SECO

In rocket propulsion, SECO is the moment when the sustainer engines (the main engines burning throughout most of the flight) cut off. This is a planned, discrete event, not a failure. For expendable launch vehicles, SECO occurs after the engines have burned long enough to accelerate the payload to near-orbital or orbital velocity, at which point continued thrust becomes counterproductive or wasteful. The timing of SECO is critical: too early and the rocket lacks sufficient velocity; too late and fuel is wasted or the vehicle overshoots its target orbit.

Different vehicle architectures reach SECO under different conditions. On a single-stage-to-orbit vehicle (rarely achieved), SECO occurs when the entire propellant load is consumed. On multi-stage rockets, the first stage sustainer engines cut off when that stage's fuel is depleted, then staging occurs and the second stage ignites. On the Space Shuttle, the Main Engines cut off at a precise calculated point during ascent, after which the External Tank separates. The flight computer monitors engine performance, propellant consumption, and velocity in real time, commanding SECO when conditions are met.

SECO is not instantaneous across all engines. Multiple engines may shut down within a fraction of a second, but not simultaneously. Engine shutdown sequence and precision matter because uneven cutoff creates transient forces and can affect the vehicle's trajectory and attitude at a critical moment in the flight profile. Ground control and onboard instrumentation confirm SECO through sensor readouts, pressure drops in fuel lines, and changes in acceleration profile.

The acronym reflects older rocket terminology in which the main engines were called the sustainer engines (sustained thrust over the bulk of the flight) as opposed to boosters (large solid or liquid engines that provide initial thrust). Although booster terminology has largely fallen out of favor in modern spaceflight, the term sustainer engine persists in some contexts, keeping SECO in use among flight dynamics engineers and mission planners despite the simpler modern term would be main engine cutoff.