We use to make tennis ball cannons when I was a kid. Sometimes you might launch potatoes out of them as well. They were made of steel soup cans with the tops/bottoms cut out and held together with electrical tape.
The bottom can would be the combustion chamber with only the top cut out, and a small whole at the very bottom side to put a match to. You'd basically squirt a bunch of lighter fluid into the top of the contraption, then whirl it around to get the fluid to mix with the air, then drop a tennis ball (or potato) down the barrel, aim, and put a match to the small whole... and boom! We could launch stuff about 100 feet or so.
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