Build an Overhead Water Rocket Launcher

This is the version with the trigger mechanism. There is a simpler version here.

Sorry, no text instructions yet, only video instructions for now.

The updated directions are in instructional video form. You can see the whole video here . Alternately, you can watch one step at a time below.

Part 1: Introduction and Overview

This video lets you know what you're getting into if you want to make the overhead water rocket launcher featured on Ask This Old House during their special kids program. It uses inexpensive, readily available materials. It peeks at how the launcher works: the bump that seals the water and air pressure in the bottle, the zip-tie assembly that holds it on, and the spring that helps prevent accidental launches. And part 1 asserts that shooting a water rocket is analogous to driving a car--potentially dangerous, but safe if done with care.

Part 2: Materials and Tools.

For printer friendly supply list, click here. You can get the things you need at a building center, plus one thing at an auto parts store. The overhead water rocket launcher uses less than $10 US of actual material.

Substitutes for the 2" of 1 1/2" pipe

Observant correspondent Andrew Baillie of Atlanta discovered that you can substitute a 1 1/4" slip joint for the 2" of 1 1/2" pipe. Also you can make one from scratch. Read More>>

Part 3: Update

I now suggest that you use a slightly larger drill bit: 9/16" (instead of 1/2") because it will be easier to pull the valve into place. 9/16" is not so common in twist drill sets, but the flat kind of drill bit called "spade bits" or "speed bits" are inexpensive even when bigger than 1/2" and they work well.
Install the Valve, Cut and Glue Pipe. You hook a bicycle pump to the launcher to pressurize the bottle. You drill a hole and install a common car tire valve. Then you can glue the plastic pipe together, which stinks (do it outside) but is not hard to do. You glue the pipe first so it will be strong by the time you are done with the rest of the launcher.

Part 4: Seal on a Bump.

I worked in Bangladesh in South Asia for an organization that promoted inexpensive water wells and hand pumps. It was there that I learned how to heat and form PVC plastic pipe. With only a candle you can create a little bump on on the small pipe that will seal in air and water while you pressurize the rocket. It's easier to make than a seal with O rings and more durable, too.

Part 5: Make the Trigger Mechanism.

An Australian water rocket enthusiast named Ian Clark came up with a simple, effective trigger mechanism that uses ordinary plastic ties (zip-ties).

Part 6: Make a Safety Spring.

A British Astronomer who--naturally--also tinkers with water rockets (sending up mini video cameras) devised a simple spring that helps prevent accidental launches. Fittingly, it's made from a 2 liter bottle

Part 7: Launch Tips

Here are launching tips like how much water and how much pressure to put in, and tipping water out of the launcher after each use. And how to modify a bottle to launch water balloons. In addition to the video version, below you can find more tips and answers, including some you won't find on the video.

Printer friendly version of the launch tips.

If the air pump locks up it's because too much water got into the launching tube and the check valve on your pump is not working. Then water leaked into the pump. The check valve is a one-way valve that is supposed to let air go out of the pump, but not go into the pump. It's on all air pumps, near where the hose goes into the pump, but it is usually not accessible for repair. You'll have to unhook the pump from the launcher and pump out the water. To prevent it from happening--short of getting a new pump--is to pump fast so there is little back flow and...

Tip out the water in the launcher after each use

If there is a leak as you pressurize the bottle you can readjust the zip ties for a better fit. If it's a hot day and you don't mind getting sprayed a little, you don't have to fix a small leak.

To adjust the launcher, loosen the hose clamp so you can slide the taped-together zip ties. You might want to mark on the pipe so you know how much you are moving them. Move them just 1/16" or so at a time, toward the bottom of the launcher to tighten the fit. If you move them too much you won't be able to get the bottle hooked on.

Very rarely, if the bottle has landed on hard pavement, the neck of the spout can crack, causing a leak.

How much water depends on what you want. You don't have to put any water in the bottle, which might be good on a cold day when you don't want to get wet. It makes a satisfying sonic boom when launched without water. Note, however that the trigger mechanism has to be well adjusted (see above) because air volume leaks out much faster than water.

If you want to get wet, you can fill the bottle up to half full with water. However, if you fill it much more than that it can be dangerous. Too much water displaces the compressed air, which is your stored energy source once the bottle is in the air. And the bottle is heavier, too, so a bottle with too much water might fall on someone before having ejected all its water.

When launching water balloons (see below) I find that a bottle 1/3 full of water works well.

How much pressure do you need in the bottle? Depending on how hard you have the bottle jammed onto the bump, you will likely need 30 or 40 psi of pressure just to get the bottle off of the launcher. And you need that much pressure to eject all the water before the bottle lands. Young kids will need some help getting the pressure high enough.

So how much pressure is too much? I talked to a bottle manufacturer who said they can guarantee the bottle will not burst up to 100 psi, but that is a new bottle, unscratched and not crushed, not left in the sun for weeks, etc. Although you can see bottles bursting at 168 psi, I stick to 70 psi maximum. If you are going to go higher, use a different kind of launcher where you are not so close to the bottle, and wear earplugs!

If your pump does not have a pressure gauge, you can unhook the air-pump use a cheap tire gauge from an auto parts store to check the pressure. With a typical bicycle tire pump, it gets pretty hard to pump after 70 psi.

If the bottle gets stuck on the launcher it could be that you need more pressure in the bottle (see how much pressure, above). Once, a bottle got stuck because the water inside was sandy. The sand lodged in between the bottle and the pipe, so even with a lot of pressure it would not launch. I was able to push off the bottle with my hand, still pointing up, of course. You can also disarm the rocket of its energy by unhooking the pump and pushing the inside core of the tire valve, which will let the air out.

What about fins on the water rocket? When we taped the Ask This Old House segment about water rockets, I was the overhead launcher construction expert. But the show’s producer, Chris Wolf, was the water rocket expert. He made some cool rockets with nose cones and fins, and they soared really high, much higher than the plain old bottles that I launch. Bottles tend to tumble around in the air, creating a huge amount of drag, so they don’t go as high. Fins keep the low profile of the nose pointing up, so the rocket goes higher.

But we found that the upside of adding fins to rockets is the downside as well. Unlike a tumbling bottle, they speed down hard and fast enough to possibly lacerate a scalp or break a windshield.

There're ideas on the internet for parachutes and other recovery devices to slow the descent of the rocket, but they're much harder to get to work than anyone thinks heading into it. So if you launch finned rockets, do it in a wide open space with no kids or cars nearby.

Launching water balloons Water balloons launched from water rockets go as high as finned rockets. Although the dense balloon would sting if it landed on you, it's unlikely that you would need stitches. I have heard of people launching tennis balls, too. The simple modification for the bottle to launch balloons is shown near the end of the video instructions. Of course you still have to launch water balloons in wide open spaces sans young kids. And it goes without saying that launching hard objects like stones could cause severe injury--or even kill someone. See Safety FAQs>>