A RETRO rocket is a type of rocket that uses a single stage and is propelled by the exhaust gases from its engine. A BOOSTER rocket is a type of rocket that provides an initial thrust to launch a spacecraft into space. A two-stage rocket is a type of rocket that uses two or more stages, each of which contains its own engine and propellant. The first stage provides the initial thrust to launch the spacecraft, while the second stage provides additional thrust to reach higher altitudes.