The Double-barrel Pneumatic Air Cannon

After restoring my mine bolt-action potato cannon, I decided that the double-barrel cannon that resided in my garage would be interesting to get working again.  It hadn’t been fired in several years, and one of the valves no longer worked.  Additionally, it leaked badly, so I had my work cut out for me.  Here’s a picture of the gun partially restored before any sort of leak testing or firing (Read on for another video at the end):

double barrel PAC on ground

The principle behind this gun, or most pneumatic air cannons,is that one side of the cannon holds the pressure (marked in red squiggly lines below) while the other side holds your projectile, water in this case – marked in blue squiggly lines.  Separating the pressure and projectile side is some sort of pneumatic valve – in this case a sprinkler valve.  I’ve put a yellow (golden?) square around these two valves.  (Be sure to see my article about spud gun range testing theory for more background)

double shot PAC marked up

Yes, two valves.  Generally most pneumatic air cannons would only have one as they only have one shot per air charge.  This design has two separate air and projectile chambers.  Basically it’s two guns attached together.  To aid in filling each chamber with air, a manifold was created with a PVC elbow and Tee.  Air goes into a Schraeder valve shown by the leftmost red arrow and is split into two by this assembly. The charging air flow is illustrated by the two other red arrows.

In order to avoid discharging both sides at once, the two parallel flows go through a one-way valve.  This way, during charging both pressures will be roughly equal, but after firing one chamber can be close to 0 PSI (gauge pressure for those that know about that) while the other will be charged to whatever it was pumped up to.  The other end can then be discharged at will.

Which brings up the next question, how does one actually get these valves to fire?  In normal operation, the valves used are powered by 24VDC, so if you happen to have a 24 volt battery laying around, that’s one option.  The way this one is done is that three 9 volt batteries are chained together in series giving a combined voltage of 27 volts.  A little more than recommended, but it seems to work well.  Both solenoids were hooked up to individual normally open pushbuttons on the positive side.  The other side was hooked up to negative as shown in the drawing below:

PAC electrical diagramHere’s how this looked in reality:

double barrel PAC electrical being assembled

I used some DIN rail, terminal blocks, and grounding blocks to make a bank of positive and common out of the batteries.  You can find this stuff at Automationdirect.com.  I would definitely recommend using an additional safety switch with this assembly.  Additionally, always keep the barrel pointed in a safe direction.  Even after firing, there can be some air pressure still in the chamber.  If there is a misfire, the valve can be manually triggered by twisting the solenoid.  Definitely useful in some situations.

As for assembling the cannon, here’s a mechanical drawing that should illustrate things:

double barrel pneumatic air cannon CAD drawingOne thing that hasn’t been discussed is putting some sort of support on the cannon.  As drawn here, two pieces of wood are cut to length and then a hole saw is used to drill a nice hole in it so it fits between the PVC pipe.  In reality, as shown in the first photo, only one of these is used.  It also doubles as a trigger assembly with a simple thru hole and counterbore for each of the trigger buttons.

Once you have everything together, it will undoubtedly leak. The way to fix this is, first of all to use enough pipe tape.  If it’s still leaking, one way to figure out where is to submerge it in some liquid.  Be careful not to submerge the electronics, but for the bottom part especially, this worked well as a diagnostic tool.

leak testing a pneumatic air cannon

Once I worked most of the kinks out, it was ready to fire.  So far I’ve only used water as a projectile, but this was mostly what it’s meant for anyway.  As with all of my posts, this is simply what I’ve done and I don’t recommend trying it yourself.  PVC is not rated for air pressure, so this adds an extra element of danger.

So regardless, here are some other views of how this cannon performed when shooting water. Special thanks to “Mario Mendoza” for helping produce these videos:

[ad#Google Adsense-square 250x250]

Evie, however, was very curious

Although I was hesitant, Evie was excited to try the cannon out.

Leave a comment ?

10 Comments.

  1. What type of pnuematic valves are you using?

  2. Be careful of the type of PVC Schedule 40 or 80 pipe you use. There are unfortunately different pressure rating on various types.
    Ex. schedule 40 cellular core DWV type PVC – is non pressure only.
    Thick wall sch. 40 (2”) is rated 280psi @ 75 degrees F (water).
    Thick wall sch. 80 (2”) rated 260psi @ 75F (water)

    *****************************************

    In ’93 I gave my 5yr old son a potato gun made of sch. 40 ABS; it had a 3×24” main fuel chamber and a 2×30” barrel with a BBQ ignitor for the trigger. I dry fired it several times by fuelling it through the 3” cleanout fitting in the butt using hair spray (propane propellant), come Christmas it was wrapped and placed next to the tree. When he unwrapped it he said “Cor, a big gun”, he couldn’t pull the ignitor very well (remember 5yr old) so he placed the butt end on the floor and promptly pulled the trigger with both forefingers and a 4’ blue flame shot out of the barrel!!! A Kodak moment as they say, his eyes were as big as saucers. I must have flooded the barrel 2 days earlier while testing, and propane being heavier than air it didn’t dissipate due to the rifle being stood upright.
    My father-in-law wasn’t very impressed with the rifle gift and neither was my wife come summer. My son got to be a pretty good shot, pinging 1 out of 3 squirrels and the same number of neighbourhood cats that wandered into the back yard to do a dump in the garden. I don’t know what she was p****d of more about, the cats or the missing potatoes?!

    And then there was the motorized go-cart with the 3hp engine when he was 8, hmmm that’s another story for another day.

    Oh and did I mention about the boat incident when he was 9…..

    Live hard friends….. enjoy life.

    • Haha, wow, I think I “only” had a BB gun to play with when I was 5! Thx for the comments.

    • Btw, if you’d like to show these inventions off to the internet or whatever, feel free to send some pics in and a description (or Dropbox it) and I’ll be happy to put it up here!

  3. Correction to my comment;
    The line “Thick wall sch. 80 (2”) rated 260psi @ 75F (water)” should read “Thick wall sch. 40 (3”)….”

    Sorry for the mistake, sch. 80 is expensive and hard to find for the average homeowner.

    Cheers

    • Hmm, yeah I used schedule 40 rated at 280 I believe. I’ve never come close to 100psi though (at least not testing it this time). Still scares me a bit as it’s rated for this pressure with water not air.

  4. Weaponized PVC Pipe Fail | JcoPro.net - pingback on April 17, 2014 at 7:02 pm

Leave a Comment

NOTE - You can use these HTML tags and attributes:
<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>

Trackbacks and Pingbacks: