2-Stage Light Gas Gun

gun_closeup

In high school I began experimenting with all sorts of potato cannons, primarily using compressed air as the propellant, but quickly got bored. After watching a documentary on the discovery channel about “Space Guns”, however, I constructed a 2-stage light gas gun using materials gathered from a local hardware store. The end result? A 6-foot long, pickle-accelerating monster that nearly destroyed my friend’s driveway!

What is a 2-stage light gas gun, you ask?

Simple cannons work by using an expanding gas, either from a compressed air chamber or a propellant charge, to push a projectile down the barrel of the gun. The maximum speed of the projectile is limited by something called the “mach speed,” or speed of sound, of the gas you are using. Using compressed air, for instance, 340 m/s is the fastest you can push something. A lighter gas, like helium or hydrogen, has a much higher mach speed (which is why it makes your voice sound funny). If you heat the gas up, the mach speed gets even higher. This is where the 2-stage light gas gun comes in!

light-gas-gun

A 2-stage light gas gun uses an initial gunpowder charge to drive a piston to quickly compress hydrogen. The now-hot, pressurized hydrogen bursts through a rupture diaphragm (or any quick-release valve) and drives the projectile down the barrel at incredibly high speeds. Seeing the relatively simple diagram made me think, hey, why can’t I build one of those?
WARNING: DON’T TRY THIS PART AT HOME

I constructed my gun entirely out of PVC and plumbing pieces from my local hardware store. Two main things drove my design: 1) Any kind of precision machining was out of the question, as I didn’t have a good workshop, and 2) I was a broke high school student.

- The main pump tube was constructed out of 3″ schedule 40 PVC pipe

-The barrel was constructed out of 1.5″ schedule 40 PVC pipe

-A spring-loaded hot-water tank pressure-release valve was used in lieu of a rupture diaphragm. It claimed to open at 150 PSI.

-An empty propane cylinder from a handheld torch, wrapped in a few layers of electrical tape to make a seal, served as the piston in the pump tube.

-An auxilary chamber hung down from the middle of the pump tube was filled with a mixture of chemicals that, after a small delay,  pressured the pump tube to about 2 atmospheres of hydrogen. This was likely the cause of the explosion when the gun fired, as will be detailed in a bit.

- A giant pickle was procured from the grocery for a projectile. The pickle was both 1) wet and 2) slightly larger in diameter than the barrel. The pickle compressed slightly inside the barrel, forming an excellent seal, while the brine served as a decent lubricant (the barrel was sprayed with a silicone lubricant beforehand as well).

The final product (and my neighbor posing with it):

full_cannon_screenshot

Results
The gun actually worked, and demonstrated the awesome effects you can get from a design like this. Unfortunately, the materials (and design) I used were not up to the task, and it lost structural integrity milliseconds after firing. It created an enormous explosion, and shot PVC shrapnel for about 100 yards in all directions. Fortunately, everyone was safely behind a barrier a safe distance away, and no one was hurt. I had a fair amount of adult supervision when I was constructing this, and I wouldn’t advise trying this at home unless you know what you are doing.

The gun firing:

cannon_slideshow1

The shrapnel:

img_0138

My Mom actually made me collect all of the pieces, lay them out, and try to root-cause the failure. Whatever happened to just grounding someone?! Anyway, I was able to narrow it down to 3 scenarios:

1) There was still a lot of oxygen in the section of the pump tube filled with hydrogen. Quickly heating the hydrogen in the presence of oxygen could have caused the explosion.

2) There was an air-gap between the top of the hydrogen-generating chemicals in the auxilary chamber, and the main pump tube. When the piston slid past the auxilary chamber,  a rich hydrogen/oxygen mixture came into contact with the expanding flame from the gunpowder and caused the explosion.

3) The PVC just exploded from overpressure from the gunpowder.

The gun actually fired, however, so I’m betting on 2, as that would still allow for the split-second delay needed for the projectile to fire.

Yes, yes, but how fast did it go?!

Well, the answer is a bit murky. The end of the barrel was 28 feet from the side of the truck it was aimed at. The event was filmed using a low-resolution, 30 frames/second mini-DV camera that my friend had at the time.  It is difficult to tell, but it appears that the pickle leaves the barrel and impacts the truck in about 1 frame of the camera. Using a little bit of math:

28 feet / (1/30 second) = 840 feet/second ~ 572 MPH

In otherwords, way faster than a pickle has any business moving on its own!