Step 1: Build Your Launcher
The air tank is used because most valve stems don't let a large volume of air through at a time. The valve stem let the air in, but not out (duh!). The first time I made this, I cut apart i bike inner tube for the stem, drilled a hole in the end cap, and hot melt glued it together. That lasted all of 10 launches. Then I found car tire valve stems at the local hardware store, so use those. Just follow the instructions on the package to drill the right size hole.
The launch pipe carries the air from the air tank into the rocket. Mine is way longer than it needed to be, but then again, I planed on make some much bigger rockets latter :) (don't, unless you have somewhere far from any cars that can be damaged. the big one I made with a Nerf ball nose but a big dent I the neighbor's car, luckily he was a good sport.) It does help you not have your ear next to the end of the pipe, and thats very good considering how loud this is. The launch pipe is 1/2" water pipe. The rockets should fit snug on it, and are fed completely onto it before launch.
I later added a threaded fitting below the valve so I could take the launcher apart for storage. If you do, use Teflon tape so that it doesn't leak.