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 a single 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, can throttle way down, and holds what Elon has called the world record for thrust-to-weight and thrust-to-cost. It is one of the most-produced and most-flown rocket engines ever built.

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. The early years were rough, and engineers had to switch 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 wide range is what allows a Falcon 9 booster to slow down for landing instead of just burning out. Combined with a thrust-to-weight ratio above 150 to 1, that throttling is the quiet engineering behind every drone-ship touchdown.

What stands out most, though, is how much the engine grew. Sea-level thrust per engine climbed from about 340 kilonewtons on the Merlin 1A to roughly 845 kilonewtons on the Full Thrust Merlin 1D, a 2.5x jump over a decade of steady improvement on the same basic design.

Each version pushed the number higher: the 1A at about 340 kilonewtons, the 1C near 350 kilonewtons on Falcon 1, the first 1D at 654 kilonewtons, and the uprated 1D at 845 kilonewtons, roughly 29 percent more than the version before it.

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 November 2025, Falcon 9 boosters had logged 518 successful landings, and a single booster, B1067, set a 31-flight reuse record in October 2025 before flying again. Spread the cost of nine engines across more than 30 flights and the engine cost per flight all but disappears. Grokipedia's figure of roughly 1,170 dollars per kilonewton of thrust, divided across about ten flights, works out to near 117 dollars per kilonewton per flight. That is the logic of steady, repeat income applied to hardware: 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 more than 140 times, with a booster recovery rate near 98.8 percent across about 524 landing tries.

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 is the engine SpaceX actually flies hundreds of times a year, while Raptor carries the longer-term Starship dreams. The two are less rivals than teammates in a relay, each running its own leg.

The Bottom Line

The Merlin proved that a simple, cheap, reusable engine could beat fancier ones on the only scoreboard that pays the bills: flights per dollar. It did not just reach orbit, it made reaching orbit ordinary.

Related

Keep reading: Polaris Program, Raptor Engine. Zoom out to the SpaceX overview, or open the Rocket Comparison Tool.