Water Powered Rocket

What you need

  • An empty two litre plastic bottle
  • A foot pump or bicycle pump with long tube and preferably an inflation needle
  • A rubber cork that fits reasonably tightly in the neck of the bottle
  • A strong cardboard box or tripod to act as a launch pad
  • Some water
  • Paper and paint to make a nose cone and fins to decorate your rocket

What you do

  1. Push the inflation needle through the centre of the rubber cork, or make a hole in the cork big enough to take the tubing from the pump. Washing up liquid might help when pushing the tubing through.
  2. Prepare the cardboard box launcher by cutting a hole in the top and in the side of the box while retaining its structure. The bottle will go upside down in the top with the pump tube coming out the side.
  3. Fill the bottle a third full of water and, while the bottle is upright and open, place the cardboard box (or tripod) upside down on top. Put the cork in quite firmly and turn the entire structure so that the bottle is upside down and balanced on the box (or tripod). The rubber tube connected to the foot pump should come out the side of the box.
  4. Start pumping and launch your rocket
  5. You can now retrieve and decorate your rocket, then relaunch it!

What's going on?

By pumping air into the bottle you are increasing the pressure inside. This pressure forces the water down which in turn forces the rubber cork out of the bottle.

This force, pushing the cork, water and air out the bottom of the bottle, is the action, and it is coupled to a reaction, the bottle flying into the air. This was what Newton was referring to in his third law where he stated that for every action there is an equal and opposite reaction.

To find out why water is best to use, you can look at conservation of momentum for the entire rocket. The whole system of water and rocket has zero momentum before launch and zero momentum after launch. The heavier the water and the faster it exits the bottle, the more downward momentum it will carry. To balance this, the lighter the bottle the faster it will go, and the more spectacular the launch.

Special Safety advice

This will be a fairly fast moving projectile, so make sure you stand back while launching it. As with all projectiles, don't use anything sharp or breakable attached to the plastic bottle.