Because the product I wanted to shoot was just over 3" in diameter, I had to go to a 4" caliber, which is the next largest commonly available size of ABS pipe. Of course, firing a product that is just over 3" out of a 4" cannon presents some problems. No amount of wadding could make up for this difference in caliber.
The solution was to make some 4" plugs. I made these to fit quite precisely in the barrel. With one end of the barrel sealed, it took a good 20 seconds for the plug to drop to the bottom because of the air resistance. I then cut a grove around the circumference of the plug and filled it with wadding, to make for an even tighter seal.
This picture shows two of the plugs I made for firing the product. The plug with
the blue cloth attached has a slot to help hold the product. The blue sling is
intended as a sort of drag chute, and also to make it easier to find. The idea is
to fire the product a long way, but I do want to be able to find the plug again.
The slot in the plug is for holding the "unit under test" that is launched.
Behind is the back of the barrel, with spark plug facing the camera, and fueling valve on the end. The wooden block is to catch the recoil. To the left of that is a bunch of AA batteries in series, which I used to power the ignition coil with.
When I first tested this cannon, I used Propane and Oxygen. The shots were loud, but I was a bit disappointed, because I expected it to be unbelievably loud. To push the plug out of the barrel takes a 5:1 expansion of the gas, and that was probably all I was getting with that fuel. For the actual launching event (with lots of co-workers there), I remembered to bring the 'good stuff' in terms of fuel. I used "bernzOmatic MPS gas". The bottle says its "Methylacetylene Propadiene, stabilized". Its also says "Burns Hotter than Propane for Welding, Brazing and Soldering". I mixed it with oxygen, of course. What a difference that made! It made a good bang! The muzzle flashes were also quite impressive. I have since however seen a documentary about the Bismark, and I thought my muzzle flash was neat until I saw the muzzle flashes from the 16 inch guns on that ship!
We launched some products in the air, but we had a hard time retrieving them - all we could find was a few plastic parts that broke off early. The actual product was way out there in the field somwhere, we couldn't find it. Thanks to the drag chute, we were able to locate the plug each time and reuse it for the next shot.
For the sake of targeted shooting, I brought a 1" thick steel plate, on which we fired products at close range. The above shot shows the cannon just after firing. You can see the cannon is hovering in the air. The pieces of wood on the left of it are to catch recoil. The mess on the right is the target area. Towards the top of it, you can see the plug (what's left of it) ricocheting off, while all the dust is probably parts of the product on impact. The product itself folded itself in several places from the impact. Behind the green patio table 'blast shield' is Ron Harding, with his camrea (also protected) taking pictures of the imact zone at close range.
|Inspecting the results|
There was a lot of people from work at the site. Plus, Jack (who's property we used) had kids, and there was a kid's birthday party at a neighbours, so it made for lots of spectators. It was possible to herd all the kids onto the garage (in the background) for the actual fueling and firing. But between shots, it seemed like there were kids everywhere. Not that there was anything explosive after a firing, but you never know.
Previous cannons I had built had lasted for many many firings, but I hadn't used MPS gas before. I initially figured this cannon should last a long time, becuase of its simple construction, and because the barrel is the same diameter as the combustion chamber, allowing for very rapid expansion. The MPS gas does appear to be a lot more powerful though. I noticed the 2" plug I had at the breech had somehow come out partway after some of the firings. On later examination of the videos that were taken, I could see quite a progressively larger flash coming out the back, even though I re-tightened the plug every time. Somehow, the pressure was enough to get it to skip threads. The only thing that kept it from coming all the way out is that it hit the wooden bracket I used to catch the recoil with.
On the very last shot, I decided to shoot the empty oxygen cylinder (one cylinder is good for about 10 firings). I also used a somewhat damaged plug, which took a bit of force to ram into the barrel. The shot itself was very disappointing - the bottle must have gone no more than 25 meters. But on close examination, the barrel had a big crack along its length. The pictures shows the crack, with the screw hole for a screw I put in to keep the plugs from sliding too far back. that was the end of that canon.
There is still room for improvement with this design. With the leakage out the back plug, obviously not all of the energy went into shooting the projectile. Also, the next product I plan on "launching" is just under 3" wide, so my next cannon will have a reinforced 3" barrel, with a 4" reinforced combustion chamber. That way, it might last for more than 10 firings.
Still, the imact was enough to completely destroy the product - more effectively than firing a wooden plug at it. The circuit boards ended up stripped bare of surface-mount components, even where nothing hit the PCB. The deceleratoin from the impact was enough to rip the components off the board.
In addition to firing the cannon, Kent Nickerson also brought some model rockets which he fired off. After this wanton destruction, we were all feeling more manly and satified. We subsequently had a nice BBQ courtesy of Jack Idsik's wife. We also reviewed the video footage just taken. Food and entertainment, who could ask for anything more?
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