Try this with a balloon
Blow up a balloon and let it go without tying it. It zooms around the room! The air rushes out of the back, and the balloon shoots forward. That is exactly how a rocket works — just with a lot more push.
Pushing and being pushed
A long time ago, a scientist called Isaac Newton noticed a rule that is always true: every push has an equal push back the other way. When you jump off a skateboard, the skateboard rolls backwards. When a rocket throws hot gas out of its bottom really, really fast, the gas pushes the rocket up.
The rocket does not push against the air. That is a common mix-up! Rockets work best in space, where there is no air at all, because there is nothing in the way.
Why rockets are so big
A rocket has to carry everything it will ever throw out of the back — all its fuel — from the very start. Most of a rocket is a giant fuel tank. When the Saturn V rocket took astronauts to the Moon, it was as tall as a 36-storey building, and almost all of it was fuel. The bit with the astronauts in was about the size of a small car.
Stages: throwing away the empty bits
Carrying empty tanks is a waste, so rockets are built in stages. When the first stage runs out of fuel, it falls away, and the next stage lights up. The rocket gets lighter and lighter and goes faster and faster.
Some rockets today, like SpaceX's Falcon 9, fly their first stage back and land it on its legs so it can be used again. That was science fiction when your parents were kids.
How fast?
To get into orbit (the next lesson explains what that is), a rocket needs to reach about 28,000 kilometres an hour — around 25 times faster than a jet airliner. It gets there in less than ten minutes. The astronauts feel about three times their normal weight pressing them into their seats.
Words to remember
- Thrust — the push a rocket engine makes.
- Stage — one section of a rocket with its own engines and fuel.
- Payload — the useful thing the rocket is carrying: a satellite, a probe, or people.