A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel—the engine of the rocket—is usually a used plastic soft drink bottle. The water is forced out by a pressurized gas, typically compressed air.
The term aquajet has been used in parts of Europe in place of the more common "water rocket" and in some places they are also referred to as "bottle rockets" (which can be confusing as this term traditionally refers to a firework in other places).
Water rocket engines are most commonly used to drive model rockets, but have also been used in model boats, cars, and rocket-assisted gliders. Water rockets are also popular in school science experiments.
Operation: 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.
Water and gas are used in combination, with the gas providing a means to store potential energy, as it is compressible, and the water increasing the mass fraction and providing greater momentum 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. It is speculated that foam acts as a compressible liquid and enhances the thrust when used with De Laval nozzles.
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 by enthusiasts 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 launch pressures vary from 75 to 150 psi (500 to 1000 kPa). The higher the pressure, the larger the stored energy.
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. Increased volume leads to increased weight, but this should be offset by a commensurate increase in the duration of the thrust of the rocket. Multi-bottle rockets can be unreliable, as any failure in sealing the rocket can cause the different sections to separate. To make sure the launch goes well, pressure tests are performed beforehand, as safety is a concern. These are very good if you want to make the rocket go high however they are not very accurate and may veer off course.
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. Methods to time the launches in correct order and at the right time vary, but the crushing-sleeve method is quite popular.
Fins: As the propellant level in the rocket goes down, the center of mass may move backwards. This reduces stability and tends to 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 can be added which bring the center of drag further back, helping ensure stability.
However, stabilizing fins cause the rocket to fall with a significantly higher velocity, possibly damaging the rocket or whatever it strikes upon landing. This is noteworthy if the rocket has no parachute or other recovery system or it has one which malfunctions. This should be 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.
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.
Safety concerns: Water rockets employ considerable amounts of energy and can be dangerous if handled improperly or in cases of faulty construction or material failure. Certain safety procedures are observed by experienced water rocket enthusiasts.
Water rocket competitions: The Oscar Swigelhoffer Trophy is an Aquajet (Water Rocket) competition held at the Annual International Rocket Week in Largs, Scotland and organized by STAAR Research through John Bonsor. The competition goes back to the mid-1980s, organized by the Paisley Rocketeers who have been active in amateur rocketry since the 1930s. The trophy is named after the late founder of ASTRA, Oscar Swiglehoffer, who was also a personal friend and student of Hermann Oberth, one of the founding fathers of rocketry.
The Guinness World Record of launching most water rockets simultaneously is in hands of Gotta Launch, when on June 19th they launched 213 of them at the same time, together with students of the Delft University of Technology.
The current record for greatest height achieved by a water and air propelled rocket is 2044 feet, (623 meters), held by U.S. Water Rockets on June 14, 2007. This altitude was calculated by averaging two flights. The first flight achieved 2068 feet, (630 meters) and the second 2020 feet, (615.7 meters). The rocket also carried an onboard video camera on both flights.
Topics of Interest
A hot water rocket (or steam rocket) is a water rocket which uses hot water as its propellant. Water is kept in the rocket under pressure, at below its boiling point at that pressure. As it exits through a rocket nozzle, the pressure drops and the water instantly boils and expands against the nozzle and this greatly increases the exhaust speed and thrust.
A nuclear salt-water rocket (or NSWR) is a proposed type of nuclear thermal rocket designed by Robert Zubrin that would be fueled by water bearing dissolved salts of Plutonium or U235. These would be stored in tanks that would prevent a critical mass from forming by some combination of geometry or neutron absorption (for example: long tubes made out of boron in an array with considerable spacing between tubes). Thrust would be generated by nuclear fission reactions from the nuclear salts heating the water and being expelled through a nozzle. The water would serve as both a neutron moderator and propellant.
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