AbstractThis paper documents a numerical model, developed for the McGill Rocket Team based on classical chemical thermodynamics coupled with the Trebble-Bishnoi equation of state, to solve for the oxidizer tank conditions (pressure, temperature, mole flowrate and liquid/vapour equilibrium) during the operation of a hybrid rocket. This model is modular and can be coupled to fluid mechanics and combustion chamber models for a more detailed analysis of a hybrid rocket engine.