The concept is simple enough: use a standard 2-liter plastic pop bottle as the main component, add some fins for stability, a nosecone for aerodynamics, fasten a parachute, and hack together a launching apparatus. Then you fill your rocket with water (about 40% full) and pressurize it with air. The launcher keeps it all together and, when you release the rocket, it flies. In fact, 100 feet is not an unusual target with these things.
We built the rocket. We built a lovely standing launcher to control pressurization and rocket release. We tested at every turn with small, controlled flights of about 20-30 feet. Everything just worked. Watch:
Today was our final test before going for serious altitude. The test - make sure the nosecone would fall off easily and release the parachute.
Here's the 'before':