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chamber diameter and length combination?

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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chamber diameter and length combination?

Unread postAuthor: cfb_rolley » Fri Jun 17, 2011 3:42 am

So I was about to start embarking on my next cannon design since my very first DWV cannon developed a crack (and therefore I had to stop using it there and then), and decided I'd go with pressure rated pipe again like my second cannon (still going strong).

I wanted to make something similar to an M82A2 style setup, and use a false magazine to hide the fan batteries etc. the problem is that the chamber would be about 65mm I.D x 400 or 500mm long. I remember being told that a chamber config like this can cause DDT, and that's bad.

Would 2 or 4 spark gaps at equal spacing along the chamber eliminate the risk of DDT while leaving my fan undamaged?

also, the barrel I.D would be about 20mm, however I could go to 32mm. is 20mm too small in relation to the chamber diameter?

cheers in advance.
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Unread postAuthor: Labtecpower » Fri Jun 17, 2011 3:48 am

I assume you are talking about a atmospheric combustion?

In that case DDT wouldn't be a problem. As far as I know, there has not been a single case of DDT in combustions using standard mixtures. (propane,butane / air)
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Unread postAuthor: cfb_rolley » Fri Jun 17, 2011 7:38 am

Definitely only an advanced combustion, yeah. Not ready for a hybrid build yet.
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Unread postAuthor: saefroch » Fri Jun 17, 2011 10:05 am

DDT is really less of a danger than most people think, though it has been induced at atmospheric conditions, it require a specialized setup. And in any case, with the nitrogen present in the air, there's no significant risk with most air-fuel mixes.
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Unread postAuthor: cfb_rolley » Fri Jun 17, 2011 6:44 pm

awesome, that's good news. how about the difference between the chamber diameter and the barrel diameter? can problems be caused by having say, a 17mm I.D barrel and a 65mm I.D chamber?
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Unread postAuthor: saefroch » Fri Jun 17, 2011 9:03 pm

Nope. In my opinion you'd be losing out by using such a small barrel on a sizable chamber, but there is no danger. Just keep in mind that the pressure=Force/Area, so at one pressure the larger the area the larger the force. You can pack a lot more kinetic energy onto a projectile with a larger diameter barrel.
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Unread postAuthor: cfb_rolley » Fri Jun 17, 2011 10:59 pm

that's good. the reason why i asked is because I'm going to have a threaded fitting to be able to change barrels to different sizes to launch different things, just was worried about going too small.
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Unread postAuthor: saefroch » Sat Jun 18, 2011 9:30 am

I suggest a sabot if you're going to launch small objects, not a smaller barrel. You'll get higher muzzle velocity using a reasonably low-density sabot over a smaller barrel.
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Unread postAuthor: cfb_rolley » Sat Jun 18, 2011 10:06 am

hmm, didn't think of that either, thanks for that one.
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