Advanced modeling and trajectory optimization of the in-air-capturing maneuver for winged RLVs

Future reusable launch vehicle concepts and their key technologies have been investigated within the DLR research project AKIRA. In this context, several return options for reusable launch vehicles (RLV) were categorized by vertical (SpaceX, Blue Origin) or horizontal landing strategies (Space Shuttle), and then systematically evaluated based on their influence on overall design and technical feasibility. In general, system dynamics, guidance, and control aspects are of special importance within preliminary design studies, in particular if complex and difficult maneuvers like the DLR-patented in-air-capturing method are considered. In this case, the unpowered winged RLV is captured during descent by an aerodynamically controlled capturing device which is connected to an aircraft by a cable. After successful capturing, the launch vehicle is towed back to its landing site. In previous studies, the technical feasibility of the in-air-capturing maneuver was mainly assessed by simulations for an aerodynamically controlled RLV and an aircraft which is assumed to be passive. In contrast to this, we consider an optimal control approach to the problem of in-air-capturing, investigating both passive and active (cooperative) RLV and aircraft operations. To study the risk of failure of the in-air-capturing maneuver, both the initial capturing approach and a subsequent second attempt for recapture after an initial miss are analyzed. For this purpose, a multi-disciplinary multibody modeling and simulation framework based on the object-oriented modeling language MODELICA is used for the consistent flight dynamics modeling of each vehicle including a rigid cable connecting the aircraft and its capturing device. The trajectory optimization results provide an overview of the dynamic behavior of the multibody system for several constraints and flight conditions. Additionally, the results show that for a successful in-air-capturing maneuver with minimum control effort and multiple recapturing attempts, an actively controlled aircraft with drag-increasing subsystems and a cooperative launch vehicle maintaining a suitable flight path angle are recommended. The obtained reference trajectories can be used for future controllability studies and control system design considering a flexible cable and disturbances.