We made this cheap-o air rocket launcher for the kids for Christmas--they are loving it!! (So are we!) The instructions were from here: www.sciencetoymaker.org/airRocket/index.html
It was a very easy, inexpensive launcher to make (like less than $10) and not even an hour of work to make it. The kids make their own rockets from things from your recycling bin (following the instructions) and then launch them! The propellant is the air from a 2L pop bottle. They really fly VERY high!!! Higher than our roof, up to tree level! It is fun for the kids and the adults alike!