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, though advanced designs with more power and usability are also possible. Most of the combustion spudguns are made out of PVC or ABS.
Though being the simplest cannon of all, a well-made pneumatic or hybrid is usually more powerful then a combustion. The pressures in a combustion usually do not rise above 50-60 psi on spray and pray cannons and about 70-90 psi on advanced cannons. Jimmy101, a member of spudfiles showed that in closed chamber experiments the maximum pressure possible with a propane mix is a little more then 100psi in a closed chamber.
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. Some combustion cannons have chambers made from clear pvc, this allows you to see the internals like the sparks and the fan (LED fans can be beautiful this way) and most importantly: a flash of fire on ignition.
Operation & Theory
In order to fire, the operator loads a projectile (usually a potato) down into the barrel, adds fuel to the combustion chamber, like spraying aerosols in the back end and screwing the cap on and then triggers the ignition source (e.g. a piezoelectric BBQ ignitor). The ignition system creates a spark in the chamber which ignites the fuel. The fuel quickly burns up and the hot gases want to expand, creating pressure. The pressure acts as force on the back of the projectile and pushes it rapidly down the barrel. Muzzle velocity and distance vary greatly mostly depending on the stoichiometry of the fuel/air mixture, the size and chamber to barrel ratio (C:B ratio) of the launcher and possible performance increasing additions like a chamber fan.
The most basic spudguns use aerosols like hairspray, deodorant or starting fluid sprayed in at the back (spray and pray), while more advanced cannons usually use propane injection. Besides the difference of low-power aerosols and pure fuels like propane there is not much of a difference between fuels, the power of propane and butane vary no more then a few percent. Achieving the right stoichiometry (right amount of fuel and air/oxygen) is usually more important for the performance. Spray and pray cannons often have poor stoichiometry and thus inconsistent shots with relatively low power. Fast burning fuels like hydrogen, acetylene and oxygen enriched mixtures are usually avoided, even if they may be a little more powerful, their fast burn rates and high chance of DDT create shockwaves that are powerful enough to rip any PVC or ABS chamber to shreds.
The ignition and combustion sequence in a clear barrel/chamber tennis ball launcher can be observed at various speeds in the videos below:
- Chamber fan
- Stun gun, flyback transformer or camera flash ignition
- Multiple spark gaps
- Fuel meter
- A good C:B ratio
- Breech loading mechanism
The above improvements will increase the power and/or usability of the gun. View those pages for more information.
Non pressure rated "DWV" pipe is used so now and then in combustion cannons and it is generally accepted that this is safe with basic spray and pray; however some of these cannons have blown up. If you want to be safe you should definitely use pressure rated pipe and fittings, especially on advanced combustions. As stated above specific fuels such as acetylene, hydrogen, and any oxygen enriched mixtures are not safe to use in a gun made out of PVC or ABS. When using a normal fuel, pressure rated pipe and fittings and have properly solvent welded all of the joints, you can be considered 100% safe.
Some people, like parents, think it is unsafe because it is an "explosion" and thus dangerous. In that case point them on the facts that it is just a deflagration and not a detonation creating no huge shockwaves, but creating just heat and pressure. This pressure is lower then the rated pressure of pressure rated pipe and fittings and that means the gun simply can't explode.
Note that cold weather makes pvc brittle and you should not use a frozen gun. However a gun that has been frozen some time but is up to normal temperature again is safe to use. Dropping the gun isn't recommended, especially frozen guns, since this might create tiny cracks; a starting point for completely cracking open and blowing up.
Laws are different in each country, state or even city, check the legal issues page for more information. In some countries combustion cannons are illegal while pneumatics (possibly being way more powerful) may be legal.