This site describes homemade water rockets constructed from plastic pop bottles. The bottles are one of those rare miracles of serendipity: although designed for something else altogether, they are nearly perfect for making water rockets. They're designed to hold liquids at high pressures, they're very lightweight, and they have a conveniently located ring molded around the neck that's handy for holding the bottle down while pressurizing it. About the only thing that could make them better is to have the fins and nose cone molded in for you.
The rockets made from these bottles are surprisingly powerful. A standard 2-liter pop bottle 1/3 full of water, pumped to 80 psi and then released, will eject all its water in less than one-tenth of a second, and at that point ( burnout ) will be only about 2 meters off the ground. Amazingly, its velocity at burnout is around 76 meters per second. That's over 170 miles per hour! This means the average acceleration during thrust is 111 g's! Yowza. Safety rule number 1: Never get in the way of one of these rockets...
For me it all started with a small plastic launcher that used to be sold by that wonderful place that every hobbyist should know about, American Science and Surplus (once known as Jerryco), for around 10 dollars. The same launcher, called a POP FLIGHT launcher, is also sold by Apogee Components, a model rocket supplier. The catalog copy said I could make a water rocket from a plastic pop bottle: that sounded like a fun little diversion, so I ordered the thing on a whim. When it arrived, I innocently took it out to my tiny San Francisco backyard, put some water in a 2 liter bottle, clamped it on the launcher, pumped it up to 40 psi or so, and yanked the string. I wasn't quite ready for what happened next...
The simplest kind of rocket doesn't really require a launcher at all, just some way to prop the bottle in position while you pump. This kind of rocket is made by simply stuffing a rubber stopper in the neck of the bottle, inserting an inflation needle through a hole in the stopper, and pumping it up until it blows! You never quite know when it's goiing to go, which of course adds to the excitement (kids love this). There's lots of good information on this sort of launch technique at the Interplanetary Water Rocket Society home page, maintained by Gordon McDonough in Santa Fe, New Mexico.
