ESA GNC Conference Papers Repository

BepiColombo is an ESA cornerstone mission to Mercury in collaboration with the Japanese Space Agency (JAXA), with Airbus Defence & Space as prime industrial contactor for the ESA contribution. The two scientific orbiters -ESA's Mercury Planetary Orbiter (MPO) and JAXA's Mercury Magnetospheric Orbiter (MMO)- are launched together in 2018 as a single composite spacecraft, including a module with electric propulsion supporting the 7 year cruise phase. On BepiColombo, three major changes in the spacecraft configuration over the mission, resulting from module separations, significantly increase the GNC complexity with respect to previous European interplanetary missions. In addition, the propulsion systems used in each flight configuration are optimised for the specific mission arc where each configuration is used: as a consequence, the GNC subsystem must be designed to control the composite spacecraft using either electric propulsion (during cruise) or different types of chemical propulsion (for either axial or transverse thrust during planetary fly-bys and Mercury orbit insertion). This variable flight configuration imposes a number of challenges on the design of the GNC subsystem. Handling multiple configurations and the required GNC modes/phases leads to a theoretical total of 83 controllers to be designed and tuned for the applicable performance and stability requirements: while commonalities have been exploited where possible, still 45 different controllers turned out to be needed. In performing the tuning, limitation of fuel consumption (due to the criticality of the mission mass budget) and control of flexible modes (due to large appendages and sloshing) have been the driving factors: the use of structured H-infinity techniques and an optimisation of the thruster modulator actuation period as a function of the configuration were needed to achieve the required performance and robustness to mechanical uncertainties on the flexible modes. A dedicated toolbox has been implemented to minimise the cost of the re-tuning needed as the S/C dynamical properties evolved with project maturity. In addition, several GNC functions have been designed to be able to cope with variable configuration. One notable example is the antenna guidance function, which commands the high-gain and medium-gain antenna mechanisms within allowable domains, defined to avoid clashes with the spacecraft structure, which change as a function of the configuration. The GNC has also been designed to exploit the resources available in specific configurations: for instance an electric propulsion-based reaction wheel offloading approach is implemented in cruise configuration when the electric propulsion is active, in order to manage the reaction wheel momentum at limited chemical fuel consumption. The transition between different configurations also is a challenging operation that requires a certain degree of on-board autonomy: module separations are autonomously detected via break-wires and on-board control procedures (OBCPs) are used to handle the required context restoration, configuration determination and synchronisation activities to allow the GNC to restore spacecraft controllability after the module separation. This paper will provide a survey of all major GNC design features needed to cope with variable flight configurations and details on the specific design solutions adopted in the BepiColombo GNC subsystem.