L3Harris finalises design of deep space power source

The company's Next-Generation Radioisotope Thermoelectric Generator (Next Gen RTG) cleared its critical design review (CDR) on 2 April, paving the way for a new era of outer solar system exploration.

The space community has relied mainly on photovoltaic power systems, a technology that was originally developed for the purpose of space applications and has found many terrestrial uses. However, these systems pose severe limitations for missions to places like the outer solar system. The available solar energy reduces with the square of the distance from the sun. For example, on Saturn the solar power density is a hundred times lower than on Earth.

Radioisotope thermoelectric generators (RTGs) convert heat from the radioactive decay of plutonium-238 into electricity. They have been in use for 60 years. Early versions continue to supply power to NASA's twin Voyager probes, which were launched in 1977 and are now travelling in interstellar space.

The Next Gen RTG is an evolution of the general-purpose heat source RTGs that supplied power to NASA's Cassini Saturn orbiter and, more recently, New Horizons probe, which carried out a Pluto flyby in 2015 and is now exploring the Kuiper Belt, a distant, doughnut-shaped region of icy debris and dwarf planets that extends just beyond the orbit of Neptune.

Unlike the L3Harris-built Multi-Mission RTGs currently powering NASA's Curiosity and Perseverance Mars rovers, the Next Gen RTGs are optimised for spacecraft operating in the vacuum of space rather than on the surface of a planet. The vacuum-optimised design allows for more efficient heat rejection and power generation in the deep space environment where missions like the Uranus orbiter will operate. As a result, the Next Gen RTG offers a higher power output at about the same weight as the Multi-Mission RTG. With the capability to generate about 250 watts of power at the beginning of its life, each Next Gen RTG will provide reliable, long-duration power for spacecraft exploring the outer reaches of the solar system.

The US Department of Energy's Idaho National Laboratory (INL) contracted L3Harris in 2021 to re-establish the key technologies from the heritage system and update the design in response to growing interest in new deep space missions. The contract is expected to end in 2027 with a production readiness review to verify that the next-generation system can be built using the materials and components that have been re-established.

"We are proving we can do it again," said Leo Gard, Space Propulsion & Power Systems Programme Manager at L3Harris. "While we didn't build the original generators, we've successfully reconstructed incomplete documentation and identified modern equivalents for obsolete components through creative problem-solving."

"Passing the CDR is an important milestone because it validates that our design meets all the technical requirements and can be manufactured," added Bill Sack, General Manager, RocketWorks and Power Systems at L3Harris. "It also demonstrates we've successfully re-established this critical capability after years of limited production."

He added: "The Next Gen RTG represents a significant leap forward in efficiency. We're delivering more power in the same mass envelope, which is critical when every kilogram matters for deep space missions."

As prime contractor on the Next Gen RTG programme, L3Harris is responsible for the main structure and overall system integration. Teledyne Energy Systems Inc of Hunt Valley, Maryland, makes the thermoelectric couples that convert heat to electricity, while BAE Systems Space and Mission Systems in Boulder, Colorado, is responsible for insulation.

Flight units could power NASA deep space probes starting in the early 2030s, including a proposed Uranus orbiter that would use two Next Gen RTGs for power and for keeping its temperature-sensitive components warm enough to operate in the frigid environment of the outer solar system. This dual-purpose capability makes RTGs indispensable for such missions.

L3Harris said that, beyond the Uranus orbiter, these power systems could enable: extended missions to Neptune and its moon, Triton; Kuiper Belt object explorers that can go beyond the range of the New Horizons spacecraft; long-duration missions to the outer planets' moons; and interstellar precursor missions that push even farther than the Voyager 1 and Voyager 2.