Flight Control Challenges of the Winged Reusable Launch Vehicle ReFEx

Abstract

The German Aerospace Center (DLR) is currently studying different technologies for Reusable Launch Vehicles (RLVs) in order to evaluate and compare their benefits. The project CALLISTO (Cooperative Action Leading to Launcher Innovation in Stage Toss-back Operations) investigates a VTVL (Vertical Takeoff, Vertical Landing) concept. In the DLR project ReFEx (Reusability Flight Experiment), in the context of which this paper is presented, a winged VTHL (Vertical Takeoff, Horizontal Landing) concept is investigated in order to develop the key technologies for future winged RLV applications, culminating in a flight experiment to demonstrate the capability of controlled autonomous return flight from supersonic to subsonic speeds. The ReFEx re-entry vehicle has a length of 2.7 m, approximately 1.1 m wingspan and its mass is expected to be around 400 kg. In order to perform controlled autonomous re-entry, the Guidance subsystem - as part of the Guidance, Navigation and Control (GNC) system - generates and adapts a target trajectory during the mission, depending on the current position and energy state of the vehicle. Using the commands provided by the Guidance and the navigation information generated by a Hybrid Navigation System (HNS), the Flight Control System (FCS) stabilizes the vehicle and ensures that it follows the target trajectory as closely as possible, even in the presence of disturbances such as wind gusts. In this paper, first the challenging stability and control characteristics of the vehicle are examined, both at supersonic and subsonic speeds, which pose certain requirements on the FCS and the flight envelope of the vehicle. Second, a flight control architecture is proposed, and its suitability to the considered problem is shown via stability analyses and time-domain simulations.