burn

In spacecraft operations, a burn is a controlled firing of one or more thrusters or main engines to alter the vehicle's velocity and trajectory. The term applies equally to a half-second pulse from a reaction control system thruster and a multi-minute main engine ignition; what matters is that the spacecraft's velocity vector changes as a result of expelled propellant. The duration and thrust profile determine the magnitude of the velocity change, measured in meters per second and called delta-v.

Burns serve distinct mission phases. An orbital insertion burn slows a spacecraft enough for it to be captured by a planet's gravity well instead of flying past. A circularization burn follows apogee, raising perigee to create a stable circular orbit. Mid-course correction burns fine-tune trajectory during coast phases, and deorbit burns lower altitude until atmospheric drag becomes significant. Lunar or planetary landing requires a final powered descent burn, while rendezvous and docking maneuvers use small precision burns from the RCS thrusters.

Precision and planning

Every burn must be calculated in advance. Propellant consumption is tracked; the spacecraft has only so much delta-v budget for the entire mission. Burn timing is critical: a main engine burn at the wrong orbital position wastes propellant for the same velocity change. Ground controllers model atmospheric drag, gravitational perturbations, and thruster performance degradation to predict actual results. Spacecraft execute burns using attitude control systems to point the engine(s) in the correct direction, then ignite on schedule. Accelerometers and optical navigation updates help confirm the burn achieved its intended delta-v.

Failures in burn execution are permanent and often mission-ending. Engine ignition can fail due to propellant contamination, blockages in fuel lines, or ignition system faults. A thruster that fires at lower than expected pressure produces insufficient delta-v. Unplanned thruster firing from a sticking valve or electrical fault may consume precious propellant. Attitude errors during a main engine burn cause the trajectory change to miss its target, necessitating unplanned correction burns. For this reason, burn sequences are redundantly planned, with backup engines and contingency propellant reserves.

The term 'burn' originates from the visible effect of high-temperature combustion in rocket engines; early spaceflight operations used it colloquially before it became standard jargon. In aviation maintenance, awareness of burn procedures is indirect but important: ground technicians verify engine health before flight, check propellant system integrity, and validate thruster firing circuits. Any degradation discovered during preflight inspection directly affects the delta-v margin available for the mission and constrains the burn profile that will be safe to execute.