Celebrating the independent kiwi spirit of invention.

AIR CANNON Page 2:
Design Considerations

Back to
Air Cannon page 1:
Pipe and Safety Considerations

On to
Air Cannon page 3:
Construction, Testing

On to
Air Cannon page 4:
Cost, Specifications, Results

On to
Air Cannon page 5:
Gallery

Explosive Decompression and Valves:

To get the water to shoot out of the cannon, we need to build up pressure in a chamber separate from the barrel, then suddenly release the pressure into the barrel – what is known as explosive decompression – in a reasonably controlled fashion, preferably. (Wadding may also be needed to ensure all the water is expelled. A potato is OK but may go a lot further than desired, so a rag is recommended.) To do this most people prefer a diaphragm valve because of the very fast switching provided, although ball valves are also sometimes used – just swing the lever fast. If the barrel is long enough it probably won't make much difference. Solenoid diaphragm valves are also quite popular, being triggered by closing an electrical circuit with the push of a button.

There are two main designs of air cannon:

1:

Two separated chambers (one pressure chamber, one barrel) with a valve joining them. Cannons of this design very often have a pressure chamber roughly the same size and shape as the barrel, as per the example above (but longer for both, of course).

Solenoid valves from garden sprinklers are quite popular as a push button trigger can be mounted whereever the builder requires. The diaphragm valve is shown at left, barrel on top, pressure chamber below. The filling valve is at the right end of the pressure chamber.

The more creative on the World Wide Web have constructed a stock behind the valve, sealed off from any high pressure requirements, with a narrow barrel – a sniper rifle design.

2:

Use a large diameter pressure chamber surrounding the barrel with a home-made diaphragm valve (or piston) across the back of the barrel (shown in blue). The barrel protrudes from the front of the pressure chamber, and the filling valve is inserted through the back wall. The large "release" valve is located on a short length of pipe or similar (such as a brass fitting) which extends slightly into the pressure chamber, just further than the filling valve.

During the relatively slow filling of the pressure chamber, air easily seeps around the edges of the diaphragm to fill both parts of the pressure chamber evenly, the front and the back. The large "release" valve shown at the left of the picture is for sudden depressurisation of the back part of the pressure chamber. The pressure in the forward part of the chamber cannot seep around the edge of the diaphragm quickly enough, so pushes the diaphragm off the barrel, allowing a very sudden (and dramatic) escape of pressurised air up the barrel. At the same time the diaphragm also seals the escape valve pipe, meaning minimal wasted pressure.

I've decided to use the second option because they look so much cooler, there's much less air behind the barrel that needs to be pressurised before we see action from the water or projectile, and the air restriction of the design is significantly lower.

Example: Even if the diaphragm opens just 5 mm around a 50 mm barrel, we have an opening of 8.4 cm² for the air to flow from the pressure chamber into the barrel. With the external sprinkler diaphragm valve, the diameter of the valve opening is likely to be only about 1.6 cm across (and probably less), giving a 2 cm² opening – a significant bottleneck for the compressed air.

Using a piston allows the cover over the end of the barrel to be suddenly withdrawn any desired amount, giving no bottleneck and GREAT performance

Example: A piston withdraws 2 cm, effectively giving a roughly 33 cm² opening around the end of the barrel. The inside of the barrel itself (Class D) has a 22 cm² cross-sectional area. Nice.

A safety consideration for choosing the second design is that the barrel is shielded by the pressure chamber. If the sudden in-rush of pressurised air is too much for the uPVC barrel, the part most likely to shatter (the bit sticking out the front of the pressure chamber) is further away from the person holding it.

One other very important consideration is the price of one of these solenoid valve things in New Zealand. I've been quoted $214 for one rated 7 bar (~100 psi). The second design above can use a much cheaper ball valve to blow the pressure in the back of the pressure chamber. (The price for the solenoid valve was from a company charging $20-$30 for a ball valve, which I found for just $5 at Chester's Plumbing.)

Pressure, Force, and Barrel Size:

The biggest consideration for barrel size is the amount of water (or the size of projectile) to be fired with it. But apart from that, there are several factors that could influence our decision.

Pressure is measured in force per area. This means that for the same pressure, a larger area will be subjected to a larger force. For example, with a potato projectile, the larger the diameter of your barrel, the more area of the back of the potato will be exposed to the pressure, and the more force will be acting to expel the potato. If the barrel diameter is doubled, the force on the spud will be four times greater.

Example: A 50 mm Class D uPVC pipe has an internal diameter of 53.35 mm. At 692 kPa (100 psi) this is a force of about 1570 newtons on the back surface of the spud (or almost twice what I weigh).

Other considerations, however, include the mass of the potato and the amount of friction between the potato and the barrel. If the length of the potato projectile is doubled along with its diameter (a not unreasonable assumption) the mass will be increased eightfold.

The friction of the potato with the barrel is dependent on the area of the potato in contact with the barrel. Doubling the diameter of the barrel and the length of the projectile (spud) will result in four times as much friction – double the circumference, double the length.

Of course, air resistance can also be considered. A small projectile will be more affected by air resistance (and even wind) than a larger and heavier projectile will be thanks to its smaller mass:cross-sectional area ratio. And there's a lot to be said for streamlining the front of a spud, and adding fins to the back of it...

Finally, the spud can't move faster than the air in the barrel. If the barrel is smaller, will the air want to move faster?

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