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Water rockets are model rockets that use water and a pressurized gas as a propellant. They are sometimes called bottle rockets, which can lead to confusion, as a bottle rocket is a type of firework. Many people build water rockets recreationally, and they are sometimes used as educational tools as well. For older students, water rockets can make interesting science projects, especially if the students are encouraged to make modifications to their rockets to alter their trajectory, maximum height, and length of time spent in the air.
One of the cheapest and most effective bodies for a water rocket is an old plastic soda bottle, hence the alternate name “bottle rocket.” A two liter (half gallon) soda bottle is ideal for this purpose, as it is large enough to potentially propel itself quite a distance. The soda bottle is partially filled with water, then a pressurized gas is added. The easiest pressurized gas is simply air, which can be pumped into the bottle with a bicycle pump or air compressor to increase the pressure. More serious water rocket enthusiasts may use other gases for this purpose.
Once primed, the water rocket is inverted, so that the water is pushing against the lid, and the lid is removed. The pressurized air forces the water out, propelling the rocket into the air. Often, water rockets are modified with fins to stabilize their flight and increase air time, and parachutes to reduce the impact of landing.
The basic design of a water rocket can be easily modified to create multistage water rockets, or very large water rockets which are capable of holding more pressurized air and water, and therefore traveling further. In most cases, a rocket launcher is built, so that the person launching the water rocket is not injured by accident. A common rock launcher is simply a length of tubing stuck into the ground with a string threaded through it, so that the string can be used to pull the seal of the rocket off while the tubing holds the rocket facing upright.
Because of the potential danger involved, bystanders should stand well clear of the launching pad. Water rockets cannot be precisely aimed, and one may get off course early in its trajectory, potentially hitting someone. A water rocket can move with surprising speed, and a collision could be very painful. Water rockets should also not be operated by young children without supervision.