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Cannon seems underpowered?

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: bluerussetboy » Mon Dec 31, 2007 5:24 pm

reminds me of a day years ago in chemistry class and the subject was phosphorus.

Smithells Cold Flame from page 609, Partingtons Inorganic Chemistry --

The glow of phosphorus is shown in the "cold flame" experiment, due to Smithells. A few pieces of phosphorus are placed in a dry receiver, which is then filled up with glass wool. The receiver is heated on a water bath, a stream of dry carbon dioxide being passed through. The phosphorus vapour carried along with the gas oxidises in the air, and a green flame appears at the top of the exit tube. This is so cool that a finger may be held in it, and it will not kindle the head of a match.


we were never allowed to try it
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Unread postAuthor: MrCrowley » Mon Dec 31, 2007 5:31 pm

Well your valve is way undersized.

So you can make a burst disk( don't ask, search)

Or a piston valve (dont' ask, search)

I'd go with a piston valve, it should allow you to keep your current barrel if you house it in a 3" tee, and it would shoot golfballs pretty darn far.
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Unread postAuthor: DYI » Mon Dec 31, 2007 6:48 pm

Well, despite advice from other members, a quick GGDT check will show you that your barrel should be about 5 times the size of your chamber for optimum performance, assuming a fixed chamber size.

If you doubled the barrel size, you would see a very sizeable increase in performance. Essentially, unless you have a barrel over 16 feet long, you will always be increasing performance by making the barrel longer. The ideal barrel length for this chamber size is about 16' for a 3" barrel, and more for a smaller diameter one.

So; to increase performance, you can do 4 things to this design.
1. Make the barrel longer
2. Increase the valve porting, and use a faster valve
3. Make the chamber bigger
4. Increase the pressure
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Unread postAuthor: noob of noobs » Mon Dec 31, 2007 9:20 pm

Don't burst disks have a higher flow and opening time than piston valves, thus supporting higher forces?
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Unread postAuthor: elitesniper » Mon Dec 31, 2007 9:23 pm

yep they do, but there insanely loud though
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Unread postAuthor: DYI » Mon Dec 31, 2007 11:00 pm

Burst disks are only loud with an inefficient cannon. In a 120 psi cannon with a proper .2:1 C:B ratio, they are relatively quiet.
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Unread postAuthor: lemmiwinks » Wed Jan 02, 2008 3:07 am

Thanks all for the responses. I'm hearing some conflicting things about C:B ratio, Is the problem that the object is leaving the barrel too early? Does anyone know for sure?

It's the 1" valve from BCARMS, and I'm running it at about 100 PSI.

I'd equate the block to about 300-400 feet.

The only way I would change the barrel now would be to make it smaller in length or make another smaller in diameter (I used tape to connect to the valve), too expensive to make a longer 3" not to mention the obscene bulkiness.

The snow I used was really tightly packed, wet snow. I figure that it may have not gone really far because it was heavy, but I don't know.

I guess that after hearing stories of potato guns shooting like a quarter mile I was kind of disappointed with this performance. What kind of performance should I be getting out of this? Thanks again!
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Unread postAuthor: jrrdw » Wed Jan 02, 2008 5:10 am

300' = 100 yards, you should be getting approx. 200 yards at 100psi. Thats a general number. There are factors that will change that one way or the other. You just said the barrel was taped on, you could be loosing performance there. The barrel could be to short, chamber to small. These things all play a part, you have to make a good build to get a good performance from your launcher. Could be a lot of mentioned things. Go back and look and ask yourself, what could i have done to make this better. I can tell you for sure, taping your barrel on a launcher of this style don't get it.
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 02, 2008 5:21 am

As was mentioned, the valve restriction is a bit of a killer, however even with your current setup performance should be quite respectable - to be honest snow isn't exactly the ideal sort of projectile when it comes to range. You want something much lighter and a lot more aerodynamic if you're going for distance.
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Unread postAuthor: dewey-1 » Wed Jan 02, 2008 5:01 pm

--
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