First of all, DON'T BUILD THIS. During all the experimentation with this thing, lots of stuff has gone wrong. This was the uninformed brainchild of an overconfident high school student. Any person in their right mind does not need one of these.
IF YOU DO BUILD THIS: Safety first, second, and third. Everyone involved wears thick long sleeved clothing, full face protection, gloves, etc. Have fire extinguishing things on hand. Shoot this in a huge empty field, preferably in the middle of the winter and very far from civilization. Have a clear area to run away if something goes wrong.
This build was originally designed to launch rockets for airsoft, but after many testing mishaps and the overall un-reliability of the system, I have decided that it is only fit for shooting for fun, not at other people. Also, it is most likely illegal in all civilized places, and generally isn't a toy. Any rockets you shoot out of this thing should be models without a payload.
I ran a few dozen rockets through this thing, with mixed results. The best rocket launched from this went somewhere in the neighborhood of 600 feet or so when launched at a 40 degree angle. We never found the rocket, and definitely alarmed the neighbors.
A few were designed to blast airsoft bb's out when they landed or ran out of fuel. The first one I made worked like a dream, but after that, they all failed more or less spectacularly, with one managing to coat the side of a tree in molten plastic. Not fun.
I can't pick out a specific launch that I would classify as the worst, but a few involved diving to the ground, or dropping the launcher and running away. Needless to say, operating this thing needs some serious safety gear.
I built it out of 4-inch diameter DWV-D drainpipe, held together with nothing but hot glue. It is approximately 4 and a half feet long, and weighs around ten pounds when empty.
This launcher is basically a more specialized version of Rwctt's launcher, made from lighter weight pipe, with a modified ignition system made from an Estes rocket engine ignitor. I do not have any construction pictures of the launcher, because at the time of construction, my camera was without a memory card. That's probably just as well. If you need instructions for something quite like this, go to the link below, his is safer, better designed, stronger, and better looking than mine.
You can find it here-----> http://www.instructables.com/id/PVC-Rocket-Launcher/