Spacecraft are launched into space by rockets—the only engines powerful enough to overcome Earth’s gravity and lift objects into space. Most spacecraft use launch vehicles with several linked rocket units, or stages.
Rockets burn fuel mixtures called PROPELLANTS. The burning fuel creates a stream of hot gases that shoots out of the rocket’s exhaust nozzle. The backward force of the gas jet gives the rocket a forward force called thrust. The rocket’s forward thrust propels the spacecraft into space.
Rocket propellants contain fuel and oxidant. The fuel needs oxygen to burn, and the oxidant provides the oxygen. Ordinary engines can take their oxygen from Earth’s atmosphere, but in airless space, a rocket has to carry its own oxygen supply.
Most rockets burn liquid propellants, and some burn solid propellants. The Space Shuttle main engines burn liquid hydrogen and liquid oxygen. Its booster rockets burn solid propellants. When a shuttle lifts off, almost 90 percent of its weight is propellant.