People my age remember red plastic toy rockets from childhood that you filled with water, pumped with air, and watched as they leaped dozens of feet into the air. Well, things have gotten far more serious in the world of water-powered model rocketry, and hobbyists are reaching heights upwards of 800′ using plastic soda bottles, thin wood or plastic for fins, tape, glue, a tire air valve, and an air source such as a bicycle pump.
Where the DIYer goes, the kit maker soon follows. We recently spied these cool-looking kits from EnVision Labs and AntiGravity Research, both of which will allow you to build towering, multi-stage water rockets using special connectors. Watch for reviews on makezine.com.
But even without a kit, you probably already have everything you need (or can easily aquire it). Consult aircommandrockets.com for advanced hydrorocketry ideas