JWSmythe's Journal: Gas combustion expansion rates and golfball cannon

I was entertained looking at "spud guns". That is, guns that shoot potatoes. My thoughts went from potatoes to more interesting, and regular sized objects.

    I found a site talking about piercing a solid wood door (by mistake) with a tennis ball. Oops. :) I was thinking more life golf balls.

    One site I found claimed that with propane and atmospheric air, they achieved double the speed of sound (680 m/s or 1522 mph). That sounded unrealistic.

    So I was wondering, what are the combustion expansion rates of various available gasses. I figured with the intelligent people on here, someone may know.

    Propane and atmospheric air doesn't seem ideal. Most of the information I read pointed out a problem. After a single shot, the had to vent the combustion chamber, or it wouldn't fire again (not enough oxygen).

    So, here's my theoretical ideas.

    Propane/Oxygen, like from a small torch set available at any hardware store.

    MAPP gas/Oxygen

    Hydrogen/Oxygen, electrolyzed from water.

    Atomized gasoline and atmospheric air.

    Atomized aviation fuel (110LL) and NOS. :) Ok, I'm going a little overboard, and would probably blow the combustion chamber.

    Any are easily accessible, and could have good results, without the need to vent the chamber after each shot.

    I guess the other obvious question would be about the volume of expansion of the gas before combustion is complete. I saw some pictures of people using hair spray with an 8' barrel. I can't imagine the combustion created enough expansion to utilize that space, so it would actually slow it down towards the end of the barrel. I know properly sized firearms use the right size barrel, so the combustion is just almost finished by the time the bullet leaves the end of the barrel. Too much flash means there was still fuel to burn. No flash means the barrel was too long.

    This is all theoretical. I live in a lovely deed restricted residential community. I know any will go "BOOM" really nicely, so the neighbors may just complain a little. I'm just bothered that I couldn't find the combustion expansion properties of the gases.

    But someday, it may be fun to make one. :) I liked model rocketry and miniature blackpowder cannons as a kid, so this is just an extension of that. How can I make something go fast. :)

speed of venting?

[zogger]

You have to introduce the gases with a valve and metering and so on, seems just a tank of air from a normal compressor would work just as fast as most anything else, then you don't have to fool around with an additional tank of oxygen, although I can see the appeal of having it "just right" all the time by restricting it to pure gasses. I just googled around, spudtech.com seems to have a lot of nifty info and parts.

I actually want to make a small catapult just for launching watermelons/pumpkins/cabbages/ bu

Re:

[JWSmythe]

I figured it would pressurize an area to say 10psi of combustable mixture, and then ignite with a few car spark plugs. I saw some other nifty ways to ignite the fuel using stun guns and bolts, but a few spark plugs in series seemed to make more sense, since they're actually designed for it. :)

I thought about air pressure. The outlet of an air compressor is kinda small, but I could see pressurizing a larger area with a larger quick release to make the air move better. 120psi

Oh, I meant the air compressor

[zogger]

Just to clear the tube and get ready for the next shot with the propane, not as the only power source.

As to the figures you need, no idea on where to find them. I googled just a bit on it though, seems like it should be out there. Personally I think I only ever did a few shots with hairspray and tennis balls like 40 years ago. One of my little brothers was more the spud gun and tennis ball fiend in our family, I was just the normal firearms shooting guy (by far, always been a hobby of mine since gradeschool