A water rocket is a type of model rocket using water as its reaction mass. The water is forced out by a pressurized gas, typically compressed air. Like all rocket engines, it operates on the principle of Newton’s third law of motion. Water rocket hobbyists typically use one or more plastic soft drink bottle as the rocket’s pressure vessel. A variety of designs are possible including multi-stage rockets. Water rockets are also custom-built from composite materials to achieve world record altitudes.
If you haven’t built and launched water rockets with your kid, you’re missing out on an awesome project. With 10 bucks or so worth of parts, an air pump, and an empty soda bottle or two, you can have a blast of an afternoon and learn some elementary rocketry. Even the most jaded kid will be blown away by the sheer power and height of a properly constructed water rocket. I promise.
How Water Rockets Work
Water rockets work exactly like their big brothers at NASA. A reaction mass is forced out of a vessel with pressurized gas, sending the payload into the air. In this case, the pressure vessel is a soda bottle, the reaction mass is plain water, and the pressurized gas is air pumped in with a bicycle pump. With these simple concepts, you can send a Coke bottle hurtling hundreds of feet into the air and impress all the other kids at the playground. Maybe it’s not as remarkable as the launch of an Apollo rocket, but I’ll bet Elon Musk’s first launch was a water rocket.
Building Your Rocket
The rocket you see above, Zeus III, is a simple-as-possible design built in about 20 minutes with a few things we had lying around the house. It’s an empty soda bottle, some plastic fins, the tip of a foam football, and some sparkly tape for decoration.
Some water rocketeers make two-bottle rockets, build self-deploying parachutes, or construct elaborate multi-stage water-rockets, but we’re keeping it simple: just a plastic bottle, a nose cone, and some fins.
Any brand of pop will do for your rocket bottle, and you can use any size, but we’ve had the best results with a standard two-liter bottle. Just make sure it’s a soda bottle. Bottles designed for non-carbonated beverages may not be able to handle the internal pressure you’re about to put them through and could fail or explode. You don’t want that.