Port-Hamiltonian System is a well-established framework for modelling the dynamics of non-linear systems. It has been used extensively for complex mechanical systems. Recent developments have extended this representation to multiphysical systems. In this paper, we determine a Port-Hamiltonian formulation of a Liquid Propelled Rocket Engine model. Starting from a classical state-space representation, we derive a new model which allows to demonstrate the global passivity feature of the system using the property of passivity inheritance by composition of several Port-Hamiltonian subsystems. The resulting representation allows to consider passivity-based control perspective for this class of complex thermodynamics system.