page_title: Water rocket
section_title: Operation
hierarchical_section_title: Water rocket / Operation
context_page_description: 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.
context_section_description: 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 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.
caption: Launch of a bottle without nose cone or fins.
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