A booster rocket (or engine) is either the first stage of a multi-stage launch vehicle, or else a strap-on rocket used to augment the core launch vehicle's takeoff thrust and payload capability. Boosters are generally necessary to launch spacecraft into Earth orbit or beyond. In all cases, the booster is dropped into the ocean once its fuel is expended, a point known as booster engine cut-off (BECO). The rest of the launch vehicle continues flight with its core or upper stage engines. The booster may be recovered and reused, as in the case of the Space Shuttle.

Strap-on boosters are sometimes used to augment the payload or range capability of jet aircraft (usually military).

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