SpudFiles

I'm thinking about working on a Coax style launcher, obviously, but I wonder about what affects the efficiency of this kind of launchers. I've visited the spudwiki about the subject, but seems incomplete and too verbose.

Nonetheless, the wiki mentions about the barrel-to-chamber ratio. Given there is a pilot volume of air behind the piston, it may either hinder (causing honking) or help (what's supposed to do) the opening of the valve. This because the surface area of the piston being pressed by the chamber in one direction is balancing against the surface area of the piston being pressed against pilot volume in the opposite direction. Correct me if needed.

I assume that this fact affects efficiency and limitations somehow, so I wonder, how should this be tuned? A smaller ratio (bigger chamber area) allows to have a larger volume of air in a shorter chamber (something I prefer), but then the pilot valve should be made larger to avoid honking, right? I think that it also makes the valve to trigger sooner and more readily, right? Yet the wiki mentions that large ratios (smaller chamber) make a snappier triggering, that means a faster moving piston, right? How come does that happen?

Something I've considered was getting rid of the pilot volume as a triggering mechanism. The wiki mention mechanically triggered valves, but I've never seen one. Can someone point to me a launcher which valve opened by mechanical action? Apart from building complexity, what are the downsides of such mechanism?

Maybe I'd build a Coax of small ratio which uses the pressure from the chamber to open and which is kept closed by mechanical locking, similar to how firearms lock their breeches. How does this system compares in opening speed to a pneumatically triggered valve?

I was also wondering about the distance the piston moves when opening the valve. How do I determine how far back the piston must travel for the most effective discharge in to the barrel? I was considering a ballpark of a barrel's diameter distance away. From what I understand, the during the opening, the piston is basically a choke for air travelling down the barrel. But if the piston keeps moving away from the barrel when the most chamber pressure is gone, I'm worried the launcher will be overly long to accommodate the piston travel length.

P.S. EDIT:
For the sake of completeness, other factors which might affect launcher efficiency, although I'm not as bother about for my make, include Piston Mass which delays the opening of a valve and (once open) delay its closing. I believe that the frequency of oscillations that affects honking is derived from this mass (heavier piston = lower frequency).

I find your considerations very interesting, and I don't have a proper answer to all of them, but I can address one;

The issue with pure mechanical action is generally that you can't go very high pressure without 'ridiculous' mechanical force to open the valve. That is what has stopped me from using (simple) designs like it. However, with a design like this, the force required to open the valve can be tuned by choosing the right spring.
This allows for high pressure firing, provided you find a spring strong enough. This would require that your input pressure always remains the same, or within certain boundaries. (because your mechanical system can only supply so much force)

I haven't seen this design being used yet (not that I have seen much 'advanced' systems), but I think with a few modifications, for example make the piston a conical shape for better airflow, I think it would work great.

If anyone has some other good examples of mechanical valves, please share!

I imagine you could use a rotating bolt/piston like assault rifles use. It's a very tough and tried mechanism like in the picture attached. (Green things are airtight seals) A sort of operating rod or "bolt carrier" would be used to unlock the piston, thus opening the valve by allowing the piston to fly away under chamber pressure. A spring closes the valve when the chamber pressure becomes too low.

An M16 bolt is overly intricate to machine. But triangular lugs like this one aren't unheard of and might be more suitable:


Something for you to notice is also the reduced diameter of the piston relatively to the chamber diameter. A smaller piston face means a smaller force needed to be tolerated by the locking mechanism.

I think it could be desirable to have the unlocking device to unlock upon pushing forward, but I'm not sure how to actually, in practice, make it lock or unlock. As it stands in the sketch it's just magic.

Mechanically actuated blow open valves.....not coaxial, but might give an idea or two. Common theme is to create a force bias using sealing diameters and to over come that bias using a mechanical striker.

Striker is the "U" shaped piece and although not completely obvious, it slides on the spool shaft. Striker spring goes between the striker and the pin through the spool shaft. Change in diameter on the shaft acts as a dash pot to dissipate excess energy of the spool (bumper). Manually reset by cocking striker. Would use a conventional sear/trigger.



Knock to crack blow open valve...steps on spool provide a degree of regulation and reset the spool, allowing chamber to re-fill.
Intended to use a paintball type tank, rifle was intended to be similar in layout to an AF Condor.

http://www.spudfiles.com/pneumatic-cann ... c7457.html



This was one of my very first projects when all I had was syringes and epoxy, these days I would use proper o-rings and a balanced piston to reduce the strain on the trigger.