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muzzle loading .50 cal

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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Should I build this

Poll ended at Wed Dec 10, 2008 2:47 pm

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Unread postAuthor: jackssmirkingrevenge » Sat Dec 06, 2008 11:32 pm

jitup wrote:what is the bolt and washer do


Since epoxy and rubber don't go together 100%, the bolt and washer are there to ensure the rubber seal isn't sucked into the barrel before firing ;)
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Unread postAuthor: jitup » Sun Dec 07, 2008 10:09 am

That is great thinking. can this same prinipal be used with hard acrilic mix, in the rare chance I am ubable to find the correct epoxy? Other than that I will start building soon. oh, I almost forgot, a blow gun is good enough to pilot this valve, correct?
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Unread postAuthor: Gippeto » Sun Dec 07, 2008 1:34 pm

With a good fitting piston, careful attention to keeping pilot volume down, and preferably modding the blowgun, YES.

http://www.spudfiles.com/spud_wiki/inde ... dification

Your total piston travel should be 1/4 of your barrel id or only SLIGHTLY more. Certainly no more than 1/2 your barrel id, or you WILL run into problems.(piston bounce)

Don't forget to use a bumper behind the piston, a rubber garden hose washer will work well. Your piston should move the 1/4ID and then contact the bumper.

Pay attention to these "little" things, build it neatly, and it will work very well.
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Unread postAuthor: Ragnarok » Sun Dec 07, 2008 1:44 pm

Gippeto wrote:Certainly no more than 1/2 your barrel id, or you WILL run into problems.(piston bounce)

Piston bounce is actually better controlled with a larger piston travel - it has to bounce back further for it to be a problem.
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Unread postAuthor: Gippeto » Sun Dec 07, 2008 2:16 pm

GGDT says that my pistons are opening many times faster than the projectile is leaving the barrel. I'm quite certain this is the case for everyone.

This would seem to indicate that irregardless of how piston bounce occurs, it will have an opportunity to affect the airflow, and thus impact projectile velocity.

From playing with ggdt, one of my "lessons learned" has been that piston bounce is best controlled (eliminated) with the pilot diameter (pilot flow), in conjunction with eliminating excess pilot volume.

With a modded blowgun for a pilot, my experiences have shown that by keeping pilot volume to a minimum, I get excellent results. (within ~20fps for under 800fps ggdt predictions)

True, I've not built multiple launchers of varying pilot/pilot volume configurations to test the theory.

If you have a link to such a comparison I would be interested though. :)
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Unread postAuthor: Ragnarok » Sun Dec 07, 2008 2:51 pm

@Don Gippeto de la Mancha: GGDT only considers the D/4 travel rule - after that, it's considered fully open.

Where in reality, the piston may shoot back further than that, GGDT doesn't take any ends to consider that as a possibility, at least as far as I know.
Much of piston bounce also depends on factors GGDT cannot account for, like the bounciness of the piston/piston bumper.

GGDT does consider purely pneumatic piston bounce caused by the shock heating of the pilot volume air - again, this is caused by a too restrictive pilot volume, as the effects of shock heating are related to the percentage volume change caused.

Personally, I aim for D/2, but sometimes use as much as D - but only where the increased pilot volume is preferred over extending the piston and increasing it's mass. HEAL's pilot volume is as physically small as it can be, but it still has a travel of D, because I can only make the pilot volume length so short, and the sealing face of the valve is deliberately a long way forwards to improve flow and cut dead volume, meaning the piston has a travel of approximately D. It does have a good bumper to catch the piston at the far end of it's travels though.

JSR wrote a interesting piece on the D/4 rule and the fact that applying it too strictly can cause piston bounce. It can be found here:
http://www.spudfiles.com/forums/t-t11354.html
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Unread postAuthor: Gippeto » Sun Dec 07, 2008 3:50 pm

I don't have intimate knowledge of the inner workings of ggdt either (In fact I'm certain that I have less of an idea than you do.), so I'll simply nod my head and say "Uh huh". :dontknow:

As to the bumper causing piston bounce theory;

I believe it is possible to a degree, but at the END of the firing cycle as chamber pressure drops, and the force acting on the piston decreases below the return force of the bumper material.

The bumper material will of course attempt to return to its original shape, and thus impart some degree of force/acceleration to the piston.

However, when I open my copper guns post firing cycle, the piston is, invariably, resting on the bumper. Not somewhere in between. The return acceleration must certainly be rather low. (Dare I suggest, not worth considering?)

I believe that pneumatic piston bounce is what needs to be considered. I feel that has been covered.

HEAL works very well, and is a testament to careful design. It is however, piloted by "no less than a 1/2" qev". IIRC :) This would be in keeping with my earlier observations.

In my opinion, the schrader as pilot is the achilles heel of Jacks designs. The flow rate is adequate to make "it" work, but not to extract the designs ultimate potential.

I have noticed with my PVC gun (my first piston gun) that heavier projectiles DO in fact travel faster.

This is a 2" piston piloted by a modded blowgun. It has IMO excessive piston travel at 1/2D, and does experience piston bounce with light projectiles. (marshmallows)

I do use slightly more than 1/4D, because I believe that it (1/4D) does not take friction into account.
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Unread postAuthor: Ragnarok » Sun Dec 07, 2008 4:13 pm

Don Gippeto de La Mancha wrote:As to the bumper causing piston bounce theory;
I believe it is possible to a degree, but at the END of the firing cycle as chamber pressure drops, and the force acting on the piston decreases below the return force of the bumper material.

Think of it like a basketball. Drop a basket ball, and despite gravity being the leading force, the ball will still bounce back up part of the way back when it hits the ground.
It doesn't wait for gravity to switch off, it converts it's kinetic energy into elastic potential energy, then releases it again straight away.

Same with anything, even a rock hard marble against concrete.

However, when I open my copper guns post firing cycle, the piston is, invariably, resting on the bumper. Not somewhere in between. The return acceleration must certainly be rather low.

I imagine that like the basketball, which will eventually come to rest in contact with the ground, the piston will bounce back off the back stop almost immediately, and then be forced back again by pressure.

The real way to deal with mechanical piston bounce, rather than pneumatic piston bounce, is a good piston bumper, into which most of the energy of the piston can be absorbed, and not re-imparted into bounce. Like dropping the basketball into a large pile of pillows.

HEAL works very well, and is a testament to careful design. It is however, piloted by "no less than a 1/2" qev".

Well, that was for length efficiency, allowing me to go straight from 28mm copper to 1/2" BSP in one fitting. I couldn't have done the same with any other size of QEV - and a bit of overkill in that department doesn't go amiss. :twisted:

In my opinion, the schrader as pilot is the achilles heel of Jacks designs.

It has it's weaknesses - but it's interesting to see that it's possible to use it with a good piston fit.

I do use slightly more than 1/4D, because I believe that it (1/4D) does not take friction into account.

No, D/4 doesn't consider air flow friction. That's why I use D/2 - it won't hurt opening time by any real amount, and might help performance.
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Unread postAuthor: Gippeto » Sun Dec 07, 2008 4:37 pm

How about I run some shots through the copper gun,(say 5?) over the chrony, as is.

Then I'll shorten the piston to allow D/2 and fire another 5.

Then compare the results.

Not definitive for every scenario, but at least emperical data.
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Unread postAuthor: Ragnarok » Sun Dec 07, 2008 4:47 pm

Don Gippeto wrote:How about I run some shots through the copper gun,(say 5?) over the chrony, as is.

Then I'll shorten the piston to allow D/2 and fire another 5.

Sounds good, but I'd recommend trying it with a couple of different ammo weights for both lengths as well.

I reckon you could also get away with just 3 shots to find an average muzzle velocity in most cases (that is, if you want) - if there's a reasonable amount of repeatability, then that should be fine.
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Unread postAuthor: jitup » Mon Dec 08, 2008 2:39 pm

?????????, uh, I'm not 100% sure what you guy are talking abut, but I was under the impression from all the other posts that I looked at you simply took 1/4 of the diameter and that should be how far the valve opens.

Oh, I just got it D/4 is 1/4 diameter and D/2 is 1/2 diameter!!! :idea:

(sorry about the brain fart, just had one HARD chem test, brain a little fried! :D )

any ways which do you think is ideal for this gun, it will be a coaxial bullpup
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Unread postAuthor: Gippeto » Mon Dec 08, 2008 5:00 pm

Opinion would seem to be divided at this point. :)

If you can wait, I'll be doing some testing in the near future.

If not...Split the difference at D/3 ?
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Unread postAuthor: jitup » Mon Dec 08, 2008 6:47 pm

LOL :lol: D/3! (hmm this may not be a bad idea)
I can wait depending on how long you mean. If mean a couple of days I will wait before I start to build that part, but a couple of weeks I am unsure. (I have been doing so much resarch to build this gun I just want to shoot it!!! :twisted: )

Anways I really apreciate your guys patience and help.
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Unread postAuthor: Gippeto » Mon Dec 08, 2008 8:10 pm

I would like to do the testing this week. I'll be testing the bb launcher tomorrow, and then with any luck I can get the testing for this completed by the end of the week.

I have "other" testing plans for the weekend. :D (Unless my wife has other plans for my time. :( )
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Unread postAuthor: jitup » Mon Dec 08, 2008 8:38 pm

Great! I look forward to hear your results 8)

BTW if any knows any good resources about how to make a diaphram valve, please post a link on the Spudinator page
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