RS-25

The RS-25 is a rocket engine with a remarkable double life: it flew on NASA’s Space Shuttle for 30 years, and today it helps launch astronauts toward the Moon. Few engines have ever been tested and trusted as thoroughly.

Quick facts

• Also known as: the Space Shuttle Main Engine (SSME).
• Fuel: liquid hydrogen (the fuel) and liquid oxygen (the oxidizer, the part that lets the fuel burn). Both are “cryogenic,” meaning chilled to extremely cold temperatures so they stay liquid.
• Thrust (pushing force): at the 109% power setting used on today’s Moon rocket, each engine produces about 418,000 pounds of thrust at sea level, rising to roughly 512,300 pounds in the vacuum of space. (Engines push harder in a vacuum, where there is no surrounding air pressure to fight against.)
• Efficiency: a specific impulse of about 452 seconds in vacuum, exceptionally high for a hydrogen/oxygen engine. (Specific impulse measures how much push you get from each unit of propellant, like miles per gallon for rockets.)
• Throttle range: 67% to 109% of rated power, so the engine can be dialed up or down in flight.
• Size: about 168 inches (4.3 m) long with a nozzle about 91 inches (2.3 m) across at its wide end; dry weight roughly 7,000 pounds (about 3,180 kg).
• Maker: originally Rocketdyne (1972); the engine line passed through Pratt & Whitney Rocketdyne and Aerojet Rocketdyne, which L3Harris acquired in 2023.

What it is and how it works

The RS-25 is a “staged-combustion” engine. That design squeezes out maximum performance by burning nearly all of its propellant in the main chamber instead of wasting any. Here is the sequence.

High-pressure pumps called turbopumps feed liquid hydrogen and liquid oxygen into two small chambers called preburners. There the propellants are partly burned in a fuel-rich mix (more fuel than oxygen) to make hot, high-pressure gas. That gas spins turbines, which are like fans driven by the gas flow, and those turbines power the very pumps that pressurize the propellants to thousands of pounds per square inch. It is a self-feeding loop.

The still fuel-rich exhaust from the preburners is then sent into the main combustion chamber, where it meets the remaining oxygen and burns completely. The resulting hot gas rushes out through a bell-shaped nozzle, which flares the flow to produce thrust. Because almost no energy is thrown away, the engine reaches its very high specific impulse of around 452 seconds in vacuum. The chamber runs at about 2,994 psi of pressure, and the nozzle has a 78-to-1 expansion ratio (a measure of how much the gas spreads out as it exits). Being able to throttle between 67% and 109% lets the rocket manage its acceleration and the stresses of flying through the atmosphere.

Why it matters

The RS-25 set the benchmark for high-performance, reusable, human-rated rocket power. As the Space Shuttle Main Engine, it was the first large rocket engine designed to be recovered, inspected, and flown again. It built an extraordinary reliability record across 135 crewed missions over three decades, with more than 3,000 starts and over 1 million seconds of combined ground-test and flight firing time. That track record is why NASA chose to carry the engine forward rather than design a brand-new one for the Space Launch System (SLS), the giant rocket that launches the Artemis missions returning humans to the Moon. The RS-25 is a bridge connecting the Shuttle era to the Artemis era of American spaceflight.

Where it is used and notable examples

• Space Shuttle (1981 to 2011): three RS-25 engines on the back of each orbiter (Columbia, Challenger, Discovery, Atlantis, Endeavour), recovered and reused across 135 missions, starting with STS-1 on April 12, 1981.
• Space Launch System (SLS) Block 1: four RS-25 engines clustered at the base of the core stage, together producing over 2 million pounds of thrust during the roughly 8.5-minute climb to space.
• Artemis I (2022): the first SLS flight, powered by four refurbished Shuttle-heritage RS-25D engines, sent the uncrewed Orion spacecraft around the Moon.
• RS-25E: a newly built, simplified-for-cost expendable version from L3Harris (24 under contract) for future Artemis flights, run at a 111% power level.
• Ground testing at NASA’s Stennis Space Center: extensive hot-fire tests, up to 113% thrust, to certify the engine for SLS service.