1. The problem statement, all variables and given/known data I am trying to derive the formula for time varying thrust given that I know the initial mass/volume of water in a water rocket. Knowns will include initial pressure, initial volume of water, and nozzle cross sectional area. 2. Relevant equations Bernoullis equation: (p/ρg) + (2V2/g) + z = c Isentropic expansion: pvγ = c 3. The attempt at a solution I derived the formula for the exit velocity of the water from the nozzle using the Bernoullis equation, https://www.dropbox.com/s/gnatjasgb2fjx5i/Photo%202015-02-05%2C%203%2050%2055%20PM.jpg?dl=0 https://www.dropbox.com/s/gnatjasgb2fjx5i/Photo 2015-02-05, 3 50 55 PM.jpg?dl=0 I know that I am supposed to derive a differential equation for the velocity and pressure but I cant seem to setup the equation using the isentropic flow formula. any help will be appreciated. thank you.