Celebrating the independent kiwi spirit of invention.

Invention: Air Cannon Water Gun or "How To Bowl The Enemy"

By Ian Mander, 27 & 28 May 2000.

Question: How do I make the Mother of All Water Guns?

Answer: One solution would be to use a pneumatic air cannon (normally used to fire potatoes anything up to a kilometre). Fill the barrel up with water, then seal the end of the barrel with cling-film plastic wrap, preventing the water from pouring out. However, you only get one shot with an air cannon, and they should NOT be used at short range. I also advise against using one at medium range. You have been warned – they pack a mighty punch.

Disclaimer: Any damage you do to property or person as a result of reading this article is on your own head. Or maybe to your own head. If construction is your goal, please read ALL the notes before beginning constuction.

Legal: In New Zealand air cannons are classed as air guns along with air rifles, air pistols, soft air pistols, paint ball guns, etc. Some key points:

• You must be at least 18 years old to own (or even have in your possession) an air gun in New Zealand unless you have a firearms licence (for which you need to be at least 16 years old). Penalty – up to $4,000 fine and/or 3 years in prison.
• You must be 18 or over to use an air gun in New Zealand unless you are under the direct supervision of someone 18 or over, or someone 16 or over who has a firearms licence.
• Don't use an air gun carelessly. Up to $4,000 fine and/or 3 years in prison.
• Don't point an air gun at anyone. (Except when playing soft air gun or paintball war games!) Up to $1,000 fine and/or 3 years prison.
• Don't endanger, frighten, or even annoy anyone by firing an air gun, or harm property. Up to $3,000 fine or 3 months in prison.
• Don't supply someone underage an air gun, like giving them one for a birthday present. Up to $1,000 fine and/or 3 months prison.
• Don't modify your air gun to make it fully automatic. (But what an air cannon that would make!)

More information about airguns can be found on the New Zealand Police web page Arms Code Section 3: Understanding firearms.

AIR CANNON Page 1:
Pipe Considerations

Introduction:

A way to launch one and a quarter litres of water (or confetti, for that matter) in half a second at anything up to 100psi. The cannon is also very useful for launching spuds huge distances, but compressed air at 100psi will pack much more power than a combustion spud gun for either of these purposes.

A pneumatic air cannon is not the same as a combustion powered spudgun, which uses a short burst from a hydrocarbon spray can as propellant, set off by a spark plug or electric barbecue lighter. A pneumatic air cannon uses compressed air instead of an explosion, and while being far more powerful, is also much safer because of greatly increased reliability. Pneumatic air cannons are also quieter than combustion spud guns, and I realise this may be good or bad, depending on the user.

If you wish to make a combustion spudgun then you might like to talk to the guys at Chester's Plumbing (Ellerslie/Greenlane branch). They've supplied several plumbers with the materials to make spudguns from common DWV uPVC pipes. The construction of that sort of cannon is not covered here, but is sufficiently straight-forward that you can go to Chester's on a Saturday morning to grab what you need, swing past a hardware store for a suitable sparker ignitor on the way home, and be done by lunch time. Just remember to let the glue dry for a full day before using it.

Also, it's not advised to fill the DWV grade barrel of a combustion spud gun with water. The mass of water can be too much for the explosion to force out, resulting in an exploding barrel or combustion chamber.

One more point...

It is illegal to use OR OWN a combustion spud gun in New Zealand without a firearms license. Even if you have a firearms license you can be fined up to $1000 or face 3 months in prison for firing one in a public place. Don't say you weren't warned. (This is another very good reason to make a pneumatic air cannon instead. :-)

Requirements:

• Pressure rated uPVC pipes. It is very important that pressure rated pipe is used, as DWV uPVC pipe will shatter at the high pressures used.

Do not attempt to make a pneumatic air cannon from DWV uPVC pipes.

• Pressure rated uPVC fittings. Read the above caution.
• uPVC pipe glue. Standard uPVC pipe glue still works fine for pressure pipes and fittings.
• Polythene or polypropylene sheet. About 3mm thick (I think). It needs to be flexible but not too flexible.
• Rubber sheet. About 1mm thick. An old car inner tube should do.
• Car tyre valve. If the inner tube comes with a working one, all the better.

Notes on Pipes:

uPVC pipes are available in New Zealand as either DWV or pressure rated.

DWV pipes are (according to the guys at Chester's Plumbing) OK for the lower pressure produced by combustion spud guns, especially if the combustion chamber is small. However, there is a high risk of having them shatter (think "grenade") if used in compressed air cannons. (I've also heard that uPVC pipes will only split rather than shatter if the pressure is produced by water rather than air, due to the non-compressibility of water. However I've never tried it.)

Pressure pipes in New Zealand come in different classes related to wall thickness, and thus the pressure the pipe can cope with. These classes are designated by the letters B thru F inclusive.

Scale drawing of 50mm uPVC pressure pipe wall thicknesses.
Note: Outside diameter remains constant across range.

* Also used in New Zealand. (Complicated, huh?)
Same as the pressure rating in bar. (1 bar = 1kg/cm² = 100kPa, or 7 bar is about 100psi.)

Pressure pipes come in the same nominal sizes that DWV pipes do, but the actual sizes are slightly different. This is quite a good idea, since it makes using DWV fittings with pressure pipes impossible. All uPVC pipes (both DWV and pressure pipes) are specified by the outside diameter.

But wait – there's more! The pressure a pipe can cope with varies depending on the temperature of the pipe. The hotter the pipe, the less pressure it can cope with. This means that painting one's cannon black is not a good idea, and not just to avoid attention from the police.

Example: All over the Internet the recommended pipe to use is the equivalent of the New Zealand Class F pipe, giving a pressure safety factor of about 2.6 if used at 7 bar. From the table above we see that a Class F pipe is rated at 1800kPa or 260psi, but this is only at 20°C. At 30°C the pipe is only rated for 1440kPa or 208psi. This is still quite acceptable. (Obviously, the higher the class of pipe, the better.)

However, in my preliminary investigations I was only able to find Class D pipes (with green writing). At 20 degrees C these can cope with 1200kPa or 173psi, but at 30 degrees C (such as on a hot summer's day in New Zealand when we have nothing better to do but blow water around the place at high pressure) a Class D pipe is only rated for 960kPa, or 139psi. This doesn't seem a very good safety factor above the 692kPa/100psi working pressure we hope to run in the cannon, especially considering the sort of use it will be getting (handheld!). Of course, the water itself will help keep it cool.

Safety:

The aforementioned pressure concerns mean we should seriously think about protecting the user from the possibility of the pressure chamber exploding. One way is to wrap the chamber with strong rope, gluing or sewing the rope in place. It doesn't matter if the rope doesn't hold the exploding pipe together, especially afterward – just as long as it stops shrapnel from harming the user. (The rope itself should remain intact. At 7 bar the pressure is 7kg/cm². An 8mm polypropylene rope has a breaking strain of just over 1000kg.) This will, of course, tend to channel any explosion out the ends of the pipe.

Example: A 100mm pressure pipe has a mean outside diameter of 114.3mm, so using 8mm rope gives an effective circumference of 384mm. A 1m long pressure chamber would thus use about 48m of rope.

The most popular protection mentioned on the Internet is loosely wrapping an old trouser leg around the pressure chamber. Old trouser legs have great placebo value, but possibly not much else (especially if the trouser leg is very old/well used). Many trouser legs will be needed to protect from shrapnel at 7 bar (100psi).

This suggestion from an interested Aqualab reader:

Do you think that exhaust pipe lagging would work? I got some recently (for the [Suburu] WRX). It's 50mm wide and lays flat. It's made of fibre-glass I think (Explains the itching after installing it), and can be obtained from most car accessory outlets. It seems to be fairly tough, and is obviously fireproof and heat retaining (it wraps around exhaust pipes after all).

The only disadvantage is that it is fibre-glass and would probably need to be covered with something else to minimise contact with skin. This is a minor detail that I did not realise before installing on the car. :-(

It would certainly be uncomfortable if it didn't work...

Notes on Fittings:

Apparently pressure rated fittings in New Zealand are good for 1500kPa for 125mm (nominal diameter) and smaller, or 1200kPa for sizes larger than 125mm.

Compared to DWV fittings, a more limited range of fittings is available for pressure uPVC pipes. However, endcaps, sockets, and 45 and 90 degree elbows are all available.

Size converters are available, but possibly only in limited sizes, and may require two different sizes being purchased, depending on the size of pressure chamber and barrel. Size converters for DWV fit into the inside of the pipe, but the internal diameter of pressure pipes varies depending on the wall thickness, meaning size converters fit on the outside of pressure pipes.

Glossary

DWV. Refers to the normal use for that sort of pipe. Stands for drain, waste, vent. Also sometimes used is SWV (I think for sewer, waste, vent).

Pneumatic Air Cannon. A "spudgun" sort of device which uses compressed air to launch rather than an expolosion.

uPVC. The material the pipes are made from. Stands for unplasticised polyvinyl chloride.

If clarification is wanted on any of the above, please do .

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