A Superposition Approach to Aerodynamic Modelling of a Capturing Device used for In-Air Capturing of a Reusable Launch Vehicle
Creators: name:  S. SINGH 
name:   S. STAPPERT 
name:   L. BUSSLER 
name:   S. CALLSEN 
name:   M. SIPPEL 
name:   S. LOPEZ 
name:   S. BUCKINGHAM 
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: The design and quantification of aerodynamics is one of the most critical yet challenging aspects of defining the flight dynamics of an aerospace vehicle. The most common practice is to use simpler approximate models based on empirical or semi-empirical methods that provide acceptable accuracy. However, with the computing resources becoming more accessible and affordable, numerical solutions using Computational Fluid Dynamics (CFD) can provide the possibility of modelling complex geometries with good accuracy. Mathematical models based on Reynolds-Averaged Navier-Stokes (RANS) equation can define both steady-state and dynamic flows while providing the possibility to examine the effect of turbulence. Nonetheless, it can still be challenging to obtain data for a wide range of operating conditions due to the overall computational effort required. Therefore, there needs to be a balance between accuracy and computational effort for effective modelling. This paper will examine a method to generate extended aerodynamic dataset for an axisymmetric vehicle using limited numerical dataset from RANS calculations. The test vehicle is a capturing device specific to the application of In-Air-Capturing (IAC), which is a unique reusable launch concept patented by DLR [1]. The complete operational cycle of IAC starts with a vertical lift-off of the launch vehicle. After Main Engine Cut-Off, the reusable winged booster stage re-enters the atmosphere in a ballistic trajectory and slows down to a subsonic glide through atmospheric braking. Between 8 km to 2 km altitude, the final IAC manoeuvre is performed [2]. Here a waiting capturing aircraft approaches the Reusable Launch Vehicle (RLV) to achieve a parallel formation with similar velocities separated by a safe distance. Then, the RLV is actively captured by a capturing device (attached to a rope) released from the aircraft. Finally, the aircraft tows the RLV back to the landing site where it lands horizontally on an airstrip. This study will focus on a detailed aerodynamic analysis of the capturing device used in IAC. According to the previously studied full-scale simulations, the device should be able to manoeuvre a distance of up to 350 m within 70 s of the formation window between the aircraft and the RLV [3]. The preliminary design of this device includes a 2 m long fuselage with a spherical head and four large fins for agility. It is capable of pitching, yawing and rolling by symmetrically deflecting horizontal, vertical and all flaps respectively [4]. However, to achieve 6DOF movement, the flaps must be deflected asymmetrically in the presence of angle of attack and sideslip angles. This entails the requirement for a large dataset to effectively define the aerodynamics of the system. While a broad dataset can be generated using theoretical methods, the resulting large data tables and compromise on accuracy is not very desirable. An alternative approach to achieve a good balance in accuracy and computation time can be used by taking advantage of the axisymmetric geometry. Numerical RANS data for symmetric flap deflections are superimposed to achieve asymmetric flap datasets. The relative deviation of this method is compared against the semi-empirical method Missile DATCOM and finally verified using additional RANS calculations. Anomalies and sources of possible inaccuracies are also identified. The research is performed under the H2020 project - Formation flight for in-Air Launcher 1st stage Capturing demonstration (FALCon), aimed at the development and testing of the IAC technology.