A
potato cannon or
spudgun powered by compressed air rather than the combustion of a flammable gas or liquid.
It should be noted that all potato cannons are
very dangerous.
Pneumatic guns can be much more powerful than their combustion-based counterparts. Several builders have reported ranges of three to four hundred yards. This increased performance is due to the higher operating pressure. Pneumatic guns operate at pressure of 40 to 100 psi. Combustion guns are believed to operate at pressures below about 30 psi.
Pneumatic guns are widely believed to be safer than combustion types, since they do not contain nasty stuff like fire and exploding flammable gasses.
This Noder however, believes that the subject is open to debate. Safer or not, pneumatic guns are certainly more predictable and consistent. Combustion guns suffer from variation due to inconsistent loads of propellent, not-enough/too-much oxygen, differences of propellents, etc.
Most pneumatic cannons share the same basic design. Compressed air is contained in a pressure chamber. A valve opens to allow air to escape into the barrel. Air pressure pushes the projectile out the barrel at high speed.
The kinetic energy of the projectile can be roughly calculated like so.
KE = 1/2 * P * Vb * (1 + (Vc / (Vc + Vb))) - Pa * Vb
- KE = kinetic energy of projectile (Joules)
- Vb = volume of barrel (meters^3)
- Vc = volume of pressure chamber (meters^3)
- P = initial absolute pressure of chamber (newtons/meter^3)
- Pa = atmosperic pressure (newtons/meter^3)
The above calculation make the following assumptions and ommisions:
- Ignores friction of the projectile in the barrel.
- Ignores friction of air moving through the valve.
- Assumes isothermic and adiabatic expansion.
- Ignores gravity.