We launch satellites into orbit and space probes to other planets by means of rockets. The rocket is the only kind of engine that can work in airless space. The problem is that no single rocket can be built that is powerful enough to reach the necessary speed (at least 28,000 km an hour).
To make a rocket more powerful, we must make it burn more fuel. But the more fuel it has to carry, the bigger and heavier it has to be and the more deadweight it has to carry in its structure. So a mammoth rocket might develop tremendous power on the launch pad, but it could never make it into space.
The 'father of astronautics', the Russian schoolmaster Konstantin Tsiolkovsky, appreciated this power-to-weight problem at the turn of this century, long before the days of space flight. He also saw the solution. Instead of using one huge rocket to launch a body into space, we use a number of smaller rockets joined together. Each rocket fires in turn and then falls away, progressively making the rocket lighter and lighter.
This principle is known as the step rocket. Each separate rocket is known as a stage: first stage, second stage, and so on. The whole rocket combination is known as a launch vehicle.
In most space launch vehicles the rocket stages are linked together end to end. This arrangement is known as tandem staging. Three stages are usual, with the third stage carrying the payload, or cargo.
On the launch pad, the lower, first stage fires first, carrying the stages above it piggy-back into the air.
When the first stage runs out of fuel, it separates and falls away. The second stage then fires, thrusting the now smaller and lighter rocket faster and faster.
In turn this second stage runs out of fuel, separates and falls away. The third stage fires until what is left of the launch vehicle is travelling fast enough to go into orbit.
In orbit the payload separates from the third stage and becomes an independent satellite.
Some launch vehicles carry additional rockets, which provide extra power at lift-off. They are attached to the first stage of the main, or core, vehicle and are called strap-ons or boosters.
Ariane 3 and 4 launch vehicles use strap-on boosters. So does that veteran workhorse, the Delta launch vehicle. This has no less than nine solid strap-ons. Six fire on lift-off, three later. The main core vehicle of Delta uses just two stages.
This sequence shows a Delta launch. The satellite depicted is EUVE, NASA's extreme ultraviolet explorer satellite. It is housed during launch inside a fairing, which gives it protection during its passage through the atmosphere.
After insertion into orbit, the fairing is jettisoned. Then the satellite's solar panels unfold and start feeding electricity to the on-board instruments. The third stage remains in orbit, part of the useless junk now accumulating in space.
Historically, the most famous launch vehicle was the Saturn V Moon rocket. It was the brainchild of the legendary Wernher von Braun, who developed the ancestor of all modern space rockets, the V-2.
The Saturn V stood 111 metres high on the launch pad and tipped the scales at some 3000 tonnes. This colossal vehicle was needed to send three-man crews to the Moon on the Apollo Moon-landing missions of the 1960s and 70s. One hundred and eleven metres of hardware left the launch pad. All that returned was a capsule a few metres high.
Ordinary launchers like Ariane and Delta can be used only once. They are called expendable launch vehicles, or ELVs. The American space shuttle is a different type entirely, because it is re- usable. It is essentially a single-stage vehicle with twin strap- on solid rocket boosters.
The boosters fire for two minutes before separating. They parachute back to Earth and are recovered to be used again.
The winged orbiter, carrying crew and payload, forms the main stage. Its engines fire all the way into orbit. It takes fuel from a separate external tank, which is jettisoned when empty. The tank is the only part of the shuttle that is not used again.
