Mr. Omer’s class recently completed a foray into water rocket design. Water rockets work as follows: The body of the rocket is an empty, plastic, two-liter soda bottle. Cardboard or balsa fins are attached to the bottom of the bottle for stability, and a fairing and nose cone are added to the top as a payload. Prior to launch, the body of the rocket is filled with water to some desired amount, typically about 33% of the volume. The rocket is then mounted on a launcher. Air is pumped into the bottle rocket to pressurize the bottle and thrust is generated when the water is expelled from the rocket through the nozzle at the bottom.
The students spent around 2 weeks preparing for launch, attaching fins and nose cones to our bottles to customize their designs. There were a wide variety of designs, with no one rocket looking the same as another. Some remained fairly basic and close to the original material, while others implemented interesting design choices that would affect flight.
Most of the rockets flew to impressive heights, with some rockets that didn’t soar so high. All but one rocket was built from a 2-liter bottle, and were filled to ~60psi before launch. The exception, a 1-liter bottle, was filled to ~30psi. The first flight was Zerich’s rocket, which blasted off to an impressive height before returning to the ground. His nose cone was crushed, but the rest of the rocket remained intact. Neveah’s was next; her under pressured rocket soaring to ~10ft. before returning to terra firma intact. Rodney’s reached impressive heights before hitting the ground intact. Michael’s reached a decent altitude, then lost a fin during lithobraking. Madina’s saw a launch pad malfunction which sent her rocket downrange into the jungles of Vietnam (That one marsh next to the gym) where Keith and Arlo retrieved it. China’s rocket followed, taking a wide and high arc before crashing into the ground and losing a few fins. Arlo’s rocket reached medium altitude, spinning like a top until it buried itself in the ground and lost a fin. The final rocket was Keith’s, which reached impressive heights before careening into the ground with a gentle glide to landing. With testing concluded, we cleaned up the area and returned to class. Flying model rockets are relatively safe and inexpensive way for students to learn the basics of forces and the response of vehicle to external forces.
On the figure we show a generic launcher, although launchers come in a wide variety of shapes and sizes. The launcher has a base to support the rocket during launch. A hollow launch tube is mounted perpendicular to the base and is inserted into the base of the rocket before launch. The launch tube is connected to an air pump by a hollow feeder line. The pump is used to pressurize the inside of the body tube to provide thrust for the rocket. We have attached a pressure gage to the feeder line to display the change in pressure in the system. This part of the system is very similar to the simple compressed air rocket.
The other part of the water rocket system is the rocket itself. Usually the rocket is made from a 2-liter soda pop bottle. Before launch, the bottle is filled with some amount of water, which acts as the “propellant” for the launch. Since water is about 100 times heavier than air, the expelled water produces more thrust than compressed air alone. The base of the bottle is only slightly larger than the launch tube. When the rocket is placed on the launch tube, the body tube becomes a closed pressure vessel. The pressure inside the body tube equals the pressure produced by the air pump. Fins are attached to the bottom of the body tube to provide stability during the flight.