This thesis aims to study hybrid rocket engines in EcosimPro environment using ESPSS libraries. In the first chapter, a brief introduction about chemical rockets will be presented focusing the attention on hybrid rocket engines pointing out the advantages and disadvantages compared with other chemical rockets. A brief section will also be dedicated to the most common propellants, especially the ones used in this work. In the second chapter, different aspects of hybrid rocket engines will be deepened. The theory behind every element composing the final model of the thesis will be presented, for example the different types of feed system, the injector and the fuel grain.