Article,
A more thorough analysis of water rockets: Moist adiabats, transient flows, and inertial forces in a soda bottle
American Journal of Physics, 78 (3): 236--243 (2010)
DOI: 10.1119/1.3257702
Abstract
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.
