MHW-RTG cutaway, image NASA

The bridge between the SNAP-era RTGs and the modern General Purpose Heat Source RTGs, the Multi-Hundred Watt RTG (MHW-RTG) was created in response to the challenges of outer Solar System exploration and the limitations of the SNAP-19 RTG which was used in the Pioneer 10 and 11 probes (more information and documentation on the SNAP-19 can be found on the SNAP RTG page).

As RTG designs developed, a number of challenges became apparent, and a number of regulatory and safety changes in regards to radiological release into the environment were coming into effect. This necessitated more care in the way that the fuel itself was contained in the case of a failure on the launch pad, a failure-to-orbit, or a re-entry event.

Additionally, missions needed more and more power, especially for deep space communications, where bandwidth and power are directly proportional. Especially with the upcoming Grand Tour mission concept, now known as Voyager, this additional power supply was badly needed for the suite of state-of-the-art instruments on the probes, and the simpler Pioneer 10 and 11 probes were forced to use four SNAP-19s.

A new option was clearly needed. This led to the Multi-Hundred Watt RTG.

#If you’re looking for a particular section, you can use these links:

Radioisotope Heat Source; Thermoelectric Generator; Outer Casing; Missions; References

Radioisotope Heat Source

As with the SNAP-19 and SNAP-27, the MWH RTG was to be powered by plutonium 238. For more information on this radioisotope, check out our page on 238Pu here. The designers quickly settled on a design that used moderately sized (3.7 cm) spheres of PuO2 encased in iridium. Each one was designed to produce 100 Wt of energy.

Fuel sphere assembly, image DOE

These were then placed in a bracket which held four spheres on the same lateral plane, with these brackets being stacked to hold a total of 24 spheres (six layers of fuel elements).

Fuel element asembly unit, image DOE

This was then placed in a cylinder wrapped in graphite insulation (for re-entry protection) and an ablative coating.

Aeroshell, image DOE

A gas management system was used to vent the helium from the 238Pu’s decay through outlets in the RTG fins. This also carried some of the waste heat away.

Gas management system, image NASA/JPL

The entire heating unit, when assembled, looked like this:

Assembled RHS for the MMW-RTG, image DOE
Assembled RHU with areoshell and interfaces with generator. Note the gas lines, for both pad cooling and helium management. Image DOE

Thermoelectric Generator

The electric generation capabilities of the MHW-RTG was based on a silicon-germanium thermocouples. This technology is one of the oldest high-temperature thermocouple material combinations. For more information on these systems in general, check out the Thermoelectric Generators page [under construction].

The MHW-RTG used 312 thermocouples (24 circumfrential rows of 13) attached directly to the outer casing of the RTG.

These SiGe thermocouples were doped with Boron and Phosphorous. The main mode of failure in them was the germanium migrating out of solution over time, but the extent to which this occurred over the lifetime of the missions is unclear. To prevent sublimation and degradation, the thermocouples were coated with silicon nitride, which eliminated the need for the xenon cover gas used in earlier SiGe-based thermocouples.

The power conversion efficiency of the thermocouples was 6.5% at beginning of life, decreasing to 5.9 % at the end of design life (14 years, which the Voyager spacecraft have more than doubled). I am unable to find information about the current conversion efficiency of these systems.

Outer Casing

RTG bring mounted to Voyager, image JPL

The outer casing of the MHW-RTG was made out of beryllium, with six fins running the entire length for increased surface area. This was required to reject 2.2 kWt of energy at beginning of life, with a beginning of life thermal gradient between the hot and cold shoe of 700 C (1273 K/1000 C BOL hot side, 573 K/300 C BOL cold side).

The top and bottom of the casing were domed for pressure containment of the He decay gas.


The MHW-RTG was used in four successful missions, the Lincoln Experimental Satellites 8 and 9, and Voyagers 1 and 2.

Voyagers 1 and 2

Voyager 1 during testing, image NASA

The most famous application of the MHW-RTG is also the first time it was used: Voyagers 1 and 2 rewrote every textbook on our solar system, and changed the way deep space exploration is done. While not the first probes to reach Jupiter or Saturn (Pioneers 10 and 11, powered by four SNAP-19 RTGs), their data from the gas giants far outstripped all that was gathered by earlier probes.

Even today, Voyager 2 continues to beam back data, leaving the solar system as its 238Pu fuel degrades into unusability. However, the recently available power of 249 watts, combined with the minimal equipment still functional on the spacecraft, allow for 160 bits/s transmission time of the data that is collected. The spacecraft are expected to continue functioning at current levels until 2021 and 2020 respectively, with a loss of 4 watts a year from the RTGs. At this point, instruments will be turned off in priority order until the final science data is collected in 2025. However, according to NASA engineering data could continue to be transmitted, which could provide clues as to the environment the spacecraft is in.

Links and References

Voyager interstellar mission: challenges of flying a very old spacecraft on a very long mission, Matsumoto 2016

Lincoln Experimental Satellites 8 and 9

LES 8/9 in orbit, image NASA

The Lincoln Experimental Satellite series were developed by the Lincoln Labs in cooperation with the Department of Defense for communications experiments both for orbit-to-ground and orbit-to-orbit communications. The 8 and 9 both had upgraded Ka-band communications hardware to solve many of the challenges associated with satellite-to-satellite communications, and were the third- and second-to-last in the LES series.

The 8th and 9th series of satellites were powered with a pair of MHW-RTGs, mounted on a central mast. Originally, they were designed to have plasma thrusters of unspecified type, but these were replaced with ammonia cold gas thrusters. The onboard Draper Labs gyroscopes were another part of the experimental suite onboard these spacecraft.

Both satellites were launched together on a Titan IIIC booster on 14 March 1978, and went into a geosynchronous orbit. They remained there until 1992, when they began drifting. LES-8 officially ended its mission in 2004, while LES-9 remains in communication with ground stations to this day.

LES References and Links

“Thirty Years of Space Communications Research and Development at Lincoln Laboratory,” Chapter 8, Ward and Floyd

Performance of Multihundred-Watt Fueled Sphere Assemblies in the Safety Verification Test, Cramer 1976

References and Additional Reading


Radioisotope Power Systems Reference Book for Mission Designers and Planners, Lee et al 2014

Radioisotope Power: A Key Technology for Deep Space Exploration Schmidt

Solitons and Their Role in the Degradation of Doped Silicon Germanium Alloys ∗ Neil Gunther