Spud gun
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A spud gun refers to a classic child's toy which is used to fire small pieces of potato using a small volume of air at low pressure, or a larger pipe-based cannon which uses higher air pressure or combustion of a gaseous fuel to launch larger projectiles (usually potatoes). These launchers are very popular as hobby, or a weekend project.
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[edit] Launcher Types
All Potato guns propel their projectile down their barrel using pressurised gas in the same manner as a firearm (although at a much lower pressure) There are three basic ways that spudguns may achieve this:
- By the combustion of a gaseous fuel-air mixture; this is generally referred to as a combustion launcher, and its 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 release of compressed gas (normally air) through a valve; such a launcher is typically referred to as a pneumatic launcher, and its power is limited primarily by the pressure of the air supply, be that from a compressor, manual pump or bottled gas.
- By the combustion of a pre-pressurised fuel-air mixture; this is called a hybrid launcher, and yields higher pressures than that of a normal combustion spudgun, limited only by the construction of the launcher (generally a few hundred psi).
PVC, copper, or ABS pipes are commonly used in their construction.
[edit] Combustion Launchers
Combustion powered spudguns typically have the least complex designs, the four basic elements of which are:
- A fuel system
- A combustion chamber
- An ignition source
- A 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 igniter). The fuel then ignites, creating hot expanding gases, and forcing the projectile out of the barrel. Distances vary greatly depending on many factors, including the type of fuel used, the efficiency of the fuel/air ratio, the combustion chamber/barrel ration, and the flight characteristics of the projectile. 100 to 200 meters is a fairly common distance, with some cannons even exceeding 500 meters of range.[citation needed]
Advanced combustion launchers may include metered propane injection to ensure proper fueling, chamber fans to mix the fuel with the air and accelerate venting of the chamber after firing, 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 usually less powerful than their pneumatic or hybrid counterparts.
[edit] Pneumatic Launchers
Pneumatic launchers are considered a little more difficult to make build due to the need of a completely airtight construction. These cannons have four basic components:
- A filling valve
- An air chamber
- A pressure release valve
- A barrel
In a pneumatic spudgun air is pumped into the pressure chamber. After the desired chamber pressure is reached, the pressure release valve is opened, allowing the gas to expand down the barrel propelling the projectile outwards.
The filling valve is usually a commonly available type such as a Schrader or Presta valve but other assemblies to pressurise the cannon such as quick release connections with ball or check valves have been used.
The pressure release valve is often one or more of a variety of commercially available types such as plumbing ball valves, irrigation sprinkler valves and quick exhaust valves. Experienced builders often make their own valves for this purpose to gain greater flow and faster actuation. The most common custom designs are piston valves.
The range of pneumatic cannons is as, if not more variable than the ranges of combustion spudguns due to the increased variation possible in the components. Typical ranges are slightly higher because of the greater power, but the maximum range of some high power pneumatic cannons has been said to be over 1000 meters (1km).[citation needed]
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, or higher.[citation needed]
[edit] Hybrid Launchers
A hybrid launcher consists of seven basic elements:
- A fuel system (usually metered propane)
- An air filling valve (see pneumatic launchers)
- One or more pressure gauges
- A high pressure combustion chamber
- A pressure-triggered main valve (burst disk)
- A barrel
- An ignition source (see combustion launchers)
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 injects several times the normal amount of fuel and appropriately more 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 with the released combustion gases. The hybrid is capable of higher velocities than a combustion or pneumatic spudgun, due to the higher pressures generated and the higher speed of the shockwave. 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; hence the use of accurate air pressure gauges and fuel meters.
[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 or spring to achieve a faster opening speed.
- Sprinkler valve (otherwise known as a Solenoid valve) - The use of irrigation sprinkler valves as pneumatic valves has become increasingly popular for spudgunning. These valves are intended to be electronically triggered causing a solenoid to depressurize a diaphragm and allow airflow through. It is also possible to remove the solenoid and instead, to actuate the valve manually with a blowgun to depressurize the diaphragm. Such modifications allow the valve to open as much as 3 to 5 times faster.
- Diaphragm Valve - A diaphragm valve is used in pneumatic cannons where the barrel is within the air chamber. It is a disk of flexible material mounted directly behind the barrel that seals it when pressure is increased behind the disk. The design is such that air leaks past the diaphragm from behind it to the chamber around the barrel, sealing the soft rubber against the valve body. Once the chamber is fully compressed the compressed air behind the diaphragm is quickly vented, causing the diaphragm to fly backwards, exposing the butt of the barrel to the compressed air inside the chamber, which rapidly fills barrel, launching the projectile.
- Piston valve - The gold standard of pneumatic spudgunning is the piston valve, due to its extremely high rate of flow and opening speed. It works in an almost identical fashion to a diaphragm valve but replaces the flexible diaphragm with a hard rubber-faced piston. The valve opening 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.
- Quick Exhaust Valve (QEV) - a commercial piston or diaphragm valve in a metal body intended for the quick venting of pneumatic cylinders. In spudgunning they are ideal barrel sealing valves with faster opening times than custom piston valves and high flow rates. They can be commonly found in sizes from 1/8" to 1.5" and sometimes even larger models. It provides an easy option for inexperienced spud-gun builders but the cost is usually greater than for any other valve type. (An exception to this is within the UK, where QEVs are relatively inexpensive.)
- Burst Disk Valve - Used in a few pneumatics but primarily in hybrid cannons, burst valves are considered the ideal pressure release mechanism as they allow an unobstructed flow of high pressure air. Burst disks have no moving parts, making them very reliable. They are very appealing for hybrid cannons because they can withstand the low pressure gas mix in the chamber before ignition, yet fail as planned upon successful ignition. The high pressure combustion gases are released into the barrel at a rate which generates greater velocities than a mechanical valve could allow. Pneumatic cannon burst discs work similarly. The disc and projectile are loaded, and the chamber is pressurized until the disc ruptures. Unfortunately, this usually does not give the operators much in the way of control over timing, although a puncturing mechanism can be used. Burst disks are of no specific material and may consist of plastic or thin sheet metal or foil.
- Blowgun - A blowgun is a small hand held device used to blow away debris from a work area and is designed to be used attached to a compressed air line. It uses a sprung poppet valve operated by a lever to allow air through its body and out through a specially shaped nozzle. In spudgunning it is used to pilot larger valves - releasing small volumes of air to allow a piston or diaphragm to fly back and release much larger volumes of air. It is also used as the primary valve for small cannons which fire BB pellets and so do not require high air flow. Blowguns can be modified to increase airflow.
[edit] The Sound Barrier
It is rare for a spud gun to be powerful enough to break the sound barrier, although there are some cases of it happening using specialised designs. The spud guns used are typically hybrids, but some pneumatic cannons have achieved the feat, typically by using a special gas, such as helium. There is also one reported case of a combustion achieving super-sonic velocities.
The difficulty in breaking the barrier arises from the speed of the particles within the gas. If the projectile is traveling at, or near to that speed, then the gases simply cannot keep up with it to provide the accelerating force. The problem is solved by increasing the speed of the particles, either by:
- Using lighter molecules, as occurs when helium is used in a pneumatic.
- Heating the gases to far higher temperatures, and thus giving them more energy. This allows hybrids and combustions to achieve supersonic velocities.
- Using steel and much higher pressures of 800 psi or more, but achieving these pressures is difficult. CO2 gas, although it can reach these pressures, is not suitable due to its high density.
It should be noted that supersonic velocities may be attained in some rare case due to an excessive "dead space" between the air release valve and projectile. The blast of incoming air can raise the pressure so rapidly in this dead space that high temperatures, sometimes sufficient to achieve supersonic velocities, can occur.
[edit] 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.
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 high brissance fuels (especially acetylene or oxygen enriched mixtures),, low temperature, and impact (such as dropping a pressurized chamber onto concrete). Higher-strength materials, such as aluminum or copper, are generally safer as they fail at higher pressures and upon failure, leak rather than burst.[citation needed]
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]
Some spud guns - generally combustions and hybrids - are loud enough to require hearing protection. Some users will also recommend use of safety goggles to protect the eyes from flying shrapnel, usually from targets shot at short range.
The use of too much wadding or ammunition 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. The use of pressure rated fittings on both the chamber and the barrel can also prevent this, because the barrel would therefore be able to withstand the pressure, is also a means for preventing this from happening.
Don't be like Skeeter, always use them correctly
[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. See Spud Gun Legality.
[edit] External links
- UK Spudgun Community, European spud gunners welcome
- GardenBallistics.co.nr - Home made cannons, pyrotechnics and ballistic inventions
- How to Build and Shoot a Spud Gun!
- PVC Ballistics
- UIUC Cannon Videos
- Spudgun Technology Center: Great source of spudgun information
- NeoSpud.com
- Gas Gun Design Tool, simulation program for pneumatic guns
- Air Cannon Barrel Length Numerical Analysis
- Indepth Potato Gun How-To and Tips
- Website with physics of spudguns
- Large Vehicle-Mounted Spud Gun
- How to Build a Spud Gun
