BUILDING A OVERHEAD WATER ROCKET LAUNCHER

My students demonstrating the water rocket an assembled crowd (not in picture) of elementary school kids. Photo by Bethany Hartman

Before you actually start building the water rocket, here is a little overview of how it works so you know why you are doing what you are doing. And rest assured that there are over 50 photographs below to take it one step at a time--although I hate to think how many trees will have to give their lives if someone actually prints it all out!

By fortunate coincidence, In North America the inside of the 2-liter beverage bottle fits perfectly on the outside of a common size (easy to buy anywhere) of plastic pipe. The bottle slides on the pipe, but is tight enough that it does not wobble very much (Bill Chalmers of London informs me that in England..."you can commonly get pvc pipe with a diameter of 21.6 mm marked on the pipe from any plumbers merchant and this only needs slight sanding as its just a shade too nice a fit." So in some places you might have to sand off some of the outside of the pipe so the bottle slides off.

The bottle is held onto the pipe with a variation of Australian Ian Clark's brilliant cable-tie method. In the picture below you see a close up of the mouth of the bottle on the pipe. The head (end) of a green "all-purpose tie" (also easy to buy anywhere), which is commonly used to bundle electrical cables together, rests on the flange (handle) of the bottle. If the other end of the tie is attached to the pipe, and if the tie is prevented from bending outward to the right then the head of the green tie will keep the bottle from flying off.

We will actually use 6 or 7 ties, and they will be attached to the pipe with a hose clamp. They will be kept from spreading outward by a short piece of larger-diameter pipe (either bought or made). By pulling and therefore sliding the larger pipe downward, the ties will release the bottle. In the picture below, the larger pipe has not yet been slid over the heads of the ties, but when it has, the bottle will be locked on. The rope attached to the larger pipe will pull it down to trigger the launch.

At the other end of the thin pipe, a common replacement valve for a car wheel allows us to pump air into the pipe, which delivers the air to the bottle.

Although the bottle fits somewhat snuggly onto the pipe, when you start pumping air into the bottle the water gushes out prematurely unless you do something to insure a seal. People using Ian Clark's cable tie method typically resort to O-rings, but I sought a simpler-to-make, more durable way. I found that simply heating the plastic pipe over a candle and pushing inward creates a slight bulge in the pipe. In the picture below, when the mouth of the bottle reaches the bulge, it will seal very well on the taper. I find this more satisfactory than machining grooves into the pipe for the O-rings and replacing broken O-rings. If you don't want to heat the plastic pipe and form a bulge, a possible alternative is to use PTFE (Teflon) pipe thread sealing tape from wherever plumbing supplies are sold. Details at this website http://homepage.ntlworld.com/telescope/Rocketweb/Launcher.htm

Before you begin making a water rocket launcher, understand that it allows you to store lots of potential energy, so anyone using it has to use it responsibly. I discuss water rocket safety here.

WHAT YOU NEED (if you do not live in North America it is possible that it might be harder to find the first item (1/2 inch, schedule 40, PVC pipe). If you are not able to find it, click here.

1 10 foot piece of 1/2 inch, schedule 40, PVC (plastic) pipe. You might be able to find this in the plumbing section of a building center/hardware store, or in the form of conduit in the electrical area. Usually the conduit pipe is gray in color instead of white.The pipe size is printed on the pipe. If you actually measured the pipe, you would find the outside is about 21mm (almost 7/8"), and even the inside is 15mm (9/16"). Choose by what is printed on the pipe, not by actual measurements. The "schedule 40" refers to the thickness of the pipe, and rest assured that schedule 40 is the most common, easiest to find kind. See picture below.

[OPTIONAL] 1 piece of PVC 1 1/2" (one and one half inch), schedule 40 pipe. You only need 2 inches of it, but some building centers only sell it in 10 foot lengths (even a 10' long piece is only a few dollars). You might be able to find a sympathetic plumber or home handyman who can give you a couple of inches. The reason it is optional is because if all else fails, I will show you in step 8 how to make a substitute piece of pipe from a cut up 2-liter soda bottle.

2 PVC 90 degree elbows for the 1/2" pipe. See picture below, but note that only one is shown.

1 end cap for the 1/2" pipe above. See picture below.

[OPTIONAL] 2 threaded fittings that also cement to the 1/2" pipe: one with internal threads, one with external threads. The idea is that you cut the long launcher in half and cenent the fitting on the ends created when you cut. These allow you to disassemble (screw apart) your rocket launcher so it is not so long, for storage or transport. If you do this, you might want to also get "pipe thread tape aka teflon tape that you wrap around the external threads to prevent leaks, although the leak is so small it doesn't really matter.

1 small can of PVC pipe cement (or borrow some).See picture below.

1 package of 8" (or close to it) general purpose ties (also called cable ties). These will be in the electrical wiring area of the building center.

1 hose clamp. The size is important. I get hose clamps with a range from 11mm to 23 mm (a little less than 1/2" to a little less than an inch). The hose clamp must be a little bigger than the pipe because that's what holds the all purpose ties securely to the pipe. Better to have the hose clamp a little too big than a little too small.

1 tire stem. This is the one thing you are not likely to find at a building center or hardware store. However, it is easy to find at any automotive parts store. Tire repair shops often throw them away. The inner core of the tire stem does not have to work, or even be there. If you tell the person at the tire repair shop they might give you some free.

A half inch (1/2", 13mm) diameter drill bit, a quarter inch (6 or 7mm) diameter drill bit, and drill. There are only two holes to drill, so if you don't have a drill find a neighbor who does. You will only take a minute of their time. A drill press is even better.

1 pliers (not needle nose)

1 something to measure with--I have tried to include both metric and inch for each dimension.

1 roll duct tape. Other kinds of tape, including electrical tape, could work as well.

About 1 yard (90cm or so) of quarter inch (6mm?) diameter rope.

1 hack saw (or even just the hacksaw blade without the frame). See the picture of a hacksaw blade in Step 1.

1 flathead screwdriver to tighten the hose clamp

1 candle (a small birthday cake candle is fine).

2 straight edges. This could be rulers or hacksaw blades or paint sticks or any combination of the above.

2-liter bottles

Bicycle/air pump (it's nice if it has a pressure gauge, but not necessary).

STEP 1: cut the long piece of pipe

First use the hacksaw (or just the inexpensive disposable hacksaw blade) to cut the 10' piece of 1/2" PVC pipe into 3 pieces:two 1' long and one 8' long. The pipe is plastic, easy to cut.

Step 2: determine where to put the bulge and create it. Read the whole step before you start.

Fit the bottle on an end of the long piece of 1/2" PVC pipe so the pipe end is about 25mm (1") from the bottom of the bottle. Use a pencil, pen or marker to mark where the pipe goes into the bottle. Or just make your mark about 280mm (11") from the end of the pipe.

In this step you will create a slight bulge which will function as a seal to keep the bottle from leaking. You will do it with heat. It is easiest if you do it somewhere that is not very drafty or shelter the candle from drafts. Light a candle and hold the pipe where it is marked about 50mm or 75mm (2 or 3 inches) above the tip of the flame. KEEP ROTATING THE PIPE SO IT HEATS EVENLY ALL AROUND. If it discolors the pipe is too close. After about two minutes the pipe will become quite soft and flexible where you are heating it. Jay Morgan, Cubmaster of pack 327 in Texas, adds that when you smell the plastic it's probably soft enough.

Before the pipe cools, grab on either side of the hot spot, push inward and you should see a slight bulge form. While still squishing the pipe, cool it under cold water or a wet towel (it helps if there are two people). Keep the pipe as straight as possible as it is cooling. If it is really important to you to have the pipe perfectly straight, you could put a snug-fitting dowel into the pipe to keep it from bending. I never get it perfectly straight.

Notice in the picture above that the bulge is not big. It only has to be big enough to keep the bottle opening from sliding past. Test it by pushing a bottle onto the pipe. It should get stuck at the bulge (I used a green bottle this time).

Step 3: Drill the hole for the tire valve and insert it

Drill a 1/2" (13mm) hole from the middle of the inside of the pipe end cap. It's a good idea to hold the end cap with a pair of pliers and back it up with scrap wood to the drill bit does not ruin the table when it comes through.

Pull the tire valve stem through the hole in the end cap, from inside the cap to outside.

Grab the stem with pliers, but do not grab on the threads. Pull hard and wiggle the stem through until the groove in the stem gets seated on the pipe. You will know when you have it seated because it will be impossible to back the stem back out anymore.

You might get a better grip on the end cap if you hold it like this.

Step 4:Glue/solvent weld the pipes

At the very least you have to glue the end cap onto the long pipe. However, for a few more pennies you can end up with a "J" shape which has two benefits. First, it tends to keep the end of the valve stem from scraping on the ground. Secondly, if the check (one-way) valve in your air pump is leaking a little, the vertical rise should trap some air, so air will leak back into your pump rather than water.

This is how it will go together. Notice that the bulge end of the long pipe (and the bottle that it seals on, green in the picture below) is not glued into an elbow. The other end will be.

Here is a closer view.

Trial fit a piece of pipe to an elbow so you know how far to apply the cement. The pipe cement has an applicator attached to the lid. scrape off some of the excess back into the can so it doesn't drip.

Slather some cement on the inside walls of the end cap. Make sure you do not apply so much that it blocks the opening to the valve.

Slather cement around the end of one of the 30cm (1') piece of pipe.

With a twisting motion, push the pipe into the end cap. You can wipe off the little bit of cement that squeezes out.

Repeat with the elbows. Make sure the end of the long pipe that DOES NOT HAVE THE BULGE is glued into the elbow. Set the pipe aside so the glue can dry.

Step 5: Start the tie/tape part of the triggering mechanism--position the ties.

In this step you will start building the part of the water rocket launcher that holds the bottle on the pipe despite the pressure, the releases it at the right time. First, you will carefully tape the all-purpose cable ties so they apply pressure evenly to hold the bottle on. Tape the ends of two rulers or a ruler and a hack saw blade (a paint-stirring stick would also work) so they are parallel to each other and the outermost edges are 100mm (4 inches) away from each other. Take care here that they are parallel by measuring on both ends (I know the rulers in the picture below don't appear to be perfectly parallel, but it is because of cameral distortion).

Push 9 cable ties under so they are held down. It will take the next 3 pictures to make it clear exactly how this should be done. Notice that the ties are all parallel and the heads are together against each other and against the ruler.

The flat part of the tie head should be facing down, the flange part should be facing up as in the picture below. For clarity the tape is not shown and the end of the ruler shows, as in a "cutaway drawing."

The picture below shows how NOT to do it! The heads of one orange tie and one blue tie are not against the ruler. Furthermore, the ties are converging at the end, not parallel as they should be.

Step 6: Continue the tie/tape part of the trigger mechanism--tape the ties.

This requires a bit of finger dexterity because in addition to the ties that the tape is supposed to stick to, it also sticks to your hand and the table. Cut off a piece of duct tape about 90mm (3 1/2") long. Lay the tape down beside and below the straight edge (a hacksaw blade in the picture below). Press the tape onto the ties so it really sticks to them. If you are using electrical tape, add more pieces until it is wide like duct tape, about 50mm (2").

Now remove the straight edges that were holding the ties down and carefully (so the assembly does not fall apart) flip the whole thing over to the other side. Cut another piece of duct tape and carefully lay it where the first piece is (although it is on the other side this time). Again push the tape on. When the tape sticks to the ties and to the other piece of tape, the assembly is sturdy.

Trim the duct tape on either side so that only about 5mm (3/16") sticks out.

Wrap the tie/tape assembly around a pipe. If it is so wide that the ties on the end overlap each other (unlikely unless you have a brand of ties with very wide heads), then you might have to cut off a tie. If there is a bit of a gap--as in the picture below--that is OK. By the way, notice in the picture below that the this particular brand of ties bends near the end (the end opposite the heads). I am going to cut them off right where they bend because it is hard to slide the hose clamp past them in the next step.

Step 7: Finish the tie/tape part of the trigger mechanism--attach the ties/tape assembly to the pipe with a hose clamp.

Thread the hose clamp onto the long pipe on the end that you made a bulge, and slide the hose clamp past the bulge. Then push a 2 liter bottle onto the same end of the pipe until it stops on the bulge. Then lay the tie/tape assembly underneath as shown in the picture below. The smooth side of the tie heads should face down, the protruding side up. If you do not know what I mean, I will elaborate in the next paragraph.

Wrap the tie/tape assembly around the pipe. The tie heads should be positioned so that they can grab onto the handle of the 2-liter bottle. IMPORTANT: The protruding heads of the ties must face the bottle. If you do not understand that, go back up and look at the second picture from the top of this page (of the a single green tie positioned against a bottle handle) .Slide the hose clamp onto the bottom of the tie/tape assembly as shown in the picture below. Tighten the hose clamp, but not too much because you will make a fine adjustment to it later.

Step 8: Cut and drill (or tape) the slide pipe/trigger. 1 1/2" diameter schedule 40 pipe is very common, but if you cannot find and, there is a workaround here.

Cut about 50mm (2 inches) of 1 1/2" diameter, schedule 40 PVC pipe.

Take a moment to see how the slide trigger/pipe works. Take the bottle off. Slide the slide trigger/pipe on the end of the long pipe and over the heads of the ties. Put the bottle back on--pressed onto the bulge as shown in the picture below.

Slide the slide trigger/pipe back over the heads of the ties. Now you should not be able to pull the bottle off. When you tip the launcher vertically so the bottle points up, there should be enough friction between the short trigger pipe and the ties so the trigger pipe does not slide down just from the pull of gravity. If the trigger pipe is so loose that it slides down by itself, the water rocket will be launching before you are ready. To increase the friction so the trigger pipe does not slide down unexpectedly, put some electrical tape (duct tape would work too) on the inside of the trigger pipe as shown below.

Cut off a piece of tape a little longer than the trigger pipe. In the picture below, a rope is attached to the pipe, which will be covered next.

Attach the tape to the inside of the trigger pipe with the ends sticking out. The sticky side is facing us in this picture.

Finally, bend the tape around and stick it to the outside. It's not pretty, but if the ends of the tape are inside the pipe, the ends of the ties might catch on them. I used 5 pieces of tape to adjust the sensitivity of trigger to what I wanted. Experiment to see what works for you.

There are two ways that you can attach a rope to the slide pipe/trigger: drill a hole, or tape it on. If you decide to go with the hole, drill a 1/4" (7mm or so) hole near the end of the pipe. Thread the end of the rope through the hole and tie it so it does not come out.

Alternately, you can tape the rope on. It is best to weave the rope through several layers of tape so it does not come off. Tape the rope to the pipe as shown in the picture below, with about 150mm (6") of the rope sticking out.

Wrap the tape around again and put the rope through again (picture is from a different angle), and so on until the rope end is used up. The first picture in the next step (9) shows how the trigger pipe goes on. You have to slide it on and over the ties before you attempt to get the bottle on.

Step 9: Adjust the launcher

Now that you have the slide trigger pipe done, it is easy to adjust the trigger mechanism so there is no leaking. Put the slide trigger pipe on the launcher. Push a two-liter bottle onto the long pipe and push it HARD onto the bulge. It helps to twist the bottle a little as you push it down.

Lock the bottle on by pushing the slide trigger pipe over the heads of the ties.

Loosen the hose clamp a little with a screwdriver. Pull the tape/ties assembly down, and while still pulling, tighten the hose clamp back up, quite tightly (this might work best with two people, or place the other end of the launcher against a wall).

Step 10: TEST IT! (do this outside--expect to get wet) Please read this brief discussion of safety issues concerning water rockets, then return to this page.

Fill a 2-liter bottle a third or half way with water. Push the bottle on to the bulge on the pipe (a little bit of water will spill--that's ok). Lock the bottle on with the cable ties and big pipe, as explained in step 8, above. If the big pipe does not stay on, line the inside with duct tape as explained at the end of step 8. Hook a bicycle pump to the tire valve. Once you start pressurizing the bottle, you must keep the launcher pointed toward the sky until the bottle has been released. Consider it to be like a loaded gun.

Start pumping air. If the pump has a pressure gauge, pump up to 70 psi (5kg/cm) or whatever you can comfortably reach, but not higher. If it starts leaking (and assuming the bottle does not have a hole in it), then you might have to further adjust the trigger--see Step 11 below. If it is a slow leak, you might actually want to keep it that way because it disarms the launcher if some knucklehead leaves it around pressurized. If the rocket gets stuck on the launcher (it's rare, but I've seen it happen with dirty water, where tiny stones get caught between the pipe and the bottle) it's ok to push it off, but make sure you know where it's aimed. Alternately, you can unhook the pump and press the core until the pressure is released.

With the launcher pointing to the sky, pull the rope.

Step 11: Readjust the launcher (if necessary).

You only have to do this if the bottle is leaking. In the picture below is a trigger mechanism that is about to be tightened. You can see the tape/tie held to the pipe with a hose clamp.

In the next picture below you see a red mark on the pipe, as close as possible to the tape. This mark will be our gauge for knowing how far we have tightened the trigger.

Slide the tape/tie assembly toward the tire valve end about 2mm or 1/16". If you slide it too far, the tie heads will not be able to reach the bottle handle to hold it on. Tighten the hose clamp again and try another launch to see if the leak is sealed. Most bottles are the same, but every once in awhile you might come across a bottle that requires a different adjustment.

I am happy to help if you are still having problems. Just as photographs are helpful to me for showing how to build this project, sending pictures of where the problem is will be helpful. Contact me.

To learn how to hurl water balloons out of site, click here

To see water rocket variations, click here.

To see the links to the best water rocket web sites I've found--including slow-motion launches, aerial photography and multi-stage rockets click here.

To read about the history of water rockets and how this design evolved, under construction

For a frank discussion of the safety issues involved with water rockets, click here

To learn about Newton's laws of motion that make water rockets work and to calculate how high they go under construction

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