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Arrow gun: Barrel length vs fps question.

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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Re: Arrow gun: Barrel length vs fps question.

Unread postAuthor: mrfoo » Tue Oct 25, 2016 5:12 am

If you're holding the arrow back until the barrel pressure reaches a certain level, you can consider your barrel to be the chamber, and your arrow to be the piston. At that point, you're probably better off working out a way to make the barrel seal properly against the arrow, forget any high speed valving and your original chamber, and trigger with a mechanical sear which releases the arrow itself.

An alternative would be to have a long piston inside the barrel (or short piston with pushrod) which itself pushes the arrow. ISTR seeing some speargun designs that worked like that, probably on here.
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Re: Arrow gun: Barrel length vs fps question.

Unread postAuthor: mobile chernobyl » Tue Oct 25, 2016 11:23 pm

Good point @mrfoo

Here's a cutaway view of a modern spear gun: http://www.nauticexpo.com/prod/imersion ... 05649.html

And another view of the stages of a typical spear gun from rest to arrow launched: http://aquatech1.narod.ru/pneumatic.gif

I think the arrow sealing against the barrel without a piston is the best bet for efficiency since in the medium of air you are typically launching a light arrow fast, vs with spear guns you are launching a heavy arrow 'fast-ish' - but how to do so with as little modification to a standard arrow as possible?

maybe 2-3 small relatively high density split rings on the 'guide shaft' aka where the arrow slides over - you could have an o-ring all the way at the 'fully knocked' position and the split rings to help: stabilize, seal, reduce arrow to guide shaft friction as it launches?
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Re: Arrow gun: Barrel length vs fps question.

Unread postAuthor: Alanstone » Wed Oct 26, 2016 2:04 am

@ mrfoo "If you're holding the arrow back until the barrel pressure reaches a certain level, you can consider your barrel to be the chamber, and your arrow to be the piston. At that point, you're probably better off working out a way to make the barrel seal properly against the arrow, forget any high speed valving and your original chamber, and trigger with a mechanical sear which releases the arrow itself."

Thanks for your input. You are dead right and that is clever thinking. I have had the idea of using this as a platform for a .50 cal and possibly for a shotgun later so this kind of thinking never crossed my mind. It not only solves a number of issues I had with pressure but simplifies the design and reduces machined parts. When I have time later today I will give this more thought and do a bit of drawing to see how this will work. Redesigning the nock insert to provide a lip for the sear could be the way to go. In my design I was designing a method to clamp the rear of the arrow on the arrow od itself. I was planning to do that design change to the launcher today but now you have me thinking in a different direction. Hopefully I will have time for both. I must say however, that when I said "holding the arrow back" I meant for the xxx thousandth of a second as the pressure equalises between the chamber and the barrel. Holding the arrow at 850 psi seems a lot more scary. My main concern with holding the arrow back (even in my design) has been that the glue holding the insert for the broadhead would fail. I haven't found a solution to this potential problem. If I assemble the arrows myself and pressure test them with a safety factor I could say they are safe but arrows are meant to be used more than once and what happens out in the field is another story. For this reason only, I would probably prefer the firing pressure to develop once the trigger is pulled. There is definitely a practical synergy between how I intended to clamp the arrow and what you are suggesting. Hopefully it comes to me :D

@ mobile chernobyl "maybe 2-3 small relatively high density split rings on the 'guide shaft' aka where the arrow slides over - you could have an o-ring all the way at the 'fully knocked' position and the split rings to help: stabilize, seal, reduce arrow to guide shaft friction as it launches?"

Thanks for your input too and for the reference to the spear gun. I get what you mean by "an o-ring all the way at the 'fully knocked' position". It's something I had realised may be a requirement in the current design. The clearance between the arrow and the barrel is 0.003". Given that, it's probably not necessary to guide the arrow. Or am I missing your point? If so, I would be grateful if you elaborate.
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Re: Arrow gun: Barrel length vs fps question.

Unread postAuthor: mrfoo » Wed Oct 26, 2016 2:53 am

I think what m-c is talking about is having one or several o-rings on the outside of the barrel towards the tail of the arrow, which will avoid air pressure leakage "out the back" of the arrow; with some tuning they might also provide a frictional lock on the arrow that's enough to hold it back a bit until pressure has built; one the arrow has slipped off the o rings you should have higher pressure in the barrel (and your arrow will have a "running start", hah).

You'll still have the issue that you'll be averaging your chamber pressure across the entire chamber + barrel + a certain amount of the arrow, so pressure will be lower than you started with, but it might well be the simplest way to go. As you're holding the arrow back some, you don't need to worry about super-fast valving, and the o-rings should stop the arrow falling off the end of your launcher before firing, too.
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