Definitions

Spud_gun_(cannon)

Spud gun (cannon)

This article is about pipe-based cannons and similar which use air pressure, or combustion of a gaseous fuel, or both, to launch large projectiles at low speed. They are in two types:

  • To fire whole potatoes. These launchers are built and fired as a hobby.
    • For the classic child's toy which is used to fire small pieces of potato using a small volume of air at low pressure, see spud gun (toy).
  • To fire other sorts of projectiles, for practical uses.

Launcher types

All spud guns propel projectiles down their barrels using pressurised gas in the same manner as a firearm (although at a much lower pressure). There are three basic ways that spud guns may achieve this:

  • By the combustion of a gaseous fuel-air mixture; this is generally called 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 explosion of a dry ice bomb placed in the pipe before the projectile, generally referred to as a dry ice bomb cannon, these are limited in power by the materials and size of the dry ice bomb but firing pressures can be around 200-300psi (14-21bar).
  • 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 spud gun, limited only by the construction of the launcher (generally a few hundred psi).

Combustion launchers

Combustion powered spud guns 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 (often using 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 ratio, and the flight characteristics of the projectile. Common distances vary from 100 to 200 metres, and there is a reported case of a cannon exceeding 500 metres of range.

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.).

Combustion launchers are usually less powerful than their pneumatic or hybrid counterparts.

Pneumatic launchers

Pneumatic launchers are considered a little more difficult to 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 spud gun, 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 forwards.

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 of a variety of commercially available types such as a plumbing ball valve, an irrigation sprinkler valve, or a quick exhaust valve. Experienced builders often make their own valves for this purpose to gain greater flow and faster actuation. The most common custom design used is the piston valve. Multiple valves arranged to be triggered together are occasionally used as an alternative to a single larger valve.

The range of pneumatic cannons is more variable than the range of combustion spud guns 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 metres (1km).

Pneumatic spud guns are generally more powerful than combustion spud guns. A typical combustion gun generates average chamber pressures of ~30 psi / ~200 kPa with peaks of around ~70-100 psi / ~500-700 kPa , while the average pneumatic gun can operate at pressures in the vicinity of 100 psi / 690 kPa. In recent times, it has become increasingly common for metal pneumatic launchers to use even higher pressures, in the region of ~300 psi / ~2100 kPa or higher.

Dry ice launchers

A dry ice cannon uses the sublimation of solid carbon dioxide to generate the gas pressure to propel a projectile.

The oldest examples simply involve dropping pieces of dry ice into a tube closed at one end and sealing the other end by jamming a projectile in. When the pressure of the gas being generated built high enough the projectile was blown out of the tube. The pressures of such devices are not very high as it only needs to build enough to overcome the friction of the projectile jammed in the barrel. Tens of psi is most likely.

A more modern example is the dry ice bomb launcher. A plastic bottle containing water has some dry ice added then is quickly sealed and dropped down a tube closed at the other end. A projectile is dropped in after it. The water accelerates the sublimation of the dry ice and the pressure from the carbon dioxide gas produced eventually ruptures the plastic bottle and launches the projectile. The rupturing pressure of a standard fizzy drinks bottle is between 200-300psi in the open air but confined in a chamber it could be higher.

Due to the operation of a dry ice bomb cannon extra safety issues are present:

  • The dry ice bomb used for propulsion can achieve bursting pressure in a matter of seconds to hours depending on the quantity of water and dry ice, it may also not achieve bursting pressure at all. These timing issues can cause the belief that it has failed to fire but attempting to unload the cannon may then provide the extra stress on the bottle needed for it to explode.
  • Piping and any reinforcement of such may be underestimating the high pressure spike when the dry ice bomb explodes and plastic especially can rupture with the bottle.
  • The recoil of such cannons can be very great due to the high pressure combined with the large internal diameter piping needed for the bottles (5-9cm) resulting in them being ill-suited for handheld firing.

Compared to the operation of other spud guns dry ice bomb cannons are similar in firing principle to a burst disk cannon of either pneumatic or hybrid type, the plastic bottle performing the task of a burst disk albeit in a less controlled manner.

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 spud gun. It uses a pre-pressurised 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 spud gun because the pressure generated is higher than that in a combustion gun (for most fuels), and the shock wave moves faster than it can in a pneumatic (for most gases), due to the higher temperature. Projectiles fired by a hybrid have broken the sound barrier.

A hybrid using a fuel and air mix at twice atmospheric pressure 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.

Primary materials

Plastics

  • PVC-U (Polyvinyl chloride, unplasticized) - Highly popular due to its availability and relatively low cost. PVC pipes are available in a wide variety of sizes and pressure ratings. In industry, however, they are illegal for compressed air applications—if they are damaged under pressure the plastic can fail explosively.
  • ABS (Acrylonitrile butadiene styrene) - Another popular plastic piping material, more expensive and less common than PVC but available in the same sizes and pressure ratings. Unlike PVC it is used in compressed air systems as it splits rather than shatters on failure. It also has a greater temperature tolerance (-40°C to +80°C compared to 0°C to +60°C for PVC).

Both PVC and ABS piping are also available in forms which are not pressure rated. Use of unrated plastic piping and fittings is a common source of cannon failure and possesses much more of a risk to a cannon operator.

Metals

  • Steel - The very high pressure rating of steel piping makes it a familiar sight on high-powered hybrid cannons. It is however much more expensive than any other common piping material. The extra weight and joining difficulties are also a consideration.
  • Copper - As a common plumbing material, copper pipes and associated fittings are readily available. They have higher pressure ratings and flow compared to similar plastic piping. The drawbacks are that copper is heavier, and up to four times the cost of PVC or ABS pipes of similar external diameters.
  • Aluminium - Aluminium is a lightweight metal with good corrosion resistance. Aluminium pipes are sometimes used as barrels on spud guns on their own and machined aluminium is a popular material for particularly unique designs.
  • Brass - Often brass fittings are used on spud guns for small parts of the construction like fuel systems, because it is one of the most common materials for small pipe fittings. Occasionally large parts of spud guns are machined entirely out of brass.

Valve types

Manual

  • 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.
  • 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 a small volume of air to allow a piston or diaphragm to fly back and release a much larger volume of air into the barrel. It is also used as the primary valve for small cannons which fire airsoft pellets and so do not require high air flow. Blowguns can be modified to increase airflow.

Electronic

  • 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 depressurise 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 depressurise the diaphragm. Such modifications allow the valve to open as much as 3 to 5 times faster.

Pneumatic

  • 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 butt of the barrel. Once the chamber is fully pressurised the compressed air behind the diaphragm is quickly vented, causing the centre of the diaphragm to flex backwards, exposing the butt of the barrel to the compressed air inside the chamber, which rapidly exhausts through the 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.
  • 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 pressurised 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.

Connections

Welding, soldering and gluing

  • Solvent welding - used for similar plastic connections using solvent fittings, the solvent temporarily dissolves the polymer chains of the plastic and the parts to be joined are brought together. On rehardening, the polymer chains from each part are entangled and so form a solid weld.
  • Metal welding - used to form strong joints between similar metals by melting the points of connection together. It is an uncommon process in spudgun construction due to the equipment necessary to make the welds.
  • Soldering - commonly used in the construction of copper pipe based spudguns, a solder with a lower melting point than the copper is melted and drawn into the gaps between pipe and fitting with capillary action, holding to pipe and fitting with a wetting action before hardening.
  • Gluing - the use of epoxy resin in small designs is common for the making of custom parts but it is rare to see glues used for structural connections. Copper glue such as JustForCopper has been used occasionally as a cold replacement for soldering in a copper based spudgun.

Mechanical joints

  • Compression Fittings - primarily seen on copper pipe spudguns, the compression fitting squeezes a metal ring against the pipe between a nut and the fitting body to form the connection. Easier than solder fittings and requiring only a spanner they are much more expensive and are of greater weight.
  • Threaded Fittings - commonly available in BSP or NPT (not interchangeable) they generally require a fitting attached to a pipe by other means to allow screwing into another threaded fitting. The exception to this is steel pipe, the ends of which can have the appropriate thread cut into them.
  • Flange joints - on large steel spudguns, pipe and fittings are sometimes bolted together by means of flanges with a gasket sandwiched between them to provide an airtight joint.
  • Cam Locks - on spudguns with interchangeable barrels a cam lock is sometimes used to connect barrels to the cannon as it provides a quick and simple solution to switching barrels. Two levers either side of the socket side of the fitting rotate internal cams to lock in or release the plug side of the fitting to which a barrel is attached.

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 this happening using specialized designs. The spud guns used are typically hybrids; but some pneumatic cannons have achieved the feat, either by using a special low density gas, such as helium, or high pressures combined with a fast valve. There is also one reported case of a combustion design achieving super-sonic velocities.

The difficulty in breaking the barrier arises from the speed of the particles within the gas. If the projectile is travelling 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.

Supersonic velocities may theoretically be attained by pneumatics with a sufficiently large "dead space" between the main valve and projectile. The incoming air can raise the pressure rapidly in this dead space, creating high temperatures sometimes sufficient to achieve supersonic velocities. This particular effect has not yet been successfully used, but has been discussed, as shock heating is a documented phenomenon in gases.

The highest projectile speed recorded from a spud gun is 933.3m/s (approximately 2.7 times the speed of sound) with a 16.6g 20mm plastic slug from a hybrid using a ~100 bar (~1450psi) pre-ignition mixture of air and propane. The highest velocity attained by a potato specifically is approximately 2700 fps (~823 m/s).

Practical uses

Although spudguns are created and used for the purpose of recreation there are other devices which work on identical principles in many other fields with more serious uses.

Entertainment

  • Promotional sports cannons: Portable pneumatic cannons which run on bottled CO2 are common at large sports games in the US where they are used to project items such as t-shirts or wrapped food into the audience. They tend to be made of higher quality materials than an average pneumatic spudgun but they use the same methods of operation.
  • Special effects cannons: In film and theatre productions pneumatic cannons are often used as a pyrotechnic-free method of material projection. These can vary from simple ball valve, manually operated models to electronically triggered designs operated from a remote control panel depending on the exact requirements.

Industry

  • Hail cannons: these are very large devices which consist of a combustion chamber and a large funnel shape mounted on top of it. A gas mix is ignited in the combustion chamber and the funnel directs the blast wave upwards. They are intended to protect crops from hail damage by disrupting hail formation with the shock waves. There has however been no scientific proof of their effectiveness.
  • Air Cannons: This can mean:
    • A pneumatic spudgun
    • Air blaster, a compressed air device to unblock clogging in large storage containers (e.g. silos) for powdery material.
    • Air cannon (mechanics), a compressed air device for creating high pressure shock waves under water
  • bird scarers: these devices are essentially automatic combustion cannons. They run off bottled propane gas and a lead-acid battery. At intervals they ignite a propane/air mix to produce a loud explosion (up to 150 decibels close to the device) to scare birds from crop fields or near runways where birds can cause a hazard to aircraft.
  • Birdstrike simulators: pneumatic cannons are used to simulate the impact of birds on aircraft. Extensive testing is required to make sure that the common occurrence of birdstrike will not break windshields, puncture fuselages or leading edges, or impair engine function.

Military

  • Combustion light gas guns are weaponised combustion cannons which burn a low molecular weight gas such as hydrogen to provide a higher specific impulse than relatively high molecular weight conventional solid propellants.

Safety

Spud guns by nature are hazardous and can present safety issues if poorly constructed or used.

Use of basic safety equipment such as goggles and ear defenders is recommended, and it is essential to follow conventional Gun safety rules. Firing potato cannons at other people is strongly discouraged, as they may cause significant injury, or in extreme cases, even death.

Legal Issues

See Article: Spud Gun Legality
In some jurisdictions spud guns are outlawed or have restrictions on their use and may require licenses and certification of the gun.

In popular culture

  • A character in the British TV series Bottom was named after this toy.
  • In the Bugsy Malone movie, a similar device called a "splurge gun" fires custard pies
  • In The Trailer Park Boys, Julian, Ricky, Bubbles, Corey and Trevor use spud guns to investigate the enormous damage to their crops of cannabis. This was because real guns would attract too much attention.
  • In the film Tremors 3: Back to Perfection, desert Jack suggested to Burt that they could build a potato gun (Spud gun) to fight the Ass-Blasters after Burt's bunker & entire armoury was destroyed.
  • In an episode of Nickelodeon's Drake and Josh, Drake buys a potato gun, accidentally hitting Josh, making him drop the weights he was lifting, causing him to break his foot.
  • Dwight Schrute on the U.S. version of The Office has mentioned owning a spud gun in more than one episode.

See also

Other types of pneumatic gun

References

External links

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