RS-56 (Rocket System-56) was an American liquid-fueled rocket engine, developed by Rocketdyne.

RS-56 was derived from the RS-27 rocket engine,[1] which itself is derived from the Rocketdyne H-1 rocket engine used in the Saturn I and Saturn IB.

Two variants of this engine were built, both for use on the Atlas II rocket series.

The first, RS-56-OBA, was a booster engine, while the RS-56-OSA was designed for use as a sustainer and produced lower thrust but at a higher specific impulse.

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