A view of a rocket from beneath, showing the nozzles Although space starts just 100 kilometres (about 60 miles) above the Earth’s surface, it is very difficult to get there. Aeroplanes cannot reach space because the air gets thinner and thinner with altitude. Their wings begin to lose lift and their jet engines stop working through lack of oxygen. So spacecraft need rocket engines which work in space where there is no air: a vacuum. In a rocket engine, two different fuels mix and react together inside a combustion chamber. Hot gases are created, and they rush out of a nozzle at high speed. The engine, and the spacecraft, are pushed in the opposite direction.
To travel out of Earth's gravity (as when travelling to the Moon or other planets), a vehicle must overcome its pull. To do this, it needs to reach a speed of at least 40,000 km/h (25,000 mph)—10 times the speed of a rifle bullet. This is called the escape velocity. Aeroplanes cannot achieve these speeds because their jet engines need oxygen to work, and there is no air in space. Only rockets can provide the necessary thrust. Once in space, the craft’s engines can be turned off. It maintains its speed because there is no air to slow it down.
It is not practical for most rockets to achieve those speeds. Normally, a spacecraft is first launched into what is called Low Earth Orbit (160–2000 kilometres or 100–1250 miles above the Earth). It is then accelerated to the escape velocity from that altitude. The thrust needed is far less than at the Earth's surface.
Space Shuttle Main Engine (SSME)
Saturn V was 50 times as powerful as a Boeing 747 jumbo jet.
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