OK folks, let's let science go to work. Your steam cannon fizzled due to lack of energy. The BTU's contained in the metal were insuficient to generate the power needed. If you look at a standard ASME Boiler Steam Table you'll find the enthalpy of the steam you generated was low because the "flash" to steam first requires you add the heat necessary to get the water to saturation. If you use atmospheric pressure as your saturation temperature you'll find that once water reahes 212 degrees you must add about 980 BTU's to convert one pound mass of water into one pound mass of steam. So all that hot metal was doing wa swapping those stored BTU's into the liquid.

Now here's how you can make your steam cannon and it works. In the end you put copper nails and stuff, put water. Between the water reservoir and the cannon ball put a ball valve. Heat the water until the pressure from thermal expansion reaches about 3000 psig (near 700 degrees). The water will achieve a condition know as "fluid" above 705.38 degrees where pressure is irrelevant to maintain subcooling. Open the ball valve (quickly) and the fluid will expand exponentially as it converts to steam. The enthalpy should convert most of the contained volume to gas (it is likely it will be a wet vapor vice a dry steam since the container would not allow boiling to occur and you are going to atmospheric pressure). This should be an isentropic process (no change in entropy). The energy stored in the fluid will be expanded behind the ball shoving rather forcefully everything down the barrel. (That means those shields you were behind might come in handy, and I'd put the ball valve on an actuator arm to remote fire it.) So the answer is flash water already containing the BTU's instead of heat transfer first to saturation before steam generation. I don't care what that faiure analysis guy says, BTU's is BTU's and you can't convert a pound mass of water at 70 degrees into a pound mass of steam without adding 980 BTU's which robs all your energy the way you're going about. Try reading the first Law of Thermodynamics, it's an eye opener.
Sincerely,
Atomsplitter
Licensed Reactor Operations Instructor

I think mythbusters is a great show, just don't do steam anymore. Everytime you take on steam it is clear you don't know what you are dealing with and taking way too many chances.

I think the problem was they were using cold water...

They need boiling water, just at the threshold of turning into steam. also, instead of just "heat the pipes as hot as you can",
do some calculations beforehand to see how hot you really need to bring your hot pipe up to in order to provide the
necessary amount of energy to turn all your energy into steam.

There is no way this thing can fail if you do it right!

i want to see it go full power...