How Water Rockets Work

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Topics: Rocket
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How Water Rockets Work
A water rocket works using the same principles as other rockets. There are three main forces in action: thrust (Fapp), drag (Ffr) and weight (w=mg). The water, which is forced out by the difference between internal and atmospheric pressure, is a reaction mass that provides the thrust. All rockets have a reaction mass, which can vary from hot gasses that are expelled when a fuel is burnt (in the Space Shuttle's SRB's for example) to water in a water rocket. The air molecules moving along the side of the rocket as it is moving create friction and result in the drag force (the drag attempts to slow the rocket down and so acts in the opposite direction as the velocity of the rocket). Weight is simply the mass of the rocket multiplied by gravity and applies to all objects within a gravitational field. Again, this force works against the rocket’s thrust, trying to bring the rocket back down to Earth.
[pic]
Free body diagram of a rocket during the thrust phase
[pic]
Free body diagram of a rocket during the coast phase
There are two components in the ‘fuel’ of a water rocket; water and air. Water, an incompressible fluid, is poured into the rocket before it is placed on the launcher and acts as the reaction mass. The air, which stores much more energy than the water because it is compressible (water is essentially incompressible and so pressurising a rocket adds no energy to the water), is then pumped in and pressurised; therefore the greater the pressure, the greater the energy stored. When a water rocket is launched, the difference between internal and atmospheric pressures forces the rocket off the pressure seal, followed by the expulsion of water and air out of the nozzle until the internal and atmospheric pressures are equalised. This action creates a downward force and, by applying Newton’s Third Law of Motion, it can be shown that in order for the total momentum of the system to remain constant and equal to zero (i.e. the

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