Combustion cannon

2008-03-09T16:35:28Z

Bigbob12345:

[[image:Basic_combustion.JPG|right|thumb|350px|Basic combustion cannon]]
A '''combustion cannon''' is a cannon powered by the combustion of flammable fuel-air mixtures. A basic combustion cannon is the simplest one to build, and is therefore often the choice for first time builders.
bigbob edit, above it says they are the simplest to build this is not true. Pnuematics are much easier to build and oprerate.

==Basic elements==

* Fuel System
* Combustion [[Chamber]]
* [[Ignition source]]
* [[Barrel]]

==Operation & Theory==

In order to fire, the operator loads a projectile (usually a potato) into the barrel, adds [[fuel]] to the combustion chamber (for example [[aerosol]]s 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 and the [[stoichiometry]] of the fuel/air mixture, the size and [[chamber to barrel ratio]] (C:B ratio) of the launcher; 100 meters is a common distance.

A common question from beginning spudders is "'''What is the best fuel"'''? This can be a rather controversial topic but see [[Common Fuels for Combustion Spudguns]] for a reasonably comprehensive discussion of many of the common spudgun fuels.

Improvements on the basic combustion launcher include optimized C:B ratios, [[fuel meter|metered propane]] injection, [[chamber fan]]s, multiple [[spark gap]]s ([[spark strip]]s), and high voltage ignition sources ([[flyback circuit]]s, [[stun gun]]s, [[camera flash]]es, etc). Combustion launchers are mechanically simple, but are less powerful 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.

'''Combustion safety '''
Non pressure rated pipe is generally excepted as safe with combustions but pressure rated pipe is alway prefferable.
specific fuels such as acetelene, hydrogen, and any oxygen inriched mixture. They create pressures not suitable for any plastic chamber and could cause an explosion.

<nowiki>*</nowiki>The mass of the projectile is high enough that the faster cannon is not "solely" decided by the speed sound in the gas and the flow of the valve, and low enough that these things still matter. This will include almost any projectile you'll ever shoot.

==Related links==
[[Miniature spudguns]]

Hybrid cannon

2008-03-09T16:26:47Z

Bigbob12345:

[[Image:hybrid.jpg|right|thumb|300px| A very badly constructed hybrid cannon(it is made of pvc never use pvc unless you want shrapnel flying everywhere]]
A '''hybrid''' is a type of spudgun that combines principles of the [[combustion cannon|combustion]] and [[pneumatic cannon|pneumatic]] spudgun. It uses a pressurized mixture of fuel and air to get more power out of a given chamber volume than a normal combustion or pneumatic gun could.
The hybrid is capable of higher velocities than a combustion or pneumatic spudgun; projectiles fired by a hybrid have broken the sound barrier. This is possible because of the higher pressure and temperature gases generated by a hybrid combustion.

A hybrid launcher consists of seven basic elements:

*High pressure combustion chamber
*Pressure-triggered main valve (usually a [[burst disk]], see below)
*[[Barrel]]
*Fuel system (usually [[fuel meter|metered propane]])
*Air injection valve (usually schrader or quick connect)
*High-voltage [[Ignition source]]
*One or more [[pressure gauge]]s

==Operation theory==
A hybrid is first injected with a measured amount of fuel - much more than would be used in a regular combustion. However, without adequate oxygen, the mixture would be far too rich to ignite, so the chamber is then with 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.

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 gauge]]s and fuel [[fuel meter|meters]] are used for this.

The ignition of a hybrid gun is more difficult than a normal combustion, as the length of a spark is roughly inversely proportional to the density of the gas. This necessitates the use of high-powered [[ignition source]] such as a [[stungun]], flyback, or other high-voltage source; grill igniters frequently won't jump a spark gap in pressurized fuel/air environments.

The gases resulting from the combustion of the fuel/air mixture are released to the barrel via a self-operating valve of some sort, most commonly a [[burst disk]] because of it's simplicity and reliability, although self-venting [[piston valve|piston]]/[[diaphragm valve|diaphragm]] valves have been used. The valve is required to contain the pressure of the mixture pre-ignition, in other words, to prevent the pressurized fuel/air mixture from exiting out the barrel before ignition. Upon ignition of the fuel-air mix, the pressure rises and the valve opens.

===Safety===
A hybrid is one of the more dangerous types of launchers to build, apart from high pressure pneumatics. It is advised to only attempt to build one with pipe that can handle the high pressure environment (300+ [[Pressure gauge|PSI]]), generally SCH 40 steel or better. SCH 80 PVC has been used, but is not safe due to recurring fatiguing problems that have been observed in PVC hybrids and multiple failures even at low mixes. As with any launcher, the entire chamber is only rated to the spec of the lowest-rated part, so care must be taken when constructing the chamber of a hybrid. [[Remotely_activated_valve |Remote ignition]] is a must if PVC is used as a construction material, and is strongly advised for the initial testing of any hybrid. Although it was originally thought that mixes higher than 4x posed an unacceptable risk of DDT (deflagration to detonation transition), further calculations have revealed that the increased temperature of higher mixes barely counters the decreased burn rate cause by higher pre-ignition pressures, and that even a 20x mix will only burn a few m/s faster than a 1x mix. If DDT did occur, it would produce an extremely violent shock and pressure spike, which would cause any PVC chamber to shrapnel, and damage most steel or iron chambers beyond repair. The standard for hybrid construction is currently SCH 40 steel pipe and fittings, which have been tested in 2" sizes at mixes of up to 11x, repeatedly without failure. Theoretical burst pressure of SCH 40 2" steel pipe is >2kpsi, and no catastrophic failure has ever been achieved, accidentally or by design, in a hybrid made from safe materials.

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Combustion cannon

Hybrid cannon