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Pushing on nothing

How Do Rockets Launch?

A rocket throws mass down very fast. The throw pushes the rocket up.

A rocketGas thrown down, hardRocket pushed up
Every push comes with an equal push the other way.

  1. Step 1 of 5

    Rockets do not push against the ground.

    Common guess Flames push off thegroundReally It pushes against itsown exhaust

    It is tempting to think the flames push off the launch pad. They do not — a rocket works just as well in empty space, with nothing to push against.

  2. Step 2 of 5

    Every push has an equal push back.

    Push something awayIt goes thatwayYou go thisway

    Step off a skateboard and the skateboard shoots backwards. You pushed it, it pushed you. Rockets run entirely on this rule.

  3. Step 3 of 5

    Burning fuel makes a torrent of gas.

    1 Fuel burns2 Hot gas expands3 Blasted out thenozzle

    Fuel and oxygen burn in the engine and turn into scorching gas. That gas is squeezed out of a nozzle at several kilometres per second.

  4. Step 4 of 5

    Throwing gas down pushes the rocket up.

    Engine throws gasdown Rocket climbsThrust just has to beat the rocket's weight

    Millions of tonnes of gas leaving downwards every second means an equally hard shove upward on the rocket. That shove is called thrust.

  5. Step 5 of 5

    It carries its own oxygen.

    Payload — the useful bitabout 4%Engines and structureabout 6%Fuel and oxygenabout 90%

    There is no air in space, so a rocket brings tanks of oxygen as well as fuel. That is why rockets are so enormous and mostly hollow — they are flying fuel tanks.

    Most of a rocket's fuel is spent lifting the rest of its own fuel. That is the tyranny of the rocket equation.

The short version

A rocket flies by throwing gas downward — the recoil from that throw is what lifts it.

Try it yourself

Blow up a balloon and let it go. It flies because air rushes out one end. That is a rocket.

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