[curiouscat]

Came across this pretty scholarly article that was fun too; they test the firing velocities of a potato cannon using a high speed camera and various propellants. Probably a strong nominee for the 2013 Ignoble Awards.

http://arxiv.org/abs/1305.0966

Authors: E.D.S. Courtney, M.W. Courtney (both working for the US Air Force, apparantly. )

My question is, whether we could have predicted acetylene as the best propellant from chemistry principles.

And could we do any better? What'd we try to use instead of acetylene if we wanted a higher velocity.



Abstract

A potato cannon was designed to accommodate several different experimental propellants and have a transparent barrel so the movement of the projectile could be recorded on high-speed video (at 2000 frames per second). Both combustion chamber and barrel were made of polyvinyl chloride (PVC). Five experimental propellants were tested: propane (C3H8), acetylene (C2H2), ethanol (C2H6O), methanol (CH4O), and butane (C4H10). The amount of each experimental propellant was calculated to approximate a stoichometric mixture and considering the Upper Flammability Limit (UFL) and the Lower Flammability Limit (LFL), which in turn were affected by the volume of the combustion chamber. Cylindrical projectiles were cut from raw potatoes so that there was an airtight fit, and each weighed 50 (+/- 0.5) grams. For each trial, position as a function of time was determined via frame by frame analysis. Five trials were taken for each experimental propellant and the results analyzed to compute velocity and acceleration as functions of time. Additional quantities including force on the potato and the pressure applied to the potato were also computed. For each experimental propellant, average velocity vs. barrel position curves were plotted. The most effective experimental propellant was defined as the one which accelerated the potato to the highest muzzle velocity. The experimental propellant acetylene performed the best on average (138.1 m/s), followed by methanol (48.2 m/s), butane (34.6 m/s), ethanol (33.3 m/s), and propane (27.9 m/s), respectively.

[Hunter2]

Probably Hydrogen/oxygen , 2:1 or Hydrogen/Chlorine 1:1. But careful probably the canon will be destroyed.