Misc Spud Gun

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.
Their popularity is primarily due to their low cost and simple construction.

Spudguns vary in size greatly, from launchers shorter than 6" (15cm) with .25" (.5 cm) barrels to rather immobile devices taller - and larger around - than a man, or even bigger if one decides that the trailer-mounted pneumatic launchers of the pumpkin-chucking competitions count.
However, most launchers are sized as to be easily man-portable and fireable, and thusly are shorter than 10' (3 meters) and not so powerful as to be uncontrollable.

All spudguns propel their projectile down their bore using compressed gas, in the same manner as a firearm (although at a pressure nearly several hundred times lower). Most enthusiasts divide them into three general categories by the means in which this gas is provided.

* By the release of compressed gas (normally air) through a valve; such a launcher is typically referred to as a "pneumatic launcher", and its pressure is limited primarily by the air compressors available.
* By the combustion of a gaseous fuel-air mixture, the heat of which causes the gasses to expand; this is generally referred to as a "combustion launcher", and it's pressure is limited primarily by the energy density of the fuel-air mixture (less than 100 psi (7 bar) with all safe fuels).
* By the combustion of a pre-pressurized fuel-air mixture; this is called a "hybrid" launcher, and yields higher pressures than that of a normal combustion spudgun, limited only be the construction of the launcher (generally a few hundred psi).

Combustion launchers

Combustion powered spudguns can be 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, 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 many factors, including the type of fuel used, the "idealness" of the fuel/air ratio, the size of the launcher, and the flight characteristics of the projectile, but 100 meters is fairly common distance.

High tech combustion launchers may include metered propane injection to ensure proper fuelling, chamber fans to mix the fuel with the air and accelerate venting, multiple spark gaps (spark strips) to decrease combustion time, 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.
Pneumatic launchers

Pneumatic launchers are a little more difficult to construct, but are generally more powerful. These guns have a few basic components:

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

In a pneumatic spudgun air is pumped into the pressure chamber. After the desired chamber pressure is reached, a valve of some sorts, usually a sprinkler or ball valve, is opened, causing the gasses to propel the projectile outwards. Pneumatic spudguns are generally more powerful than combustions; a typical combustion gun generates chamber pressures of 10-70 PSI / 69 - 483 kP , while the average pneumatic gun can operate at pressures in the vicinity of 100 psi / 690 kP.
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 combines principles of the combustion and pneumatic spudgun. It uses a pre-pressurized mixture of fuel and air to get more power out of a given chamber volume.
In order to fire, the operator first readies the pressure triggered valve, then several injects times the normal amount of fuel and air. When the ignition source is triggered, the pressure from the combustion causes the main valve to open and propels the projectile out of the barrel. The hybrid is capable of higher velocities than a combustion or pneumatic spudgun, due to the higher pressures generated and the higher speed of sound in the propelling gasses. 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.
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 modify their valves for direct pneumatic operation. Such modifications allow 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 airflow 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.
Safety

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

All launchers constructed from PVC have the potential to violently shatter, as PVC is a brittle material that will shatter into sharp (and fast moving) shards if placed under high stress. 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 Brisance fuels (especially acetylene or oxygen enriched mixtures), and impact (such as dropping a pressurized chamber onto concrete).
Higher-strength materials, such as aluminium, are generally safer as they fail at higher pressures and in a less spectacular fashion.

For a hybrid design, the piping used needs to be able to handle the high pressure (roughly 100 psi / 690 kP per atmosphere initial pressure), and remote ignition is frequently used.
Hybrids are generally used at a 4x propane/ 4 atmosphere mix or less, since higher ratio mixes increase the likelihood of DDT (energetic materials) (deflagration to detonation transition) occurring, which would cause an extreme pressure spike and possibly cause the gun to explode.

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 more brittle when cold, which increases the chance of it shattering. The definition of "cold" is not set.

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 - generally combustions and hybrids - are loud enough to require hearing protection.

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.

# 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.