In the last decade, the number of space-based applications increased dramatically. To cover such a demanding market, launch technologies adapted and new launch solutions were developed, to increase the efficiency and the cost effectiveness of the access to space. In particular, reusability became the focus of multiple activities devoted to the design and analysis of current and future launchers solutions. Worldwide, Space X was able to make the reusability of first stages look like a routine operation with more than 50 successful landings and recoveries, and reuse up to 6 times. ULA and Blue Origin are developing their next generation launchers to be partially reusable, and also China is testing technologies to achieve reusability for micro-launchers. In Europe, several initiatives have been started in the last years to analyse and test critical technologies and system that will enable reusability. The RETro-propulsion Assisted Landing Technologies (RETALT) project is an H2020 activity, funded by the European Union and coordinated by DLR, aiming at developing key technologies to enable the recovery of vertical take-off vertical landing launchers making use of retro-propulsion, in the field of aerodynamic and aerothermodynamics, flight dynamics and GNC, and structures and mechanisms. In this context, mission engineering is a critical process of the design-for-reusability chain, and it is a discipline of excellence of DEIMOS Space. In this paper, the mission engineering process developed and applied to RETALT is presented, as well as the results obtained.