Merlin Engine Relight System
When a Falcon 9 launches a satellite, the bottom half of the rocket peels away, flips around, and flies back to land upright on a pad or a ship at sea. The thing that makes that possible is the Merlin engine’s ability to restart in mid-flight.
Quick facts
- Engine: SpaceX Merlin 1D, an open-cycle (fuel-rich gas-generator) engine. A gas-generator engine burns a little propellant in a small chamber to spin the pumps that feed the main one.
- Propellants: RP-1 (a refined kerosene, basically a very pure jet fuel) and liquid oxygen (LOX). Its cycle is similar to the Apollo-era Rocketdyne F-1.
- Thrust: about 845 kilonewtons (190,000 pounds of force) at sea level and about 981 kilonewtons (221,000 pounds) in the vacuum of space, with a thrust-to-weight ratio near 184.
- Throttle range: down to about 40% of maximum thrust.
- Layout: the Falcon 9 first stage uses 9 Merlin engines arranged in an “octaweb” pattern.
- Igniter: TEA-TEB, a chemical pair that bursts into flame the instant it touches oxygen.
What it is and how it works
A liquid rocket engine cannot simply be switched back on like a car. The fuel and oxygen flowing into the chamber have to be set alight reliably, every time. SpaceX does this with TEA-TEB (triethylaluminium and triethylborane), a pyrophoric mixture, meaning it catches fire on its own the moment it meets air or oxygen, with no spark plug or torch needed. Think of it as a chemical match that lights itself.
TEA-TEB gets used up each time it fires, so only a limited subset of the engines carry their own onboard supply (with backup igniter canisters for reliability). Those are the engines available for in-flight restarts. On the ground, the very first ignition is fed by equipment at the launch pad instead.
After the booster separates from the upper stage, grid fins, which are small waffle-shaped steering surfaces, help it flip around and aim back home. The flight computer then commands a sequence of restarts, or “burns.” The boostback burn (typically about three engines) reverses the booster’s course toward the landing site or the droneship at sea. The entry burn (about three engines, high in the atmosphere) slows it through the most violent part of reentry, where heating and aerodynamic stress peak. Finally, the landing burn fires a single engine, throttled down toward 40%, so the booster arrives at the pad at essentially zero speed. The engine constantly trims its thrust and swivels (gimbals) so the booster’s velocity hits zero exactly as it touches down. For reliability, each engine has its own triple-redundant controller, a set of three small computers that vote on every decision, which makes a successful restart far more likely.
Why it matters
In-flight Merlin relight is the core technology behind reusable rockets. Without the ability to restart its engines after launch, a Falcon 9 booster could never slow down for a vertical landing, and SpaceX could not land the same first stage and fly it again dozens of times. Reusing boosters this way is what drove the dramatic drop in launch costs. The challenge is steep: an engine must reignite after being exposed to the vacuum of space, searing reentry heat, and violent shaking as it crosses the sound barrier, then throttle deeply enough to set the booster down gently. Roughly 6 to 10% of the rocket’s propellant is held in reserve just for these recovery burns.
Where it is used and notable examples
- Falcon 9 first stage: the most-flown example, routinely returning to the launch site or a droneship using boostback, entry, and single-engine landing burns.
- Falcon Heavy cores: the side and center boosters all run the same burn phases. One notable failure came when a center core’s relight underperformed (reported TEA-TEB depletion in two of three engines), causing it to miss the droneship, a reminder of the system’s tight margins.
- Grasshopper and F9R Dev test vehicles: early low-altitude hover-and-land flights that first proved Merlin could relight and throttle for a controlled descent.
- Falcon 9 v1.1 ocean tests: “soft landing” attempts over water that showed a booster could reignite and slow itself before landing legs and grid fins were mature.
One clarification: “Merlin Engine Relight System” is a descriptive label rather than an official SpaceX product name. The capability is more precisely the Merlin 1D’s in-flight restart and TEA-TEB ignition system that enables Falcon booster recovery. The exact engine positions carrying onboard TEA-TEB and the precise count used per burn vary by source and Falcon version.
