For predicting the length of coolant liquid film extending inside a combustion chamber of bipropellant thruster, we develop an original theoretical model, newly incorporating the three-dimensional film dynamics and evaporation. The large velocity difference between the slow liquid film and fast combustion gas initially induces Kelvin-Helmholtz instability as roll wave, followed by transverse Rayleigh-Taylor instability as ripple wave. Superposing the two types of instabilities produce conical cusps as the origin of ligaments on the liquid film, ejecting entraining droplets, which reduce net coolant film flow rate. The conical cusps simultaneously enlarges the heat transfer area of the film subjected to the hot combustion gas. Implementing both effects of the entrainment and wet area expansion into the heat balance between the latent heat of the film and convective heat transfer from the combustion gas, we successfully predict the film coolant length of corresponding combustion test results, presenting that the film length shortens according to the increment of combustion pressure.