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my private coin challenge

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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my private coin challenge

Unread postAuthor: hyldgaard » Tue Sep 25, 2007 4:34 am

Hi, I'm a long time reader but first time poster, so please bare with me and my not so good english :oops:

Well, here's the idea: in a couple of weeks im going to visit my parents, and therefore have acces to my fathers shop, wich means im going to craft some kind of a new spudgun :) the spudfiles coinchallenge inspired me to try it out for my self, but with my current spudguns its not even close to be possible.
Before i get too much into this project, i would like to hear your oppinions if this is even possible, because i have some rather annoying limitations. first of all, pressure rated pvc is out, i live in Denmark and that is apparently not the thing here, so im going for copper. second, my soldering abilities is nothing to brag about either, så copper chamber is out too(i think, can't imagine how to do it without soldering?). third, hybrid is out of the question too, because this is my fourth(ish) spudgun, and i don't trust my crafting abilities enough to build a hybrid. yet. :twisted:
And the last thing, my dads compressor(?) only goes to about 80psi, so high pressure pneumatic is not possible either.
So, any suggestions how this could be possible for me? or is it totally out of the question with my limitaions?
looking forward to any reply, thanks :)
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Unread postAuthor: jackssmirkingrevenge » Tue Sep 25, 2007 5:02 am

80 psi is more than enough to penetrate a coin with a nail dart if you have a launcher of the right dimensions. What sort of valves do you have access to? If you think you can make a piston valve (which is not too hard if you can get the right fittings) then you'll get enough power, same goes if you can get your hands on a Quick Exhaust Valve or sprinkler valve. Don't worry about your soldering abilities, you can use epoxy adhesive instead, it will definitely hold 80 psi easily.

There are plenty of copper cannons on here that you can use for inspiration, have a look here. What you want is as long a barrel and as big a chamber as you can manage for maximum power. Anything between 10-25mm diameter pipe is a suitable calibre for the barrel.
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Unread postAuthor: hyldgaard » Tue Sep 25, 2007 5:45 am

genrally, valves are very hard for me to acces, as i the ones i can find in Denmark are very expensive, and i don't have alot of money as im under education :( if 80psi is more than enough with a good valve, then what about a basic small calibre combustion with some kind of burst disc? that would sure be alot easier for me as it would be possible to fire it without my dads compressor, and a whole lot cheaper too :)
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Unread postAuthor: Mitchza89 » Tue Sep 25, 2007 6:39 am

Bore wise, Don't go too small. The smaller the bore, the more psi you need to get good results. If you want cheap, a burst disk is the way to go. I havn't personally used them but I've seen a few that have.

All the best and good luck with the build :)
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Unread postAuthor: jackssmirkingrevenge » Tue Sep 25, 2007 6:41 am

A combustion with a burst disc wouldn't make much difference. A pneumatic with a burst disk on the other hand would be very powerful, cheap and easy to make. If you show us what kind of fittings you can get as fat as unions are concerned we can point you in the right direction ;)
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Unread postAuthor: hyldgaard » Tue Sep 25, 2007 6:54 am

well, as i am really bad with english/danish name for fittings and whatsoever, i found a danish site with the things i can affort, with PICTURES :D http://primavvs.dk/shop/kobberroer-fittings-16842s.html
i was thinking a copperbarrel with an inner diameter of 8mm, is that too small? i have this thing for small darts, dont know what it is :D
To be honest, i would prefer a combustion as i would be possbile for me to fire it were i live now, without a compressor, but if i have to go pneumatic for this to happen, thats just the way it is :)

jackssmirkingrevenge wrote:A combustion with a burst disc wouldn't make much difference.
i've seen significant performanceincreases on my earlier combustions by adding burstdiscs, but maybe something else made the difference, wasn't excactly top of the line spudguns :P
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Unread postAuthor: spanerman » Tue Sep 25, 2007 7:02 am

id go for a 8mm bore burst disk pneu......you can use your dads compresser, then use a bike pump when you back home....my £8 one does 120psi easy...
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Unread postAuthor: Mitchza89 » Tue Sep 25, 2007 7:07 am

Hmm I'm a bit doubtful if 8mm will be enough which is a bummer (I'm a fan of darts too :D) I'd probably go with 15mm copper as a barrel. A well made dart with a long enough barrel should be great.
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Unread postAuthor: hyldgaard » Tue Sep 25, 2007 7:08 am

well, i guess pneumatic it is then :) what proportions should i make it in to be sure to have enough power to pierce a coin with a dart? supposing id go with the 8mm bore :)

edit: a bit too fast deciding the barrel to be 8mm :) anyone with personal experience on this whole 8mm-barrel-piercing-coin-with-dart-thing? i just want to be rather sure its powerfull enough :)
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Unread postAuthor: jackssmirkingrevenge » Tue Sep 25, 2007 7:21 am

I'd say at least a metre of barrel length. I would really suggest getting a larger calibre, because since you cannot up the pressure, bigger bore is the logical step. 15mm would give you almost 4 times the power potential of 8mm for a given pressure. You also need a relatively large volume of air, for a 1 metre 8mm barrel I would suggest a chamber of at least 200mL volume. Then what you'd need is a tee, basically you thread the barrel to one and the chamber to the other. In the vertical portion of the tee you thread a schrader valve. The chamber will also need its own schrader. Find a material that bursts at 80 psi for your disk and put disks between the barrel and the tee and the chamber and the tee. Then fill the tee to 40 psi and the chamber to 80 psi. To fire, just empty the tee <i>et voila</i> :)
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Unread postAuthor: hyldgaard » Tue Sep 25, 2007 7:27 am

jack, i cant do anything but admire your knowledge 8) i thank you very much for your help so far, but i have a couple of questions still :D
if im willing to up the barrels length to about 2m, will an inner diameter of 13mm be enough? and for the camber volume you mention, how did you get that?
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Unread postAuthor: jackssmirkingrevenge » Tue Sep 25, 2007 7:47 am

if im willing to up the barrels length to about 2m, will an inner diameter of 13mm be enough? and for the camber volume you mention, how did you get that?


2 metres of 13mm barrel should be more than enough :)

I calculated a chamber:barrel volume ratio of 4:1 (basically 1 metre of 8mm barrel has a volume of 50mL, so 4 times that is 200mL for the chamber) which is a bit exagerated in terms of efficiency but it will suit your high power inspirations ;) really a 1:1 ratio is usually enough but since we're not talking about huge volumes here, why not go a bit overboard :D
For your 2 metre 13mm barrel, I would say at least 250mL of chamber volume but I would be comfortable going up to one litre.
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Unread postAuthor: hyldgaard » Tue Sep 25, 2007 7:55 am

I thank you all very much for your input, you've been a great help :) now back to the drawing board, i'll return when the build is in progress/done :)
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Unread postAuthor: jackssmirkingrevenge » Tue Sep 25, 2007 8:00 am

Good luck with it, if you think it's too complex you can make it much simpler just by having one burst disk between the chamber and the barrel but it would be somewhat unpredictable and therefore less safe than a triggered burst disk.
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Unread postAuthor: hyldgaard » Tue Sep 25, 2007 9:27 am

found a qev at a uk site and i think they ship to denmark, so i think its gonna be a qev instead of triggered burstdisc. and i guess ill be using a blowgun from the same site as pilot. im a bit excited about this project as its my first pneumatic, but i have very high expectations :D
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