Science Toy Maker has a bunch of great tutorials for young engineers, including this one on how to build an overhead water rocket launcher:
Part 1: Introduction and Overview, lets you know what you’re getting into if you want to make the overhead water rocket launcher featured on Ask This Old House during their special kids program. It uses inexpensive, readily available materials. It peeks at how the launcher works: the bump that seals the water and air pressure in the bottle, the zip-tie assembly that holds it on, and the spring that helps prevent accidental launches. And part 1 asserts that shooting a water rocket is analogous to driving a car–potentially dangerous, but safe if done with care. If YouTube is blocked in your school, try this SchoolTube link Part 1
Part 2: Materials and Tools. PRINT FRIENDLY SUPPLY LIST CLICK HERE You can get the things you need at a building center, plus one thing at an auto parts store. The overhead water rocket launcher uses less than $10 US of actual material. NEW! Observant correspondent Andrew Baillie of Atlanta discovered that you can substitute a 1 1/4″ slip joint for the 2″ of 1 1/2″ pipe.Details here. Also, some interesting variations for using an old bicycle inner tube for the valve, and using tape for the bump if you can only find metal pipe:
http://waterrocketlaunchers.com/
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