Login    Register
User Information
Username:
Password:
We are a free and open
community, all are welcome.
Click here to Register
Sponsored
Who is online

In total there are 61 users online :: 5 registered, 0 hidden and 56 guests


Most users ever online was 218 on Wed Dec 07, 2016 6:58 pm

Registered users: Bing [Bot], Google [Bot], Google Adsense [Bot], MSNbot Media, Yahoo [Bot] based on users active over the past 5 minutes

The Team
Administrators
Global Moderators
global_moderators.png CS

Considerations to have when building a Coaxial Piston Cannon

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.
Sponsored 
  • Author
    Message

Considerations to have when building a Coaxial Piston Cannon

Unread postAuthor: Dig_Gil » Sun Sep 11, 2016 6:39 am

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).
  • 0

User avatar
Dig_Gil
Private First Class
Private First Class
 
Posts: 33
Joined: Wed Jan 29, 2014 6:33 am
Location: Somewhere between Moscow and Azores
Country: Antarctica (aq)
Reputation: 0

Re: Considerations to have when building a Coaxial Piston Ca

Unread postAuthor: Doodmens » Sun Sep 11, 2016 11:49 am

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.

WhatsApp Image 2016-09-11 at 18.40.38.jpeg


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!
  • 0


Doodmens
Specialist
Specialist
 
Posts: 52
Joined: Tue May 19, 2015 7:44 am
Country: Netherlands (nl)
Reputation: 5

Re: Considerations to have when building a Coaxial Piston Ca

Unread postAuthor: Dig_Gil » Sun Sep 11, 2016 2:29 pm

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)
MechaCoAx_concept1.png
Mechanically Actuated Coaxial Launcher Concept Sketch

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:
Image

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.
  • 0

User avatar
Dig_Gil
Private First Class
Private First Class
 
Posts: 33
Joined: Wed Jan 29, 2014 6:33 am
Location: Somewhere between Moscow and Azores
Country: Antarctica (aq)
Reputation: 0

Re: Considerations to have when building a Coaxial Piston Ca

Unread postAuthor: Gippeto » Tue Sep 13, 2016 3:05 pm

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.

Image

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.

Image
  • 0

"It could be that the purpose of your life is to serve as a warning to others" – unknown

Liberalism is a mental disorder, reality is it's cure.
User avatar
Gippeto
Donating Member
Donating Member
 
Posts: 2394
Joined: Sat Jan 19, 2008 10:14 am
Location: The Great White North...Canada eh!
Reputation: 11

Re: Considerations to have when building a Coaxial Piston Ca

Unread postAuthor: jackssmirkingrevenge » Tue Sep 13, 2016 9:59 pm

pneumatic-cannon-database/topic7457.html

Image

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.
  • 0

User avatar
jackssmirkingrevenge
Donating Member
Donating Member
 
Posts: 24225
Joined: Thu Mar 15, 2007 11:28 pm
Country: Holy See (Vatican City State) (va)
Reputation: 66

Return to Pneumatic Cannon Discussion

Who is online

Registered users: Bing [Bot], Google [Bot], Google Adsense [Bot], MSNbot Media, Yahoo [Bot]

Reputation System ©'