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Q:  How much water do you put in a water rocket?

A:  The simple answer is something less than half full. Since the compressed air stores the energy, and the water provides momentum, both are needed. Fill perhaps 40-50% of the motor volume with water. We call this the Load Fraction.

The longer answer is, it depends.

The following factors effect how much water is optimal:

  • Empty weight of the rocket,
  • cross-sectional area (or diameter) of the rocket,
  • nozzle diameter,
  • operating pressure, and
  • drag coefficient.
  • So how do I determine the optimal load fraction?

    By conducting experiments, we are rocket scientists after all! Launch your rocket, and measure how high it flies, trying a few different load fractions. Choose the load fraction that gives the best result. This might be a lot of work, and measuring the flight may be hard to do accurately.

    Is there another way?

    Yes, conduct your experiments using a water-rocket simulator like our java-based applet. Here you can "launch" many times at different load fractions and see how high it goes. Better yet, you can vary all the parameter discussed above to optimize for the best rocket performance.

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    updated 11-Dec-2009
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