How Rockets Work

The challenge of leaving the ground

Getting into space is one of the hardest things humans have ever done. Earth's gravity constantly pulls everything down, and to break free of it — or even to circle the planet — you need to move incredibly fast and carry a vast amount of energy. The machine we invented to do this is the rocket. Yet the science behind how a rocket works is surprisingly simple, and you can demonstrate the key idea with a balloon.

Understanding rockets means understanding one of the most important rules in all of physics: a rule about pushing.

Newton's third law: the heart of every rocket

In the 1680s, Isaac Newton described three laws of motion. The third law is the secret of the rocket:

For every action, there is an equal and opposite reaction.

This means that whenever one object pushes on another, the second object pushes back just as hard, in the opposite direction. When you jump, you push down on the ground, and the ground pushes you up. When you step off a small boat, the boat slides backwards as you step forward.

A rocket does exactly this with gas. It burns fuel to make a huge amount of hot gas, then forces that gas out of a nozzle at the back at tremendous speed. The rocket pushes the gas backwards (the action), so the gas pushes the rocket forwards (the reaction). This forward push is called thrust. The faster and the more gas the rocket throws out the back each second, the greater the thrust.

Why rockets work in empty space

A very common belief is that rockets work by pushing against the air. This is wrong — and the truth is one of the most beautiful things about rockets.

A rocket does not push against the air outside. It pushes against its own exhaust. The action–reaction pair is entirely between the rocket and the gas it ejects. That means a rocket works even better in the vacuum of space, where there is no air to get in the way and slow it down. The early rocket pioneer Robert Goddard was once mocked by a newspaper for suggesting rockets could work in space; decades later the same paper printed an apology after astronauts reached the Moon.

Why rockets must carry oxygen

To make thrust, a rocket has to burn fuel, and burning needs oxygen. On Earth, a fire or a car engine takes oxygen from the air around it. But there is no air in space — and even high in the atmosphere there is very little.

So a rocket carries its own oxygen supply, called an oxidiser, alongside its fuel. For example, many rockets burn liquid hydrogen or kerosene as fuel and carry tanks of liquid oxygen to burn it with. This is the big difference between a rocket and a jet plane: a jet breathes air, so it can only fly in the atmosphere, while a rocket brings everything it needs and can fly through the emptiness of space.

Stages: dropping dead weight on the way up

Fuel is heavy, and as a rocket burns it, the empty tanks become useless dead weight that the engines must keep dragging upward. The clever solution is to build the rocket in stages — separate sections stacked on top of each other.

The bottom stage fires first, doing the hardest work of lifting the whole rocket off the pad. Once its fuel is used up, the entire empty stage is dropped away, so the rocket suddenly becomes much lighter. Then the next stage ignites and pushes the now-lighter rocket even faster. By shedding mass step by step, a staged rocket can reach far higher speeds than a single rocket of the same size ever could.

Going fast enough to stay in space

Reaching space — usually defined as about 100 km up — is only half the problem. To stay there in orbit, you have to go astonishingly fast sideways.

An object in orbit is really falling around the Earth. Gravity is constantly pulling it down, but it is moving sideways so quickly that the ground curves away beneath it just as fast as it falls. The result is that it keeps circling instead of coming down. To pull off this trick in low orbit, a spacecraft must travel at about 28,000 kilometres per hour — roughly ten times faster than a rifle bullet.

To leave Earth's gravity altogether and travel to the Moon or beyond, a rocket must go even faster, reaching the escape velocity of about 40,000 km/h. These enormous speeds are why rockets need such powerful engines and so much fuel.

Try it yourself: build a balloon rocket

You can demonstrate Newton's third law — the exact principle behind every space rocket — with simple materials.

You will need: a long piece of string, a drinking straw, a balloon, sticky tape, and two chairs or door handles.

1. Thread the string through the straw, then tie or hold each end of the string to two chairs across the room, pulling it tight and level.
2. Blow up the balloon but do not tie it — pinch the neck closed with your fingers.
3. Tape the balloon to the straw while still pinching it shut, so the balloon's open neck points to one end.
4. Let go. The air rushes out of the back of the balloon, and the balloon zooms forward along the string.

The air shooting out one way (the action) pushes the balloon the other way (the reaction) — exactly how a rocket's exhaust pushes the rocket forward. Try changing how much you inflate the balloon, or angle the string upward, and see how the "rocket" performs. You have just done real rocket science.

Want to learn more? See how people live once they reach orbit in Living in Space, and discover the worlds rockets carry us toward in Planets of the Solar System.