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Type of model rocket using water as its reaction mass

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.

The bottle is partly filled with water and sealed. The bottle is then pressurized with a gas, usually air compressed from a bicycle pump, air compressor, or cylinder up to 125 psi, but sometimes CO2 or nitrogen from a cylinder are used. Water and gas are used in combination, with the gas providing a means to store energy, as it is compressible, and the water increasing the propellant mass fraction and providing greater force when ejected from the rocket's nozzle. Sometimes additives are combined with the water to enhance performance in different ways. For example: salt can be added to increase the density of the reaction mass resulting in a higher specific impulse. Soap is also sometimes used to create a dense foam in the rocket which lowers the density of the expelled reaction mass but increases the duration of thrust. The seal on the nozzle of the rocket is then released and rapid expulsion of water occurs at high speeds until the propellant has been used up and the air pressure inside the rocket drops to atmospheric pressure. There is a net force created on the rocket in accordance with Newton's third law. The expulsion of the water thus can cause the rocket to leap a considerable distance into the air. In addition to aerodynamic considerations, altitude and flight duration are dependent upon the volume of water, the initial pressure, the rocket nozzle's size, and the unloaded weight of the rocket. The relationship between these factors is complex and several simulators have been written to explore these and other factors. Often the pressure vessel is built from one or more used plastic soft drink bottles, but polycarbonate fluorescent tube covers, plastic pipes, and other light weight pressure resistant cylindrical vessels have also been used.

Typically a single polyethylene terephthalate (PET) carbonated soft drink bottle serves as the pressure vessel. Multi bottle rockets are created by joining two or more bottles in any of several different ways; bottles can be connected via their nozzles, by cutting them apart and sliding the sections over each other, or by connecting them opening to bottom, making a chain to increase volume. This adds complexity and the increased volume leads to increased weight but this should be offset by an increase in the duration of the thrust of the rocket. Multi stage rockets are much more complicated. They involve two or more rockets stacked on top of each other, designed to launch while in the air, much like the multi stage rockets that are used to send payloads into space.

As the propellant level in the rocket goes down, the center of mass initially moves downwards before finally moving upwards again as the propellant is depleted. This initial movement reduces stability and can cause water rockets to start tumbling end over end, greatly decreasing the maximum speed and thus the length of glide (time that the rocket is flying under its own momentum). To lower the center of pressure and add stability, fins or other stabilizers can be added which bring the center of drag further back, well behind the center of mass at all times. Stabilizers of any sort are normally placed near the back of the bottle where the center of mass is found. The increase in stability which well designed fins give is worth the extra drag, and helps to maximize the height to which the rocket will fly.

Stabilizing fins cause the rocket to fly nose first which will give significantly higher speed, but they will also cause it to fall with a significantly higher velocity than it would if it tumbled to the ground, and this may damage the rocket or whomever or whatever it strikes upon landing. Some water rockets have parachute or other recovery system to help prevent problems. However, these systems can suffer from malfunctions. This is often taken into account when designing rockets. Rubber bumpers, Crumple zones, and safe launch practices can be utilized to minimize damage or injury caused by a falling rocket. Another possible recovery system involves simply using the rocket's fins to slow its descent and is sometimes called backward sliding. By increasing fin size, more drag is generated. If the center of mass is placed forward of the fins, the rocket will nose dive. In the case of super roc or back gliding rockets, the rocket is designed such that the relationship between center of gravity and the center of pressure of the empty rocket causes the fin induced tendency of the rocket to tip nose down to be counteracted by the air resistance of the long body which would cause it to fall tail down, and resulting in the rocket falling sideways, slowly.

Some water rocket launchers use launch tubes. A launch tube fits inside the nozzle of the rocket and extends upward toward the nose. The launch tube is anchored to the ground. As the rocket begins accelerating upward, the launch tube blocks the nozzle, and very little water is ejected until the rocket leaves the launch tube. This allows almost perfectly efficient conversion of the potential energy in the compressed air to kinetic energy and gravitational potential energy of the rocket and water. The high efficiency during the initial phase of the launch is important, because rocket engines are least efficient at low speeds. A launch tube therefore significantly increases the speed and height attained by the rocket. Launch tubes are most effective when used with long rockets, which can accommodate long launch tubes.

The Guinness World Record of launching most water rockets is held by Royal College, Colombo when on 10 November 2017, they launched 1950 of them at the same time. The current record for greatest altitude achieved by a water and air propelled rocket is 2723 feet (830 m), held by the University of Cape Town, achieved on 26 August 2015, exceeding the previous 2007 record of 2044 feet (623 m) held by US Water Rockets. The rocket also carried a video camera as payload as part of the verification required by the competition rules.

A steam rocket, or "hot water rocket", is a rocket that uses water held in a pressure vessel at a high temperature, and which generates thrust through this being released as steam through a rocket nozzle.