booster

A booster is a rocket (or rocket engine) used either in the first stage of a multistage launch vehicle or in parallel with longer-burning sustainer rockets to augment the space vehicle's takeoff thrust and payload capability. Boosters are traditionally necessary to launch spacecraft into low Earth orbit (absent a single-stage-to-orbit design), and are especially important for a space vehicle to go beyond Earth orbit. The booster is dropped to fall back to Earth once its fuel is expended, a point known as booster engine cut-off (BECO).
Following booster separation, the rest of the launch vehicle continues flight with its core or upper-stage engines. The booster may be recovered, refurbished and reused, as was the case of the steel casings used for the Space Shuttle Solid Rocket Boosters.

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