Control Design and Analysis of a Capturing Device performing the In-Air Capturing of a Reusable Launch Vehicle
Creators: name:  S. SINGH 
name:   M. SIPPEL 
name:   S. STAPPERT 
name:   S. LOPEZ 
name:   S. BUCKINGHAM 
name:   M. SIMIOANA 
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: In the recent years, the development of multiple reusable launch systems has provided a means to achieve higher launch frequencies while keeping the costs in check. The popular recovery methods that are widely studied and utilized are mainly Vertical Take-off Vertical Landing (VTVL) and Vertical Take-Off Horizontal Landing (VTHL). However, some other non-conventional recovery methods, like partial recovery of launcher parts using inflatable devices and capture of rocket stages with helicopters (mid-air retrieval) have also shown scope for cost savings. One such innovative method, called In-Air Capturing (IAC) has been proposed by DLR [1]. The idea involves the winged stage being caught mid-air using an aircraft and towed back to the launch site without the need of additional propulsion system for landing [2]. This provides potential for considerable cost reduction as the propulsion system is one of the most expensive components of a launch vehicle [3]. The operational cycle of IAC starts with the launcher lifting off vertically and ascending until Main Engine Cut-Off (MECO). At MECO, the winged first stage separates from the launch vehicle and re-enters the atmosphere in a ballistic trajectory, in the course of which it decelerates from supersonic velocity to a subsonic glide. Meanwhile, a capturing aircraft is waiting at the downrange rendezvous area, loitering until the RLV arrives. Between 8 km to 2 km altitude, the final IAC manoeuvre is performed [4]. The aircraft first approaches the RLV to achieve the correct formation for capture. Both aircraft have the same flight path angle and similar velocities in a parallel formation. Then, the RLV is actively captured by an Aerodynamically Controlled Capturing Device (ACCD) released from the aircraft. Finally, the aircraft tows the RLV back to the landing site where it lands horizontally on an airstrip. This paper will examine the control design and simulation of the capture phase of IAC. During this phase, the ACCD attempts to autonomously navigate its way to the RLV, while the towing aircraft and RLV remain in a parallel formation [5]. The goal of the ACCD is to establish contact with the RLV within 60s of the formation flight and therefore, connect the two vehicles (aircraft and RLV) with a rope. This manoeuvre requires a high degree of agility and accuracy to allow for multiple capture attempts in case of failure. To be able to effectively design and test a control system for the current configuration, detailed models of the most critical subsystems and external disturbances are included in the dynamic modelling [6]. These include the flexible rope dynamics as well as wake from the towing aircraft. The control design is first designed assuming a rigid rope model. Then, the controller is applied to the model with a flexible rope and the two system dynamics are compared. Along with the challenges, possible adaptations and improvements to the controller design are finally 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