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First cannon a failure, need advice for 2nd attempt.

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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First cannon a failure, need advice for 2nd attempt.

Unread postAuthor: AJChambers91 » Mon May 26, 2008 11:57 pm

ill admit that i didnt put quality time and effort into this first one but i still dont understand why it wouldnt shoot. it is about 4 feet long and is just one single piece of pvc pipe. had bbq sparker attatched to screws near the bottome that made small gap inside of pipe. ANYWAY, this time around i want to make a cannon that will fire well but i dont really know where start. i know i want combustion and i know i want to keep it simple. Can someone help me out?[/img]
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Unread postAuthor: Lentamentalisk » Tue May 27, 2008 12:10 am

did the sparker spark when you clicked it when it was in the cannon?
what were you using as fuel?
what diameter pipe did you use?
did you fuel it through a cleanout cap in the back?
can you show us a picture?
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Unread postAuthor: TurboSuper » Tue May 27, 2008 5:06 am

What were you fueling it with? I remember it took me 4 days or so to get my first cannon working. Getting the right fuel mixture can be a [female doggy], even if the sparker is working.

I'd recommend adding a chamber fan to improve reliability. If you're really feeling ambitious, go for metered propane :wink:

About a chamber fan, if you don't know already:

-Just a computer fan mounted inside the chamber;
-No, the combustion process won't blow it into little itty-bitty pieces;
-You can run it off 9-18V reliably.
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Unread postAuthor: XxtriviumxX » Tue May 27, 2008 5:48 am

Remember not to use to much fuel... :wink:

You do need oxygen to attain ignition...

Some pics would really help... :)
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Unread postAuthor: AJChambers91 » Tue May 27, 2008 7:28 am

k ill post a pic for u guys after school...the diameter was about 1 1/2". we tried a lot of fuels and different amounts.
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Unread postAuthor: jimmy101 » Tue May 27, 2008 12:40 pm

Most likely problem is you were using way too much fuel. Gaseous fuels have "combustion limits", too much or too little fuel won't ignite. You might want to take a look at these two pages;
http://www.spudfiles.com/spud_wiki/inde ... n_Spudguns
http://www.inpharmix.com/jps/Combustion_fuels.html

Second most likely problem is the right amount of fuel but it is poorly mixed. A 1.5" diameter chamber will mix pretty slowly. Put some packing peanuts in the chamber and invert it several times before firing to try to mix the fuel with the air. Better yet, install some kind of fan. (It is kind of hard to get a fan small enough to fit in 1.5" pipe.)

Since this is a "mono gun" (chamber and barrel are the same piece of pipe) you'll need a way to reproducably position the spud in the pipe. It will make a big difference if sometimes the spud the 3/4 of the way from the muzzle to the breach and other times it is only half way.
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Unread postAuthor: AJChambers91 » Tue May 27, 2008 5:49 pm

ok in this picture the nice blues spark is visible Image
this is the entire cannon Image
this is the ignitor wiring with screws Image


After doing some research today during study hall i realised that the C:B ratio should be 1.5:1 and in this case it is nowhere near that and probably never would have worked. i think i understand the basics of spud cannons a little better now...im thinking of making my next one with following basic dimensions:

2"diameter 40" long barrel Volume:125.66 cubic inches
4" diameter 15" long chamber Volume:188.49 cubic inches

After a little algebra (never thought id need it while sittin in class) i found some dimensions that agreed with each other. would someone please share their opinion on these?
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Unread postAuthor: XxtriviumxX » Wed May 28, 2008 3:16 am

**Quoted from the wiki**

The goal of an optimal ratio is to have the barrel end at the exact point where the projectile stops accelerating.

The ratio will vary somewhat depending on the weight, friction, diameter and blow by of a particular projectile, as well as the fuel, ignition type and perhaps other variables. In most cases though, the optimal C:B is expected to be in the 0.6 to 0.8 range.


What you have is a bit off... Though a larger chamber volume, makes a louder "bang" :D
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Unread postAuthor: BigGrib » Wed May 28, 2008 3:51 am

So looking at the pictures i'm just curious what did you use to seal the chamber after fueling, i dont see any threaded adapters of any sort. were you just slapping an endcap on there and calling it good or what??
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Unread postAuthor: TwitchTheAussie » Wed May 28, 2008 3:56 am

Thats a problem right there BG. I'd say find a chamber and use that as a barrel mate. Use a chamber fan and a fuel metering system with propane and your good to roll.
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Unread postAuthor: AJChambers91 » Wed May 28, 2008 11:00 am

BigGrib wrote:So looking at the pictures i'm just curious what did you use to seal the chamber after fueling, i dont see any threaded adapters of any sort. were you just slapping an endcap on there and calling it good or what??


Kind of. i would put the cap on and i knew that it would stay on even if it did go so i held the back of it up against a tree.
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Unread postAuthor: AJChambers91 » Wed May 28, 2008 11:04 am

ok then ill tone down the size of the chamber and ill just be sure to use pressure rated pipe. propane guns do look amazing and i am definately gonna build one someday soon, but not as my first.
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Unread postAuthor: starman » Wed May 28, 2008 12:03 pm

Put a 2" female clean-out adapter and plug on the back. You'll need only a very small amount of fuel in there. Small chambers are harder to fire because the fuel % gets harder to nail. You don't have as much leeway.

Also push the potato only down to around the 20 - 24" mark on your ruler there. This will give you some simblance of proper c:b ratio.
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Unread postAuthor: jimmy101 » Wed May 28, 2008 8:48 pm

AJChambers91 wrote:After doing some research today during study hall i realised that the C:B ratio should be 1.5:1 and in this case it is nowhere near that and probably never would have worked.

Actually, with a monogun you can set it to be any CB ratio you want so it will work. The only time a combustion spud gun fails completeley is (1) fuel ratio outside the combustion limits and (2) the barrel is much too long for the chamber (CB<~0.3 or so) and the spud never clears the barrel.

i think i understand the basics of spud cannons a little better now...im thinking of making my next one with following basic dimensions:

2"diameter 40" long barrel Volume:125.66 cubic inches
4" diameter 15" long chamber Volume:188.49 cubic inches

After a little algebra (never thought id need it while sittin in class) i found some dimensions that agreed with each other. would someone please share their opinion on these?

Algebra (and mathematics in general) are your friend. :D

The dimensions look fine. You might also calculate the chamber dimension for 3" pipe since 3" pressure rated pipe and fittings are much more common in most hardware stores than is 4" stuff.
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Unread postAuthor: AJChambers91 » Thu May 29, 2008 10:36 am

alright thatnks for the help everyone! :)
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