MECO

Main Engine Cut-Off, or MECO, is the point during a rocket or spacecraft ascent when the primary engines shut down after their fuel or oxidizer is depleted. For orbital launch vehicles, MECO marks the end of powered flight and the transition to coast, separation, or orbital insertion sequences. It is a discrete event, not a gradual process: engines either run or they do not, and MECO is the instant they do not.

During ascent, flight controllers and onboard systems monitor engine parameters continuously. Fuel depletion is the normal trigger for MECO, but engines may cut off early if sensors detect abnormal pressure, temperature, thrust imbalance, or other failures. On multi-stage vehicles, each stage has its own MECO. The Space Shuttle Main Engines, for example, shut down when the external tank was nearly empty, typically around 8.5 minutes after launch. Solid rocket boosters on the Shuttle had a predetermined burn time and simply burned out; their conclusion was not called MECO in the same operational sense.

MECO in procedure and planning

Flight plans are built around MECO events. Engineers calculate the target velocity, altitude, and trajectory needed at MECO to achieve the desired orbit or flight path. Deviations from planned MECO conditions trigger abort procedures or orbital makeup burns. On crewed vehicles, MECO is a critical milestone communicated explicitly: "MECO" is a call-out in the flight log and mission transcript. Ground teams and crew listen for engine shutdown signatures in vibration, acceleration, and telemetry data.

Predictability and margin are central to MECO operations. Vehicles carry reserve propellant to extend engine burn beyond nominal MECO if needed, and guidance systems are designed to command shutdown at the right moment. Early MECO due to sensor error or component failure requires crew response or automatic abort logic. Late MECO results in excess velocity and requires corrective action downrange.

The term originated in spaceflight and remains specific to rocket and spacecraft propulsion. It has no direct equivalent in aircraft operations, where engines throttle and glide rather than cut off sharply. In the aerospace industry, MECO is a term of precision: it identifies a measurable event, not a phase or condition, and its accuracy directly affects mission success and safety.