Potato cannon
Spud guns, potato cannons, or produce accelerators are all recreational launching devices usually made of polyvinyl chloride (PVC) water pipe. There are two common means to launch projectiles from a spudgun. The first uses the energy stored in a compressed gas and the second, more popular method, uses the energy stored in combustible fuel vapors to heat the air and cause it to expand. There is a quite rare mix of the two, a "hybrid", that combusts a pressurized fuel-air mixture. The object of these varieties is to propel "non-destructive" projectiles in the direction of a designated (safe) target for fun. Common projectiles include: mandarins, potatoes, tennis balls, and golf balls. They are a particularly popular home-made class of projectile launchers because of their low cost to build: a simple hairspray powered gun costs no more than $20 in PVC and glue, less if parts can be scrounged.
Combustion launchers
Combustion powered spudguns have four basic elements:
- Fuel system
- Combustion chamber
- Ignition source
- Barrel
In order to fire, the operator loads a projectile (usually a potato) into the barrel, adds fuel to the combustion chamber (for example aerosols or propane), and triggers the ignition source (e.g. a piezoelectric BBQ ignitor). The fuel should then ignite, create hot expanding gases, and force the projectile out of the barrel. Distances vary greatly depending on the type of fuel used, the size of the launcher, and the "idealness" of the fuel/air ratio; 100 metres is a common distance.
High tech combustion launchers may now include metered propane injection, chamber fans, multiple spark gaps (spark strips), and high voltage ignition sources (flyback circuits, stun guns, camera flashes, etc). Combustion launchers are mechanically simple, but are weaker in typical situations compared to a pneumatic launcher of the same size. Potato guns are often painted to increase their aesthetic value and to cover unsightly primer stains. Krylon Fusion, a type of paint specifically made for plastics, is the most common type of spray paint used on potato cannons.
Pneumatic launchers
Pneumatic launchers consist of five basic elements:
- Compressed gas storage reservoir (chamber for short)
- Filling valve (bicycle Schrader valves, quick-disconnects, etc.)
- Main valve (ball valves, sprinkler valves, piston valves, quick-exhaust valves, and burst disks are the most common)
- Barrel
- Pressure gauge (for safety & repeatable launches)
In order to fire, the operator loads a projectile into the barrel, fills the chamber with a compressed gas (air compressors, hand pumps, and regulated bottled gas are common filling methods), and activates the main valve thus dumping the air in the chamber to the barrel.
The expansion of the compressed gas propels the projectile down the barrel. The barrel length determines the amount of energy transferred to the projectile and ultimately the velocity of the projectile.
High performance launchers use techniques such as modifying valves for faster opening times, using homemade valves, and maximizing the amount of airflow through the valve.
Hybrid launchers
A hybrid launcher consists of seven basic elements
- High pressure combustion chamber
- Pressure-triggered main valve (usually a burst disk)
- Barrel
- Fuel system (usually metered propane)
- Air filling valve
- Ingition source
- One or more pressure gauges
A hybrid is a type of spudgun that combines principles of the combustion and pneumatic spudgun. It uses a pressurized mixture of fuel and air to get more power out of a given chamber volume. In order to fire, the operator first inserts a burst disk, usually consisting of aluminium foil or plastic, between the chamber and barrel. A cam coupling or union is used to hold the burst disk in place. The operator then loads projectile into the barrel, adds fuel to the combustion chamber, and then adds pressurized air. The amount of air depends on how much fuel was added, and is calculated to achieve a stoichiometric fuel/air ratio in the chamber. when the ignition source is triggered, the pressure from the combustion ruptures the burst disk and propels the projectile out of the barrel. The hybrid is capable of higher velocities than a combustion or pneumatic spudgun, projectiles fired by a hybrid have broken the sound barrier (Source). A hybrid using twice as much fuel and air as a comparable combustion gun is said to be using a 2X mix, higher mixtures can be used and will produce even higher pressures. The fuel and air needs to be measured and matched carefully to ensure reliable operation, pressure gauges and fuel meters are used for this. A pressure gauge with a check valve can also be added to record the peak chamber pressure of the gun.
SAFETY:
A hybrid is the most dangerous launcher that can be built. It is advised to build with pipe that can handle the high pressure environment (200+ PSI) such as Sch. 80 PVC or metal. Remote ignition is a must, and the limit of a 4x propane/ 4 atmosphere mix should never be breached. If a higher mix is attempted, DDT (deflagration to detonation transition) can occur, causing an extreme pressure spike. Most pipe cannot handle the spike, and will shatter, launching plastic shards every where. Therefore utmost care must be taken with these launchers.
Performance issues
Some tricks to improve the performance of a pneumatic launcher include:
- Oiling the barrel (the less friction and resistance, the faster the potato will shoot)
- Lubricating the main valve (if it's a ball valve) with dish soap. It is bad to use oil, as it will cause deterioration of the ball seal. Lubricating the valve makes it easier, and therefore faster, to open the valve)
- Obtaining higher pressure piping and a higher pressure pump. Most PVC pipes have a maximum pressure of 280 lbf/in² (1.9 MPa), larger pipes normally have lower ratings. Higher pressure launchers can theoretically shoot potatoes farther.
- Be sure to file down the end of the barrel so that when you load a potato it is trimmed by the sharpened edges to the correct size. This kind of modification is called a Muzzle Knife.
To improve the performance of a combustion launcher try:
- Patching the potato with polyethylene plastic - often you can cut patches from the same bag the potatoes came in - will improve both accuracy and range. To use a patch, place the patch (about twice the barrel diameter) over the muzzle then load the pre-sized projectile. Be careful about chamber pressure - patching can raise pressures considerably.
- Blowing out the barrel and combustion chamber after each shot to remove the burnt gasses will promote more even and predictable combustion.
- Use less fuel than you expect - a little goes a long way and you will not get ignition with an over-rich mixture.
Safety issues
Improperly designed or assembled spud guns can be unsafe. Using proper assembly techniques and correct materials (pressure rated pipe and fittings) is essential. This is especially important in pneumatic guns, and even moreso in a hybrid. It should be noted that PVC becomes brittle in low temperatures, and can be unsafe (especially for pneumatic use) when it's below freezing. The use of a spud gun which has developed visible structural defects (ie. cracks) or has not been constructed properly is dangerous and can result in serious injury. Safety is paramount while operating a spudgun; treat them with the same respect you would a real gun. Some spud guns are loud enough to require hearing protection.
Legal issues
The United States Bureau of Alcohol, Tobacco, and Firearms does not classify spud guns as firearms. However, certain states and most cities have outlawed them. Check with your local law agency before shooting a spudgun to determine their legality. Never fire a spudgun within city limits, there are too many dangers involved by doing so. If authorities tell you not to use a spudgun in a certain area, it is in your best interest to follow their directions.
Vandalism that involves the use of spudguns contributes to the illegality of the hobby, so if you know someone who intends to use a spudgun for such purposes, alert authorities immediately. The use of dangerous, explosive and/or flammable projectiles is illegal and not condoned by the spud gunning community.
All spud guns have been classified as illegal in Western Australia.
All combustion spud guns are considered firearms in Germany.
Pneumatic spud guns are illegal to possess if you are under 16 in New York.
In the UK, pneumatic spud guns with projectile energy greater than 4 joules (3 ft·lbf) are classified as section 1 firearms and require a license.
In Walnut Creek, California, potato cannons are considered legal.