This is the water rocket launcher i made out of 1/2 inch (1.25 cm) diameter PVC pipe, a BIC pen shaft, hot glue, a 1/2 inch (1.25 cm) tee, 2 end caps, duct tape, zip ties(used for bundling cables) a hose clamp, a 1 1/4 inch (3.175 cm) slip connector, string and 2 - 2 liter PLASTIC soda bottles. Don't use glass bottles for two reasons. First, it won't fit on the pipe. Second, the bottle goes very high, but a glass bottle has a lot of weight and it'll shatter, injuring someone. A plastic bottle doesn't hurt because an empty bottle has almost no mass. This can go like 50-100 ft (15-30.5 m) and even higher (10 times as high)if you add fins. I chose not to because if a finned bottle lands on someone it may injure someone because the fins give it mass. Thanks to slater harrison for the plans.