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A booster rocket (or engine) is either the first stage of a multistage launch vehicle, or else a shorter-burning rocket used in parallel with longer-burning sustainer rockets to augment the space vehicle's takeoff thrust and payload capability. (Boosters used in this way are frequently designated "zero stages".) 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). The rest of the launch vehicle continues flight with its core or upper-stage engines. The booster may be recovered and reused, as was the case of the Space Shuttle.
The SM-65 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.
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, cancelled in 1969. Strap-on boosters are sometimes used to augment the payload or range capability of military jet aircraft.
NASA's Space Shuttle was the first manned vehicle to use solid-fueled boosters as strap-ons. The solid boosters consisted of stacked segments, and were recovered and reused multiple times.
In a new development program initiated in 2011, SpaceX developed reusable first stages of their Falcon 9 rocket. After launching the second stage and the payload, the booster returns to launch site or flies to a drone ship and lands vertically. After landing multiple boosters both on land and on drone ships, a landed stage was first flown again in March 2017: Rocket core B1021 was used to launch both a re-supply mission to the ISS in April 2016 and the satellite SES-10 in March 2017. The program is intended to reduce launch prices significantly.
Rocket boosters used on aircraft are known as jet-assisted take-off (JATO) rockets.
Various missiles also use solid rocket boosters. Examples are: