Plausibility of coaxial cannon

[lucky_13]

Hello, new to the site, but I've built a couple cannons before. Here's one: A breech-loading turret mounted cannon.


Here's my newest project - I have a new idea that has some pretty specific requirements. I won't go into the "why" yet, keeping secrets keeps me motivated to work on my projects.

Basically, I need a co-axial, piston valve cannon with the main chamber made of 2.5" nominal PVC, schedule 80. The cannon will have a 2" barrel, but the inner barrel of the cannon will be some diameter less then the 2" barrel. This is the "X" in the drawing.



What I need to know is the largest inner barrel I can use and still have the piston valve function. I'm thinking 1.5".
Also, I'd like some input on the pilot valve- would a push-button valve (blowgun, w/e) work for this setup?

I'm looking at a 5' barrel - given the cannon is 2.5" diameter, would 10" of cannon length be enough to fire a nerf round a considerable distance?

I'm currently stationed at Fort Lee, VA with the Marine Corps, otherwise I'd be out building instead of asking questions. But I figure with you guys' help, I'll have a functioning cannon on the first try.

[Dig_Gil]

Because it seems to be hard for people in this forum to answer these kinds of questions, I'll give you my two cents. Firstly let's make something clear:

In a coaxial cannon you have a tube (where the projectile flies through) which is the barrel and another tube (enveloping the barrel, where air pressure is contained) we call the air chamber. It seemed to me that you called "barrel" to 2 different things. Also, allow me to use metric units, because that 1.5-slash-slash versus 5-slash confuses me.

The force with which air pressure pushes a piston is given by this equation:
Force = Area * Pressure
Area refers to the area of the piston exposed to pressure along its movement.
When the piston is fully inside the cannon, the "valve" is closed/the barrel is blocked. At this point, the pressure can only affect a "donut" section of the piston. Getting the area of that section is made this way:
Piston_Face_Area = PI*Piston_Radius^2
Hidden_Piston_Area = PI*(Barrel_Outer_Diameter/2)^2
Donut_Section_Area = Piston_Face_Area - Hidden_Piston_Area

The force with which the piston is pushed away is (by crude approximation):
Force_on_Piston = Donut_Section_Area * (Pressure_In_Chamber - Pressure_in_Pilot)
I say crude approximation because it neglects the fact that area exposed by pilot side isn't affected by the barrel. I want to make the equation short...
If the resulting number is positive, then you have a valve being opened and that means you're doing something right!

Of course that when air is being contained behind the piston, in pilot space, the pressure is equal and the equation gives a value of zero and rightly so.
The faster you can dump the pilot volume of air, the lower will pressure in pilot be at any given moment and thus a faster moving piston. To dump the pilot faster you need a bigger triggering valve (or pilot valve as we call it). A small donut section will have the piston to require more pilot to be dumped before the Force number becomes positive enough to counteract friction and the force of pilot pushing the piston into the barrel.

[TL;DR]
Most things will work, but if you can:
Max pilot valve, Min barrel diameter.

[mrfoo]

There's another few things to play with, actually. The bigger / higher flow your pilot valve, the harder it will be to trigger. So you can play tricks with the pilot volume and pilot-side piston to improve matters.

Firstly, there's pilot volume. Obviously, the smaller the volume, the faster it can be dumped for any given pilot valve. There's also the added bonus that it means less "wasted" air.

Next up, pilot piston area. Assuming we're using the same air source (and thus the same pressure) for the air chamber and the pilot volume, it stands to reason that the pilot piston area should be larger than the "donut" area exposed to chamber pressure when the valve is closed, in order to keep the valve closed. On the other hand, it should probably not be much bigger than the "donut" area, which is where your "small barrel" comes from.

If you had a very high pressure air source, or an air amplifier, you could push the pilot volume pressure up to much higher than the air chamber pressure. Again, this means you can have a smaller pilot volume and pilot piston area, and faster dumping.

This does tend to mean that your air chamber (or, at least, the valve piston) shouldn't be massively larger than the barrel. Quick back of a fag packet calcs tell me that if you have a 14mm OD barrel with a 60cm OD piston, you're not going to save much pilot side; you might as well do things simply and have a cylindrical piston.

Remember, although the chamber side and pilot side of the piston have to be connected in some manner, they don't necessarily have to be one piece.

Once the piston(s) start moving, unless the pilot valve is dumping air faster than the moving piston is compressing it, the pilot volume will tend to act as an air spring, slowing the piston. This can be avoided by, counter-intuitively enough, *increasing* the pilot volume in a way that creates a "dead space" the piston can never reach and which therefore never needs to be dumped. Probably the best way to do this is to have an annular space around the pilot area; the moving piston will compress air into this space but will *not* be pushed by it once the piston has moved past. Leaving a small, unvented, volume after that for the piston allows the air spring effect to buffer the piston.

Then there's flow from the air chamber to the barrel. It needs to reverse direction, the "mouth" of the barrel is a flow restriction. Flaring the barrel mouth and shaping the piston sealing face will help.

A few thoughts.

[mark.f]

The faster you can dump the pilot volume of air, the lower will pressure in pilot be at any given moment and thus a faster moving piston. To dump the pilot faster you need a bigger triggering valve (or pilot valve as we call it). A small donut section will have the piston to require more pilot to be dumped before the Force number becomes positive enough to counteract friction and the force of pilot pushing the piston into the barrel.

[TL;DR]
Most things will work, but if you can:
Max pilot valve, Min barrel diameter.

I though I would go ahead and post here even though it's an old topic.

Your assessment that having a minimum barrel diameter will increase performance somehow is a little flawed. First off, that leads to less porting on the valve. Secondly, with a smaller barrel OD, the valve will pilot with a higher pressure in the pilot still, leading to incomplete opening of the valve and possible "honking" or bouncing of the piston.

The significant advantage of a barrel (outlet) sealing valve which kind of lead to them overtaking chamber (inlet) sealers, even though most of the popular early piston valves were chamber sealers, is that as you increase porting for a certain piston size (think trying to get the most flow out of a certain size of valve or chamber ((in a coaxial cannon)) ), the pilot needs to be dumped more completely before the piston unseats, leaving less of an air spring behind it which could cause it to open slower or less completely. It does require more attention to construction of the piston (or a suitably large pilot valve if you're sloppy and don't care), but considering you need a complete seal with o-rings to do inlet (chamber) sealing, it makes them very attractive.

So, I guess my tl;dr version is: maximize barrel OD, increase pilot valve flow, decrease pilot volume... to the point practical, obviously, 1.75" PVC pipe doesn't exist, and if your barrel and piston OD are too close, your piston simply won't unseat reliably.