  We study the thermodynamics of the water rocket in the thrust phase, taking
into account the expansion of the air with water vapor, vapor condensation and
the energy taken from the environment. We set up a simple experimental device
with a stationary bottle and verified that the gas expansion in the bottle is
well approximated by a polytropic process $PV^\beta$= constant, where the
parameter $\beta$ depends on the initial conditions. We find an analytical
expression for $\beta $ that only depends on the thermodynamic initial
conditions and is in good agreement with the experimental results.
