Although store-bought water rockets are 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. Soda bottles 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 issues surrounding 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) 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.
