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MECO: what main engine cutoff means in a rocket launch

MECO stands for main engine cutoff. It is the moment a launch vehicle's main engines are commanded to shut down. When a launch commentator calls "MECO", the main engines have finished their burn, and the next events come quickly: a separation, then the next engine lights.

This page covers where MECO falls in an ascent, why an engine is cut off when it is, how a shutdown differs from a throttle-down, and what happened on flights where it did not go to plan. Every fact about a real flight has a NASA source, listed at the end of the page.

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What MECO means

A rocket engine does not simply burn until its tanks are empty. The vehicle's computers decide when it stops, and they send the command. MECO is that command for the main engines, and the instant the thrust drops to zero.

Which engines count as "main" depends on the vehicle:

  • Falcon 9. MECO is the shutdown of the first stage engines, a little over two minutes after liftoff. The second stage's engine shutdown is a separate event later in the flight.
  • Space Shuttle. The solid rocket boosters burned beside the main engines for the first two minutes and then separated. The three main engines kept firing, and MECO came at the end of the climb to orbit, about eight and a half minutes after liftoff.
  • SLS. The RS-25 engines on the core stage are the main engines. MECO is when they shut down at the end of the core stage burn.

So MECO is not one fixed point in a launch. It is the end of the burn by whatever engines the vehicle calls its main engines.

Where MECO falls in an ascent

On a two-stage rocket like Falcon 9, the order of events after liftoff is:

  1. MECO. The first stage engines shut down. NASA's timeline for the Sentinel-6B launch puts this a little more than two minutes after liftoff.
  2. Stage separation. Shortly after MECO, the first and second stages come apart.
  3. Second-stage ignition. The second stage engine starts. On Falcon 9 the first stage then begins its own boost-back burn toward a landing.
  4. SECO. The second stage engine cuts off. Launch commentary calls this SECO, for second engine cutoff. On the Sentinel-6B flight, the first second-stage cutoff was planned for about eight minutes after liftoff, with the vehicle in a temporary parking orbit.

A second stage can restart, so a flight can have more than one SECO. On the same Sentinel-6B timeline, the second stage engine fires again about 44 minutes after its first cutoff, before the satellite separates.

The order is the same on a three-stage rocket, with one more cutoff and separation in the middle. On Apollo 13, the Saturn V's first stage shut off two minutes and 40 seconds after liftoff, at an altitude of about 42 miles. The empty stage was jettisoned, and a few seconds later the second stage's five J-2 engines ignited.

Why an engine is cut off when it is

Cutoff on a guidance target

The usual reason for MECO is that the vehicle has done what that burn was for. The guidance system flies toward a target, usually a velocity and a position, and it commands the cutoff when the vehicle gets there. The cutoff time on the countdown clock is a prediction, not the rule.

NASA's first assessment of the Artemis II launch describes exactly this. At main engine cutoff, when the core stage's RS-25 engines shut down, Orion was travelling at over 18,000 miles per hour. It had reached its insertion velocity for orbit, in the place it was meant to be.

Apollo 13 shows the same logic under stress. The second stage's center engine shut down more than two minutes early. To make up the lost thrust, the other four engines burned for an extra 34 seconds, and the third stage burned an extra nine seconds. The vehicle reached a 114 by 115 mile orbit, very close to the plan. The burns did not stop on the clock. They stopped when the vehicle reached the target.

Longer burns cannot always close the gap. On Apollo 6, two of the five second-stage engines shut down early. The three remaining engines burned longer, and so did the third stage, but the vehicle reached a 108 by 222 mile orbit instead of the planned 115 mile circular orbit.

Cutoff on propellant depletion

The second reason is that a stage is running out of propellant. A stage carries more propellant than its nominal burn needs, so a stage that performs a little below prediction can still finish. On Apollo 13, the third stage burned nine seconds longer than planned and still had enough propellant left for the burn that sent the crew toward the Moon.

When the margin is not enough, propellant ends the burn before guidance does. On STS-93, the Space Shuttle flight that launched the Chandra X-ray Observatory, a pin knocked loose during ignition punched holes in hydrogen cooling tubes in one engine's nozzle. The engine's controller compensated by increasing the flow of oxidizer. Because the engines used more oxidizer than expected, main engine cutoff came early, and Columbia reached a lower orbit than planned. The orbiter's own maneuvering engines made up the difference.

Shutdown versus throttle-down

A throttle-down reduces thrust while the engine keeps burning. A shutdown stops the engine completely. MECO is a shutdown.

The Space Shuttle's main engines show the difference. They could be throttled between 65 and 109 percent. Late in the climb, the engines were throttled back to limit the acceleration to 3 g. Throttling down changes how hard the engines push. MECO ends the burn. After MECO, thrust is zero until an engine is lit again. Some engines can be restarted, like the Falcon 9 first stage for its boost-back burn and the second stage for its second burn.

An engine can also be shut down to protect the vehicle. On Apollo 13, excessive lengthwise oscillations, the so-called pogo effect, caused the engine controller to shut down the second stage's center engine. That cutoff was not in the plan, but the controller still ordered it.

Further reading

The rocket propulsion primer covers the control system that throttles, shuts down and restarts an engine. Specific impulse explains how much thrust an engine gets from each kilogram of propellant a stage carries.

Sources

  1. NASA, NASA, European Partners Set to Launch Sentinel-6B Earth Satellite: the Falcon 9 launch timeline, from main engine cutoff and stage separation to the second stage's two burns.
  2. NASA, The Space Shuttle: booster burn and separation, the eight-and-a-half-minute climb, external tank jettison, and the main engine throttle range and 3 g limit.
  3. NASA, NASA on Track for Future Missions with Initial Artemis II Assessments: SLS core stage main engine cutoff at insertion velocity.
  4. NASA, NASA's Artemis II Moon Mission Daily Agenda: separation of Orion and the upper stage after main engine cutoff.
  5. NASA, 50 Years Ago: Apollo 13 Off to the Moon: first stage cutoff time and altitude, the early center engine shutdown, and the longer burns that made up for it.
  6. NASA, 55 Years Ago: The Flight of Apollo 6: two early second-stage engine shutdowns and the orbit that resulted.
  7. NASA, 25 Years Ago: STS-93, Launch of the Chandra X-Ray Observatory: the hydrogen leak, the higher oxidizer use and the early main engine cutoff.

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