This launcher has not been built yet, but I'll eventually get around to it... it's made specifically for PETE bottles, and incorporates a rubber seal that expands to grip the inner neck of the bottle.
This is not the best rendering... but the basic idea is here... the lever coming out the side is the launch "trigger". It pokes through threaded lamp rod that runs vertically through the launch stand (basically, steel bar bent 90 deg. at the top and driven into the ground as a stake) and a rubber seal... capped with a washer and nut. As the leaver is pushed back, it is forced downward because it rides on a section of PVC pipe with a ramps fashioned into it. This squeezes the rubber seal, causing it to expand and grip the bottle. The air is pumped through the lamp rod from the bottom. Pulling a line attached to the leaver will launch the rocket..
