Combustion Cannon Query

[kozak]

Hi, I'm new to the spud cannon game, and inspite of reading all the forums and sites I could find, I'm at a standstill.

My intention is to build two cannons, one "rifle" style handheld cannon and a mounted "howitzer" style cannon. I'll explain further.

Rifle.

I intend the barrel to be about a metre long, thats 3 feet for the US. Barrel width of 40mm (1.5") The combustion chamber will be a T-section of 100mm (4"), with a BBQ igniter or lantern igniter in the T and the rear screw cap for fuel spray. I intend using hairspray or autostart for this one, though may progress to propane if the cannon seems sturdy enough.

Howitzer.

The barrel length will be 2 metres (6 feet). I'll have a width of 50mm (2") and a chamber width of 100mm (4"). Again, a T section will be used for the chamber. The top will contain a standard auto spark plug, connected to an ignition coil and car battery for the firing system. The rear plug will feature a copper valve which can be quick released from a propane tank for quick filling. This will avoid the need to open the chamber, eliminate waste of fuel and make everything easier.

Investigation.

I went into my local hardware store, Bunnings, and did some snooping. I haven't checked plumbing stores yet, but expect their prices to be higher.
Anyway, Bunnings had pressure rated PVC piping, which is fine, but its not the pipe that is going to be subjected to the most pressure, which I believe is a common misconception in Spud Gun construction.

I'm not worried for the pressure rating of the pipe, my chamber will be a T-section. The only pipe I will be using is the barrel, which gets exposed to practically no pressure at all. Simple physics of a standard firearm dictate the majority of pressure exerted in a controlled explosion occurs in the chamber. Therefore I need some clarification.

I intend using a 4" T-section PVC for both chambers, but as yet have been unable to locate a pressure tested section, or for that fact, even any identification markings on the fitting. The product label on the box says DWV, and as far as I can tell it is regular, not that cell core stuff. I've found pressure tested pipe, but this is useless without other pressure tested components.

My question is, has anyone used DWV fitting for a combustion cannon, and if so, has anyone experienced any problems with them? They aren't the cheapest items, and whether this has any bearing on its quality or rating I am unsure. Maybe its because Australia is shit when it comes to hardware.

I obviously don't want to blow my hands or my face up. Anyone have any advice on the use of DWV fittings? I don't want pipe advice, I've sorted that out. Just fittings. Better yet, someone in Australia who has made one and can give me advice on where to buy parts or what to use available here. Thanks.

[FreakyShotGlass]

As far as I believe DVW should be fine for a combustion, especially for a hairspray one, but I am more of a pneumatics man.

[TurboSuper]

Yeah, my cannon is made of DWV ABS and have never had a problem, but PVC will fragment like crazy if it does fail, unlike ABS which just splits.

[Jumpin Jehosaphat]

DWV ABS has failed on people in the past. I've never personally heard of pressure rated PVC failing on a combustion launcher.

If you do ever find a pressure rated 4" T (or any other 4" fitting for that matter) it's going to be hella expensive.
For your chamber, I recommend using a section of 3" pipe with an end cap that you can take off to vent the system after firing.
A T will be a very small chamber considering the cost. Check out cannons people have done here, there are some brilliant ideas.