Control Design and Analysis of a Capturing Device Performing In-Air Capturing of a Reusable Launch Vehicle

Abstract

The innovative "In-Air-Capturing (IAC)" recovery method involves winged rocket stages being captured mid-air and towed back to the launch site. This patented approach by German Aerospace Center (DLR), shows potential for substantial cost reduction by eliminating the need for an additional propulsion system during descent. An important operation in IAC involves a capturing device attached to a rope, which is released from a towing aircraft. This device must autonomously navigate its way to the winged launcher stage in the vicinity of the aircraft. This process known as capture phase of "In-Air Capturing" is expected to last about 70 s. The device must overcome disturbing forces originating from the flexible rope and wake of the aircraft to establish connection with the launcher stage. This paper studies the modelling and simulation of the capture phase. The main focus lies on the flexible dynamics of the rope and the control of the capturing device. First, extensive sensitivity studies are performed to select the best configuration for the rope. Then, a robust feedback controller including active damping is proposed to mitigate the vibrations from the rope and allow the capturing device to maneouvre with agility and accuracy. Using open loop tests, it is confirmed that the system remains stable. The rope is also proved to be strong enough to support the towing loads from pulling a large launcher stage. Closed loop tests show that the device is able to perform multiple maneouvres within the capture window. However, the device was unable to maneouvre when exposed to the strong regions of wake. A longer rope is proposed to mediate the effect of wake in future simulations.