Booster rocket - Definition and Overview

A booster in space-related applications is usually a solid rocket booster: a solid fuel rocket of which two or more are attached to the main rocket to provide the main thrust in the initial phase of the rocket's flight. They form the first stage, though the second stage also starts operating at the beginning. After the first stage is discarded, the second continues. There are also liquid fuel boosters (see also Ariane 4), these are called liquid boosters.

Spacecraft aren't the only vehicles to utilise rocket boosters. Missiles (being atmosphere-bound rockets) often do also. For example, see 2K11 (SA-4) or S-200 (SA-5).