WASHINGTON — A new version of a 60-year-old rocket engine, with performance and cost improvements, is expected to make its debut in 2025 on the Vulcan Centaur rocket.

At a Nov. 27 briefing, executives with United Launch Alliance and Aerojet Rocketdyne, an L3Harris Technologies company, said they expected that the RL10C-X engine, the latest upgrade to the RL10, to make its first flight on a Vulcan launch some time in 2025.

A major change for the RL10C-X is how it is manufactured. “It relies heavily on additive manufacturing,” said Jim Maus, vice president of program execution and integration at Aerojet. The current RL10 uses additive manufacturing to produce its injector, but the RL10C-X will use additive manufacturing to produce the entire thrust chamber.

That change offers cost reductions “that has been one of the enablers for us continuing to be viable in the marketplace,” he said, but did not quantify those reductions.

The engine also uses a carbon-silicon nozzle that he said improves its specific impulse, a measure of efficiency. Other elements of the engine, including its turbomachinery, are unchanged from existing versions of the RL10.

Maus said the RL10C-X is going through certification now, and he expected it first fly on a Vulcan Centaur some time in 2025. Gary Wentz, vice president of government and commercial programs at ULA, confirmed that timeline. “We are targeting in the 2025 timeframe,” he said, “subject to integrating it with the vehicle and assigning a mission to it.”

ULA selected the RL10C-X in 2018 to power the Centaur upper stage on Vulcan in what it described at the time as a “competitive procurement,” although versions of the RL10 have been used on Atlas and Delta launch vehicles for decades. ULA ordered 116 RL10C-X engines in 2022 for future Vulcan launches.

Aerojet has a contracted backlog of “north of 150” RL10 engines, Maus said, including the RL10C-X and older versions. While that backlog is dominated by the ULA order of RL10C-X engines, he said deliveries of the older RL10 design are projected to continue to 2026, with overall RL10 production ramping up from a current 16 to 18 engines a year to 40 a year.

After the retirement of the Atlas 5 and Delta 4 Heavy, the RL10 will be used by Vulcan as well as the Space Launch System. That rocket’s Interim Cryogenic Propulsion Stage uses a single RL10 while the Exploration Upper Stage (EUS), set to make its debut on Artemis 4 in the late 2020s, will use four RL10 engines. Maus said NASA has yet to decide if will use the new RL10C-X engine on the EUS.

The companies held the briefing to mark the 60th anniversary of the first use of the RL10 on an Atlas-Centaur rocket. The engine has been used on a variety of vehicles since then, from the Saturn 1 to the DC-X suborbital experimental vehicle.

Maus credited that longevity to the performance of the engine, whose high specific impulse comes from its use of liquid hydrogen and liquid oxygen propellants and an expander cycle that drives the engine’s turbomachinery without having to burn propellants.

“We continue to deliver very high reliability, very high performance, and the engine itself is very versatile,” he said. “It’s great to be at 60 years, with a very long and bright future ahead.”

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...