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Coaxial mortar?

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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Coaxial mortar?

Unread postAuthor: warhead052 » Wed Jun 08, 2011 8:34 pm

Would it work to have a coaxial mortar? I dont mean by launching the object with a exhaust valve (which can still be on it) but by dropping something like a concrete filled caulk tube and having that be the pilot by smashing the piston down? Would it work, or would you have to use the exhaust valve? If you have a coaxial cannon can you try that for me?
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Unread postAuthor: Lockednloaded » Wed Jun 08, 2011 8:38 pm

Maybe with a co-axial QDV, but there is just too much force holding the piston shut
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Unread postAuthor: warhead052 » Wed Jun 08, 2011 8:43 pm

What about spring loaded, to where the spring is stronger than the air pressure pushing down and no air pressure behind it? I am not talking pressures at like 100 psi, just around 25 to see if it would even open it, then work on stronger/heavier projectiles.
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Unread postAuthor: Gaderelguitarist » Wed Jun 08, 2011 9:48 pm

Why not just have a nail glued to the caulk tube and use a burst disk.
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Unread postAuthor: Technician1002 » Wed Jun 08, 2011 11:29 pm

A QDV will work, but it requires a heavy projectile and a poor air seal on the projectile so it falls freely. Due to the poor projectile air seal, the range is limited. With too much air seal, it won't trigger the valve. A golf ball it too light.
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Unread postAuthor: warhead052 » Thu Jun 09, 2011 9:59 am

Gade, I would not know how to even assemble something like that. I have built a burst disk mortar but its pretty much pump till it pops...

Tech, Can you test it in your QDV cannon? I don't have one, and I kinda spent my money on the burst disk mortar so I cant buy more parts atm.
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Unread postAuthor: Hubb » Thu Jun 09, 2011 12:08 pm

The problem you are faced with, as Tech explained, is that the mass of the projectile being dropped into the barrel is going to have to gain enough momentum to overcome the pressure being applied to the piston. In your concept, there is not going to be a pilot and the projectile will have to push against the present air, which is basically going to generate an air spring. This air spring is probably going to push the piston back into place before any air can dump to launch the projectile.

In other words, it ain't gonna happen.

Tech's QDV will somewhat work around this, as there will be no air pushing the piston directly into the exhaust (barrel), but the design is going to have to be just right in order for it to work.

There have been designs over the years that do just what Gaderelguitarist recommended, and those are fascinating, but then you would have to replace the burst disk after every shot.

A while back, I built what I call a Real-Action-Mortar for paintball, in the sense that you dropped the projectile into the barrel and it fired once the projectile hit the bottom, but it was nothing like you proposed. It worked pretty good, but it was finicky as well.
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Last edited by Hubb on Thu Jun 09, 2011 12:09 pm, edited 1 time in total.
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Unread postAuthor: SP00K » Thu Jun 09, 2011 12:08 pm

A burst disk mortar is unbelievably cheap. Put a pressure gauge on it and stop pumping once it reaches its max.

I made one and it worked fantastically (although it wasn't coaxial, it doesn't change much). I used a small piece of wood with a hole through it for a screw, washer, and bolt to be my piercing mechanism for whatever projectile I dropped in to pop the burst disk.
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Unread postAuthor: Technician1002 » Thu Jun 09, 2011 4:14 pm

I've tested it in my QDV. Most of the time the projectile is not heavy enough to do the job or I have to use a long slender projectile to do the job.

That is why I mentioned that a golf ball is not heavy enough to do the job. It might work if I use low pressure to reduce the force on the o rings. I was able to launch a 1 inch dowel 3 feet long, but it didn't go far due to the huge blow by. I have to use the 2 inch barrel for testing as larger barrels and projectiles won't reach the 2 inch valve. This limits the size of projectiles I have tested. I'm thinking of trying later with a soda can in the 3 inch barrel by placing a short dowel in the 2 inch valve with a sabot in place. This may trigger the valve and provide a good air seal for a good can launch from the 7 foot barrel. I will have to wait until I am in a safe location to launch soda cans again so don't expect any tests soon.
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Unread postAuthor: warhead052 » Thu Jun 09, 2011 4:20 pm

Thats fine tech. I can wait. Have you tried the concrete filled caulking tubes?

Hubb, can you link me to your mortar? I have looking around the site and have not really found any mortars.
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Unread postAuthor: Technician1002 » Thu Jun 09, 2011 4:50 pm

I have not tried launching cement. The recoil would be too much. The heaviest item launched to date has been a large Gatoraid from the 3 inch barrel. The recoil gave me a bruise.
[youtube]http://www.youtube.com/watch?v=XsSTxCP8rvY&[/youtube]
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Unread postAuthor: biged » Thu Jun 09, 2011 4:57 pm

Not really a coax, but it has a 3-inch piston, pressure tested the chamber to 800psi. The burst pressure of the push to connect hose is 290psi, so I use it in 90-250 psi range.

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Go figure I stumbled across some free steel tubing with the same OD of the 4-conduit. I may have to make another tube...

///ed///
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