I wonder what they use in those potato and pumpkin cannons?

The only one I ever saw used a 2" iron pipe with a cap on the end tapped
for a spark plug. Some butane was sprayed into the pipe from a lighter
refill canister, which mixed with the air of course, then the potato was
jammed in almost to the bottom to compress the mix. Then, a 9V battery
was applied to an ignition coil hooked to the spark plug. Next time I
saw the potato it was hundreds of feet in the air. I didn't do this
myself BTW, and I didn't want to be anywere near it when it was
fired--I'm not crazy. I saw this experiment at an
all-engineering-students frat house back in my college days.

-Adam
adam at airraidsirens dot com