Reusable launchers re-entry controlled dynamics simulator
Creators: name:  A. DE OLIVEIRA 
name:   M. LAVAGNA 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLIGHT DYNAMICS
GNC
AVIONICS
SIMULATIONS
VALIDATIONS

description: Reusable Launch Vehicles (RLVs) atmospheric re-entry and precision soft-landing on Earth is very challenging [1]. Whereas the origin of this technology comes from the Apollo era [2], its cost efficiency has been demonstrated recently by the successful SpaceXs Falcon 9 program [3]. Indeed, during this phase, the vehicle is subjected to fast system dynamics changes induced by external loads such as lift and drag, wind and gusts, and also to actuation commands to answer the landing constraints satisfaction and the vehicle integrity preservation. All those involve uncertainties and nonlinearities as well as coupling effects with high frequencies, which lead to vehicles instability and therefore give reason why for a highly performant Guidance, Navigation and Control (GNC) system implementation. More particularly, one of the specifications for re-entry GNC is the real-time implementation of the guidance algorithm which is not trivial due to the high computational power needed to solve the highly nonlinear problem. In the last decade, convex optimisation [4, 5] has been demonstrated as a key technology to solve the entry problem onboard based on the current flight conditions and to enable a pinpoint landing. This method consists in transforming the nonlinear Optimal Control Problem (OCP) into a convex one, easier and faster to solve, without the loss of any problem constraints. This paper describes a RLV re-entry dynamics simulator with closed-loop guidance and control integration, from design, to validation and critical discussion on some representative simulation cases. This simulator covers a vertical take-off vertical landing (VTVL) vehicle first-stage booster atmospheric re-entry and soft pinpoint landing. It includes the 6-DoF re-entry dynamics of a rigid-body model with varying mass, evolving in the terrestrial atmosphere with varying environmental parameters, uncertainties and disturbances (atmospheric density, ambient pressure, wind), and subjected to external forces (gravity, aerodynamics). To steer the spacecraft towards a controlled atmospheric re-entry and a soft pinpoint landing, the vehicle is equipped with a Thrust Vector Control (TVC) system controlled by gain-scheduled PID controllers. In addition to this, to enhance the RLV control authority during the atmospheric re-entry, the insertion of two pairs of planar fins for increased aerodynamic effectiveness is particularly studied and their impact on other systems is analysed. Finally, a successive convex optimisation algorithm has been implemented in a closed-loop fashion with the control system, the actuators, and the vehicles dynamics. This guidance algorithm computes onboard the fuel-optimal reference trajectory to be followed by the vehicle, based on the current flight conditions. Therefore, the guidance scheme is updated several times during the descent to adapt the computed optimal reference trajectory to the current flight conditions. The performance assessment of the mentioned technologies is here presented. Two different scenarios, one considering TVC only and the other adding fins control authority, are evaluated and compared. The interactions that exist between the vehicles dynamics, the atmospheric environment, the actuation system and the designed guidance and control algorithms are particularly analysed. Moreover, a controllability analysis is performed to evaluate the control authority needed from each actuator during the different phases of the RLV descent and landing and its impact on the trajectory: obtained results serve to address the RLV re-entry dynamics simulator design according to the guidance and control attainable performances. The results demonstrate that the configuration with TVC and fins enables an enhanced controllability, especially in case of wind perturbations. This study lies within the ASCenSIon (Advancing Space Access Capabilities - Reusability and Multiple Satellite Injection) project, an Innovative Training Network (ITN) funded within H2020.