Re-entry and Flight Dynamics of a Winged Reusable First Stage
Creators: name:  S. STAPPERT 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: REUSABLE SYSTEMS
SPACE ACCESS
FALCON

description: Reusing launch vehicle stages has the potential to reduce launch costs and is hence of high interest in the context of defining the next generation of European launch vehicles. Winged reusable first stages have been a focus of the system launcher analysis group at DLR in the past years [1]. Using wings and other aerodynamic surfaces, the aerodynamic forces during re-entry are used to safely decelerate the vehicle, thus rendering the reignition of any engines and the use of propellants unnecessary. Thus, the performance of the vehicle in terms of payload mass is less impacted compared to propulsive methods like vertical landing. Furthermore, the returning winged first stage could be caught and towed to a landing site by a capturing aircraft. This In-Air-Capturing method is patented at DLR and about to be demonstrated on subscale level in the Horizon 2020 project FALCon [2]. Nevertheless, the design of a winged first rocket stage capable of fulfilling its main objective, accelerating the launch vehicle from ground up to stage separation, while also being able to do an autonomous and controlled atmospheric re-entry offers its challenges. Traversing from hypersonic to subsonic velocities, the vehicle has to be controllable at a vast range of flight conditions. Understanding the flight dynamics behind the re-entry of a winged vehicle in an early design stage is necessary to identify challenges with regards to controlling and actively steering such a high-performance vehicle in order to arrive at a feasible and robust design that does not fail to converge in later design iterations. Past preliminary investigations have identified the possibility of instabilities in roll and yaw movement at high velocities and high angle of attack. This work focuses on a more sophisticated analysis of the vehicle dynamics using linearized dynamic equations with 6 degrees of freedom. In the context of this work, two reusable winged stages were investigated. First, the winged first stage of a reusable launch system designed at DLR was used to investigate the re-entry and flight dynamics for a nominal trajectory [3]. This winged stage is part of a 3-Stage-To-Orbit system capable of carrying up to 14 tons to GTO. The stage features a double-delta wing and aerodynamic control surfaces, such as ailerons, elevons and a rudder to aerodynamically maneuver the vehicle. Further, an RCS system for the exoatmospheric part of the flight is used. Second, the booster stage of the SpaceLiner vehicle, a hypersonic point-to-point passenger transporter under investigation at DLR, is subjected to a similar flight dynamics analysis [4]. To investigate the re-entry dynamics, linearized dynamics are used to determine the open-loop stability and maneuverability of the vehicle in the pitch, roll and yaw axis at certain trajectory points. Further, a preliminary active control system is designed based on these linearized dynamics, using either aerodynamic control surfaces or the RCS system to actively steer the vehicle to commanded values. The closed-loop dynamic behavior of those actively controlled systems is then investigated for the complete trajectory. In the end, the impact of wind and gusts onto the movement of the vehicle throughout the flight is investigated. The results are used to identify the challenges of the RLV designs and offer potential design improvements with regards to aerodynamic performance, dynamic stability and control.