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Improving guns with slow pressure build up

A place to ask general spud cannon related questions.
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Improving guns with slow pressure build up

Unread postAuthor: btrettel » Sun Jan 25, 2009 1:25 pm

I'm not sure if this was discussed elsewhere because it seems somewhat obvious, but I couldn't find any discussion on it.

Slow pressure build up is undesirable because by the time any decent pressure is built (if it ever is built) the projectile has moved down the barrel, reducing your "effective" barrel length. This is common knowledge.

Low barrel friction is desirable because energy losses to friction are minimized. Again, common knowledge.

However, a higher friction barrel will build up more pressure. That is especially if the amount of static friction the barrel-projectile combination can take is high. But, this is undesirable as it would decrease performance in the end.

What is necessary then is high static (not moving) friction to build up pressure and low dynamic friction (for when the projectile is accelerating). The easiest way to achieve this is by holding the projectile in a short high friction sleeve initially (something like an O-ring might work). The pressure will build higher and any energy losses will be minimal because no energy is lost to static friction because there is no movement. This is a simple way to improve the efficiency of slow valves. Don't expect any miracles with reasonably fast valves though, but I would imagine this would help their performance a bit.

The main problem with this approach is that it'll only work on solid projectiles. It's somewhat like the valveless launcher in that respect.

If anyone's wondering, this wasn't my idea. I read about it here and found it odd that no one was using it.
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Last edited by btrettel on Sun Jan 25, 2009 10:25 pm, edited 2 times in total.
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Unread postAuthor: jimmy101 » Sun Jan 25, 2009 3:06 pm

Actually, similar ideas have been proposed before, but you are right, it is rarely used.

One way to get high static friction without the high dymanic friction, and still be able to use rigid ammo, is to use an expanding sleeve. Below is a years old idea for use on a BBMG. The blue thing in "configuration 3" is a short length of Tygon tubing with ID less than the ammo's OD. When pressure behind the round rises enough the tubing expands releasing the ammo.
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Unread postAuthor: btrettel » Sun Jan 25, 2009 3:26 pm

That's an interesting approach. I actually remember that drawing from what I was skimming your website, but I didn't read further because I wasn't interested in BBMGs. Perhaps I should read a page more closely before closing it.

Has anyone tried that idea?
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Unread postAuthor: jimmy101 » Sun Jan 25, 2009 4:08 pm

I tried it with a cloud BBMG. Didn't get any chrony data but the BBMG was putting BBs through about 6 layers of corragated cardboard then putting significant dents in a steel garage door. (oops)

The same gun without the Tygon fires steel bbs at about 300 FPS which won't penetrate that much cardboard and still dent sheet steel. So the concept was working. The problem was it fired very inconsitently, you never quite knew when it might spit out a BB. Probably need to do a careful study using different types of tubing, or figure out a way to control the elasticity of the tubing.

I considered passing the hose through a compression T. The side inlet to the T would be hooked to either a pressure or vacuum source so the pressure on the outside of the tubing could be controlled.

BTW, it turns out that a generic Hot Melt Glue Gun makes an excellent flaring tool for Tygon tubing.

EDIT: Engrish
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Unread postAuthor: Ragnarok » Sun Jan 25, 2009 4:52 pm

If I might point out the obvious solution to make slow valves more efficient?
Make them faster! The simple truth of the matter is that it's far easier to make a slow valve quick than it is to faff around with any of this.

The only people who use truly slow valves are not interested in investing effort into this kind of solution - because to be honest, if they were, they'd have used a fast valve in the first place.

Jimmy's point about BBMGs is the only one that makes logical sense, because in every other situation, the effort is better invested in increasing valve speed - and once your valve is fast enough, additional gains become increasingly moot.
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Unread postAuthor: jimmy101 » Sun Jan 25, 2009 5:24 pm

Don't over simplify things ragn...

I agree that, in general it, is easiest (and best) to simply make the valve faster, but there are times when you don't want a fast valve.

After all, there are situations where a person might want a slow valve. For example, in a pumpkin chucker. A fast valve will obliterate the ammo.

Or, situations where there is no valve, like a combustion gun. Being able to increase static friction without increasing dynamic friction would be a huge help in most combustion guns.
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Unread postAuthor: Ragnarok » Sun Jan 25, 2009 5:44 pm

jimmy101 wrote:After all, there are situations where a person might want a slow valve. For example, in a pumpkin chucker. A fast valve will obliterate the ammo.

And forcing the ammo through a "choke" that's tight enough to make a meaningful difference is going to be any nicer on it?

Or, situations where there is no valve, like a combustion gun.

Yes, agreed, but equally, where there's no valve, there's no slow valve to be making up for.

Indeed, there are situations where being able to control static and dynamic friction separately would be a boon to the operation of the cannon. However, this situations are almost all going to be where there is no valve, rather than where there is a slow valve.
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Unread postAuthor: jackssmirkingrevenge » Sun Jan 25, 2009 6:20 pm

Another way to approach this would be to incorporate a burst disk into the design, or a mechanical detent.
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Unread postAuthor: btrettel » Sun Jan 25, 2009 7:19 pm

@Ragnarok: This and anything with similar goals are no replacement for a fast valve. Perhaps I shouldn't have said "valve"--I did post it in the general forum for a reason but neglected to mention it'd work on anything really. The problem is a slow build up of pressure to be more specific. Sometimes you don't really have a choice in the matter (like in a spring-powered gun), and other times it might be a good idea even though your valve is adequate (though I haven't run the numbers so I don't know).

Burst disks work too, but I'm interested in something that doesn't require maintenance.

I like the mechanical detent idea. It gives you direct control of how much force is required, which is good if you want to try different amounts.

Edit: I changed the thread title.
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Unread postAuthor: TurboSuper » Sun Jan 25, 2009 7:32 pm

Mabye some type of springed latch which can only be pushed far enough back by a certain pressure?

I also had the idea of making a coaxial spud-cutter on the chamber side- you push the spud halfway into the barrel from the chamber side, then let the pressure do the rest. I'm not sure if that method would work or not, but if it did it would make for a good semi-auto spud cannon.
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Unread postAuthor: Combustion Monkey » Sun Jan 25, 2009 8:23 pm

I've been wrestling with ideas like these for a few days now. The best i have found so far is the burst disk. They work very well but I don't like them. I too would like some thing mechanical. I had some decent luck with building Up the breach end of my coaxial golf ball barrel with tape for an inch or so.

I think that some sort of snug fitting rubber hose mounted in the end of the barrel would be ideal for my setup.

As for the self cutting coax spud barrel. I don't think it would work. My gun is a coax with a chamber now something over 350ci. I have had very green fully cut spuds contain the ignition. Not a fun experience hearing that thump and knowing that all that pressure is just sitting in there.
But if you partially baked the potato and put some butter on the leading edge for lube it may work :wink: .
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Unread postAuthor: TurboSuper » Sun Jan 25, 2009 9:29 pm

Oh, well I was thinking more for a pneumatic since it would keep up a constant pressure...you'd need one helluva combustion to make it work.

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Unread postAuthor: Combustion Monkey » Mon Jan 26, 2009 4:23 am

It would be interesting to do some testing to see how much pressure it would take to make it work though. How would you do it in a pneumatic, just keep adding pressure until the spud let go?
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Unread postAuthor: btrettel » Mon Jan 26, 2009 1:37 pm

Yes, the idea would be to keep adding pressure until whatever you're shooting left the "stop". The problem with that approach is that the pressure the projectile shot at would likely vary unless each set up was very consistent. It'd be more convenient than a burst disk though, so it might be worth trying as the simplest valveless design.

You could probably make it more consistent with some sort of physical piece preventing movement in addition to the compression fit. Seems like an interesting idea, but it'd probably be a pain to build. The valveless designs previously mentioned would probably work better because this approach would probably eventually bend whatever physical device blocks the barrel unless it was designed well. Though it might work well--I don't know!
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Unread postAuthor: Combustion Monkey » Mon Jan 26, 2009 3:59 pm

I was thinking of trying a two O-ring rig for my golf ball barrel. The idea is that i would inset two o-rings inside the barrel right at the breach about 1/4 inch apart. They would be just big enough to let the ball pop through under pressure but still hold it firmly in the breach. I need to see if I can hit the parts bin at work to find some.
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