A more thorough analysis of water rockets: Moist adiabats, transient flows, and inertial forces in a soda bottle

Although water rockets are widely used to illustrate first year physics principles, accurate measurements show that they outperform the usual textbook analysis at the beginning of the thrust phase. This paper gives a more thorough analysis of this problem. It is shown that the air expansion in the r...

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Bibliographic Details
Published in:American journal of physics 2010-03, Vol.78 (3), p.236-243
Main Author: Gommes, C. J
Format: Article
Language:English
Subjects:
Analysis
Containers
Inertia
Physical, chemical, mathematical & earth Sciences
Physics
Physik
Physique
Physique, chimie, mathématiques & sciences de la terre
Rakete
Raketenphysik
rocket propulsion
Soft drinks
Wasser
Water rockets
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Summary:Although water rockets are widely used to illustrate first year physics principles, accurate measurements show that they outperform the usual textbook analysis at the beginning of the thrust phase. This paper gives a more thorough analysis of this problem. It is shown that the air expansion in the rocket is accompanied by water vapor condensation, which provides an extra thrust; the downward acceleration of water within the rocket also contributes to the thrust, an effect that is negligible in other types of rockets; the apparent gravity resulting from the acceleration of the rocket contributes as much to water ejection as does the pressure difference between the inside and outside of the rocket; and the water flow is transient, which precludes the use of Bernoulli's equation. Although none of these effects is negligible, they mostly cancel each other, and the overall accuracy of the analysis is only marginally improved. There remains a difference between theory and experiment with water rockets. © 2010 American Association of Physics Teachers.
ISSN:0002-9505
1943-2909
DOI:10.1119/1.3257702