boostphase

Boost phase is the powered portion of a rocket's ascent, from ignition at liftoff to the burnout of the active propulsion stage. In orbital launch vehicles, the boost phase accelerates the vehicle from standstill to the velocity needed for orbital insertion, and may include one or more stage burns, along with stage separations and engine restarts.

During this phase the vehicle experiences high thrust-to-weight ratio and rapid acceleration as it climbs through denser atmosphere. The flight path is guided by a combination of thrust-vector control and inertial navigation, typically following a gravity-turn trajectory to minimize aerodynamic loads and fuel use. The engines are mounted on the first stage (and any boosters) and, in multi-stage designs, the boost phase ends with stage burnout and separation, followed by ignition of the next stage.

Telemetry, tracking, and range safety are active throughout the boost phase. Vehicle health monitoring, engine performance data, and structural loads are continuously assessed to detect anomalies. The phase typically includes the period of maximum dynamic pressure (Max-Q) and major attitude changes as the vehicle transitions from vertical ascent to a more horizontal flight path.

The term is used across spaceflight and, in broader terms, can apply to any mission where propulsion dominates the early flight. It is a critical window for achieving the planned orbit, and failures or delays during boost phase can jeopardize the entire mission. Examples include traditional expendable rockets and modern partially reusable launch systems.