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This paper presents the GNC concept solution developed for the recovery and landing of Vertical Take-off Vertical Landing (VTVL) launch vehicles in the context of RETALT (RETro-propulsion Assisted Landing Technologies), a European Union Horizon 2020 project with the objective of investigating and developing launch system reusability technologies based on the use of retro-propulsion. The project aims to increase the Technology Readiness Level (TRL) of the recovery technologies up to 5 for structures and mechanisms, Thermal Protection Systems (TPS), Aerodynamics and Aero-thermodynamics, and up to TRL 3 for GNC. One of the great technical challenges related to the reusability of launchers is the recovery Guidance, Navigation and Control (GNC) system, of which DEIMOS Space is in charge for RETALT. In particular, the design of the powered- descent and landing GNC offers a difficult challenge, since it must allow the system to perform a precision landing in a fast- dynamic environment, with extremely limited fuel margins, and with significant unknown dispersions accumulated during prior phases. To tackle this, state-of-the art algorithms based on hybrid Navigation techniques for state estimation, as well as online convex optimization and successive convexification for the design of the guidance GNC sub- function are explored. The Control algorithm operates in distinct modes dependent on the GNC phase and available GNC actuators, and it is based on modern robust control methods in order to provide analytical guarantees over the control performance in the presence of uncertainties and unmodelled dynamics. The proposed GNC solutions were integrated and tested in a high-fidelity simulator and the performance were preliminary assessed, demonstrating the capability to successfully steer the vehicle to the desired landing site. |
