Water Rocket photo

Instructions to Build a Water Rocket

Introduction

Water rockets are a heap of fun. Their performance is amazingly good. Other people have reported getting their rockets to go as high as 90 metres or even 200 metres for a two-stage rocket. They are certainly the fastest and most spectacular of any of the toys from the mechanical toys page. And they drive you to continually improve the rocket so that it goes higher, or comes back with a parachute, or has two stages. Some water-rocket competitions have been organised in the United States. Water Rockets can be made at home with no special equipment and are cheap as well.

Basically, a water rocket is a plastic fizzy drink bottle (PET bottle), that you fill half way with water, pressurize with air, and invert. When you release it, the pressurized air forces the water out which propels the rocket. Certain water rocket designs are said to fly as high as 90 metres. Clifford Heath has written up a water rocket design that can be made very easily. He uses a nozzle that he claims improves the rocket's peak altitude. Clifford had troubles with the stability of the rocket until he made a special fin that extended some 200 mm behind the PET bottle. The design I have written about here is essentially the one that Clifford created. Unlike Clifford, I haven't had any problems with stability but that may be because I am using 1.5 litre bottles instead of the two litre bottles that Clifford uses.

See Gary Ensmenger's Water Rocket Playground for other rocket designs, rocket-building techniques, water rocket links and a device that allows you to build multi-stage rockets. Bruce Berggren's Water Rockets! page includes simulations to estimate how high your rocket will go and Clifford has a similar page for simulations. Drag is the biggest factor in water rocket performance so it is worthwhile reading about drag on model rockets at Model Rocket Drag Analysis, NAR R&D Report. Another good water rocket resource page is Dave Johnson's Water Rocket Annex. Finally, you can join the water rocket mailing list and details of how to join are given on Clifford's page.

Rick launching a water rocketHere is a photo of my friend Rick launching the rocket. You can see the water falling to the ground. It is hard to estimate how high the rocket goes when you are standing beneath it but we think it went to 50 metres, give or take 10. We used a 1.5 litre bottle in our rocket. Everyone else seems to use 2 litre bottles which are relatively rare in The Netherlands but as soon as we get one we'll make a rocket out of it.

Close up of Rick's laughing face
Here is a close-up of Rick's face as he launches the rocket. See him laughing.

Launch pad photo Launch pad close up photo Pre-launch
preparations
Here are some photos of the launch pad after we turned the hand-held launcher that Rick is using above into a remotely-operated one. Basically, the hand-held launcher is cut in half and the two pieces are soldered together with an elbow joint. Then it is mounted in a wooden frame. The garden connector is operated with a string. The close-up photo shows the elbow joint and the string passing behind a nail. 120 mm nails in the wooden frame allow it to be easily staked to the ground. The last photo shows last-minute adjustments to the launch pad by the engineering crew (John). Don't worry, the rocket was not pressurized.

How to Build the Launcher

What You Will Need

For a hand held launcher you will need The hand held launcher is probably not so good as one that you can operate remotely for safety reasons and because you get a bit wet every time you use the hand-held launcher. You also have to close your eyes as you launch to avoid the initial jet of water from the rocket and so you miss the best part of the flight which is where the rocket accelerates to full speed. If you want to convert the launcher into one that sits on the ground then you will also need the following. You will need these tools

How To Build It

Start with the launcher. The figure below shows how it should look when it is finished. The instructions below describe how to build the hand held launcher shown in the photo with Rick. The list of instructions that follow it describe how to build a wooden support so that you can launch it remotely with a string. Diagram of the launcher
  1. Remove the inside of the tyre valve with the tyre-valve-inside-remover tool.
  2. Cut all of the rubber off the car tyre valve. Make sure every bit of rubber is removed or you will not be able to solder it to the end cap (which is what you are going to do soon). Use a sharp knife and wire brush to clean the rubber off. You may be able to burn small, hard-to-remove pieces of rubber away with a propane gas torch and the wire brush.
  3. Drill a hole through the centre of the end cap the same diameter as the tyre valve. The valve must be a close fit in the hole so that you can solder it.
  4. With a wire brush clean one end of the copper pipe till it is bright with the colour of fresh metal.
  5. Smear some plumbers flux on the cleaned metal and on the end cap and valve. Put the end cap over the pipe and insert the valve into the hole that you drilled. Solder the pieces together with a propane gas torch and plumbers solder as shown in the photo. Try not to get solder on the threads of the valve. Make sure the solder fills all of the gaps.
    Photo of the valve soldered into
     the end cap
  6. File all burrs off the copper pipe at the other end to the valve. Smear some vaseline onto the pipe. Warm up the piece of garden hose with hot water so that it is soft and slip about 25 mm of it over the end of the pipe. Using the screwdriver, clamp the hose firmly in place with the hose clamp.
  7. Fix the female garden connector to the free end of the garden hose. Cut the hose short with the knife, if you need to, so that the hose connector sits close to where the end of the copper pipe is.
The launcher is essentially complete. You can operate it by hand but I recommend that you follow the next set of instructions and make it so that you can launch it remotely. A diagram of the wooden support is shown in the figure below. The cyan parts are the 120 mm nails.

Diagram of the wooden support for
the launch pad

  1. Remove the core of the tyre valve and the hose so that they don't melt when you solder the pipes in the next step.
  2. Cut the copper pipe in the middle. Clean the pipe ends with the wire brush. With the flux and propane gas torch solder the two pieces of pipe into the elbow fitting.
  3. Put the tyre-valve core and the hose back on the copper pipes.
  4. Wrap the string around the hose connector so that the ends come out on either side. The string has to be fixed tightly enough that you can use it to pull back the hose connector without it slipping off.
  5. You need to build a wooden support for the launcher. With whatever pieces of wood you have handy, build the support with nails and butt joints as shown in the figure.
  6. Use the three clamps to fix the copper pipe to the wooden support: two at the bottom and one on the vertical part. Put a long nail through the side pieces, in front of the pipe. The string on the garden hose will pass behind the nail so that it may be pulled to launch the rocket. The 120 mm nails will probably split the wood when you hammer them in so drill holes for them first. The holes should be about 0.5 mm diameter less than the nails.
  7. Put the four remaining nails through the wooden support so that they point downwards. They make an easy way of staking the launcher to the ground so that it will not topple over when you pull the launch string.
The launcher is complete! A 6mm rod may optionally be placed inside the copper pipe so that it extends 200 mm or so into the rocket when the rocket is attached to the launcher. This launch tube helps guide the rocket as it takes off although, so far, I haven't really needed one.

How to Build the Rocket

It is now time to build the rocket. You need to prepare a nozzle from the male garden hose fitting. The nozzle can be screwed onto any rocket you build so one nozzle can be used with all of your rockets.

What You Will Need

How To Build It

Here is how to build the nozzle and one way of building a rocket. Of course, half the fun of water rockets is trying to improve your rockets so that they go higher. The rocket construction that I give here was designed by Clifford Heath. I haven't changed it in any way. I have had good flights with it and it has been robust enough to survive many hard landings. Because it is also to easy to make (no glueing) this is a good one to do first.
  1. Remove the rubber washer from inside the male garden hose fitting.
  2. Note: If you have a metal-working lathe with a collet chuck then you are going to find this step easy. Otherwise it is going to take a lot of work with a hacksaw, vice and files. Cut off the threaded part of the garden hose coupling. Be careful not to scratch the inside face of the coupling as you do it (that is, don't scratch the inside surface that the rubber washer normally lies against. Cut the diameter of the coupling down to the point where it will fit snuggly inside a PET bottle top.
  3. Smooth off the face of the coupling with very fine abrasive paper. This face will mate with the mouth of the rocket and to make the contact air tight it must be perfectly smooth. Put the abrasive paper on a flat surface and rub the garden hose coupling over it with a circular motion.
  4. Cut a hole through the centre of a PET bottle top. The hole must accept the shoulder of the hose coupling. For me the hole needed a diameter of 19 mm. If you use a twist drill the hole may not be properly round so use an 18 mm drill bit and carefully shave off extra plastic with a sharp knife. Remove what remains of the plastic seal from inside the bottle top.
  5. Make up a gasket from the thin plastic. Cut a circle of the material to the same diameter as the hose coupling and then cut a hole in the middle of it. The gasket helps to make a good seal between the coupling and the mouth of the PET bottle as well as protects the coupling from being scratched as you screw it on and off the bottle.
  6. Put the hose coupling, the bottle top and the gasket together as shown in the photo. No glue or sealant is necessary. This becomes the nozzle for the rocket and can be screwed onto any rocket that you care to make.
    Photo of the nozzle
	  assembly
  7. With a hacksaw, cut one of the PET bottles into three parts as shown in the figure. The top part will become the nose cone, the middle will become the fin and the bottom can be discarded.
    Diagram
	  of how to cut the first PET bottle
  8. Push the nose cone over the base of the second PET bottle. Tape it in place with some duct tape.
  9. Using the duct tape, tape the struts to opposite sides of the second PET bottle. The struts should point down and extend at least, say, 100 mm below the mouth of the second PET bottle.
  10. Tape the cylindrical fin section from the first PET bottle to the struts with the duct tape. The rocket is complete and should now look something like the one shown in the figure. Note that the longer and further back that you make the fin the more stable the rocket will be.
    Diagram
	  of the rocket after assembly

Testing and Launching the Rocket

WARNING: There is enough energy stored inside a pressurized PET bottle to seriously injure yourself if it explodes. And the rocket flies with enough speed to break bones if it should become unstable in flight and head back to earth. Please adhere to the safety precautions given below. You need a car foot pump to pressurize the rocket. It needs to be sturdy to withstand the amount of use it will get. And it needs to have an in-built pressure gauge so you know how far it has been pumped.

Pressure Test

The pressure test will allow you to check that there are no leaks in your system and that the bottle will withstand the pressures required.
  1. Fill the rocket completely with water. It must be full of water so that it cannot explode if the PET bottle ruptures.
  2. Attach the rocket to the launcher. Keep yourself and other people clear of the launcher.
  3. Pressurize the bottle to 3 bar (45 psi) and check all joints and connections carefully for leaking water. Make sure that the pressure reading on the pump's pressure gauge does not fall; it should stay constant if there are no leaks.
  4. Increase the pressure to 7 bar (110 psi) and check again for leaks. You will not pressurize the rocket beyond 6.5 bar (100 psi) when you launch it.

Launching the Rocket

Launch your rockets only in large open areas where there are no people around. The best rocket flights go straight up and then come straight back down. If the rocket is unstable it can fly in any direction. It will often fly horizontally. Rockets have been reported to fly as high as 90 metres so if it is windy they may land a large distance from the launch pad. Your launch string should be at least 10 metres long.
  1. Push the launch pad into the ground so that the nails hold it stable. Make sure the rocket sits level.
  2. Fill the rocket 40% with water.
  3. Attach the rocket to the launcher and make sure that it is securely held and pointing directly up.
  4. Pressurize the rocket. Use 3 or 4 bar (50 psi) for your first launch in case the rocket is not stable. Do not go higher than 6 bar as the bottle may be at risk of exploding. Keep yourself (and everyone else) as far from the bottle as you can while you are pumping it.
  5. Warn everyone that you are about to launch and check that everyone is paying attention to the launch.
  6. Count down to 1 and pull the launch string (from 10 metres away).
If the rocket does not go straight up it may be unstable. To improve the stability of the rocket you must either increase the length of the cylindrical fin or move the fin further back (by increasing the length of the struts).

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Last modified: Thu May 14 08:55:52 1998
©  Malcolm Goris   <mgoris@csc.com>