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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).

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 vapourised. 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.)

More testing to come as time allows.

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