Instructions to Build a Water Rocket

Introduction

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.

Here 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.

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

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

• 75 mm of garden hose
• 600 mm of 15 mm diameter copper pipe (a plumber gave me this for free). The outside diameter of the pipe should be slightly larger than the inside diameter of the garden hose. Do not use a pipe with a smaller diameter, it will not make an airtight seal when it is assembled.
• An automotive hose clamp of the right size for the garden hose.
• a 15 mm end cap (for the pipe)
• a car tyre valve (I got mine from a car tyre seller and the guy gave it to me for free)
• a garden hose coupling, the female part that fits on the hose

• elbow fitting for the pipe
• three saddle brackets for holding the pipe
• some string
• a plank of wood
• some nails to knock the wood together
• five 120 mm nails

• plumbers solder and flux
• a propane gas torch
• a wire brush
• a hack saw
• a sharp knife to cut the rubber off the tyre valve
• a hammer
• an electric drill and drill bits
• some vaseline
• a screwdriver
• a tool to remove the inside of the tyre valve

How To Build It

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.
   
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.

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.

How to Build the Rocket

What You Will Need

• two 1.5 or 2 litre PET bottles including their tops
• the male part of a garden hose coupling, the part that screws onto a tap
• some ordinary packing tape. product like silcon sealer, seems to be the best glue to use.
• a piece of plastic to use as a gasket for the mouth of the bottle. Some PET bottle tops have their plastic gaskets placed inside without glue. These would be ideal. I used a piece of teflon sheet about 0.5 mm thick but that might be hard to get hold of.
• some material to use as struts for the fin of the rocket. They need to be at least 250 mm long each. I used anti-static tubes that are made to protect integrated circuits in transit. Any tube will do but ones made for PLCC-packaged integrated circuits have the lowest profile. You should be able to get one or two tubes for free from any place that buys lots of integrated circuits. If you can't get integrated-circuit tubes try using some light-weight pieces of dowel or anything similar.

How To Build It

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.
   
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.
   
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.
    

Testing and Launching the Rocket

Pressure Test

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

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).

Return to the Mechanical Toys Home Page