The rocket produces its 'push', or propulsive thrust, by burning substances to produce a jet of hot gases. In rocket terminology, the substances that are burned are called propellants. They include the fuel and a substance called the oxidizer, which provides the oxygen to burn the fuel. The original Chinese rocket and the typical firework rocket burn a solid propellant - gunpowder. This is a mixture of sulphur, powdered charcoal and saltpetre (potassium nitrate). The sulphur and carbon act as fuel, while the saltpetre acts as oxidizer. Space rockets can also use solid propellants - the boosters for the space shuttle, for example, are solids. Solid- propellant space rockets do not use gunpowder, but a much more powerful mixture, consisting in the shuttle solid rocket boosters (SRBs) of powdered aluminium (fuel) and ammonium perchlorate (oxidizer) in a synthetic-rubber binder. The solid rocket engine is essentially very simple - a tube filled with ring-shaped propellant with a nozzle at the end. The most powerful space rockets, however, use liquid propellants. A combination of kerosene (fuel) and liquid oxygen (oxidizer) was used in many early rockets, and is still used in US Delta rockets and the Soviet A-2 Soyuz launch vehicles, for example. A more potent combination is liquid hydrogen and liquid oxygen, used in the main engines of the space shuttle, for example. Liquid hydrogen and liquid oxygen are known as cryogenic propellants because they have a very low temperature (-253 degrees C and -183 degrees C, respectively). Other rockets use a form of hydrazine (such as UDMH, unsymmetrical dimethyl hydrazine) as fuel, and nitrogen tetroxide as oxidizer. The European Ariane uses these propellants for its first two stages, for example, as does the space shuttle for its OMS (orbital manoeuvring system) and RCS (reaction control system) engines. These propellants are termed hypergolic, which means they ignite spontaneously when they are mixed together. In other rockets the propellants have to be ignited before they burn.
