Project 20 from my book SNIP, BURN, SOLDER, SHRED is available for free over at the Art of Manliness:
Although store-bought water rockets were once an almost universal part of childhood (at least in suburban middle America, where I grew up), DIY water rockets are almost unheard of in the United States, despite being common in the United Kingdom. The key to the DIY water rocket is the ubiquitous plastic soda bottle. (Often called PET bottles in the UK, these are basically made of polyester, just like your favorite pantsuit.) A soda bottle can easily withstand 150 pounds of pressure per square inch—significantly more than a car or bike tire, and more than enough to launch it into flight.
The following design dispenses with many of the concerns regarding valves, firing mechanisms, pressurization, and so on, because the unit is impossible to over-pressurize, thus eliminating the risk of a rocket explosion. It’s quick and easy (no more than 5 minutes to construct), practically free, fun to make and launch with your kids (while teaching them some basic principles of physics), and, depending on the quality of your bike pump, can arc a bottle 30 feet at speeds safe enough to shoot at your pals, or send a rocket soaring a hundred feet skyward.…
Weekend DIY Dad Activity: How to Make a Quick-n-Easy Water Rocket | The Art of Manliness
Here’s what it all looks in time-lapse: