The air-powered water rocket

This is fabulous home made rocket using a plastic drink bottle

Pump air in with a bike pump and increase the air pressure inside the bottle. Finally, the pressure is high enough for the rocket to take off, firing water out of it’s mouth and streaking up into the sky. This rocket can go 10-20 meters off the ground. It’s serious.

BEWARE: This rocket is quite powerful. Make sure you have an adult around to supervise.

You need:

  • A plastic (PET) bottle (empty soft drink bottle – any size)
  • A bike pump
  • A needle to pump up a basketball OR a bike valve (cut it from an old bike tyre).
  • A rubber stopper (that fits the mouth of the bottle)
  • Tie wire or coathanger wire (OR thick cardboard)
  • Straw
  • Strong sticky tape
  • Pliers, Wire cutter
  • Water

There are two types of launch pads you can make:

Click to enlarge

Launch Pad Type 2

Click to enlarge

Launch Pad Type 1

What to do

Make your rocket launch pad:

This is the trickiest part. You need to make something sturdy enough to keep the rocket standing up straight, about 5 – 10cm off the ground. The fins or the stand must hold the rocket high enough off the ground for the pump to attach underneath.

Type 1: Use cardboard cutouts of fins, attached to the bottle with sticky tape. (The fins will travel up with the rocket when it launches.)
Type 2: fold a wire into a stand. OR make a stand out of an upside down icecream bucket with a hole in the top for the bottle. (These stands stay on the ground while the bottle flies up.)

Click to enlarge

Click to enlarge

a) Stopper with bike pump valve b) Stopper with ball pump needle

To set your rocket up

  1. Fill the bottle about one third full of water.
  2. Push the rubber stopper into the mouth of the bottle firmly.
  3. There are two ways to attach the pump:
  4. a. Drill a hole through the stopper, and glue a bike valve over the hole. OR b. Poke the bike pump needle through the rubber stopper. It must be a tight fit. Place your bottle-rocket the right way up on its stand (with the bottle upside down)!

To launch your rocket

  1. Pump air into the bottle.
  2. Keep pumping. Sooner or later the rocket will take off! (You get no warning. Don’t EVER lean over the rocket!).

Why it works

Air fills the bottle-rocket as you pump. As you squash more and more air into the bottle the pressure increases. Eventually the air pressure will be strong enough to push the rubber stopper out.

When the rubber stopper is forced out, the air pushes the water out too.

As the rubber, water and air come out the bottom of the bottle, the bottle is pushed in the opposite direction. When the rubber-water-air-mix goes down, the bottle-rocket goes up! Because the bottle-rocket doesn’t weigh very much it goes a long way up. We put water in the bottle because water is much heavier than air. So the rocket will go further.

Why it doesn’t work

Everything depends on getting a good seal with the rubber stopper. If the rubber stopper doesn’t fit tight enough, water will leak out when you pump. Or the bottle will fly off too early and not go far.

  • When nothing happens: Look for air bubbles when you pump. You should see air bubbling in. Your pump may not work, or it may leak air out before it gets to the bottle.
  • When your bottle doesn’t go far: You may have the wrong amount of water in the bottle. Try with more or less water
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