RL-10

The RL10 is a rocket engine that does not fire on the launch pad. It switches on high above the planet, in the vacuum of space, giving payloads the final push to reach their destination.

Quick facts

• Type: cryogenic upper-stage engine using an “expander cycle” (cryogenic means it burns propellants chilled to extreme cold so they stay liquid).
• Propellants: liquid hydrogen (the fuel) and liquid oxygen (the oxidizer, the substance that lets the fuel burn).
• Thrust in vacuum: roughly 22,300 to 24,750 pounds-force (about 99 to 110 kilonewtons) per engine on today’s versions, with earlier variants producing as little as about 15,000 pounds-force (about 67 kilonewtons).
• Specific impulse (a measure of fuel efficiency): up to about 465.5 seconds, among the highest of any operational chemical rocket engine.
• First ground test: 1959. First flight: November 27, 1963, on an Atlas-Centaur rocket.
• Restartable: yes, designed for multiple restarts in space.
• Flight heritage: more than 500 RL10 engines have flown.
• Makers over time: Pratt & Whitney, then Pratt & Whitney Rocketdyne, then Aerojet Rocketdyne, then L3Harris Technologies (which acquired Aerojet Rocketdyne in July 2023). In January 2026, L3Harris agreed to sell a majority stake in its space-propulsion business, including the RL10, to AE Industrial Partners, reviving the “Rocketdyne” name; that deal was expected to close in the second half of 2026.

What it is and how it works

The RL10 is an American liquid-propellant engine that burns liquid hydrogen (LH2) and liquid oxygen (LOX). First flown in 1963, it was the world’s first operational hydrogen-fueled rocket engine, and it remains one of the most reliable and long-serving rocket engines ever built. Its job is to power an upper stage, the top section of a rocket that takes over after the booster falls away, lifting a payload from a temporary “parking” orbit to its final orbit or trajectory.

What makes the RL10 unusual is its “expander cycle.” A rocket needs pumps to force propellants into the combustion chamber, and those pumps need power. Many engines burn extra fuel in a separate device just to spin those pumps. The RL10 does something cleverer. It routes the cold liquid hydrogen through cooling channels wrapped around the hot combustion chamber and nozzle. The heat boils the hydrogen into a high-pressure gas, and that expanding gas spins the turbine that drives the pumps. Only afterward is the hydrogen injected into the chamber and burned.

Think of it like a tea kettle that powers itself: the same flame that you want to use also boils water into steam, and that steam could spin a small turbine. This makes the engine very efficient and clean-running, because nothing is wasted. The trade-off is size. There is only so much waste heat available to drive the pumps, so the expander cycle works best on smaller upper-stage engines rather than the giant engines that lift a rocket off the ground. Burning hydrogen and oxygen produces a very fast exhaust (high specific impulse), and the engine can shut down and restart several times in space to perform multiple burns on a single mission.

Why it matters

The RL10 is the workhorse upper-stage engine of the U.S. space program, flying for more than six decades and earning one of the longest and most reliable track records in history. Its high efficiency makes it ideal for the final, energy-critical phase of a mission, whether that is placing a satellite into geostationary orbit or sending a probe toward the Moon or the planets. It powered the Centaur, the first rocket stage to use liquid hydrogen, proving that cryogenic hydrogen propulsion was practical and enabling most of NASA’s major interplanetary launches. The newest variants, the RL10C-X and RL10E-1, use a 3D-printed copper-alloy thrust chamber that cuts part count dramatically and shortens fabrication from about 20 months to roughly 4 to 6 months, a notable example of modernizing a legacy design.

Where it is used

• Atlas V: one RL10C-1, or two RL10A-4-2 engines in the dual-engine Centaur stage.
• Delta IV: the RL10B-2, with an extendable carbon-carbon nozzle and the family’s highest specific impulse (about 465.5 seconds).
• ULA Vulcan Centaur: two RL10C-1-1 engines; the Heavy variant is associated with the more powerful RL10C-X.
• NASA SLS (Interim Cryogenic Propulsion Stage): a single RL10B-2, used on Artemis I and early Block 1 flights.
• SLS Exploration Upper Stage: designed for four RL10C-3 engines, but that program was cancelled in February 2026 in favor of using Centaur V.