ICARUS: In-Air Capturing Apparatus for Recovering Unpropelled Stages

Kurzfassung

With the advent of Reusable Launch Vehicles (RLVs) on the European launch market, the German Aerospace Centre (DLR)’s In-Air Capturing (IAC) recovery technique promises a competitive reduction in launch costs. Central to this IAC procedure is the Aerodynamically Controlled Capturing Device (ACCD), which is deployed from a Towing Aircraft (TA) by means of a tether. It intercepts a returning RLV, and provides a structural interface between the TA and RLV during tow-back. Once in the vicinity of a landing site, the ACCD releases the towing connection, and the RLV lands autonomously. Whereas the aerodynamic shell of the ACCD has been defined and studied in the past, an in-depth exploration of its electromechanical design space has not been performed previously. This work proposes a systems engineering approach to fill the corresponding design gap, based on the definition of key requirements and risks characterizing the ACCD’s functionalities, and its operational environment. First, the behaviour of the ACCD as part of a larger towing system is analysed. For this, a two-dimensional (2D), quasi-steady-state Towing Model is developed, enabling a swift exploration of the ACCD’s design space, as well as a preliminary estimation of extreme tow-back loads. This Towing Model shows that the ACCD’s towing position relative to the TA is highly sensitivity to the ACCD’s Centre of Gravity (CoG); in order to stay clear of the TA’s disturbing wake zone, a maximum allowable CoG position of 1 m behind the vehicle’s nose is defined. Additionally, a significant pitch control authority is demonstrated, indicating substantial manoeuvrability. Finally, a study of extreme tow-back conditions results in the definition of a 300 kN axial towing design load - representing a 72% increase compared to previous estimates. Next, the ACCD’s relative navigation system is studied, used to estimate the position and attitude of the RLV. A comparison between state-of-the-art systems is made, and the so-called VisNav solution is proposed - with a sensor attached to the RLV, and active beacons mounted on the ACCD. In order to identify a representative configuration for this system, a geometric VisNav Model is developed, analysing the visibility of beacons from the sensor’s Point of View (PoV). Based on comparative studies performed with this model, a design with three asymmetric rings of twelve beacons is proposed. Combined with Global Navigation Satellite System (GNSS) and Inertial Navigation System (INS) data, this configuration enables relative position and attitude estimation with an accuracy of 20 cm and 1°. A representative electromechanical design for the ACCD is then proposed, based on results from the foregoing design space exploration. On the one hand, the selection of Commercial-off-the-Shelf (COTS) avionics is covered, as well as a preliminary dimensioning of the vehicle’s power system. Here, the ACCD’s actuators are identified to be the main drivers of its electronic design in terms of Size, Weight and Power (SWaP) footprint. Additionally, the feasibility of a battery-powered design is demonstrated. On the other hand, a Computer-Aided Design (CAD) study of mechanical subassemblies is performed, which are further analysed in terms of extreme operational loads. Most noticeably, a separation of the docking and release systems is proposed, as combining both into a single mechanism is deemed infeasible, due to the continuous presence of significant towing loads. Based on the presented design, a Bill of Materials (BoM) for the ACCD is established, yielding a total functional mass of 159.14 kg. Because of the substantial impact of the vehicle’s wings and docking system on its overall CoG, an additional 16.3 kg trim mass is required to reach the desired CoG position. Compared to previous studies, the final estimate for the ACCD’s total mass has increased by 33%. Concluding the study, compliance with the vast majority of the postulated design requirements is demonstrated, while a number of future improvements and complementary studies are identified. These include an aerodynamic redesign and shortening of the ACCD’s shell, to eliminate the need for a trim mass. Additionally, prototyping of this unique vehicle is highly recommended, in order to further study its behaviour, and validate the theoretical design proposed in this work.