Firing problems HELP!

[blaineanator]

Hey guys so i built my first cannon its 4 inch combustion with 3 inch barrel. a BBQ grill sparker and a PVC screw back pretty basic. i tried metering propane and butane multiple times with all failures. the dimensions of the combustion chamber are
16" tall 4" wide so pie radius squared times 16 is 201 inches converted to cm is 508 cm. 3% of that is 15.5 which equates to about 16ml. i bought a syringe at Kroger used for cooking and tried filling it to 16 i pumped it in screwed the cap on and waited a few minutes for it to diffuse. no boom tried 5 times with pro and butane..... whats going on

[jimmy101]

I think your math is off.

A cylinder that has 4" diameter and 16" long has volume pi*(2^2)*16=201 cubic inches. For propane you want 4% of the chamber volume, which is 8 cubic inches = 131 ml.

For butane you want 3% of the chamber volume, 98 ml.

[Fnord]

508 cm^3 is half a liter... Just by looking at it, do you really think your chamber is that small?

Off the top of my head I'd say your chamber is probably closer to 3.5.

[blaineanator]

well i know that inch to cm conversion is 2.54 so 200 X 2.54= 508...

[blaineanator]

a cc equals a ml

[blaineanator]

ok i added roughly 100 ml of butane and it shot surprisingly but not near as far as axe would propel it....

[pneumaticcannons]

well i know that inch to cm conversion is 2.54 so 200 X 2.54= 508...

This is a common mistake.... we are talking about volume not length. so if you want to convert 200 cubic inches then you would have to multiply 200 by (2.54^3) or about 16.4..... about 3,277.4 mL. Online Conversion is useful tool

But before you redo your math, re-measure. A sch 40 pvc pipe that has a "4 inch diameter" is not going to have a 4 inch inner diameter.

[jimmy101]

What pneumaticcannons said ...

You have to check your units.

well i know that inch to cm conversion is 2.54 so 200 X 2.54= 508...

Add units and see if it still makes sense;
(2.54cm/inch)*(200inch³)=508inch²*cm
which is a not a very useful unit, though it is a unit of volume (length cubed).

The conversion factor's units have to work out.
(2.54cm/inch)³*(200inch³)=508cm³ = 508ml (since 1ml=1cm³)

Length: 2.54cm/inch
Area: 2.54²cm²/inch²
Volume:2.54³cm³/inch³

[jimmy101]

ok i added roughly 100 ml of butane and it shot surprisingly but not near as far as axe would propel it....

Meter them both and they will perform exactly the same. The only advantage Axe has is that since it is injected has a fairly high velocity stream it tends to mix better. Use propane (or butane) and wait a couple minutes for mixing (or better yet, use a chamber fan which will mix almost instantly) and the propane (butane) will not only work the same it is cheaper and won't muck up your spark gap.

[blaineanator]

Ok so i measured some more its ACTUALLY 17.5 inches tall and 4 inches wide (diameter) so can someone tell me what 3% would be in ml. I have propane and butane in my garage.

And if i meter it right by simply waiting a few mins allowing it to mix will cause the projectile to fly much further??

[jimmy101]

It will make the projectile fly as far as possible with that particular gun and ammo.
(17.5in)(3.14)(2^2in^2)(2.54^3/in^3)(0.03)(ml/cm^3)= 108 ml for 3% mix

[blaineanator]

thanks so i have a 35 ml cooking syringe all i do is fill it up 3 times and pour that in then cap it off and wait? like..... 2 mins or 10-20?

[Crna Legija]

Put a ball of foil in the chamber when you meter fuel close it up and shake for a minute then fire at will.

[blaineanator]

thanks! ill try that

[blaineanator]

it worked but still only launched my orange about 50 yards how do i achieve the magical 100+????