UAV Pre-Study for In-Air-Capturing Maneuver

Abstract

Implementation of reusable launch vehicle (RLV) missions is a major goal in current aerospace research. One conceptual idea to return a booster stage is the so-called "in-air capturing" (IAC), where a winged stage is captured by an aerial vehicle and towed back to the final destination. This approach has the advantage that a towed winged stage does not need a propulsion system or fuel reserves to arrive to a destination point, compared to alternative approaches as demonstrated by SpaceX. The concept of capturing the RLV in air is based on earlier IAC missions for satellite photo capsules and the probe-and-drogue refueling method. A key difference is that in an air-to-air-refueling procedure a highly dynamic fighter tries to connect to a trailing drogue or rigid boom from a sluggish tanker. Opposite to this procedure, it can be assumed that in IAC case two aerodynamically sluggish aircraft need to be coupled. The challenge is to build up a formation which enables a connection between a gliding RLV and a dynamic coupling device trailing from a large aircraft. To investigate this IAC approach, the German Aerospace Center (DLR) built a scaled demonstration system with smaller unmanned aerial vehicles (UAV) to research different aspects of IAC flight tests. Based on the assumption that at an IAC approach would involve two large aerodynamically sluggish systems, a third highly dynamic vehicle should be introduced to enable a safe and reliable connection. Therefore, the trailing system, which is known from the air-to-air refueling, was modified by DLR with aerodynamic control surfaces and an independent flight control system. This enables a dynamic and independent motion of the coupling device relative to the towing aircraft, as well as the RLV. This paper gives an overview about IAC investigations of DLR, which are validated in experimental flight test demonstrations with scaled unmanned systems. The focus of this work is on building up the formation, from the rough approach with GNSS up to the final approach, where the global, absolute localization is supported by an image based relative position estimation of the coupling device. A major aspect in the formation implementation is the active coupling device (ACD). Therefore, the paper will show the construction of the ACD, its functionality and operation during the formation flight and a validation of its behavior at the flight demonstration. The first flight test results show that our research is heading in the right direction and further tests are expected to provide comprehensive results for the validation of the IAC concept. The presented investigations and results base on work from the DLR funded AKIRA project and the subsequent project FALCon funded by the European commission in the H2020 Program. FALCon has the aim to increase the TRL of AKIRA investigations and results.