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A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

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Benefit of water

Discussions about rockets, construction materials, adhesives, nozzles, nosecones and fin design.
mjn
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Benefit of water

Postby mjn » Wed Aug 02, 2017 10:08 am

Using the 3D printed Gardena nozzle we launched 2 liter soda bottles at 100psi. Curiously we got straighter and higher launches using just compressed CO2. It made me curious as to the benefit of using water.
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anachronist
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Re: Benefit of water

Postby anachronist » Thu Aug 03, 2017 11:07 am

The nozzle doesn't really matter -- the total impulse is the same regardless of the nozzle diameter. Smaller nozzles provide less thrust for a longer time, bigger nozzles provide more thrust for a shorter time.

The advantage to water is its reaction mass. However, CO2 can store a lot more energy than compressed air, so you may be observing better performance with CO2 by itself than with air+water.

For a given initial pressure (say 100 psi or 7 bars) there is a "sweet spot" combination of initial water fill and total weight. Too much water, and the rocket is too heavy to start with, and it exhausts its pressure and reaction mass before it can reach a high velocity. Too little water, and you sacrifice reaction mass, which also sacrifices velocity available before the residual air thrust occurs. The optimal fill for a 2-liter soda bottle rocket is about 0.5 liters.

For the same water fill and same initial pressure, you should observe best performance with CO2+water instead of air+water, and you'll likely find that CO2+water is better than CO2 by itself.

I've seen some people also use soapy water, shaken up inside the rocket to create foam, claiming somewhat better performance. This is likely due to the residual air thrust having some reaction mass inthe foam.