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CO2 Stokes 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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Re: CO2 Stokes Mortar

Unread postAuthor: Cthulhu » Sun May 21, 2017 10:15 pm

I simply folded some tin foil a couple times then cut it into a disk about the diameter of the bottle cap (which I had cut a hole into) then I placed the burst disk inside and tightened the cap on to form it. I will try again with a larger dismayed though.

I tested the mortar today, couldn't get it to fire the 12g due to the lack of weight on the projectile and drag from the material it was made of (foam). I guess I'll shove some books in the nose of the shell and lather some baby oil on it.

I'm starting to see why a home made shell would be nice, I'll be looking for valves within the upcoming days.
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Re: CO2 Stokes Mortar

Unread postAuthor: jackssmirkingrevenge » Mon May 22, 2017 6:11 am

You'll be wanting a rubber ring of some description in order to ensure a good seal, something like this:

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Re: CO2 Stokes Mortar

Unread postAuthor: Cthulhu » Tue May 23, 2017 4:32 pm

I've realized the reason I may not be getting the CO2 to puncture is that the fit of the projectile in the barrel may be enough to cause resistance as the air beneath it searches for a way out. Sounds like plenty of projectile modification for me.
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Re: CO2 Stokes Mortar

Unread postAuthor: jackssmirkingrevenge » Wed May 24, 2017 5:47 am

A lightweight projectile that is a tight fit will certainly have difficulty generating enough momentum to puncture the disk. Consider a solenoid "hammer" at the base of the barrel to do the job, perhaps with a detector that actuates it the moment the projectile is in place.
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Re: CO2 Stokes Mortar

Unread postAuthor: WizardNoodle » Wed May 24, 2017 6:45 am

Or try Exacto knife blades glued to the bottom of the mortar.
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Re: CO2 Stokes Mortar

Unread postAuthor: Cthulhu » Wed May 24, 2017 7:16 am

Or possibly just add extra weight to the projectile and add a hole for the air to escape through.

But I've already done that, now my problem is is that the cartridge keeps missing the nail at the bottom.
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Re: CO2 Stokes Mortar

Unread postAuthor: hectmarr » Thu May 25, 2017 1:03 pm

You can create an emptiness to give velocity to the projectile in the direction of the perforating element of the rupture disk, I mean inwards. I opened it to see the method to do this, without taking up much space. It is another alternative to those that have influenced you here. Question to try. Very nice project by the way.
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Re: CO2 Stokes Mortar

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Re: CO2 Stokes Mortar

Unread postAuthor: Cthulhu » Sat Nov 04, 2017 9:46 pm

IMG_20171104_214025.jpg
My bottle "shell"
Sorry for the extremely long wait, I took a break from this project but I'm back at it now!

I finally found some Schrader valves, and I installed them into mountain dew bottles (they are a pretty good fit in 3" PVC and are made of a thicker plastic).

However, the burst disks are still not working. I tried an O-ring but it was too thick and would not let me screw the cap back on. I tried taping the aluminum foil, I tried glueing, I tried alternate burst disk materials, but nothing seems to work!
On top of these struggles I'm worrying whether it will even have the necessary force to leave the barrel, but perhaps adding some water into the bottle like a bottle rocket may help...
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Re: CO2 Stokes Mortar

Unread postAuthor: Anatine Duo » Sun Nov 05, 2017 5:51 am

I want to see this work so keep at it!

Try using a gasket from inside the bottle cap for a burst disk. It should fit perfectly and seal. If it doesn't rupture nicely maybe alter your firing pin as suggested above (blade/s instead of nail, or a serrated tube)

Also you can cut a hole in that gasket to use as a sealing washer... much thinner than the o-ring.

The water rocket idea sounds good too, probably more fun to watch.

I made a launcher with the 12g CO2 using a nail as firing pin. The canisters came out as rockets which were fun and dangerous (unpredictable)
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Re: CO2 Stokes Mortar

Unread postAuthor: jackssmirkingrevenge » Sun Nov 05, 2017 6:35 am

Have you tried using adhesive tape only?
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Re: CO2 Stokes Mortar

Unread postAuthor: Cthulhu » Sun Nov 05, 2017 12:12 pm

Thanks for the advice guys, I made a lot of progress this morning, but it's still not functioning properly.

I found out that the mountain dew bottle caps have a little extruding ring on the inside that prevents a gasket or O-ring from fitting inside. I simply opted to use coca-cola caps instead.
My sealing rubber is actually a rubber pad meant to go under furniture feet, but it's fits perfectly and seems to make a good seal if the burst disk is cut well.

Sadly, even though I've been able to pressurize the bottle, the disk refuses to burst. Instead the firing pin simply pops a hole in it and the pressure instantly leaks out with a "pssssss" noise. I'm pretty sure this is from low pressure, but every time I try to pump higher the disk explodes before I'm able to remove the pump.

Then again it could be because the air pressure has no mass to push on, so perhaps placing some water in the bottle is needed... :scratch:
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Attachments
IMG_20171105_105953~2.jpg
The mountain dew cap (on the right) has a beveled ring on the inside that prevents O-rings or gaskets from fitting
IMG_20171105_105637~2.jpg
My makeshift gasket/O-ring, a burst disk, and the cap. You can see how the burst disk didn't actually rupture but was just punctured.
IMG_20171105_104553~2.jpg
The new configuration.

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Re: CO2 Stokes Mortar

Unread postAuthor: mark.f » Sun Nov 05, 2017 12:22 pm

On the topic of mortars:

You can make the stand adjustable for angle, sure. Another option is to make the firing pin adjustable for length. That way, you can adjust range without adjusting the launcher angle and, for a pneumatic version like this at least, make these adjustments a lot quicker (think a sliding firing pin with a collet or something similar to hold in place).

Good for firing through canopy openings. :)

And yeah, the burst disk is your best bet. The opening of a CO2 cartridge is tiny.
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Re: CO2 Stokes Mortar

Unread postAuthor: Cthulhu » Thu Nov 09, 2017 6:45 pm

Well, I've done some further tests and I'm nearly there, but something just doesn't add up.
I am using 4 layer thick aluminum foil and a cruddy electric car tire compressor.
At 60 psi the burst disk actually popped, but only slid up the barrel a few inches before sliding back down.
At 80 psi the burst disk sprung a minor leak, but I quickly dropped it into the mortar and it slid even further up the barrel!
This is both good and bad news. With a nice burst disk this could possibly fire at a very high pressure, but I don't think either the disk or bottle for the matter could handle the necessary pressures to even leave the barrel.
Perhaps this is due to the projectile being too heavy? Or perhaps the hole in the cap is too small for optimum flow? Or maybe it's time to fill it with a bit of water?
A lot of factors to test gentlemen. Stay tuned and please give me any feedback you may have!
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Re: CO2 Stokes Mortar

Unread postAuthor: jackssmirkingrevenge » Thu Nov 09, 2017 7:00 pm

How loose is the bottle in the tube?

Consider using a x-acto blade (or several) instead of a pin to burst the disk.
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Re: CO2 Stokes Mortar

Unread postAuthor: Cthulhu » Thu Nov 09, 2017 8:51 pm

jackssmirkingrevenge wrote:How loose is the bottle in the tube?


It's got a little less than half of an inch wiggle room. It allows it to slide does quickly with no air resistance but I can see how that may be preventing it from working.

jackssmirkingrevenge wrote: Consider using a x-acto blade (or several) instead of a pin to burst the disk.


I can try that, do you think it would really help to burst the disk any better though?
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