In this work, a bi-level Multidisciplinary Design Optimisation was implemented on the trajectory and the performances of an hybrid propulsion system for a spaceplane mission. The test case used as a reference for dimensions and aerodynamics is the CFASTT-1, a reusable launch vehicle developed for a SSTO mission whose concept is similar to the most recent ongoing project Skylon. The engine modelisation was carried out through HyPro, a medium-fidelity modular model which employs the equations of fluid dynamics to design the main components of a hybrid engine in all its configurations, airbreathing (ejector ramjet, ramjet and scramjet) and rocket. Due to its complexity, though, HyPro is too computationally expensive for the MDO process, so Surrogate Models were used instead.