Physics

How do you build a water rocket?

Andrew Hanson, from the National Physical Laboratory, explains how to build you very own water rocket, which is capable of being launched and flown in the air. With some basic, everyday household items and tools you’ll be able to make your own rocket in only an hour or so.

Safety Warning: Before you start, please be aware that water rockets can be a dangerous activity – which requires you to fire a highly pressurised rocket at high speed into the air. Please take the time to act safely, and never fire any rockets towards other people or animals.

Things you will need;

  • A 2 litre plastic fizzy drinks bottle (for safety reasons, the bottle must be a fizzy drinks bottle)
  • A tennis ball (or similar size and weight ball)
  • Corrugated plastic (or corrugated card, however plastic is better)
  • Duct tape

First things first; get your fizzy drinks bottle and make sure it’s empty! It’s also very important to use a fizzy drinks bottle, and not a normal still drinks bottle, as fizzy drinks bottles are designed to handle high pressures, and bottles which are not designed for fizzy drinks are liable to explode.

Think about a dart; you have something really really heavy at the front and the feathers at the back. Once it’s flying, those two centres align into the direction of flight, and keep the thing on board and glide gracefully. So the first thing we need is to put some weight at the front. We use tennis balls, so let’s strap one to the front using duct tape. Other types of tape may also be suitable, but duct tape is best.

So now the rocket is heavier at the front, and the centre of gravity is close to the base of the bottle. Before adding the tennis ball it was in the centre of the bottle. Next thing we need are fins, to provide us with that centre of drag which we will make using corrugated plastic, called corflute – the same stuff which is used to make estate agent signs. If you can’t get hold of any corflute, you can use corrugated card – however if it gets wet (which, seeing as we’re building a water rocket is likely) it may not work very well.

Using a sheet of corflute, if you cut diagonally you can create two triangular fins from each sheet. For this particular rocket I’m going to be making four fins to give us some fair amount of drag at the back. You can have great fun, discussing and deciding where you want your fins to go and what size they should be, but the trick is is to place them as far back on your rocket design as possible. The other trick is to get them true to the front of the rocket. If you have them at an angle, your rocket will rotate, or rifle, as it’s known in the trade. Making sure they are kept true will provide greater stability to the rocket whilst in flight.

Using duct tape, attach the fins to the bottle, making sure they are spaced apart correctly and there you have it. Your very own water rocket. Feel free to decorate and paint it, and then make as many as you want!

You can find out more specific details, and an incredible range of resources on NPL’s section on Water Rockets on their website.