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Chambers and barrels...

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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Chambers and barrels...

Unread postAuthor: WitSkaapie » Wed May 23, 2007 8:10 am

ok guys... i know this is a really noob question, but forgive me

i'm busy building my spudgun and i want to ensure that my C/B ratios are perfect...

so, i need a little help.

i found ( thanks to this site ) that the ideal ratio is between 0.6-0.8:1

however, is the barrel the lower volume or is the chamber the lower volume...

ie: is the chamber the fixed value ( 1 ) and the barrel volume the variable ( 0.6-0.8 ) ???

thanks for all your help guys...
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Unread postAuthor: hi » Wed May 23, 2007 8:51 am

the chamber is the lower volume. its smaller because the fuel expands. Thats why its a C:B ratio, which is Chamber to Barrel.

one thing that you might want to consider is that the ratio .8:1 is the ratio that you can have on a connon without the projectil slowing down in the barrel. so if its power you are after, then i would go for a more like 1:1 because you would gett a little more power.
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Unread postAuthor: jimmy101 » Wed May 23, 2007 11:48 am

There is really no way to predict what the optimum C:B is for a combustion gun. Way too many variables, like barrel diameter, projectile mass and friction, fuel, number of sparks .... All of which affect the optimum C:B ratio for a particular gun. The oft quoted value of 0.8:1 (chamber to barrel) is really based on single gun and ammo.

The good news is that the optimum is a pretty wide peak. Generally, the difference between the optimal and fairly close to the optimal is pretty small in terms of muzzle velocity (or kinetic energy). Bottom line, if the optimal really is 0.8 but you build a gun with a C:B in the range of perhaps 0.6 to 1.0 you won't be able to tell the difference. Even with a chronometer you probably wouldn't be able to tell the difference.

Bottom line, don't try to "ensure that my C/B ratios are perfect". Anything in the range of perhaps 0.6 to 1.0 is AOK. Much better to spend your time (and $$$) getting a fan, fuel meter system etc.

Hi said;
one thing that you might want to consider is that the ratio .8:1 is the ratio that you can have on a connon without the projectil slowing down in the barrel. so if its power you are after, then i would go for a more like 1:1 because you would gett a little more power.


Hmmm, that doesn't make much sense. If greater than 0.8:1 the spud is slowing down then you can't get "more power" at 1:1.
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Unread postAuthor: Captain Barbosa » Thu Jun 14, 2007 12:56 pm

So anything over a 1:1 ratio is just wasting fuel? I remember people at Spudtech used to recommend 1.5:1 a lot, even though the BurntLatke tests said 0.8:1. (My old name was Lord of the Rings Junkie...)
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Unread postAuthor: jimmy101 » Thu Jun 14, 2007 2:25 pm

Captain Barbosa wrote:So anything over a 1:1 ratio is just wasting fuel? I remember people at Spudtech used to recommend 1.5:1 a lot, even though the BurntLatke tests said 0.8:1. (My old name was Lord of the Rings Junkie...)


Yep, wasting fuel and PVC and making the gun bigger than it needs to be.

Some people think that the old 1.5:1 is a good ratio for a "squirt-and-screw" (spray-and-pray) cannon. I suspect that is just because that fueling method is so irreproducable that how big the chamber is really doesn't matter.

If you have any kind of metering system then C:B's in the 0.6 to 1 range are probably best.
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