big 4X chamber concept

[Moonbogg]

Hey all. I felt the desire to share my concept for the chamber of my next cannon project. This cannon has a name (Cobra Striker Extreme) and I am starting to take my next project seriously now. The caps will be bolted with 5/16" or 3/8" bolts, all aluminum construction. The piston housing will be bolted to the front cap from the inside. The piston housing has 3/4" wide slots for venting. The pressure, so far, is planned to act on the o-ring only (not shown). I haven't fine tuned the pressure differential faces yet, but I plan on having very little piston area exposed to chamber pressure (likely the o-ring only) to allow me to use a low pilot pressure, which should aid in rapid opening.

The cannon will operate using compressed air and propane at a mixture of 4x.

Ideas, comments all welcome please.

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[qwerty*]

thats a really nice canon pic!
but why did you decide to make this cannon all metal?
you should considder making your combustion chammber larger.
I really admire that you are keeping it original and making a combustion cannon rather than rigging it up to propane or air!
Nice job, 8) keep it up!

[MrCrowley]

but why did you decide to make this cannon all metal?

If it was made from PVC it would probably explode...
A 4x mix generates around 400PSI in the chamber, you generally want a metal chamber to handle those pressures.

I really admire that you are keeping it original and making a combustion cannon rather than rigging it up to propane or air!

Actually, that's exactly what he is doing, hooking it up to propane AND air.

You know a combustion cannon may use propane for a fuel, right?
Just confused to why you applaud him for making a combustion (though he's really building what we prefer to call a hybrid) and not hooking it up to propane.

I think you got a little confused so you might want to quickly read the below links.
Combustion Cannon
Hybrid Cannon

[jackssmirkingrevenge]

Interesting piston location, should provide better flow. Is the space behind it pressurised/sprung or is the o-ring floating to allow pressure into the pilot but not out?

[qwerty*]

thanks for the cool links
I am currently building a basic combustion cannon and im using a bbq lighter for ignition because I have easy acess to them.
after what you have told me, I may use propane instead of the classical hairspray. If I build it with an extra long barrel will I have increased accuracy?

[jackssmirkingrevenge]

If I build it with an extra long barrel will I have increased accuracy?

A few words on the subject:

Long barrels are associated with long range accuracy because a long barrel usually means high velocity and this results in a flatter trajectory. The projectile gets to the target quicker and therefore has less time to be affected by gravity, wind and other environmental factors which have to be compensated for.

The other side of the coin however is that with a longer barrel, there's more time in between you pulling the trigger and the projectile leaving the muzzle, so any movement during that time is more likely to have a negative effect on your accuracy.

In short, the ideal in terms of accuracy is to achieve as high a velocity as possible from as short a barrel as possible.

[Gun Freak]

Certainly an original idea! Looks very promising!

[Technician1002]

I like the design. On the air side of things, this is almost exactly like the design I am playing with for the 4 inch piloted QDV. The concept is sound and should work for you. I would add a small tube from the piston pilot area so an initial shot of air can be used to set the piston into position and create the initial seal. This would prevent fuel accumulation in the pilot that could diesel when fired. I would increase the space behind the piston slightly to provide space for a bumper of some kind to keep peak forces on the piston to reasonable levels. Other than that, it looks fantastic. I like the close ratio on the piston valve. It should pop open.

[Moonbogg]

Thanks for replies. Specifically to JSR and Tech, I could use some help here in deciding the best way to pressurize the pilot. This will be my first piloted cannon ever so I have no experience with them. Like tech said, I thought of having an air line going to the pilot to pressurize to desired levels. Upon firing, another line could lead to a pop-off valve whos opening pressure could be the same as the pilot pressure, so when the piston pops open, the air will dump out of the pilot right away. I wonder how it would work if I just left the air pressurized in the pilot, maybe the air will just act like a spring and stay in the pilot? Keeping the air trapped in the pilot could act as an adjustable gas spring assuming it doesn't leak from the violent opening.
Then there's the floating o-ring/check valve method for pressurizing the pilot and chamber at the same time, which is common and seems to work well. I STILL don't understand just how the o-ring check valve configuration works. If the o-ring slides forward allowing air to pass through a hole, i'm afraid that when adding propane to the chamber that some will enter back through that same hole into the pilot area.

[Gun Freak]

Yeah because the thing with a floating o-ring check valve is that the actual check valve part doesn't take place until the o-rings are seated in the other position by a pressure change, ie piloting, so I dont think that the concept can't really be used for fueling in this case.

[Technician1002]

The configuration would be a small tube from the back of the pilot to the rear of the cannon. To prevent fuel in the pilot area, the charging would be in 3 steps.

1 add some air to set the piston and initial chamber pressure to hold it in place. The piston can have a small leak to enable this. By using floating o rings, in one direction it could seal normally. In the other direction, some intentional damage to one of the o ring groove side walls will provide the leakage path needed to make this work as a check valve.

2 add the fuel charge either directly into the chamber or through the pilot. This is your full metered charge.

3 add air up to the operating pressure through the pilot. This will purge the pilot of fuel.

With the chamber at pressure, when fired the piston will have a high pressure gas shock to arrest the piston. With a proper size tube from the pilot to the rear, high flow will not be possible to feed a pop off valve. No worries, the tube will act as a secondary buffer and restriction to flow so the combination will act as a dashpot. The valve will remain open as long as the chamber pressure is above the original pre fire pressure and close as the chamber pressure drops. A pop off on the pilot can be added if desired to reduce the pressure the valve closes, but I don't think it would be fast enough to make much difference. The main disadvantage to a pop off on the pilot would be an accidental discharge while bringing it up to pressure.

[Moonbogg]

The configuration would be a small tube from the back of the pilot to the rear of the cannon. To prevent fuel in the pilot area, the charging would be in 3 steps.

1 add some air to set the piston and initial chamber pressure to hold it in place. The piston can have a small leak to enable this. By using floating o rings, in one direction it could seal normally. In the other direction, some intentional damage to one of the o ring groove side walls will provide the leakage path needed to make this work as a check valve.

2 add the fuel charge either directly into the chamber or through the pilot. This is your full metered charge.

3 add air up to the operating pressure through the pilot. This will purge the pilot of fuel.

With the chamber at pressure, when fired the piston will have a high pressure gas shock to arrest the piston. With a proper size tube from the pilot to the rear, high flow will not be possible to feed a pop off valve. No worries, the tube will act as a secondary buffer and restriction to flow so the combination will act as a dashpot. The valve will remain open as long as the chamber pressure is above the original pre fire pressure and close as the chamber pressure drops. A pop off on the pilot can be added if desired to reduce the pressure the valve closes, but I don't think it would be fast enough to make much difference. The main disadvantage to a pop off on the pilot would be an accidental discharge while bringing it up to pressure.

That really sounds good. My only concern is that before the piston's o-ring is slammed back, some fuel will find its way inside and may possibly ignite despite being sealed off with the o-ring. I mean, even if it does, the cannon will just either not shoot or will shoot poorly. So not a huge deal anyway as it can always be tweaked.

Another thing I want to run by you guys with piston experience. The piston housing material is in question. With a 2" UHMW or possibly even aluminum piston at 4X, would you feel comfortable with an aluminum piston housing? I was considering taking on 2-3 extra pounds and making it out of 4140 alloy steel. That should be able to take the repeated shock from the piston. It should slam pretty hard even with a thick bumper I think and I have doubts that a reasonably thick aluminum housing will hold up. I don't have the room to make it super thick, so I was considering a stronger material for that highly stressed part at the expense of some weight.

Also, the UHMW piston...do you think that the force acting on the o-ring will cause the UHMW to shear at the o-ring sealing face? Thats where all the force is going to cause the piston to move back, and I wonder if the UHMW will warp, bend or outright shear at that location.

[ramses]

If I build it with an extra long barrel will I have increased accuracy?

A few words on the subject:

Long barrels are associated with long range accuracy because a long barrel usually means high velocity and this results in a flatter trajectory. The projectile gets to the target quicker and therefore has less time to be affected by gravity, wind and other environmental factors which have to be compensated for.

The other side of the coin however is that with a longer barrel, there's more time in between you pulling the trigger and the projectile leaving the muzzle, so any movement during that time is more likely to have a negative effect on your accuracy.

In short, the ideal in terms of accuracy is to achieve as high a velocity as possible from as short a barrel as possible.

Also, longer barreled guns tend to have a longer sight radius, which allows the user to use iron sights more effectively. A longer barrel has a lower resonate frequency, and so deflects more during the shot. If the barrel is tuned, that should help make inconsistent velocity less of a factor (low velocity projectiles would be released with the muzzle deflected further "up" than a higher velocity projectile. Longer barrels do that more.

[velocity3x]

Ideas, comments all welcome please.

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The chamber being 16" long will leave a lot of unsupported material extending beyond the lathe chuck jaws. Also, how do you plan to bore the inside of the chamber? The bore is 16" deep. Have you devised a plan to make such an extremely deep bore? Even boring from both ends will require deep boring.

[Fnord]

Ever considered adapting a toolie-style design to this? With the piston impacting in the rear (hey!)you'll be able to add more structural support and bumper than the somwhat thin housing in the current design, and it will also be easier to service the whole chamber.

You also may be able to cut down on the weight a little with a toolie design