This paper presents the Guidance, Navigation and Control solution currently in development by DEIMOS Space for RETALT (Retro Propulsion Assisted Landing Technologies), an EU Horizon 2020 project for studying launch system re-usability technology. The general high-level GNC architecture is presented, with a more in- depth discussion on the navigation and landing phase guidance solutions. The navigation solution is based on a Considered Kalman Filter and a sensor suite that includes an INS/GNSS coupled system as baseline. Navigation simulation results are presented, which demonstrate very good performance. The guidance strategy is based on direct optimal control methods via on-board optimization, which is the only available solution able to satisfy the demanding requirements for a booster recovery mission as such. Within this methodology two approaches are identified, namely single convex optimization and successive convexification, for which a trade-off is performed. The former has been preliminarily selected for the RETALT landing guidance due to its lower computational complexity but still high fidelity. High- fidelity simulation results, however, demonstrate that the fidelity achievable with this approach is not sufficient to satisfy the mission requirements, and therefore a more complex solution based on successive convexification is required.