This open access book takes space propulsion system, gel propulsion system, and pumped liquid rocket engine as research objects and establishes and describes the theory, dynamic model, and numerical calculation method of working process of liquid/gel rocket engine. The first part of this book establishes a mathematical model for the working process of the space propulsion system, including a mathematical model for gas cylinders, an electric explosion valve, a pressure reducing valve, a storage tank, a liquid pipeline, an orifice and filter, an electromagnetic valve, a filling pipeline, and...