Merlin 1D

The Merlin 1D is the rocket engine that powers SpaceX’s Falcon 9 and Falcon Heavy. Cheap to reuse and flown more times than any engine in history, it is a big part of why getting to space costs less than it used to.

Quick facts

• Maker: SpaceX
• Type: Reusable liquid-fuel rocket engine
• Propellants (the things it burns): RP-1, a refined kerosene very similar to jet fuel, and liquid oxygen, which is oxygen chilled until it turns into a liquid so the engine has something to burn the fuel with
• Sea-level thrust (pushing force at ground level): about 845 kilonewtons, roughly the force it would take to lift 86 tonnes
• Throttling (turning the power up and down): down to about 40 percent of full power
• Used in clusters of: nine, on the first stage of Falcon 9 and Falcon Heavy

What it is and how it works

An engine like this has one job: shove burning gas out the back very fast so the rocket is pushed forward, the same way letting go of an inflated balloon sends it shooting across a room.

To do that, the Merlin 1D has to force fuel and oxygen into its combustion chamber (the space where the burning happens) at enormous pressure. It uses a turbopump, a spinning pump driven by a small turbine, to do the forcing. The turbine is spun by a gas generator, a tiny side burner that burns a little of the same fuel and oxygen just to make hot gas. This setup is called an open-cycle gas generator, “open” because that small stream of gas is used once to spin the pump and then thrown overboard rather than reused.

The fuel and oxygen meet through a pintle injector, a single central nozzle (picture a showerhead with one large pin in the middle) that sprays and mixes them as they enter the chamber. This is a simpler, sturdier design than the many small holes other engines use.

Two features make the Merlin special for reuse. It can gimbal, meaning the whole engine swivels to steer the rocket, and it can throttle, easing its power down to about 40 percent. Together those let a returning booster slow itself and balance upright as it lands, so it can be flown again.

Why it matters

Before engines like the Merlin 1D, rocket boosters were thrown away after a single flight, like scrapping an airliner after one trip. Because the Merlin can throttle low and steer precisely, the booster it powers can fly back and land in one piece, ready to launch again. That made genuinely cheap, reusable rockets practical instead of just a dream. The proof is in the numbers: by 2024, one individual Merlin engine had flown 22 missions, more than any rocket engine ever built.

Where it is used

• Falcon 9 first stage: nine Merlin 1D engines fire together at liftoff. The first stage is the bottom section that does the heavy lifting off the launch pad, and it is the part that lands for reuse.
• Falcon 9 upper stage: a single MVac version, short for “Merlin Vacuum.” The upper stage is the section that keeps going after the first stage separates, and the MVac is tuned to work in the near-vacuum of space rather than at ground level.
• Falcon Heavy: its first stage is three Falcon 9 cores strapped together, so the same nine-engine Merlin booster is used three times over, for 27 engines at liftoff.