cannon trouble

[fruitcannon]

Hi, this is my first post here, and i've been having some issues with this spud gun i built today. Its my third build, and the first to not work.

Basically its a 1 1/2 inch barrel, 28 inches long, with a 3" chamber, 18 inches long. all ABS plastic, sch. 40. We used a replacement bbq ignitor( red button) and screwed two normal screws into the middle of the chamber. Were getting a good spark about 50% of the clicks, with no spark the other 50%.

Were using aquanet for propellant, based on price and availabily.

problem is when we spray it in and screw on the end cap, we can't get anything to happen, but when we pull off the cap and ignite it (pointing away) we get a nice fireball. This is my first cannon with a 3" chamber, in the past i've used 4" with no issues.

We think it's a problem with not enough 02 getting around the spark.

1st, are there some screws that work better than others?

2nd, is there another alternative for a cheap propellant?

3rd, would putting the screws in the end cap improve our combustion?

4th, would shortening the chamber to 12" be of benefit?

and last, am i totally off on my reasoning and the problem is something else, and if so I would dig it if we could get some help!

thanks and sorry for the long post

[fruitcannon]

its pretty basic.

[Shrimphead]

You might have too much fuel, that's probably why it fires when you take it off and some air can get it. Try putting a little less in.

[fruitcannon]

we tried several different lengths of spray time, one being a very short burst, and we got nuthin. does the location of the spark in the chamber have an effect on how well it can ignite?

[Shrimphead]

You might want to try putting a chamber fan in to help circulate the fuel and air. This should solve and ignition position problems, if any.

[carlbelcher]

The location of the spark won't matter as long as there is the fuel/air mixture. You might try a different fuel like spray deodorant. I would also suggest going with a stun gun or at least a lantern lighter for ignition, BBQ lighters have proven problematic.

[ProfessorAmadeus]

Lantern lighters suck. After 3 shots they get to wet to even work. Try 3 seconds of fuel and lower and the chamber might have to much condecation on the sides if you try to shoot to fast.

[frogy]

I disagree about lantern lighters sucking, they make giant sparks %100 in my ABS cannon, all you have to do is pull them up before you spray and don't spray on them, then push them down before firing....

TO: fruitcannon:
The best ratio for a combustion gun is 1.5:1.... This ratio is the volume of your chamber compared to the volume of your barrel.... known as C:B ratio, this means volume of your chamber should be 1.5 times more than the volume of your barrel for good combustion\launches

Your chamber is 18" of 3" abs
Area = Pi * (Radius * Radius)

Volume = Area of pipe * length of pipe

Volume of chamber: (3.14 * (1.5 * 1.5)) * 18
= 127.17

Volume of barrel: (3.14 * (.75 * .75)) * 28
= 49.445

49.445 * 1.5 = 74.1675

That means your chamber volume should be close to 74.1675

OR

127.12 / 1.5 = 84.746666666666666666666666666667

Your barrel's volume should be equal to 84.746

To get this....

Volume of chamber: (3.14 * (1.5 * 1.5)) * 10.4978768...
= 74.1675

Volume of barrel: (3.14 * (.75 * .75)) * 47.981127
= 84.746666666666666666666666666667

This means make your chamber 10.5" or your barrel 48" to achieve a 1.5:1 chamber to barrel ratio, only do one of the other...

That should help, but I also suggest moving the bolts closer together in your ignition so it's close to %100 sparking, also try Right Gaurd Original, and a chamber fan helps greatly...

[Shrimphead]

Having a good c:b ration doesn't really affect how well it ignites, only how efficiently the ignition is used to launch the projectile. And the optimum c:b ratio varies depending on which fuel you are using. The ratio for propane is .6:1-.8:1. I would suggest trying butane (from lighter refills), spray deodorant, starting fluid, propane, etc. Try to stay away from hairspray, it gums up your chamber.

[plumpthumb]

Actually, if I may intercede here, to make the issue clear it sounds like it is a matter of three things: fuel to air ratio (probably most important), dump the hairspray and try something like Oust room deoderant (pricey but powerful-or use Glade or such),and air out the chamber well between shots. Just my humble opinion.

[djt]

amen frogy i do the same exact thing and i have never had a problem. they are more reliable than bbq sparkers i think. the only kind of bad thing is its a twisting motion instead of a button to make the spark which makes it difficult to hold and ignite sometimes.

[fruitcannon]

over the 4th of jult i realized that the problem was with using hairspray for the propellant. Funny, because it always worked in the past in my larger cannons. Now i'm hunting for a better fuel. This oust room deoderant might be worth looking into. Anyone have any relativly inexpensive fuels they swear by?

[fruitcannon]

Hey ya'll thought i'd post a follow up considering this cannon i built and was having trouble with now kicks ass.

I shortened the chamber 4 inches, bought longer screws, and bought a can of right guard no scent deoderant.

I have to make sure to spray away from the screws, and it can only be an extremely short burst of spray into the chamber. But it's loud as hell and launches at least a hundred yards. I'm happy.

[frankrede]

over the 4th of jult i realized that the problem was with using hairspray for the propellant. Funny, because it always worked in the past in my larger cannons. Now i'm hunting for a better fuel. This oust room deoderant might be worth looking into. Anyone have any relativly inexpensive fuels they swear by?

Equate body seoderant, 2$ at walmart. Sane stuff as rightguard except cheaper and i like it better. Its powerful too.

[MrCrowley]

lighter fluid. one small squirt is enough and really powerful