A difference this time was that the newer products I wanted to launch were just a tiny bit narrower than the old products. The old product just barely wouldn't fit down a 3" barrel, so I was forced to use a 4" barrel. But the newer product fit snugly down a 3" barrel.
I also wanted to make some improvements to my cannon design. Specifically, I wanted to address:
For extra strength, I made the barrel of the new cannon out very short pieces of 4" ABS
pipe the same length as the ABS pipe part for joining two pieces. The joining part
goes around the pipe, so where it is joined, the pipe is twice as thick. With the joining
pieces touching each other, the entire combustion chamber had a double thickness wall.
Of course, along its axis, this arrangement has some weakness between the connectors,
but the, but the 'hoop stress' (around the circumference) of
a cylinder is twice what the axial stress is, so the relative weakness in the axial
direction between the joining pieces didn't matter that much. When a normal piece of
pipe fails due to pressure, it always fails by a crack opening length wise, not hoop-wise.
Another weakness had been the 2" cleanout plug I had used. I used a 2" cleanout on my previous cannon because there is no adapter that goes straight from 4" to anything smaller than 2". This time around, I used a 4" to 3" adapter, and a 3" to 1.5" adapter. I figured the 1.5" plug should be able to hold more pressure than the 2" plug. I hadn't had a 1.5" plug fail on me yet.
I wrapped some steel wire around the cleanout part, so that the outside threads could
hopefully not expand as much, and not let go of the 1.5" plug. I also wrapped the wire
around the start of the 3" barrel, which is only single wall thickness. I figured the
part nearest the combustion chamber would be most vulnerable. The further from the barrel,
the more the gas has expanded before it gets there, so there is less pressure further
from the combustion chamber.
For extra measure, I made a hardwood bracket to fit around the breech. This was designed to provide even support against the cannon's recoil against the ends of the pile, and especially against the 1.5" cleanout plug, to prevent it from coming out if the threads wouldn't hold it.
The coke can did not hold up too well - it couldn't handle the acceleration. All the coke shifted towards the bottom of the can, which then ruptured and expanded to the full 3" barel diameter of the cannon. The can subsequently didn't contain much coke anymore, and an empty shredded aluminium can doesn't fly too far. Bending the flap of metal that had opened to the side back in, bottom half of the can fit quite snugly in the 3" barrel. The top of the can was undamaged, not even the pop tab had opened.
Also, the wooden plug, which I put behind the coke can, held up quite well. The twisted cloth seal I put in the grove must have worked fairly well, because only the part of the plug behind the seal was blackened from the firing. The blue cloth extending from the plug is intended as a bit of a drag chute, and to make it easier to find. The plug did fly about 10 times as far as the shredded can.
We started out by launching product into the air. Problem with that is that it is very difficult to locate the test specimen. Somewhere out in that farmers field is some moderately damaged piece of handheld electronics.
Plus, its kind of inefficient to waste all that momentum on drag. And then there's no guarantee the device even lands on something hard. Its much better to have the impact zone as close to the muzzle as possible.
Above you can see a typical test setup. The cannon is supported by several pieces of firewood. Behind the breech is a heavy (15 kg) steel weight to absorb some of the recoil from the cannon. At right is a 1" thick steel plate, propped up with some firewood. For scale, the thin part of the barrel is 6 foot long.
The results were quite good. Except for the plastics, which can deform a lot without breaking, the units under test ended up in fragments about the size of a quarter. To avoid drawing unwanted attention to the product under test, I can't really show recognizable pieces of it.
We also fired a couple 3" caliber cans of vegetables. fortunately, 3" is a very common caliber for canned goods, and the cans fit very snugly down the barrel. The first can we shot was at a 45 degree angle, and we weren't able to find it. Its somewhere in the field now.
The second vegetable can was shot at the steel plate. We expected this to be quite messy, but with the plate angled up slightly, the vegetables were dispersed so finely that we were unable to find any traces of them, save for slight traces of mashed carrots on the can. Also note just how many places the can ripped.
Despite the bracket on the back, and the wire wrapped around the outside of the thread, and the use of a 1.5" plug instead of a 2" plug, the plug still ended up slipping its threads. This very much suggests that the MPS gas makes for a much more powerful explosion than propane or propylene.
This canon also didn't produce very large fireballs at the muzzle and wasn't as loud. This probably because the proportions of this canon were such that most of the energy was used for accelerating the unit under test. In fact, on some shots, the 'ping' against the steel plate was more prominent than the explosion from firing.
