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The water rocket

Rockets and landing · level 2, lesson 5 of 6

A question to start

A plastic bottle, some water and a bicycle pump can make a rocket that flies higher than a house. How does it work, and why do you need the water?

The idea

A water rocket is a plastic fizzy-drink bottle, partly filled with water, turned upside down on a launcher. You pump air into it. The air is squashed tighter and tighter, pressing hard on the water. When you let it go, the squashed air shoots the water out of the neck, very fast, and the bottle flies up.

(Build it only with a grown-up and a proper launcher, outside, away from people. Use only bottles made for fizzy drinks, which are strong, and never pump them up too far.)

squashed airwaterwater rushes out

Why water? Try it with no water, only air: the bottle hardly rises. Air is very light, so throwing it out gives only a small push. Water is about 800 times heavier than the same amount of air. Throwing out heavy water gives a much bigger push.

How much water? Too little, and there is not much to push out. Too much, and there is little room for squashed air, and the rocket is heavy. People who fly water rockets find that about a third full works best.

Fins at the bottom keep it flying straight, like the feathers on an arrow. A nose cone on top helps it slip through the air.

Worked examples

1. A test. Fly the same bottle with a quarter, a third and a half full of water, pumped to the same pressure. Measure how high each goes (or time how long it stays up). The third-full one usually wins.

2. A real rocket is like this. A real rocket throws out hot gas instead of water, much faster: thousands of metres per second instead of tens.

Try it

  1. What pushes the water out of the bottle?
  2. Why does a water rocket go higher with water than with air alone?
  3. What happens if you fill the bottle almost to the top?
  4. What do the fins do?
Answers
  1. The squashed air inside.
  2. Water is much heavier than air, so pushing it out pushes the bottle harder.
  3. There is little room for squashed air, and the rocket is heavy, so it does not go high.
  4. They keep the rocket flying straight.

Common mistake

"More water always means a bigger push." The water is pushed out by the squashed air. With too much water there is too little air to push it, and the extra water is just more weight to lift.

Where it leads