Merlin Engine

The Merlin is SpaceX's family of liquid-fueled rocket engines, and it is the workhorse that made cheap access to orbit real. Each one burns RP-1, a refined kerosene, with liquid oxygen (LOX), and SpaceX designed the engine in-house in the early 2000s. Nine sea-level Merlin 1D engines fire together in a circular "octaweb" cluster on the Falcon 9 first stage, while one vacuum-tuned version, the Merlin Vacuum, powers the upper stage.

What is the Merlin Engine?

The Merlin is a gas-generator rocket engine, an "open" design that bleeds off a little fuel to spin its pumps and then dumps that gas overboard. It is the engine behind Falcon 1, Falcon 9, and Falcon Heavy. The current Merlin 1D produces about 845 kilonewtons, or 190,000 pounds of force, of thrust at sea level per engine and can throttle way down.

Why the Merlin Engine Exists

The reasoning was about money before it was about technology. SpaceX needed a first-stage engine and did not want to pay the high price of the ones already out there, so the company built a cheap, simple engine instead. A gas-generator cycle is less efficient than the exotic staged-combustion engines, but it is far easier and cheaper to build. SpaceX also borrowed a pintle injector, the part that mixes fuel and oxidizer, from the Apollo Lunar Module Descent Engine, a proven design from the 1960s. In the early years engineers switched key parts from cast aluminum to tougher Inconel, a nickel alloy, so they could survive the heat.

How it Works

The pintle injector does double duty. It lets the Merlin throttle from full power down to 40 percent, and that range lets a Falcon 9 booster slow down enough to land instead of burning out. The engine is also light for its push, with a thrust-to-weight ratio above 150 to 1.

Sea-level thrust per engine climbed from about 340 kilonewtons on the Merlin 1A in 2006 to 654 kilonewtons when the Merlin 1D debuted on Falcon 9 v1.1 in 2013, then to about 845 kilonewtons with Falcon 9 Block 5 in 2018, about 2.5 times the original from the same basic design.

The upper-stage engine differs in one key way. Swap the sea-level nozzle for a large bell shaped for vacuum, and the same core gains about 66 seconds of specific impulse (Isp), a measure of fuel efficiency in seconds, rising from 282 seconds at sea level to 348 in vacuum. That bigger nozzle is where the engine earns its keep on the long climb to orbit.

The Economics of the Merlin Engine

Reuse is the part that changes the math. By October 2026, SpaceX's counter showed about 666 Falcon booster landings, and booster B1067 flew a record 37th mission on August 25, 2026. Spread the cost of nine engines across more than 30 flights and the engine cost per flight shrinks to a small share of each launch. The same logic runs through the rest of the business: high production volume, more reuse every year, and a falling cost per unit feeding a launch business that also lifts SpaceX's own Starlink fleet.

Current Status of Merlin

The Merlin is now one of the most-produced and most-flown rocket engines ever built. SpaceX turns out up to roughly 500 a year, and the nine-engine Falcon 9 first stage delivers more than 7.6 meganewtons of combined sea-level thrust at liftoff. In 2025, Falcon rockets flew a record 165 times.

What Comes Next

Elon has been open about the fact that the Merlin still holds records its successor may not match. He has called it the world-record holder for thrust-to-weight and thrust-to-cost, and has said he worries the methane-burning Raptor, while more efficient, could come up short on exactly those two measures. For now, the Merlin flies on more than 100 SpaceX launches a year, while Raptor powers the longer-term Starship program.

The Bottom Line

The Merlin proved that a simple, cheap, reusable engine could beat fancier ones on flights per dollar, and it has now flown on more than 700 Falcon launches.