A potato cannon or spudgun powered by compressed air rather than the combustion of a flammable gas or liquid.

It should be noted that all potato cannons are very dangerous.

Pneumatic guns can be much more powerful than their combustion-based counterparts. Several builders have reported ranges of three to four hundred yards. This increased performance is due to the higher operating pressure. Pneumatic guns operate at pressure of 40 to 100 psi. Combustion guns are believed to operate at pressures below about 30 psi.

Pneumatic guns are widely believed to be safer than combustion types, since they do not contain nasty stuff like fire and exploding flammable gasses. This Noder however, believes that the subject is open to debate. Safer or not, pneumatic guns are certainly more predictable and consistent. Combustion guns suffer from variation due to inconsistent loads of propellent, not-enough/too-much oxygen, differences of propellents, etc.

Most pneumatic cannons share the same basic design. Compressed air is contained in a pressure chamber. A valve opens to allow air to escape into the barrel. Air pressure pushes the projectile out the barrel at high speed.

The kinetic energy of the projectile can be roughly calculated like so.

KE = 1/2 * P * Vb * (1 + (Vc / (Vc + Vb))) - Pa * Vb

  • KE = kinetic energy of projectile (Joules)
  • Vb = volume of barrel (meters^3)
  • Vc = volume of pressure chamber (meters^3)
  • P = initial absolute pressure of chamber (newtons/meter^3)
  • Pa = atmosperic pressure (newtons/meter^3)

The above calculation make the following assumptions and ommisions:
  • Ignores friction of the projectile in the barrel.
  • Ignores friction of air moving through the valve.
  • Assumes isothermic and adiabatic expansion.
  • Ignores gravity.

A device similar to a potato cannon, in that it fires potatoes large distances. It is far superior, and in this node I'll give you a quick guide on how to build one.

The principle behind a pneumatic air cannon is pretty simple. You have one tube of PVC pressure pipe that holds compressed air (usually to the order of 100 PSI). It is separated from another pipe by a valve. When the valve is opened, the air is released into the second pipe, firing the projectile (usually a potato, but other things can be used).

They are relatively cheap to make. Mine cost about NZ$60 (US$30), but your mileage will vary widely depending on your location, and your supplier. All parts are available at your local plumbing supplies store, except for the tyre valve, which you should get at a mag wheels store.

The cannon described here is a basic "artillery" style one. There are many advances made on it, but this is the cheapest. I measured the range of mine as 130 metres!

What you need

  • 2 one metre lengths of 50mm PVC pressure pipe. This must be able to sustain pressures of at least 125 psi, else it will explode!
  • Valve mechanism. This consists of a 25mm ball valve, and adapters to connect it to the 2 pipes.
  • 1 endcap. Again pressure pipe
  • Car tyre valve
  • Really good solvent.
  • A pump. Not a foot pump The best sort is a 'T' type, that you push down on to operate. A bicycle pump probably won't do the trick unless you're really strong. One's designed for car tyres are best. Alternatively you could use an air compressor.

Construction

Drill a hole near the end of one of the pipes, and attach the valve, so that air can be pumped into the pipe.
Glue the endcap on the end of this pipe. This is the pressure chamber.

Attach the adapters to the pressure chamber and the barrel (the other 50ml pipe). You'll have to work this our yourself as there are heaps different possibilities as to how this works. Basically, you want the valve to be in-between the two pipes.

Once the glue is set, it's ready! You are now ready to fire potatoes and other vegetables large distances.

Operation

Get a potato of a size that it covers the entire end of the barrel. Jam it down to the bottom of the barrel with a broomstick.

Pump up the pressure chamber as much as you can.

Open the valve. You should hear a thoomp sound, followed by the scream of an innocent bystander you hadn't seen.

Getting the Most from Your Cannon

Now that you have your cannon, it's time to tweak it. The design you have is very basic, and there are many advances on it you can make.

Firstly, sharpen the barrel. This makes the potatoes cut better, producing a better seal.

Check all your joins. If there are any leaks, fix them with glue. If they are threaded joins, get some plumbing tape to seal them.

One of the most common things I've seen on the internet is to use a solenoid sprinkler valve instead of a ball valve. This opens much faster, and is electrically operated so you have a trigger. Keep in mind this will double the cost of your cannon.

Make the pressure chamber sit under the barrel. You can achieve this by attaching some elbow joins around the valve. This combined with a solenoid valve means you can carry it around like a gun!

You can look around for more information on different designs. The internet is rife with information, as there are many cannon fanatics out there.

Different Ammo

After a couple of weeks, you'll find that potatoes don't offer the same entertainment that they used to. Now it is time to use different ammo.

When looking to created more destruction, ice is your friend. A frozen orange will cause massive damage at high speed. Get a small portion of 50 ml pipe, and stick and orange or other food product in there. Freeze it over night. It will fit in your cannon, and go through walls.

While on the subject of ice, frozen water balloons are wicked as well. Lube it up, as the rubber will create a lot of friction.

Water (in liquid form). This is fun, half fill your barrel up and it will vaporise the water, giving you a refreshing mist on a hot summers day.

Confetti. Stick some confetti in there and shower your neighbourhood with happiness.

Safety

OK guys be sensible. These things are pretty safe, provided no one is in front of the barrel . I'm not exaggerating, this could kill someone if fired at close range.

Don't do what I did and do you're first test fire straight up in the air. It comes back down and it comes back down fast.

Keep it out of reach of irresponsible morons. The only exception to this rule is if the moron in question is you.

Oh yea, and these are illegal in lots of places, so don't get caught with it by a police officer. If they don’t know what it is they may mistake it for a really weird bong.

Have fun and don't damage/maim/hurt any thing I wouldn't

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