The Aerojet Rocketdyne RS-25, also known as the Space Shuttle Main Engine, is a liquid-fuel cryogenic rocket engine that was used on NASA's Space Shuttle and is currently used on the Space Launch System. Designed and manufactured in the United States by Rocketdyne, the RS-25 burns cryogenic liquid hydrogen and liquid oxygen propellants, with each engine producing 1,859 kN thrust at liftoff. Although RS-25 heritage traces back to the 1960s, its concerted development began in the 1970s with the first flight, STS-1, on April 12, 1981. The RS-25 has undergone upgrades over its operational history to improve the engine's reliability, safety, and maintenance load. The engine produces a specific impulse of 452 seconds in a vacuum, or 366 seconds at sea level, has a mass of approximately 3.5 tonnes, and is capable of throttling between 67% and 109% of its rated power level in one-percent increments.

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