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propane newbie

A place to ask general spud cannon related questions.
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Re: propane newbie

Unread postAuthor: lockmanslammin » Tue Oct 07, 2014 10:19 am

I think lubing the barrel would reverse the "ammo to barrel friction" improvement gained by double beveling the barrel, and decrease performance.

If it were a pneumatic cannon, beveling the barrel might help because the pressure is already at max so a sealed and slippery projectile is what your aiming for. Not so in a combustion cannon. The way I understand it, double beveling causes the tater to be tighter, increasing friction, which is highest before the potato starts to move. I don't know the sciency sounding explenation, but something like, static friction is higher than kinetic friction. This allows the pressure via combustion too build a bit before the potato starts to move.

I suppose in a small way it would be like adding a weak burst disc into the cannon. I'm sure it (double beveling) helps get a better seal to so that can't hurt. make sense?

Chris

/edit/ oh and one more thing... You can get away from the weak end cap entirely by using adaptor bushings and a large pvc ball valve. That way exhausting after each shot is much simpler, and everything is pressure rated. And in case you need to get into the chamber you just unscrew the whole assembly.
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Re: propane newbie

Unread postAuthor: Brian the brain » Tue Oct 07, 2014 12:28 pm

If you're using an inline design where you go from a large diameter chamber to a smaller diameter barrel..and fill from the back of the chamber...you could have the barrel extend into the chamber right?

Then load the tater from the reservoir-side and press it onto the breech to the point it is partially cut and leave it.

The spud becomes it's own burst disc.
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Re: propane newbie

Unread postAuthor: mobile chernobyl » Tue Oct 07, 2014 1:09 pm

That would be great for a 1x or possibly 2x mixture ratio! Calculating (if you found it neccesary) the failure mode of said potato and therefore the "burst pressure" may be a bit harder haha

eliminating burst discs and pistons all together and using this method, possibly more calculated such as the folding skirt projectile Utron uses, or one of the multiple methods shown on Spudfiles of shear failure projectiles is the future!
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Re: propane newbie

Unread postAuthor: Chris spud » Tue Oct 07, 2014 5:35 pm

Brian the brain wrote:If you're using an inline design where you go from a large diameter chamber to a smaller diameter barrel..and fill from the back of the chamber...you could have the barrel extend into the chamber right?

Then load the tater from the reservoir-side and press it onto the breech to the point it is partially cut and leave it.

The spud becomes it's own burst disc.



Ok, I need to understand this. A burst disc bursts under pressure - it helps to get maximum pressure before spud starts moving, a good thing, more acceleration. If I extend the barrel through the reducer into the chamber, doable, then you sharpen the end inside the resevoir and load the spud from there..but, you don't push in the spud completely, only partially - so the spud is acting like a burst dick - pressure builds until the potato is cut all the way and start to move. I would do this, but I've already blew off the screw-on cap once. I might try to source a large ball valve as someone suggested. In South Africa, you do get blue PVC piping that's rated to 16bar - around 235psi..i'd feel more safe with this, problem is you buy 6m lenghts at a time
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Re: propane newbie

Unread postAuthor: Chris spud » Wed Oct 08, 2014 10:14 am

RE the burst disc idea, double bezel barrel has the same effect -it cuts the tater slightly bigger than the inner diameter of the spud. You can feel the difference when you push it back - a lot more friction. Have any of you timed hang time for spud if you shoot straight up? I think it's an easy way of comparing performance. The faster the spud leaves the barrel, the more hang time. Getting 11s, but i think I can do better
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