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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).
Drop-away engines 
The Atlas rocket used three engines, one of which was fixed to the fuel tank, and two of which were mounted on a skirt which dropped away at BECO. This was used as an Intercontinental ballistic missile (ICBM); to launch the manned Project Mercury capsule into orbit; and as the first stage of the Atlas-Agena and Atlas-Centaur launch vehicles.
First-stage boosters 
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Strap-on boosters 
The Titan III, used by the United States Air Force as an unmanned heavy-lift vehicle,was developed from the Titan II launch vehicle by adding a pair of strap-on solid rocket boosters (SRB). It was also planned to be used for the Manned Orbital Laboratory program, which was cancelled in 1969.
Use in aviation 
Rocket boosters used on aircraft are known as Jet-Assisted TakeOff (JATO) rockets.
Various missiles also use solid rocket boosters. Examples are;
- 2K11 (SA-4) which uses SRBs as a first stage, and then a ramjet.
- S-200 (SA-5) which uses SRBs as the first stage, followed by a liquid fuel rocket.
- Surface-launched versions of the turbojet powered Boeing Harpoon use an SRB.