclear Laboratory Cumbria Lab by a team including personnel from the University of Leicester, and announced on May 3rd. This material was isolated out of the UK’s nuclear weapons stockpile of plutonium for nuclear warheads, which is something that we’ll look at more in the post itself.

A quick note on nomenclature and notation, since this is something that varies a bit: the way I learned to notate isotopes of an element (or nuclei of a given element with different numbers or neutrons) is as follows, and is what I generally use. If the element is spelled out, the result is [Element name][atomic mass of the nucleus](isomer state – if applicable); if it’s abbreviated it’s [atomic mass](isomer)[element symbol]. An isomer shows the energetic state of the nucleus: if gamma rays are absorbed by the nucleus, then a nucleon can jump to a higher energy state, just as electrons do for lower-wavelength (and lower energy) photons, and for this post it may not matter, but will come up with this element in the future. This means that Plutonium 238 will become 238Pu, and Americium 242(m) becomes 242(m)Am – I chose that because it’s a long-lived isomer that we will return to at some point due to its incredible usefulness in astronuclear design and reactor geometry advantages.

In this post, we’ll take a brief look at the fuels we currently use, as well as this “new” fuel, and one or two more that have been used in the past.

Radioisotope Power Source Fuels: What Are They, and How Do they Work?

Radioisotopes release energy in proportion to the instability of the isotope of whatever element is being used. This level of instability is a very complex issue, and how they decay largely is determined by where they fall in relation to the Valley of Stability, or the region in the Chart of the Isotopes where the number of protons and the number of neutrons balances the strong nuclear and electroweak forces in the nucleus. We aren’t going to go into this in too much detail, but the CEA did an excellent video on this topic which is highly recommended viewing if you want more information on the mechanics of radioactive decay and its different manifestations:


For our purposes, the important things to consider about radioactive decay are: how much energy is released in a given amount of time, and how easy is that energy to harness into kinetic energy, and therefore heat, while minimizing the amount of unharnessable radiation that could potentially damage sensitive components in the payload of the spacecraft. The amount of energy released in a given time is inversely proportional to the half-life of the isotope: the shorter the half-life, the more energy is released in a given time, but as a consequence the energy is being released at a faster rate and therefore the fuel will produce useful energy levels for a shorter period of time. How much of that energy is useful is determined by the decay chain of the isotope in question, which shows the potential decay mechanisms that the isotope will go through, how much energy is released in each decay, and how what the subsequent “daughter” isotopes are – with their decay mechanisms as well. The overall energy release from a particular isotope has to take all of these daughters, granddaughters, etc into consideration in order to calculate the total energy release, and how useful that energy is. If it’s mostly (or purely) alpha radiation, that’s ideal since it’s easily converted to kinetic energy, and each decay releases a proportionally larger amount of energy on average due to the high relative mass of the alpha particle. Beta radiation is not as good, but still easily shieldable (which converts the energy into kinetic energy and therefore heat), and so while not as ideal for heating is still acceptable.

For an RTG designer, the biggest problem (in this area at least) is gamma radiation. Not only is this very hard to convert into useful energy, but it can damage the payload and components of a spacecraft. This is because of the way gamma radiation interacts with matter: the wavelength is so short that it is absorbed most efficiently by very dense elements with a high number of nucleons (protons and neutrons), after which that nucleon jumps to a higher energy state (an isomer), and usually just spits it back out at a lower energy state. This process is repeated until there isn’t enough energy in the short wavelength photon to actually do anything meaningful.

Diverted particle (blue) emitting a photon (green) through brehmsstrahlung. Image Wikipedia

Unfortunately, radioactive decay isn’t the only way to produce gamma rays, you also have to deal with one of the key concepts in radiation shielding which always ties my fingers in knots whenever I try and type it (my tongue just says “nah, not even going to try”): Brehmsstrahlung. This is also known by the name “braking radiation,” and is close to the concept of “cyclotron radiation.” Basically, when a massive, high energy particle is diverted from its original course, two things happen: it slows down, and that slowing down means that energy has to be conserved somehow. In the case of quantum mechanics, the elastic collisions between the particle and whatever force acts on it (in practice the electromagnetic field it interacts with) creates kinetic energy (heat) if it hits a particle, and in every case (including magnetic containment of the particle) a photon is produced. This photon’s wavelength is proportional to the energy the particle originally has, the degree that its direction is changed, and how much energy is lost through mechanisms other than elastic collisions with other nucleons – and it’s never able to be fully slowed by elastic collisions for very complex quantum mechanical reasons. The amount converted to brehmstrahlung is proportional to the initial velocity of the particle in the case we’re talking about (magnetic containment and diversion of radiation isn’t the subject of this blog post after all). This means that if you have a high energy alpha particle emitter, and a low energy alpha emitter, the high energy alpha emitter will create more gamma radiation, meaning that it’s more problematic in terms of energy efficiency of a radioisotope for heat production.

Another concern is what chemical form the radioisotope is used in the fuel element itself. This defines a number of things, including how well any heat produced is transmitted to where it’s useful (or if the heat isn’t transferred efficiently enough, the thermal failure point of the fuel), what kind of nuclear interactions will occur within the fuel, the chemical impact of the elemental composition of the fuel changing through the radioactive decay that is the entire point of using these materials, how much the particles are slowed within the fuel itself (and any subsequent brehmsstrahlung), how much of the radiation that comes from the radioisotope decay is shielded by the fuel itself, and a whole host of other characteristics that are critical to the real-world design of a radioisotope power source.

Now that we’ve (briefly) looked at how radioisotopes are used as power sources, let’s look at what options are available for fueling these systems, which ones have been used in the past, and which ones may be used in the future.

How Are Radioisotopes Used in Space?

The best known use of radioisotopes in space is the use of radioisotope thermoelectric generators, RTGs. RTGs are devices that use the thermoelectric effect, paired with a material containing an isotope going through radioactive decay, to produce electricity. While these are the most well known of the radioisotope power sources, they aren’t the only ones, and in fact they are made up of an even more fundamental type of power source: a radioisotope heating unit, or RHU.

These RHUs are actually more common than RTGs, since often the challenge isn’t getting power to your systems but keeping them from freezing in the extreme cold of space (when there isn’t direct sunlight, in which case it’s extreme heat which is the problem). In that case, a small pellet of radioisotope is connected to a heat transfer mechanism, which could be simple thermal conduction through metal components or the use of heat pipes, which use a wick to transfer a working fluid from a heat source to a cold sink through the boiling and then condensation of the working fluid. Heat pipes have become well known in the astronuclear community thanks to the Kilopower reactor, but are common components in electronics and other systems, and have been used in many flight systems for decades.

RTGs use RHUs as well, as the source of their heat to produce electricity. In fact, the design of a common RHU for both RTG and spacecraft thermal management requirements was a focus of NASA for years, resulting in the General Purpose Heating Unit (GPHU) and its successor variations by the same name. This is important to efficiently and reliably manufacture the radioisotope fuel needed for the wide variety of systems that NASA has used in its spacecraft over the decades. While RTGs are the focus of this technology, we aren’t interested in either the power conversion system or the heat rejection system that make up two of the four main systems in an RTG, so we won’t delve into the details of these systems in particular. Rather, our focus is on the RHU radioisotope fuel itself, and the shielding requirements that this fuel mandates for both spacecraft and payload functionality (the other two major systems of an RTG). Because of this, for the rest of the post we will be discussing RHUs rather than RTGs for the most part (although mentions of RTG implications will be liberally scattered throughout).

Another potential use for radioisotopes in space seems to be something that is rare in discussions of space systems, but is common on Earth: as a source of well-characterized and -understood radiation for both analysis and manufacture of materials and systems. Radioisotope tracking is common in everything from medicine to agriculture, and radioactive analysis is used on everything from ancient artifacts to modern currency. The ISS has had experiments that use mildly radioactive isotopes to analyze the growth of living beings and microbes in microgravity environments, for instance, a very common use in agriculture to analyze nutrient uptake in crops, and a variation of a technique used in medicine to analyze everything from circulatory flow to tumor growth in nuclear medicine. X-ray analysis of materials is also a common method used in high-end manufacturing, and as groups such as Made in Space, Relativity Space and Tethers Unlimited continue exploring 3d printing and ISRU in microgravity and low gravity environments, this will be an invaluable tool. However, this is a VERY different subject, and so this will be where we leave this biological analysis and technological development technology and move to the most common use of radioisotopes: to provide heat to do some sort of work.

RHU Fuel: The Choices Available, and the Choices Made

Most RHUs, for space applications at least, are made from 238Pu, which is an isotope of plutonium that is not only not able to undergo fission, but in fairly minute quantities will render Pu meant for nuclear weapons completely unusable. In the early days of the American (and possibly Soviet) nuclear weapons program, small amounts of this isotope were isolated from material meant for nuclear weapons, but as time went on and the irradiation process became more efficient for producing weapons (which is very different from producing power), the percentage of 238Pu dropped from the single digits to insignificant. By this time, though, the Mound Laboratories in Miamisburg, Ohio had become very interested in the material as a source of radioactive decay heat for a variety of uses. These uses ranged from spacecraft to pacemakers (yes, pacemakers… they were absolutely safe unless the person was cremated, and the fact that the removal of said pacemakers couldn’t be guaranteed is what killed the program). This doesn’t mean that they didn’t investigate the rest of what we’ll be looking at as well, but much of their later focus was on 238Pu.

The advantages of 238Pu are significant: it only undergoes alpha decay with an exceptionally small chance of spontaneous fission (which occurs in the vast majority, if not all, isotopes of uranium and above on the periodic table), it has a good middle-of the road half-life (87.84 years) for long term use as a power source for missions lasting decades (as the majority of outer solar system missions require just to get tot he target destination and provide useful science returns), it has a good amount of energy released during alpha decay (5.59 MeV), and its daughter – 234U – is incredibly stable, with a half-life so long that it basically won’t undergo any significant decay until long after the fuel is useless and the spacecraft has fulfilled its mission centuries ago (245,500 years). The overall radiation power for 238Pu is 0.57 W/g, which is one of the best power densities available for long lived radioisotopes.

Additionally, the fuel used PuO2, is incredibly chemically stable, and when 238Pu becomes 234U, those two oxygen atoms are easily able to reattach to the newly formed U to form UO2 – the same fissile fuel form used in most nuclear reactors (although since it’s 234 rather than 235, it would be useless in a reactor), which is also incredibly chemically stable. Finally, the fuel itself is largely self-shielding once encased in the usual iridium clad to protect the fuel during handling and transport, meaning that the minimal amount of gamma radiation coming off the power source is only a major consideration for instruments looking in the x-ray and gamma bands of the EM spectrum causing noise, rather than significant material or electronic degradation.

238Pu is generated by first making a target of 237Np, usually through irradiation of a precursor material in a nuclear reactor. This target is then exposed to the neutrons produced by a nuclear reactor for a set length of time, and then chemically separated into 238Pu. Due to the irradiation time and energy level, the final result is almost pure 238Pu (close enough for the DOE and NASA’s purposes, anyway), which can then be turned into the ceramic pellet and encased in the clad material. This is then mated to the system that the spacecraft will use the power source for, usually just before spacecraft integration into the launch vehicle. Due to the rigid and strict interpretations of radiation protection, this is an incredibly complex and challenging process – but one that is done on a fairly regular basis. The supply of 238Pu has been a challenge from a bureaucratic perspective, but a recent shakeup of the American astronuclear industry due to a GAO report released last year offers hope for sufficient supply for American flight systems.

This is far from the only radioisotope source used for RHUs – in fact, it wasn’t even the first. Many early US designs used strontium 90, as did many Soviet designs. The first RTG to fly, SNAP-3, used this power source, as did multiple nautical navigation buoys, Soviet lighthouses along their northern coast, and many other systems. This isotope has a half-life of 28.79 years, which means that it’s more useful for shorter-lived systems than 238Pu, but is still a long enough half-life that it’s still a useful radioisotope source. The disadvantage is that it decays via beta decay (at 546 KeV), to 90Y, which is less efficient in converting the radioactive decay into heat. However, this isotope goes through another decay as well. 90Y only has a 2.66 day (!) half-life, ejecting a 2.28 MeV beta particle, resulting in a final decay product of 90Zr, which is radiologically stable. This means that the total energy release is 2.826 MeV, through two beta emissions. The overall energy release in the initial decay is attractive, at 0.9 W/g, however, so either sufficient shielding (of sufficiently high thermal conductivity) to convert this beta radiation into kinetic energy, or a different power conversion system than one that is thermally based is a potential way to increase the efficiency of these systems.

Finally, we come to the one that scares many, and has a horrible reputation in international politics, polonium 210. This is most famous for being used as a poison in the case of Alexander Litvinenko, a Russian defector who was poisoned with Po in his tea (and a whole lot of other people being exposed), but much of the effectiveness is due to chemical toxicity, not radiotoxicity. 210Po has an incredibly short half-life, of only 138.38 days. This is only acceptable for short mission times, but the massive amount of heat generated is still incredibly attractive for designers looking for very high temperature applications. Designs such as radioisotope thermal rockets (where the decay heats hydrogen or other propellant, much like an NTR driven by decay rather than a reactor) that have their efficiency defined purely by temperature can gain significant advantages thanks to this high decay energy. 210Po decays via alpha decay into lead 206, a stable isotope, so there are no complexities from daughter products emitting additional radiation, and a 5.4 MeV alpha particle carries quite a lot of energy.

Other isotopes are also available, and there’s a fascinating table in one of my sources that shows the ESA decision process when it comes to radioisotope selection from 2012:


241Am: The Fuel in the News

Americium-241 is the big news, however, and the main focus of this post. 241Am is produced during irradiation of uranium, which goes through neutron capture during irradiation to 241Pu, which is one of the isotopes that degrades the usefulness of weapons-grade 239Pu. It decays in a very short time (14 day half-life) into 241Am, which is not useful as weapons-grade material, but is still useful in nuclear reactors. According to Ed Pheil, the CEO of Elysium Industries, the Monju sodium cooled fast reactor in Japan faced a restart problem because the 241Pu in the reactor’s fuel decayed before restarting the reactor, causing a reactivity deficit and delaying startup.

241Am decays through a 5.64 MeV alpha emission into 237Np, which in turn goes through a 4.95 MeV alpha decay with a half-life of 2.14 x 10^6 years. This means that the daughter is effectively radiologically stable. With its longer half-life of 433 years (compared to 88 years for 238Pu), 241Am won’t put out as much energy at any given time compared to 238Pu fuel, but the difference in power output as the mission continues will allow for a more steady power supply for the spacecraft. A comparison of beginning of life power to 20 year end of life power for a 238Pu RTG shows a reduction in power output of about 15%, compared to only about 3.5% for 241Am. This allows for more consistent power availability, and for extremely long range or long duration missions a greater amount of power available. However, in the ESA design documentation, this extreme longevity is not something that is examined, a curious omission on first inspection. This could be explained by two factors, however: mission component lifetime, which could be influenced by multiple factors independent of power supply, and the continuing high cost of maintaining a mission control team and science complement to support the probe.

Depending on what power level is needed (more on that in the next section about the ESA RTG design), and how long the mission is, the longer half-life could make 241Am superior in terms of useful energy released compared to 238Pu, and is one of the reasons ESA started looking at 241Am as the main focus of their RTG efforts.

Why the focus on weapons material in the description of production methods? Because that’s where the UK’s National Nuclear Laboratory gained their 241Am in the latest announcement. The NNL is responsible for production of all 241Am for Europe’s RTG programs, but doesn’t HAVE to produce the material from weapons stockpiles.

Fuel cycle using civilian power reactors, Summerer 2012

The EU reprocesses their fuel, unlike the US, and use the Pu to create mixed-oxide (MOX) fuel. If the Pu is chemically separated from the irradiated fuel pellets, then allowed to decay, the much shorter half-life of 241Pu compared to all of the others will lead to the ability to chemically separate the 241Am from the Pu fuel for the MOX. This could, in theory, allow for a steady supply of 241Am for European space missions. As to how much 241Am that would be available through reprocessing, this is a complex question, and one that I have not been able to explore sufficiently to give a good answer to how much 241Am would be available through reprocessing. Jaro Franta was kind enough to provide a pressurized water reactor spent fuel composition table, which provides a vague baseline:

However, MOX fuel generally undergoes higher burnup, and according to several experts the Pu is quickly integrated into fuel as part of the reprocessing of spent fuel. This could be to ensure weapons material is not lying around in Le Hague, but also prevents enough of a decay time to separate the 241Am – plus, as we see in 238Pu production, where the materials are fabricated in one place, separated in another, and made into fuel in a third, Le Hague and the Cumbria Laboratory are not only in different locations but different countries, and after this process the Pu is even more useful for weapons, this bureaucratic requirement makes the process of using spent nuclear fuel for 241Am production an iffy proposition at best. However, according to Summerer and Stephenson (referenced in one of the papers, but theirs is behind a paywall) the economical separation of 241Am from spent civilian fuel can be economical (I’m assuming due to the short half-life of 241Pu), so it seems like the problem is systemic, not technical.

241Am is used as RHU fuel in the form of Am2O3. This allows for very good chemical stability, as well as reasonable thermal transfer properties (for an oxide). This fuel is encased in a “multilayer containment structure similar to that of the general-purpose heat source (GPHS) system,” with thermal and structural trade-offs made to account for the different thermal profile and power level of the ESA RTG (which, as far as I can tell, doesn’t have a catchy name like “GPHS RTG” or “MMRTG” yet). Neptunium oxide most often takes the form of NpO2, meaning that a deficit of oxygen will occur over time in the fuel pellet. The implications of this are something that I am completely unable to answer, and something that is definitely distinct from the use of 238Pu, which then becomes 234U, both of which have two oxygen atoms in their most common oxide state. However, considering there’s a stoichiometric mismatch between the initial material, the partially-decayed material, and the final, fully-degenerate state of the fuel element. I know just enough to know that this is far from ideal, and will change a whole host of properties, from thermal conductivity to chemical reactivity with the clad, so there will be (potentially insignificant) other factors that have impacts on fuel element life from the chemical point of view rather than the nuclear one.

ESA’s RTG: the 241Am RTG

ESA has been interested in RTGs for basically its entire existence, but for reasons that I haven’t been able to determine for certainty, they have not investigated 238Pu production for their RTG fuel to any significant degree for that entire time. Rather, they have focused on 241Am. This comes with a trade-off: due to the longer half-life, at any given time less energy is available per unit mass (0.57 W/g for 238Pu vs 0.1 W/g for 241Am), but as previously noted there are a couple of threshold points at which this longer half-life is an advantage.

Mass to power design envelope, Ambrose et al 2013

The first advantage is in mission lifetime (assuming the radiochemistry situation is of minimal concern): the centuries-long half-life could allow for unprecedented mission durations, where overall mass budget for the power source is of less concern in mission design than longevity of the mission. The second comes to when a very small amount of power is used, and this is the focus of ESA’s RTG program. The GPHS used in American systems produces 250 Wt at the time of manufacture in no more than 1.44 kg. Sadly, it’s very difficult to determine what the ESA fuel element’s mass and specific composition, so a direct comparison is currently impossible. Based on a 2012 presentation, there were two parallel options being explored, with different specific power characteristics, but also different thermal conductivity – and more efficient thermal transport. One was the use of CERMET fuel, where the oxide fuel is encapsulated in a refractory metal (which was unspecified), manufactured by spark plasma sintering. This is a technology that we’ve examined in terms of fissile fuel rather than radioisotope fuel, but many of the advantages apply in both cases. In addition, the refractory metal provides internal brehmstrahlung shielding, potentially offsetting the need for robust clad materials, but this is potentially offset by the need to contain the radioisotope in the case of a launch failure, which requires a mechanical strength that may ensure that the reduction in radiation shielding requirements are rendered effectively moot. The second was a parallel of American RHU design, using a multi-layered clad structure, using refractory metals, carbon-based insulators, and carbon-carbon insulators. This seems to be the architecture which was ultimately chosen, after a contract with Areva TA in France and other partners (which I have not been able to find documentation on, if you have that info please post it in the comments below!).

Cross-section of 1st generation ESA RTG, Ambrosi et al 2013

ESA has two RTG designs that they’ve been discussing over the last two decades: an incredibly small one, of only 1 W of electricity, and a larger one, in the 10-50 We range. These systems are similar in that they are both starting from the same design philosophy, but at the same time different materials and design tradeoffs were required for the two systems, so they have evolved in different directions.

BiTe thermocouple unit, Summerer 2012

The 10-50 We RTG combines the 241Am RHU with a composite clad, surrounded by a bismuth telluride thermocouple structure. This is very similar to lead telluride TE converters in many ways, but is more efficient in lower operating temperatures. PbTe is also under investigation, developed and manufactured by Fraunhofer IPM in Germany, but BiTe seems to be the current technological forerunner in ESA RTG development. This seems to be commercially available, but sadly based on the large number of thermopiles (another name for TE converters) available, it is not clear which is being used.

The 1-1.5 We RTG is one that doesn’t seem to be explored in depth in the currently published literature. This power output is useful for certain applications, such as powering an individual sensor, but is a much more niche application. Details on this design are very thin on the ground, though, so we will leave this design at that and move on to the experiment performed – again, very little is available on the specifics, but the experiment WAS described in previous papers.

Gen 2 ESA RTG, Ambrosi 2015

This design went through an evolution into the model seen in the public announcement video. This is called the Gen 2 Flight System Design, and likely was introduced sometime in the 2015 timeframe. At the same time, the Radioisotope Heating Unit itself went to TRL 3.

On the fuel element fabrication side, after 241Am was selected in 2010 two phases of isotope production occurred. The first was from 2011 to 2013, and the second from 2013 to 2016.

This was the first test of that batch of RTG fuel to produce electricity, which is a major achievement, for which the entire team should be congratulated.

The Experiment in the News: What is All the Fuss Actually About?

Sadly, there’s no publicly available information about the Am-fueled test in the news. The promotional video and press release from NNL/UL provided only two pieces of information: 241Am fuel was used, and they lit a light bulb. No details about how much 241Am, its fuel form or clad, the thermocouples used, the power requirements of the light bulb… this was meant for maximum viral distribution, not for conveying technical information.

The best way to look at the test is by looking at preceeding tests. RTG design from the ground up takes time, and in the case of ESA and the NNL, this process was only funded for the last ten years. They have had to pick a fuel type, continue to go through selection on thermocouple type, and will be working to finalize the design of their flight system.

Electrically heated breadboard experiment, Ambrosi 2012

In 2012, an electrically heated breadboard experiment was conducted. The test used a cuboid form factor, rather than the more common cylindrical form, and was conducted in a liquid nitrogen cooled vacuum chamber. It was designed around a theoretical fuel element of Am2O3 which would provide 83 Wt of power to the thermocouple. This thermocouple, in the proposal phase, was either a commercially available BiTe or bespoke PbTe thermocouple, contained in a cover gas of argon. The maximum output of electrical power was 5 We, which is less than the minimum size of the “normal” 10 We RTG design. Depending on the fuel element used, it’s possible that the experiment was carried out at 83 Wt/ 5We to allow for maximum comparison between the electrically heated version and the radioisotope powered version, but the lack of RI-powered experiment data prevents us from knowing if this was the case, or whether the experiment was performed at 10 We (166 Wt?) due to manufacturing and fuel element design constraints.

The electrically heated design in the breadboard experiment demonstrated that in the 5-50 We power output range, a specific power of 2 We/kg is feasible. There are possibilities that this could be improved somewhat, but it provides a good baseline for the power range and the specific power that these units will provide. This paper (linked below, Development and Testing of Americium-241 Radioisotope Thermoelectric Generator: Concept Designs and Breadboard System, Ambrosi et al) notes that only at small power outputs can 241Am compete with 238Pu systems, but extended mission lifetime considerations were not addressed.


ESA and the University of Leicester continue to look at expanding the use of RTGs in the future. The focus for the thermocouples in the first flight design was in bismuth telluride TEGs as of 2015. They are also looking into Stirling convertors as well, continuing in the current drive to move away from the

It takes time to do the things that they’re attempting, there are few specialists in these areas (although the fundamental tasks aren’t difficult if you know what you’re doing, it takes a while to know the ins and outs of any sort of large-scale chemistry), and it requires a lot of research to verify that each step of the way is both safe and reliable without compromising efficiency.

I have reached out to Dr. Ambrosi at University of Leicester for additional information about this test. If I hear anything from him, I will add the information about this particular test to the page on this NTR system, which should release soon.

Conclusions: 241Am, Is It the RTG Fuel of the Future?

As with most things in astronuclear engineering, the choice of an RHU fuel is a very complex question, and one which has no simple answer. 238Pu remains the preferred long-duration RTG fuel for space missions in terms of specific power, but its expense, and the requirements as far as infrastructure for fabrication and manufacture provide a high cost barrier for new entrants into the use of RHUs. For Europe, this barrier to entry is considered unacceptable, and that has kept them out of the RTG-flying world community for the entirety of their history.

241Am, on the other hand, is available to nations that conduct reprocessing of spent nuclear fuel, such as signatories to the Euratom treaty (as well as the UK, who are withdrawing voluntarily from the treaty as part of Brexit… don’t ask me why, it’s not required), where reprocessing of spent nuclear fuel is normal practice. Similar challenges to bureaucratic roadblocks to significant production of 238Pu by the US can be seen in European production of 241Am, but the existence of significant reprocessing capabilities make it theoretically far more available. 241Am is also available commercially in the US, meaning that at least in one country the regulatory barriers to possession, and therefore cost, are significantly lower than 238Pu.

This choice, however, comes at a cost of halving the specific power available to RTG systems due to the lower specific power of the fuel, at least as the design is historically described. Optimization calculations by ESA and its partners, primarily the University of Leicester, show that in the 5-50 We range of electric output the impact on mission mass is minimal, and for very low power applications (1-1.5 We) it is superior. The increased availability, and lower cost of acquisition of sufficiently pure 241Am, offset the advantages of 238Pu from the European perspective, as it integrates into current industrial capabilities, which outweigh the engineering advantages of 238Pu for the organizations involved. Even so, this is an exciting development for deep space exploration nerds, and one that can’t be overstated.

While this was a fascinating experiment, and I will be trying to find more information, the significance of this experiment boils down to one thing: this is the first time, outside the US or Russia, that an RTG designed for spacecraft use produced electrical power. It opens up new mission opportunities for ESA, who have been hampered in deep space exploration by their lack of suitable RHU fuel, and offers hope for more missions, more science, and more discoveries in the future.

References

Isotope information for 241Am, Periodictable.com https://periodictable.com/Isotopes/095.241/index.html

Isotope information for 238Pu, Periodictable.com https://periodictable.com/Isotopes/094.238/index2.full.dm.prod.html

Isotope information for 90Sr, Periodictable.com https://periodictable.com/Isotopes/038.90/index2.full.dm.prod.html

Isotope information for 210Po, Periodictable.com https://periodictable.com/Isotopes/084.210/index2.full.dm.prod.html

241Am ESA RTG Design

Development and Testing of Americium-241 Radioisotope Thermoelectric Generator: Concept Designs and Breadboard System; Ambrosi et al 2012 https://www.lpi.usra.edu/meetings/nets2012/pdf/3043.pdf

Americium-241 Radioisotope Thermoelectric Generator Development for Space Applications, Ambrosi et al 2013 https://inis.iaea.org/collection/NCLCollectionStore/_Public/45/066/45066049.pdf

Nuclear Power Sources for Space Applications – a key enabling technology (slideshow), Summerer et al, ESA 2012 https://www.euronuclear.org/events/enc/enc2012/presentations/L-Summerer.pdf

Space Nuclear Power Systems: Update on Activities and Programmes in the UK, Ambrosi (University of Leicester) and Tinsley (National Nuclear Laboratory), 2015 http://www.unoosa.org/pdf/pres/stsc2015/tech-15E.pdf