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1/2 inch gun

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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1/2 inch gun

Unread postAuthor: blowgunsgalore » Thu Jul 03, 2008 6:43 pm

hey everyone
this is my second cannon and i am wondering what size tank i should use if i am using a half inch wide and 4 foot lang barrel. i want it to be able to shoot over 100 yrds with some power behind it
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Unread postAuthor: Sticky_Tape » Thu Jul 03, 2008 7:15 pm

I am pretty sure the volume of your barrel would be 9.42 cubic inches so try toget your tank size as close to that number as possible.

radius of barrel X radius of barrel X 3.14 X lenth of barrel in the same unit of messurement as you messure the radius of barrel should give you the volume of the barrel or chamber.
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Unread postAuthor: sirspuddest » Fri Jul 04, 2008 12:53 pm

My combustion uses a 36" barrel of 1.5" pipe, and a 14" chamber of 3" pipe, and it will vaporize a potato at over 100 yards, and put a D cell battery through 3/4" plywood, so I would go with something similar to that.

That's roughly a 1.5:1 C:B ratio.
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Unread postAuthor: Sticky_Tape » Fri Jul 04, 2008 1:12 pm

Sir spuddest the best c:b ratio is .6 - .8 : 1 for combustion
1 : 1 for pneumatic.
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xnt rnm ne z ahtbg
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Unread postAuthor: sirspuddest » Fri Jul 04, 2008 1:21 pm

Sticky_Tape wrote:Sir spuddest the best c:b ratio is .6 - .8 : 1 for combustion
1 : 1 for pneumatic.


Oh, my bad. :P

I just went with the ratio given on the SGTC website.

So, if I were to cut a few in. off my chamber, my gun would perform better?

I would a never guessed that! :o
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Unread postAuthor: Carlman » Fri Jul 04, 2008 1:28 pm

No it will not perform better, those ratios are the optimum ratios. like after that point the gains of adding chamber volume is not noticeable really.

Also the 0.8:1 is for a metered propane gun 1.5:1 like you have is pretty much optimum for a spray and pray (excluding some external factors).
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Unread postAuthor: jimmy101 » Sun Jul 06, 2008 12:36 pm

Sticky_Tape wrote:Sir spuddest the best c:b ratio is .6 - .8 : 1 for combustion
1 : 1 for pneumatic.

Nope.

0.6~0.8 is not the best ratio (besides the range is actually closer to 0.6~1.0). That ratio range optimizes the efficiency of the chamber. Since 99% of experienced spudders really don't give a crap about efficiency, they ignore the "optimal" CB range. They want maximum performance given whatever their design constraints are.

If you are designing your gun starting from the barrel (ID and length) then the optimal CB ratio depends on the ammo. A CB of 1.5:1 is good starting point. It is likely that a larger CB will improve performance but above ~1.5 you are usually in the diminishing returns domain. Going from CB 1.5 to 3 might only increase the performance (muzzle velocity) by a few percent.

For very light and low friction ammo the optimal CB may be less than 0.8 but the ammo has to be pretty light for that to be the case. Very light ammo also shows a very pronounced "mine sized chamber affect". Make the chamber too big and the performance drops off significantly. Heavy ammos don't show "mine sized affects" until the CB gets extremely large.

Easiest thing to do is enter your barrel, fuel, spark and ammo parameters into HGDT and do a sensitivity study on the chamber length starting with a convenient chamber diameter. There is a good chance you'll find the "optimal" chamber is impractically large, so decide what an acceptable trade off is between performance and practicality.

Attached is a HGDT study for a 0.5"x48" barrel assuming a 25g shell and 3" ID chamber. The CB 0.8 gun has a 1.1" long chamber. A CB 2 gun greatly outperforms the CB 0.8 gun, ~200FPS vs. ~130FPS. Larger CBs are even better. If you change the ammo mass or friction the results will change significantly. It's up to the builder to decide where the best tradeoff is between performance and chamber length.
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Unread postAuthor: Mr.Sandman » Sun Jul 06, 2008 12:54 pm

if i were you i would just hgdt it cause it would be easier to use and you would not have to wait for replies. although it can be wrong some times it is usually pretty accurate. personally i would work out the formulas by hand but thats just me. anyways jimmy101 is correct so just listen to him.
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