Spud gun

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Spud guns, potato cannons, potato guns, or produce accelerators are all names for a recreational projectile launching device usually made of PVC, copper, or ABS pipes.

There are two common means to launch projectiles from a spud gun. 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 create an exothermic reaction between the fuel and oxygen which causes the mixture to expand. A modification of a basic combustion cannon, known as a "hybrid", combusts a pressurized fuel-air mixture, yielding higher muzzle velocities. The object of these varieties is to propel "non-destructive" projectiles in the direction of a designated targets for fun. Common projectiles include: mandarins, marbles, 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 aerosol powered gun costs no more than US$30 in PVC and glue, less if parts can be scrounged.

A pneumatic potato cannon
Enlarge
A pneumatic potato cannon

Contents

[edit] Combustion launchers

Combustion powered spudguns are the least complex to build. They have four basic elements:

  • Fuel system
  • Combustion chamber
  • Ignition source
  • Barrel

In order to fire, the operator loads a projectile into the barrel, so there 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 at about 300 ft/s (200 mph). Distances vary greatly depending on the type of fuel used, the size of the launcher, and the "idealness" of the fuel/air ratio; 100 meters 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). Despite their ease of construction, combustion launchers are less powerful than their pneumatic counterparts. Spud guns are often painted to increase their aesthetic value and to cover unsightly primer stains.

[edit] Pneumatic launchers

Pneumatic launchers are a little more difficult to construct, but the results are much better. These guns have a few basic components:

  • A pressurized air chamber
  • A pressure release system, typically a solenoid sprinkler or ball valve
  • A barrel

The concept behind a pneumatic potato launcher is relatively simple. Like combustion guns, pneumatic spudguns use pressurized gas to force the projectile out of the barrel, but there are no explosions involved. Instead, air is pumped into the pressure chamber, usually with a common bike pump. Some variations use pressurized air tanks, and PSI (Pounds per Square Inch) meters. When the desired pressure is reached within the chamber, a release mechanism, usually a sprinkler or ball valve, is triggered, causing the gases to propel the projectile outwards. Pneumatic spudguns are generally more powerful; a typical combustion gun can generate chamber pressures of 10-50 PSI, while the average pneumatic gun can easily generate pressures well over 80 PSI. With a little patience, pressures of over 100 PSI can also be made. If you have an air compressor, it will make the launching faster. However, there is a tradeoff : The increased pressure of pneumatic guns make them more susceptible to bursting than combustion guns. On the plus side to this you do not need to worry about the PVC melting. But with the proper PSI measuring equipment, this risk can easily be avoided.

[edit] 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
  • Ignition source
  • One or more pressure gauges

A hybrid is a type of spud gun 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. 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, due to denser fuel/air mixtures. Projectiles fired by a hybrid have broken the sound barrier. 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.

[edit] Valve Types

Ball Valve - Made out of either plastic or metal, ball valves are considered inferior by many enthusiasts due to their slow opening times. For those on a tight budget or in low-power setups, ball valves are ideal. Some choose to modify their valves by attaching a pneumatic actuator. This setup can achieve better performance than manual opening.

Solenoid valve ("sprinkler" valve) - The use of irrigation sprinklers as pneumatic valves has become increasingly popular. These valves are usually electronically triggered, although some spud gun enthusiasts choose to "mod" their valves. This involves removing the solenoid and tapping the valve with a manual release, such as a quick-exhaust valve (QEV) or a blowgun. This effectively widens the exhaust port, allowing the valve to open much quicker.

Piston Valve - The gold standard of pneumatic spud gunning is the piston valve, due to its extremely high rate of flow and opening speed. The valve porting is generally as wide as or wider than the barrel diameter, so there is very little constriction of airflow. Piston valves also open much faster than either ball or solenoid valves. However, construction of this type of valve is inherently complex, and some choose to order pre-built valves through the internet.

Burst Disk Valve - Used in pneumatics and primarily in hybrid cannons, burst valves are the ideal pressure release mechanism because they allow full air flow instantaneously. They have no moving parts, making them extremely reliable. This makes them appealing for hybrid cannons because there are no sensitive valve parts that would be destroyed by the deflagration. 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. When the ignition source is triggered, the pressure from the combustion ruptures the burst disk and propels the projectile out of the barrel. Pneumatic burst discs work similarly. The disc and projectile are loaded, and the chamber is pressurized until the disc ruptures. Unfortunately, this does not give the operators much in the way of control over timing. A sheet of aluminum foil withstands approximately 15 psi before rupturing, subsequent sheets can be added to increase total pressure.

[edit] Safety

While all spudguns have the capability to seriously injure or possibly kill their operater or bystanders, when built correctly and used in a responsible manner they can be operated in relative safety. Spudguns can be dangerous and should be treated in the same way real firearms would be.

All launchers constructed from PVC have the potential to "grenade," or violently shatter the chamber. PVC is a brittle material that will shatter if placed under high stress. In chamber failure, PVC piping fragments into shards that are propelled by the pressurized gas behind them. There are many reasons that PVC piping can fail under pressure; manufacturing defects, microscopic stress fractures, use of cellular core pipe, over pressurization, use of powerful fuels (acetylene, MAPP gas, oxygen, etc...), and impact (such as dropping a pressurized chamber onto concrete) are some of the many possible scenarios in which a spud gun can fail with drastic consequences. Higher-strength materials, such as aluminum, are generally safer as they can withstand much greater pressures before failing.

For a hybrid design, piping needs to be able to handle the high pressure environment (200+ PSI) such as Sch. 80 PVC or metal, and remote ignition is nearly always used. The limit of a 4x propane/ 4 atmosphere mix is usually observed, since higher ratio mixes can cause DDT (deflagration to detonation transition) to occur, causing an extreme pressure spike, possibly shattering the gun and causing injury. [citation needed]

Improperly designed or assembled spud guns can also 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 more so in a hybrid. PVC becomes brittle in low temperatures, and can be unsafe (especially for pneumatic use) when it is below freezing. The use of a spud gun which has developed visible structural defects (e.g. cracks) or has not been constructed properly is dangerous and can result in serious injury. Some spud guns are loud enough to require hearing protection.

The use of duct tape around the combustion chamber in two layers will prevent shrapnel from flying in all directions should the chamber decide to explode.The duct tape should be stretched as it is wrapped around the chamber.

Any projectile that does not form an adequate seal can be wrapped in toilet paper and then stuffed into the tube. The use of a stick to push the projectile down into the chamber will indicate the necessary resistance that will be given when fired.

The use of too much wadding while loading a spud gun can also cause a failure. As the pressure from the chamber is released into the barrel, excessive wadding can prevent the projectile from firing, instead compressing the barrel, and thus causing a failure of the joint between the barrel and the valve. If this occurs, the entire barrel may detach with a great deal of force, as illustrated in this example. Using a reasonable amount of wadding prevents this from happening.

When using a carbon steel cannon with hydrogen gas as fuel, a process called embrittlement occurs, in which the hydrogen reacts with the carbon to create methane, leaving the metal structurally weak. Embrittlement leaves the cannon severely vulnerable to failure. Hydrogen is also highly discouraged as a fuel because of the extreme risk of accidental ignition.[1]

[edit] Legal issues

In many countries spud guns are outlawed or have restrictions on their use and may require licenses and certification of the gun.

  • United States: Not federally regulated (unless the ATF finds that it's classified as something else, such as a destructive device), however legislation varies widely by state, county, and township. Check with local authorities to make sure spud guns are legal in that area.
  • Australia: legal in most states and territories
  • New Zealand: All combustion spud guns are considered firearms.
  • Germany: All combustion spud guns are considered firearms.
  • United Kingdom: pneumatic spud guns with projectile energy greater than 4 joules (3 ft·lbf) are classified as section 1 firearms and don't require a license. In recent years, with the rise of spud gun use, there has often been much debate as to whether or not spud guns should actually be classed as Light Air Weapons. If spud guns were to be reclassified, then their maximum lawful projectile energy would be 12 ft/lbf, not 3.
  • Canada: No federal or provincial law regulates Spud Guns.

Other notes:

  • Flaming, explosive, black powder, or living projectiles can often make a legal spud gun illegal in many jurisdictions.
  • Many heavily-populated areas have ordinances on projectiles and loud noise.
  • While combustion cannons may be legal in a given area, stun guns, commonly used for ignition, are illegal in many states.
This list is incomplete; you can help by expanding it.

[edit] External links

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