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. It is an
example of Newton's third law of motion. 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 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 lo