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A few simple things I need clarification on

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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A few simple things I need clarification on

Unread postAuthor: niglch » Mon Jul 30, 2007 9:48 pm

The first thing I'm wondering about concerns the diameter of barrels. 4" chambers with 2" barrels and 3" chambers with 1.5" barrels seem to be the most popular choices, but does one get better performance than the other assuming chamber:barrel ratios are equal, or will the two guns have almost the same performance? What about 4" chambers with 1.5" barrels and other combinations?
This leads into my next question. In the Burnt Latke test data for russets launched from a 1.5" barrel, the best C:B ratio appears to be around .7:1 using metered propane as the fuel. This is mentioned a lot on the forums, but would this ratio also be optimal using good spray & pray fuels such as Right Guard or Axe assuming you have a good fuel to air mix? (This might be impossible to truly test, but does anyone have some idea?)
My last question is about triggers. I already made one spud gun, but the problem is that the BBQ igniter is only held in place by electrical tape making it hard to press the button without quickly ruining the tape holding it in place. What are some better ways to mount the trigger on the gun?

Thanks for your help
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Unread postAuthor: hi » Mon Jul 30, 2007 9:59 pm

4'' chamber with 1.5'' barrel will work just fine. it will theoreticly give you more speed.

for spray and pray combustions, about .9:1 or 1:1 is good.

.7:1 is as small as it can be to use up all the energy. any smaller it will waste energy and any bigger and it will loose energy.

i personally like making them with ablut .9:1 for the extra bang.
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Unread postAuthor: Scotty » Mon Jul 30, 2007 10:01 pm

You would usually get less muzzle energy from a smaller i.d. barrel, people make there choices based on practicality i.e. they don't want to carry around a 3 metre barrel.
You could try resin to fix your igniter to your gun- or good old zip ties, you know the ones cops use as handcuffs en masse.

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Damn you Hi- just beat me! :)
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Unread postAuthor: jackssmirkingrevenge » Mon Jul 30, 2007 11:42 pm

A 2" barrel has a calibre that's 177% the area of a 1.5" barrel, meaning that for a given barrel length and C:B ratio, the 2" barrel should be more powerful in the same proportion.
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Unread postAuthor: niglch » Tue Jul 31, 2007 12:22 pm

Hmmm, ok, I'm still a bit confused. Perhaps examples will help.
Say you have 2 guns with 4" x 12" chambers. Both have barrels 48" long, but one is 2" wide and the other is 1.5" wide. Obviously the gun with the thicker barrel has a more efficient C:B ratio, but would the gun with a 1.5" barrel shoot farther and faster because gas would be forced more quickly down the barrel and be expected to shoot even faster if the barrel length is increased?

Of course, making an efficient C:B ratio with this size chamber and a 1.5" pipe could result in a ridiculously long barrel.
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Unread postAuthor: jackssmirkingrevenge » Tue Jul 31, 2007 1:00 pm

You need to define what you want in terms of power - a large heavy projectile will have more energy than a small fast one, but by virtue of its velocity and smaller surface area, the latter will penetrate better, and due to its flatter trajectory, be easier to shoot accurately.
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Re: A few simple things I need clarification on

Unread postAuthor: jimmy101 » Tue Jul 31, 2007 1:46 pm

niglch wrote:In the Burnt Latke test data for russets launched from a 1.5" barrel, the best C:B ratio appears to be around .7:1 using metered propane as the fuel. This is mentioned a lot on the forums, but would this ratio also be optimal using good spray & pray fuels such as Right Guard or Axe assuming you have a good fuel to air mix? (This might be impossible to truly test, but does anyone have some idea?)

The optimal CB ratio for a gun is pretty independent of the fuel. Besides, RightGaurd, Axe etc are really just propane anyway (there is only a small percentage of other things in most spray cans). So, if "you have a good fuel to air mix" (you have a chamber fan for example) then the optimal CB is still goinig to be in the 0.7~0.9 range.

niglch wrote:My last question is about triggers. I already made one spud gun, but the problem is that the BBQ igniter is only held in place by electrical tape making it hard to press the button without quickly ruining the tape holding it in place. What are some better ways to mount the trigger on the gun?

Try to fit your piezo sparker into various sizes of PVC pipe and fittings. IIRC, my lighter fits pretty snuggly in 1/2" (3/4"?) T fitting. Get a couple of end caps and pipe and make a handle. The grip is the vertical of the T. Strap the top of the T to your gun with a pair of big-ass hose clamps. Pass the sparker wires out the back of the T and on to your gaps.
Image
You might think that the T would want to roll against the chamber but it really doesn't. If you want, you can always add a dab of PVC between the fittings on the handle and the chamber.
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Re: A few simple things I need clarification on

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Unread postAuthor: niglch » Tue Jul 31, 2007 3:32 pm

jackssmirkingrevenge wrote:You need to define what you want in terms of power - a large heavy projectile will have more energy than a small fast one, but by virtue of its velocity and smaller surface area, the latter will penetrate better, and due to its flatter trajectory, be easier to shoot accurately.


By power I meant range and speed. I want a gun that will shoot as far as possible (while of course still being safe to fire). I'm not very concerned about the actual size or weight of the projectile.
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Unread postAuthor: Scotty » Tue Jul 31, 2007 3:54 pm

Speed and range is different to muzzle energy.

A 1gram projectile at a range of say 10 metres might hit the target with an energy of 5 joules and a velocity of 300fps, A 10 gram projectile might hit with 25 joules and velocity of 100fps. (just as an example)

Max range comes with ammo that is weighted properly for YOUR guns power, you have to find a "happy medium".

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Unread postAuthor: jackssmirkingrevenge » Tue Jul 31, 2007 10:18 pm

niglch wrote:By power I meant range and speed. I want a gun that will shoot as far as possible (while of course still being safe to fire). I'm not very concerned about the actual size or weight of the projectile.


In that case, I would go for a smaller calibre long barrel - and something other than potatoes as ammunition :)
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