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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! [insert link]

Multi-Mission RTG Use for Dragonfly

While the MMRTG 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

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