How a rocket works

Blow up a balloon and let it go. Air rushes out of the neck one way, and the balloon shoots off the other way. A rocket does the same thing with far more force. It burns fuel to make very hot gas, and throws that gas out of its back as fast as it can. Throwing the gas backward pushes the rocket forward. That is the whole idea. It is one of Isaac Newton’s laws of motion: every push comes with an equal push back.

A rocket does not push against the ground or against the air. It pushes against its own exhaust, so it works in empty space, where there is nothing else to push on. Fire needs oxygen, and space has none, so a rocket carries its own oxygen as well as its fuel. The first rocket to burn liquid fuel this way was built by an American teacher called Robert Goddard. This is a real photograph of him with it on 16 March 1926. It flew for two and a half seconds and came down in a cabbage patch.

The hard part is speed. To stay in orbit round the Earth a spacecraft must travel at about 28,000 km an hour, which is 8 km every second. Reaching that speed takes an enormous amount of fuel, and the fuel has to lift itself as well. The Saturn V, the rocket that sent people to the Moon, weighed more than 2,800 tonnes on the launch pad. About nine tenths of that was fuel and the oxygen to burn it. The part that went on to the Moon was about one and a half per cent.

An empty tank is dead weight. So a big rocket is really several rockets stacked one on top of another, called stages. The bottom stage fires first. When its tanks are empty it drops away, and the next one lights, pushing a much lighter rocket than before. The Saturn V had three stages. The first burned for about two and a half minutes, lifted the rocket 68 km, and then fell into the ocean.

This is the Saturn V leaving for the Moon on 16 July 1969 with three men on top. It stood 111 metres high, about as tall as a 36-storey building. In its first stage, five engines burned nearly 14 tonnes of fuel and oxygen every second. Together they pushed upward with a force about a fifth greater than the weight of the whole rocket. That small margin is why it seemed to rise so slowly at first.

For most of the space age, the big first stage of a rocket was thrown away after one flight. That has changed. This is the first stage of a Falcon 9 rocket on 1 October 2026, landing upright on its own flame after sending four people on their way to the International Space Station. The first landing like this was on 21 December 2015. By November 2026 there had been more than 650, and one booster had flown 37 times.