The Merlin engine is a family of rocket engines developed by SpaceX for use on its Falcon 1, Falcon 9, and Falcon Heavy launch vehicles. The engine uses RP-1 and liquid oxygen as rocket propellants in a gas-generator power cycle. The Merlin engine was originally designed for sea recovery and reuse, but since 2016 the entire Falcon 9 booster is recovered for reuse by landing vertically on a landing pad using one of its nine Merlin engines.

The Merlin engine has evolved over time to enable greater thrust, reliability, and reusability for SpaceX's ambitious space exploration goals. The latest version of the first stage Falcon 9 rocket can return to Earth and be flown again many times. The Merlin-1D++ engine is used in the Falcon 9 and Falcon Heavy rockets and has a thrust of 854 kN at sea level and a specific impulse of 283 seconds. The engine has a throttle capability of 100% to 57% and a restart capability. The nozzle is gimbaled with a 16:1 expansion ratio. The engine has a chamber pressure of 10.8 MPa.

The Merlin engine is incredibly powerful and one of the most efficient engines ever built. SpaceX builds the Merlin engine in-house and it contains a combination of rocket-grade kerosene called Rocket Propellant 1 and liquid oxygen. The engine has a thrust-to-weight ratio of 184 and a specific impulse of 311 seconds in vacuum. The engine has a throttle range of 57% to 64% and a chamber pressure of 9.7 MPa. The Merlin engine has played a crucial role in enabling SpaceX to achieve its ambitious space exploration goals, including the development of reusable rockets and the eventual colonization of Mars.
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