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No air needed

Rockets and landing · level 2, lesson 3 of 6

A question to start

An aeroplane's propeller pushes on the air. A boat's propeller pushes on the water. A car's wheels push on the road. In space there is no air, no water and no road. So what does a rocket push on?

The idea

A rocket does not push on anything outside it. It pushes on its own exhaust: the hot gas it makes and throws out of its engine. The gas pushes back on the rocket. That is all a rocket needs.

In fact, rockets work better in space than in air. In the air, the air around the engine gets in the way of the gas coming out. In space there is nothing in the way, so the gas rushes out freely and the push is a little stronger.

pushes airpushes waterpushes its own gas

People did not always believe this. In 1920 a New York newspaper said that the rocket pioneer Robert Goddard did not seem to understand that a rocket would need something to push against in space. The newspaper was wrong. In 1969, as Apollo 11 flew to the Moon, it printed a correction.

Worked examples

1. Turning in space. Spacecraft turn by firing small thrusters, puffing gas out sideways. The gas goes one way and the craft turns the other.

2. An astronaut on a spacewalk. An astronaut floating in space who throws a tool away drifts slowly the other way. (That is why astronauts are always tied on.)

Try it

  1. What does a car push on to move? A boat? An aeroplane?
  2. What does a rocket push on?
  3. Why do rockets work a little better in space than in air?
  4. An astronaut floating in space throws a spanner forwards. What happens to the astronaut?
Answers
  1. The road; the water; the air.
  2. Its own exhaust gas.
  3. There is no air in the way of the gas coming out of the engine.
  4. They drift slowly backwards.

Common mistake

"The flames push on the air behind the rocket." The push comes from inside the engine, where the hot gas presses on the engine and is thrown out. The air behind does not help.

Where it leads