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Reinventing the underwater gun

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: iknowmy3tables » Fri Feb 29, 2008 3:59 pm

hmm whould this increase preformence, well I'm gonna try to use my minimal knowledge of ballistic physics. okay lets see the projectile in this situation is surrounded by dense everything else seams very similar in the action, the projectile surrounded by dense air seem like it may me a negative factor since the air is more dense thus the speed of sound is lower,

also you couldn't fill it quickly or else the projectile would move out of place, I think it would be better to use a normal gun with a barrel condom or some test tube film to seal it from water
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Unread postAuthor: Jared Haehnel » Sat Mar 01, 2008 8:47 am

If you plumb it so the barrel and the chamber fill at the same rate there shouldn't be a problem as long as chamber pressure and barrel pressure remain equal nothing is going to move.


Edit: If you can get the object to fit pretty well when you open the valve the pressure ahead of the object will go away very quickly leaving the pressure behind the object to propel it forward.... at least in theory.... the air ahead of the object won't be nearly as dense air the air behind it during the "firing" of the gun....

This is all based on nothing more then speculation so feel free to correct me if I'm wrong.
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Unread postAuthor: Rokmonkey » Sat Mar 01, 2008 10:55 am

Hmm, I'm fairly certain it is every 33 feet deep it is one atmosphere, so if you plug the end of it, I think you are safe for a good amount of time, but you definitely want to fill the barrel as well, with an equalization hole, or you are going to have some SERIOUS pressure problems on that barrel, not only the pressure from the compressed air in the chamber but also the water pressure on the chamber, That's some serious pressure. I'd do a test first, put maybe 50 PSI in a small chamber and sink it, see if it explodes.
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Unread postAuthor: SNDM » Sat Mar 01, 2008 11:20 am

Make sure you get a *really*fast openingf valve, otherwise you porbably will just shoot that as its opening.
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Unread postAuthor: Jared Haehnel » Sat Mar 01, 2008 11:24 am

That wouldn't make sense..... If I am understanding correctly.

The PVC chamber is a pressure vessel and is likely to contain more pressure inside than the outside water pressure is exerting on it. I seriously doubt the PVC pipe will burst underwater in depths that any of us could safely dive to.
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Unread postAuthor: Rokmonkey » Sat Mar 01, 2008 11:33 am

Really, depends how deep you go, It will burst at a depth probably lower than 33 feet, but before most think it would., I was over thinking it there, thats for sure, water pressure is weird on air pressurized systems, I've seen it first hand, trying to pump air down a hose, even 15 feet is a challenge compared to 5 feet.
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Unread postAuthor: Brian the brain » Sat Mar 01, 2008 11:47 am

How about letting a gas ram do the job for you?
Think about a real speargun, without the rubber band
The push rod would fire the projectile..
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Unread postAuthor: Jared Haehnel » Sat Mar 01, 2008 11:52 am

Yep, it would.
Water wrecks havoc when its, where it is not suppose to be. The idea would be that the only time water is introduced to the chamber and barrel is after valve or burst disk is open and after the pressure of air is lower then that of the water. At this point the projectile is well on its way.

This is assuming of course the barrel and chamber are held level.
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Unread postAuthor: homedepotpro » Sat Mar 01, 2008 12:53 pm

you barely hear it under water. the concussion is minimal especially at depth. I really like the barrel valve idea though. it eliminates the need to keep replacing burst disk.
Edit: sorry i was wrong about the 9' thing.
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Unread postAuthor: iknowmy3tables » Wed Mar 05, 2008 6:09 pm

Jared Haehnel wrote:That wouldn't make sense..... If I am understanding correctly.

The PVC chamber is a pressure vessel and is likely to contain more pressure inside than the outside water pressure is exerting on it. I seriously doubt the PVC pipe will burst underwater in depths that any of us could safely dive to.

he's right, you've got it backwards, the pressure inside the chamber decreases as you go deeper since the outside pressure increases. so you got it in reverse, and only if you sent it to extreme under water depths then you might manage to implode but not explode the chamber
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Unread postAuthor: DYI » Wed Mar 05, 2008 9:44 pm

That's roughly correct...
The pressure isn't any lower inside the chamber, the pressure outside it is higher. The burst pressure of a pipe is rated assuming that the outside of the pipe is exposed to 14.7 psi. Put that pipe in a vacuum, and the rating decreases by 14.7 psi. Put it in a 114.7 psia environment, and the rating increases by 100 psi (assuming that it doesn't implode).

The whole concept is called differential pressure. It's what makes triggered burst disk guns work - in essence, the burst pressure is not what internal pressure the vessel can survive, it's how much higher the internal pressure can be than the external pressure before the vessel fails. The implosion pressure is the same thing, in reverse.
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Unread postAuthor: LikimysCrotchus5 » Wed Mar 05, 2008 10:08 pm

Yes, just to support most of the people here, there is no need to worry about the internal pressure, and it would implode, not explode

If there is any knowledge that i learned and retianed in 8th grade science, it is that high pressure tends to move and equal out with low pressure

So saying that, the outside water pressure would be acting on the outside of the pipe, and if that pressure increases dramatically, to to point of the pipe failing, then the pipe would implode, where the high pressure from the outside would push in to equilize with the low pressure.

In the triggered burst disk system, you pump the small chamber between the disks to half the pressure in the chamber. Then, what you do is release the ball valve, where the pressure decreases dramatically. Because pressure needs equilize, and high pressure moves from low pressure, the pressure in the chamber is overwhelming and cannot be contained once the pressure drops down back to atmospheric pressure (14.7 psi). So the burst disk ruptures, causing the projectile to be forced out and the pressure to equilize.

Get it? :D
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Unread postAuthor: DYI » Wed Mar 05, 2008 10:29 pm

Your understanding is rather limited. The pressure doesn't *need* to equalize, the disk has been exposed to enough force to make it fail because the differential pressure has surpassed the burst pressure of the disk. Since the particles in air are always moving, when the disk does fail, air particles will continue on their trajectories, but will no longer be blocked by the disk, and will move through at speeds of ~500m/s (discounting shockwave formation).

What needs to be understood is that, in this case, pressure is simply the concentration of molecules in a volume of gas. As the concentration increases, so does the pressure. By this logic, a vacuum doesn't "suck" anything, ambient pressure simply "pushes" it. So high pressure doesn't exactly equalize with low pressure, but once two vessels have a flow channel between them, the concentration of gas particles in each will approach equivalence at whatever rate the particles can move through the flow channel.
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Unread postAuthor: sjog » Wed Mar 05, 2008 10:33 pm

Early spear guns were powered w/co2
small diameter barrel to fit spear snug
one spear w/line and barb then 'drivers' shot after that to subdue large fish
Hans Hass was the guy who wrote book.I read it in many years ago.
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Unread postAuthor: LikimysCrotchus5 » Wed Mar 05, 2008 10:48 pm

DYI wrote:Your understanding is rather limited. The pressure doesn't *need* to equalize, the disk has been exposed to enough force to make it fail because the differential pressure has surpassed the burst pressure of the disk. Since the particles in air are always moving, when the disk does fail, air particles will continue on their trajectories, but will no longer be blocked by the disk, and will move through at speeds of ~500m/s (discounting shockwave formation).

What needs to be understood is that, in this case, pressure is simply the concentration of molecules in a volume of gas. As the concentration increases, so does the pressure. By this logic, a vacuum doesn't "suck" anything, ambient pressure simply "pushes" it. So high pressure doesn't exactly equalize with low pressure, but once two vessels have a flow channel between them, the concentration of gas particles in each will approach equivalence at whatever rate the particles can move through the flow channel.


Well, thats 8th grade science for ya' :D

And also on the part about the vacuum, doesn that work with bernoulli's principle, where there is faster moving particles there is low pressure which is why things are pushed by the outside greater forces rather than sucked?
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