The objective of this study is to develop a propulsion system analysis framework for a rocket-propelled aircraft tested with vertical take-off and landing. This is the first phase of a long-term project at the Technical University of Munich, where I conducted my thesis, which will lead to the construction of a real prototype. The project consists in the generation of a group of simulators capable of modelling the proper functioning of the propulsion system. The primary focus of this work is the study of modelling fixed and variable area injectors, to fully understand the differences in performance. The first step was to write a trajectory simulator in the MATLAB-Simulink working environment, which evaluates the thrust required to complete the mission, which consists of taking off to a predefined altitude, maintaining it for a predefined hovering time, and finally soft-landing.