SpudFiles

I am working on building my first hybrid, and I have never worked with metal pipe before, so I have some questions. If I get threaded pipe and fittings, can I just screw it in tight, or will I have to weld it? Also, will sch 40 aluminum be able to withstand the same pressures of sch 40 steel?

You don't have to weld threaded fittings. Steel is generally stronger than aluminum and can contain more pressure. I myself like aluminum since its lighter and still very strong. It all depends on the cannon. What size cannon are you going to make?

Welding or brazing is an option where screwing it together isn't an option, such as the valve built inside a propane tank.

Thanks for the help. I wont actually be beginning this cannon until I can get the money (im 14), which should be by next month, but I am currently drawing up several designs on a computer imaging program. I am planning on having the chamber be about 50cc (2in stainless steel), and the barrel being 1inch diameter about 50 in long. I am going to be using either a 4 or 5x mix. With these specifications, HGDT predicted a velocity of about 800ft/s and a peak chamber pressure of about 400psi.

May want to re check the figures. 50cc is tiny. I have a model airplane engine with a larger cylinder volume.

yea they make those kiddy 4 wheelers with like 50cc engines. But don't bother welding aluminum or stainless steel, unless you have TIG welding experience or can oxy-fuel weld. However, don't be discouraged about using aluminum, it is strong stuff, just think, they use it on aircraft frames/bodies. Plus, unless you have liquid mercury near your cannon, aluminum won't rust (at least in this lifetime). Anyways, apples to apples, strength wise, steel wins.

Whoops, I meant 44 ci (cubic inches). I think I put cubic centimeters on my previous post. Will 2" aluminum withstand 500 psi?

kjjohn wrote:Whoops, I meant 44 ci (cubic inches). I think I put cubic centimeters on my previous post. Will 2" aluminum withstand 500 psi?

Aluminum comes in several thicknesses and grades of alloy. My answer is it depends on the aluminum.

2" sch40 aluminum, assuming it is the standard 6061-T6 will easily handle 500psi.

Aluminium rusts EXTREMELY quickly. However it's oxide layer is passivating, as in it doesn't chip away. When iron rusts the layer is quite weak and chips away, allowing the material underneath to rust. However, the aluminum oxide layer is actually very strong and will not flake away, and protects the metal. This is used in anodizing.

However, if there is something that could affect the oxide layer then it will completely corrode. Also, you can buy stainless and aluminum wires and gasses for mig welders.

Aluminium is great, go for it. Try use threads, or hold the ends in with bolts (similar to some piston cannons).

oh, and something to add to inonicknames' post, if you weld aluminum, you may "activate" the aluminum using the heat and electric current, and this will affect the oxide.

Thanks guys, I have one more question. Like I said before, I have never made a metal cannon before, and I was wondering exactly how to wire the ignition terminals in without contacting the sides of the chamber. I have several ideas that would work, but I think that the pressure would just blow the wires out or leak. What is the usual way to wire one of these up? Also, on McMaster, aluminum and stainless steel pipe doesn't have pressure ratings. The fittings do, but not the pipe. I am sure that they are still rated for fairly high pressures, but I just want to make sure.

you could put o-rings under the terminals, and use heat shrink wire wrap (radioshack) around the bare parts of the wires/terminals. They should not leak, as long as you use the proper tools, such as a tap and tap wrench, to thread the holes for the screws (if you're using screws). Thats the same thing i worried about when i built my first electronic combustion. And you are somewhat right about the pipe, as the pressure rating on the fittings match up to the pipe, but you really don't need to worry about buying any special high pressure fittings, anything rated at 150 psi (10 bar) will do just fine. Plus, make sure you have a pipe wrench big enough to fit around the pipe.

some people make epoxy plugs in side male end caps
and have there spark gap(or spark plugs from cars)there.

Just run wires through the chamber to a spark strip or use spark plugs. Nothing different than with PVC cannons.