The startup Rocket Factory Augsburg (RFA) is developing the RFA ONE orbital launch vehicle. A significant hurdle for its closed-loop guidance is the need to fully deplete the third stage for insertion during the first flight, which results in lengthy burn-arcs and complexity not managed properly by traditional guidance algorithms. Additionally, new testing methods introduce uncertainty and component failure risks, demanding an adaptable guidance system.This thesis therefore aims to enhance RFA's closed-loop guidance system to ensure optimal orbit insertion in all feasible scenarios. This was achieved by redesigning the closed-loop guidance system to make it more robust for extended thrust-arcs.Furthermore an in-flight estimation algorithm for the key performance parameters was implemented. Finally the guidance can now fall back to alternate target orbits in case of insufficient performance margins. It was shown that the new guidance is able to successfully complete the maiden flight with the large amount of expected uncertainty.