When we want to launch a satellite into space, we install it in a rocket and fire it into the heavens. Why do we use rockets to launch spacecraft? Why don't we use, for example, the jet engines we use to power aircraft instead?
The main reason we can't use a jet engine is it needs air to work. It draws in air as it travels through the atmosphere, and uses this to burn its fuel. Or rather it uses the oxygen in the air to burn its fuel.
Because there is no air in space, jet engines can't work there. Nor can petrol or diesel engines. Like jets, these engines burn their fuel in oxygen they take in from the atmosphere. Also, jet, petrol and diesel engines are nowhere near powerful enough to launch bodies into space against the pull of Earth's gravity.
At present we know of only one kind of engine that can work in space, the rocket. This is because a rocket carries not only fuel, but also the oxygen to burn the fuel. It does not depend on outside air. Therefore it can work in the vacuum of space. And, by selecting the right fuels, rockets can be made powerful enough to beat gravity and launch bodies into space.
The theory behind the rocket engine, or rocket motor as it is usually called, is simple. A fuel and an oxidizer - a substance that provides oxygen - are burned in an open-ended chamber. Burning, or combustion, produces large amounts of hot gases. The gases shoot backwards out of the open end of the chamber.
As the gases shoot out backwards, the motor is thrust forwards. This follows from the principle of reaction: that a force in one direction is balanced by an equal force in the opposite direction. The English genius Isaac Newton first stated this principle as his third law of motion: "to every action there is an equal and opposite reaction." Rockets are often called reaction engines.
The fuel and oxidizer in a rocket are called its propellants, because when they burn they produce the gases to propel the rocket. The propulsive force developed by a rocket is called its thrust.
The simplest kinds of rocket use solid propellants. The original rocket, invented by the Chinese about 900 years ago, used gunpowder as a solid propellant. We still use this in our fireworks rockets today. It is a combination of powdered charcoal, sulphur and saltpetre, or potassium nitrate. Charcoal (a form of carbon) and sulphur make up the fuel, and saltpetre is the oxidizer.
In a firework rocket, the nose contains chemicals to produce coloured stars and streamers. The gunpowder is packed into a cardboard tube, which forms the combustion chamber. At the rear of the tube is a twist of chemically treated 'touch paper', which acts as a fuse. The tube is attached to a long stick, which helps the rocket travel straight.
To set off the rocket, you light the touch paper. This ignites the gunpowder and produces a stream of hot gases. This shoots out of the open end of the tube and propels the rocket into the sky.
The solid-propellant rockets used for space flight have much the same basic design as the firework rocket. They consist of a tube packed with propellant, with a shaped nozzle at the rear, through which the hot gases escape. But instead of the propellants burning from the rear forwards, they burn radially, or outwards from the centre. The propellant charge has a hole through the middle. This forms the space for combustion to take place. The charge is set alight by an igniter. Once ignited, it cannot be put out.
The solid propellants used in space rockets are much more powerful than gunpowder. They typically consist of a mixture of powdered aluminium and ammonium perchlorate (the oxidizer) in a binder of synthetic rubber. This propellant mix is used, for example in the solid rocket boosters fitted to the space shuttle. Mostly, however, space launch vehicles are powered by liquid- propellant rockets. These are easier to control than solid ones. They can be throttled up or down, or shut-off and restarted.
Liquid-propellant rockets are much more complicated than solid ones. They have separate tanks to carry the fuel and oxidizer. These propellants are pumped separately into a combustion chamber, where they mix and are ignited. The hot gases produced escape at high speed through the shaped exhaust nozzle.
Naturally, the combustion chamber and nozzle get very hot as the propellants burn - 3,000 degrees Celsius or more. To prevent the metal walls melting, they are cooled by incoming fuel. This is channelled through the walls of the chamber and nozzle or surrounding pipes before it is fed into the combustion chamber.
This arrangement is called regenerative cooling. It also improves the efficiency of combustion by preheating the fuel.
Liquid rockets contain many subsidiary systems to make them work. For example, The turbopumps that pump the propellants are themselves powered by gas, produced in a gas generator. Gas is also fed into the propellant tanks to keep them pressurized as they empty. Numerous valves are located in the propellant delivery system to control the flow to the combustion chamber, and thus the thrust developed by the rocket.
The commonest liquid propellants used in early space rockets were kerosene as fuel and liquid oxygen as oxidizer. This combination was used in the first stage of the Saturn V and is still used in the vehicles that launch Russia's Soyuz and Progress spacecraft.
A much more powerful combination of liquid propellants is liquid hydrogen fuel and liquid oxygen oxidizer. These are known as cryogenic propellants because they have a very low temperature: liquid hydrogen -253 degrees C, liquid oxygen -183 degrees C. The space shuttle main engines use these propellants. When they burn, they do so with a bluish, nearly transparent flame. The resulting gas is water vapour and is non-polluting. This certainly can't be said of the filthy exhausts from the shuttle's solid boosters!.
