No matter what propellants rocket designers use, they cannot design a single rocket powerful enough to launch bodies into space. The body of the rocket and its payload (fuel and spacecraft) are always too heavy for the power it can produce. So what rocket designers do is join several rockets together, end to end. This combination is called a step-rocket or a multi-stage rocket. Each separate rocket is called a stage. In a three-stage step rocket the first-stage rocket fires first to give the others a boost. When it has used up its fuel, it falls away. Then the second stage fires, providing a further boost in speed to the now lighter vehicle. In turn it falls away, and the third stage fires. With no dead weight to carry, the third stage boosts its payload (cargo) to such a speed that it can enter orbit around the Earth or escape Earth's gravity completely and travel into the depths of the Solar System. A three-stage rocket combination is the most common, used in the US Delta and European Ariane launch vehicles, for example. Both these vehicles also feature strap-on booster rockets, which provide additional thrust for lift-off. The space shuttle uses a single main stage (orbiter's engines) with two strap-on boosters.
