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AIR CANNON Page 4:
Cost, Specifications, Results

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Air Cannon page 1:
Pipe and Safety Considerations

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Air Cannon page 2:
Design Considerations

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Air Cannon page 3:
Construction, Testing

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Air Cannon page 5:
Gallery

Cost

All prices are in New Zealand dollars (and include GST).

Pipes:

40mm barrel, 65mm pressure chamber. These I got for free. They happened to be lying around at work and would never be used. I don't know what these sizes would cost, sorry, but I've been put off the idea of making a larger cannon with a 100mm pressure chamber because it costs about $50 per metre and now has to be bought in 6 metre lengths. (It would take ages to pump up, too.) Pity, because a 65mm pressure pipe is almost exactly the right size for tennis balls (and coke cans – just imagine a full one of those hitting you).

However, for potatoes a 40mm barrel is just right (or at about the upper limit). Anything larger would make finding big enough potatoes very hard.

Valves:

The ball valve I got for free from a friend who works for an air compressor company. They cost about $5 depending on size.

The car tyre valve I got for free from a friend who owns a tyre wholesale company. I don't think they cost very much.

Fittings, Miscellaneous:

65mm end cap $9.52. Sadly the first time I put the cannon together it didn't work at all. I learnt from that sad experience to tape it up and test before gluing. Add $10.33 for a socket and another $11.55 for the second cap. (Price rise?)

65mm – 50mm reducing socket $13.94, 50mm – 40mm bush $2.76. Both for the front of the cannon.

1/2" x 3/8" brass hex nipple (for the ball valve to mount on) $4.53, 15mm brass lock nut $1.43. Also bought was a "15mm polywasher" (to seal under the lock nut) for $0.18 which didn't make it into the finished version of the cannon.

Thread seal tape was supplied to me by an extraordinarily good friend who just happened to have a roll (or ten) to spare.

The diaphragm was made from a peanut butter jar lid that fits the inside of the 65mm pressure pipe perfectly, with the rubber lining each side from an old inner tube provided by the above mentioned tyre valve supplier.

Pressure pipe glue cost $8.18 for a 125mL pot (with brush) although I could have used the glue at my work. We use it for DWV pipes but it claims to be OK for use with pressure pipes also. However it wasn't a pretty blue colour like the stuff I bought.

Total:

$32.18 not counting mistakes and unused/unnecessary parts. (This is how much it would cost me to make another one, now that I know what I'm doing.)

$62.42 including everything. (I got many of the bits for free, which admittedly helps a lot.)

Again, all prices are in New Zealand dollars (and include GST).

Specifications

Barrel:

Internal diameter: 42.6mm
Length: 880mm
Volume: 1.25L (useful to know when planning on filling barrel with water in a remote location)
External diameter: 48.4mm

Pressure Chamber:

Internal diameter: 66.3mm
Length: 840mm
Volume: 1.40L (allowing for barrel in centre)

Total Weight:

Still to come.

The relative volumes of the barrel and pressure chamber mean that when a potato is ejected the pressure in the cannon has dropped to about 0.53 of the original pressure when the spud leaves the barrel. Seems to work. (See Results below.)

Using a concentric design has the advantage that the cannon is very compact, and due to the fast switching action of the peanut butter jar lid (thus providing no bottlenecks for air pressure), the cannon should be able to hold its own quite well against much larger separate-pipe designs.

Results

Pressure 40 psi:

Projectile: Small potato.
Distance at 45° firing angle: 130 m (very roughly, from pacing it out).
Height at 90° firing angle: 82 m (calculated from the very impressive 8.2 second "float" time, ignoring air resistance).
Muzzle velocity: 40.2 m/s (calculated from height, ignoring air resistance).

Not bad for a cannon still held together (at that stage) with brown parcel tape (and hence the 40 psi maximum pressure).

Pressure 70 psi:

Projectile: Small old potato.
Height: Undetermined – the potato was almost vaporised. I'll use a higher quality (and not so old) potato next time.

Lolly Scramble:

Projectile: Paper-wrapped lollies.
Pressure: 30 or 40 psi.
Result: The lollies were fired in a reasonably tight bunch (shotgun blast?) over the heads (like, way over the heads) of the participating children.
Notes: Only one bag of lollies was used, with a sock pushing them out of the barrel. A good lolly scramble would be wider spread with more lollies, but the air cannon certainly gets the distance on them.

Youngest Tester:

Projectile: Potato.
Pressure: 40 psi.
Distance at 45°: 110 m.
Age of tester: 8 years, 4 months.
Notes: He was pretty chuffed with that shot, having done most of the pumping as well. This illustrates the ease of use of the cannon. Being so compact, reasonably light weight and with so little kick, even a kid can use it. (FWIW one of his hobbies is making bombs.)

Long term update:

Date: 21 January 2002 (posted 26 January 2016).
Cannon status: Inoperable.
Notes: I believe the diaphragm has twisted inside the pressure chamber, meaning it doesn't seal well enough to slide back and forth. (I used to use a pencil to push it back into place against the back of the barrel.) It's also possible the centre of the diaphragm is cracked. Even reinforced with a layer of rubber each side it may not have been strong enough either for the pressure it was holding back from the barrel, or for being slammed against the ball valve tightening ring when the cannon was fired. Any fixing would result in a much shorter cannon. Some other time, maybe.

Date: 26 March 2017.
Cannon status: Semi-operable.
Notes: The diaphragm can move a little, and sometimes moves to sit against the back of the barrel. This is evidenced by a sort of bonging noise as the diaphagm hits the back of the barrel and causes a resonance. The cannon can then be pressurised, which I have done once with the cannon unloaded ('twas a pity to waste all that pressure) and once loaded with a rag. The rag was satisfyingly expelled about 11 metres horizontally. It would be nice if it was reliable.

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