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A place to ask general spud cannon related questions.
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Unread postAuthor: Khyren » Tue Jan 22, 2008 8:33 pm

Finally after a couple years I join you guys. I made my first combustion about 3 years ago. It's all steel and everything is 1/4" thick. The first design was 76" of 2 5/8" ID pipe. But that didn't work at all so I got some 6.75" ID pipe that is 24" long and attached it to the back of 42" of the original barrel/chamber. Yes, I know that the CB ratio is something like 3.79:1. For those interested in safety, all the joints that needed to be welded were beveled and then triple welded with a MIG. 1 "hot" weld to fill the bevel and get good penetration and then 2 proper temped welds on either side of the "hot" weld yet right next to one another so that they cover the "hot" weld.

After I got the larger pipe on there, I drilled and tapped 3 holes in the top for spark plugs. To fire it, I was taking one out and lighting it with a fireplace lighter. And that way it stayed up until now.

I have the original stand that I made for it that gives me 45* of up elevation if I bottom it out. But have since added the ignition system from my old 1978 bronco. There's a mount on one side that hold the distributor and on the other side is the ignition box. Beside the distributor mount is a bolt which I have the coil mounted to. I have 3 separate plug wires run to the plugs and I spin the distributor by hand in a clockwise motion so that the back one fires first. Yes I know that the 3 other plugs aren't really being used yet. I plan to finish the distributor thing this week. There will be a motor to drive the dist and fire them in sequence at high speed. So that in theory, the back will start an explosion and then the next one will make another just ahead of it so that they compound one another. I was really shooting for combustion multiplication but I'm not sure of how that is really going to work.

As for fuel. Fuel is added through a small piece of 3/4NPT galv pipe. Right now I have an old oxy/propane torch kit that only has propane left. So I'm just spraying that in there with the nozzle for 5 seconds, closing it up and firing.

Ventilation isn't really the best right now and it's got to be the most annoying point.

Ammo has mostly been tennis balls because they fit nicely... But recently I discovered that certain water bottles do really well too.

Improvement plans:
1) The previously mentioned motor for the distributor.

2) A propane and oxygen metering system using the old propane torch kit.

3) Add a pressurized tank for ventilation.

4) The propane tank will be on on side of the chamber, the oxygen will be on the opposite of the propane, and the pressurized vent tank will be on the opposite side of the plugs. They will all tie into the NPT port that is in the back and have individual valves. The oxy and propane will be opened at the same time so that they mix coming into the chamber.

Questions:
1) Should I put the spark plugs on bottom because I know that propane is a heavy gas? With the way that I made the stand, putting them on bottom is a matter of turning the cannon over.

2) I know the psi and metering chamber volume for the propane to be 4.2%. But since this is all steel, can I go higher on the ratio?

3) Is there a way to calculate the metering chamber volume for the oxygen that is needed?

This cannon isn't really designed to be portable so it's fed power from my truck. The pressurizing of the tank for venting won't really be an issue and neither is power for the ignition and the motor. I will get some pics of this as soon as the weather clears up and will try to get video as soon as possible.

Khyren
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 22, 2008 11:04 pm

The 4.2% ratio refers to concentration of propane in air, which has about 20% oxygen in it. If you're using pure oxygen, you're going to have to put a lot more propane, for a bigger boom :D in which case, all-steel construction is a major bonus for your health.
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