When I was a kid, I loved to build and fly model rockets (I still do), but the hobby can be costly as the solid propellant "engine" cartridges are fairly expensive. While it's possible to pack one's own engines; 1) I lack a place
do such a thing, 2) I lack the skills
do such a thing.
I found the solution to the expense in a makezine
podcast. It features a compressed air rocket launcher, using PVC pipe with an electric irrigation valve, and a standard bike pump. The same assembly is featured in Make: Vol 15.
I recently put this launcher kit together. It was very easy and took me maybe 30 minutes. Assembling the launcher is very well documented in many places (including the kit instructions).
This instructable focusses on assembling the rockets. The kit has ok instructions but I found the templates, and techniques described produce a rocket that's good for maybe 6 launches before the rocket becomes so maimed that one has to build another.
My own template (developed based upon the original) improves upon the "design" and construction techniques to produce a rocket that can take 10 or 15 launches worth of punishment before becoming a crumpled wad of tape and paper.