Acetylene

Acetylene (chemical formula C₂H₂) is a flammable gas used for welding and cutting. Its low DDT runup distance, high flame front speed, and extreme reactivity generally make it a poor spudgun fuel. The molecule contains an unstable and energetic Carbon - Carbon triple bond that is easily broken.

Acetylene is a very reactive gas, with an unusually wide range of flammability, 2.5 to 82%, in air. It can decompose spontaneously with no air present if the pressure of the gas exceeds just 15 psi. It is more prone to detonation than any other gas except perhaps hydrogen. Acetylene also has an extremely low ignition energy, and even the static charges developed in plastic pipes can ignite it.

In addition to these dangers, the gas doesn't even contain as much energy per volume as propane, it's gross calorific value is 56 MJ/m³ as opposed to propane's 101 MJ/m³. However, since acetylene requires only one half as much oxygen for complete combustion (compared to propane) the net energy per unit volume chamber is very similar.

2C₂H₂ + 5O₂ = 4CO₂ + 2H₂0

Moles oxygen / mole acetylene = 2.5

C₃H₈ + 5O₂ = 3CO₂ + 4H₂0

Moles oxygen / mole propane = 5

The potentially destructive power of acetylene comes from the extremely rapid release of energy that creates very high pressure spikes. Acetylene can create slightly higher pressure levels than propane, but the hazards associated with acetylene make it unusable in spudguns. Acetylene is particularly hazardous in hybrids.

Sources

Acetylene is available as pressurized cylinders from welding suppliers. To stabilize the pressurized acetylene gas, acetone and a porous material (frequently crushed brick) is added to the cylinder.

Another common source of acetylene is Calcium Carbide (chemical formula CaC₂). Calcium carbide is a stable solid and is readily available as BangSite®. When calcium carbide is mixed with water, acetylene gas is produced.

Big-Bang Cannons (noise makers)

Big_Bang Cannons are noise makers that use acetylene, produced by the reaction of calcium carbide and water, that is ignited with a flint sparker. These cannons have been produced in the US for nearly 100 years. Because of the hazards of acetylene combustion this type of cannon is never fired with any type of a projectile in the barrel.

Safety and use in spudguns

The combination of calcium carbide and water is considered too hazardous for use in spudguns for two reasons.

1. In a closed chamber it is possible for the acetylene produced to auto-ignite.

2. The potential for detonation (instead of deflagration) is too great with acetylene.