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Pressure Building Barrel

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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Pressure Building Barrel

Unread postAuthor: usdiver63 » Tue Feb 07, 2012 9:37 pm

Greetings All,

Ok lets get to it. Now what I started with was your basic 4" chamber to 2" barrel, with a threaded coupler for portability and interchangeable barrels. Now the first works great but i found myself wanting more punch and further distance. Already knowing a little bit about plumbing and pipe reduction, i decided to make another barrel. this one dropping from a 2" to a 1" and still using the 4 inch barrel. This thing sounds like a shotgun just went off and i would recommend wearing hearing protection.

A general rule of thumb that i have found with pressure increase regarding reduction is for every inch dropped you get 1.5 x the original force. Feel free to correct if i am wrong, i want your ideas and feed back.

So from a 4" to a 2" is a 3x multiplyer of force and a 2" to a 1" is a 1.5x increase so if you work the numbers your adding a multiplyer of 4.5x to your total output of force.

Feel free to correct and give feed back. Image[/img]
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Unread postAuthor: Gun Freak » Tue Feb 07, 2012 9:52 pm

Interesting thoughts :lol:

Smaller barrel = less area for pressure to act on = less force.

No such thing as a "pressure boost".
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Unread postAuthor: usdiver63 » Tue Feb 07, 2012 9:59 pm

hmm. i see that you are correct sir so then what should it be called?
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Unread postAuthor: Zeus » Tue Feb 07, 2012 10:01 pm

I'd call it a good thought but ineffective.

This got me thinking though, how about a well formed venturi?
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/sarcasm, /hyperbole
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Unread postAuthor: Gun Freak » Tue Feb 07, 2012 10:01 pm

Let's see... I'd call it "getting less net force by using a smaller barrel" :)
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Unread postAuthor: mobile chernobyl » Tue Feb 07, 2012 11:52 pm

taking a fluid from lower velocity to high velocity through a nozzle results in a lower pressure in the high velocity field vs the low velocity field.

what you do have is marginally faster fluid.

look up dem bernoulli's equations

At first I thought you were working on some hashed out 2 stage gun and it got my hopes up :(
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Unread postAuthor: Technician1002 » Wed Feb 08, 2012 12:24 am

One aspect not mentioned is when using GGDT, the barrel burns out after the chamber burns out. Due to the higher barrel turbulance the barrel burnout could provide higher in barrel pressures somewhat like the ones seen on hybrids with copper barrels that get damaged from DDT in the barrel.

On an air cannon, a smaller barrel on a proper size valve is lower power. On a combustion, you have another factor at play, so I don't know. It is time to get a chronograph and document the results.

In regards to the sound, a smaller volume barrel (changing CB ratio) is louder. Normally it is associated with a shorter barrel, but a reduced diameter barrel is again reducing the barrel volume.
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 08, 2012 12:40 am

Interesting idea. Reminds me of the work of Herr Gerlich, but not quite, as you're squeezing the gasses and not the projectile.

I guess the measure of its effectiveness would be:

- comparing the velocity of the same projectile with the equivalent length of just 1" barrel.

- comparing the velocity of the same projectile in a lightweight sabot in the equivalent length of just 2" barrel.

Have you got a chrony and some spare time :)
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Unread postAuthor: cfb_rolley » Wed Feb 08, 2012 4:53 am

I think JSR hit the nail on the head there. what I'd like to see though, is muzzle velocity and distance tested using say, a 2" barrel and a 1" barrel but both with the same volume, and the projectile being of the same diametre as the barrel it's being fired from, with equal length and density (2"x2" potato and 2"x1" potato?).

There's probably something already done like this, but I haven't been around spudding much recently.
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Unread postAuthor: Technician1002 » Wed Feb 08, 2012 5:36 am

I know what my results are in the air cannon catagory. Bigger is better.

Note on the slug in the prior post, the 1 inch slug would be about 1/4 the mass of a 2 inch slug. For power compairison, both slugs should be the same mass. To test for just a pressure rise, the same length should be used as mentioned.

AA battery in sabot in 2.5 inch barrel.
Image

For comparison, AA batteries shot from a 1/2 inch barrel.
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