ESA GNC Conference Papers Repository

Liquid-fuel sloshing is a major source of disturbance for space vehicles and, as such, represents an important mission, system and AOCS design and performance driver, particularly for those missions requiring high accuracy pointing, fast tranquillization time to achieve high mission availability, or tight center-of-mass (COM) knowledge and stability. In fact, these high performance requirements, negatively affected by propellant sloshing disturbances, are often in combination with competing requirements for fast re-pointing slew maneuvers, which excite fuel sloshing. This is e.g. the case for several cosmic vision missions such as Euclid, Plato, LOFT, Athena, Juice; GEO missions as MTG and Geo-HR, as well as all the agile LEO missions (SAR and optical) as NGSAR and Jason-CS/Sentinel 6. Additionally, de-orbiting requirements have been recently introduced, implying that nearly all future missions will carry much larger liquid tanks for their entire operational life. In all these cases, the disturbances produced by the large amount of fuel, excited e.g. by a re-pointing or deorbiting maneuver, interact with the solid body dynamics, its structural flexibilities and its control system, potentially leading to increased actuators commands and associated fuel consumption, degraded satellite pointing performances, increased tranquillization times and reduced mission availability, undesired COM displacements, and therefore representing a major driver in the system design. Therefore, accurate and validated models and tools are needed to predict and mitigate the impact of microgravity propellant sloshing for a large number of mission scenarios. For these reasons Airbus Defence & Space has developed a novel tool, which couples the AOCS Offline Simulator Environment (AOSE) simulator, the in-house high fidelity simulation environment used for AOCS design and analysis, to the Final Phase Simulator (FiPS®), which incorporates different codes for computational fluid dynamics (CFD). Coupling CFD with an AOCS simulator elegantly allows for high accuracy predictions of the interaction between the dynamics of the sloshing liquid propellants and the closed-loop dynamics of the solid spacecraft, including its structural flexibilities and its AOCS control system. Instead of relying on classical linear analytical models or mechanical analogies, which are approximated and not valid during all stages of flight, this method allows for a direct link between the vehicle dynamic environments simulated by means of the AOSE to the fluid flow equations solved by the CFD code within the FiPS tool. This approach therewith enables including propellant sloshing effects into the AOCS design and analysis for high performance missions in microgravity conditions. Especially the possibility to include flexible body dynamics is an important feature of the AOSE/FiPS coupled tool, as the interplay of structural flexible modes like solar array or appendices with the sloshing modes can cause large nonlinear resonances which are only seen when the fluid dynamics is coupled to the flexible body dynamics. Furthermore, a benchmarking study to evaluate the state-of-the-art and the improvements required by current sloshing modelling methodologies in low-gravity environment to support high-fidelity simulation for high-performance missions has been conducted by Airbus DS under ESA contract. Two benchmark case-studies have been defined: one based on the AS400 Functional Avionics Platform developed by Airbus DS and representative of a wide range of applications in the framework of highly-stable system; the second is a scaled test-case for an experiment on-board the International Space Station (ISS). The analysis is founded on state-of-the-art analytical models and numerical simulations, making use of the CFD tool Flow3D and the in-house CFD-in-the-loop FiPS. The present paper discusses approaches and results achieved in the study, focusing on the improvement required w.r.t. modelling, accuracy and computational costs to cope with high-performance missions and presents the Airbus DS coupled tools: FiPS and AOSE/FiPS. As the lack of long-duration microgravity experimental data is considered crucial to improve the understanding of the fluid-mechanics in microgravity and therefore to enhance the mathematical formulation and modelling of the sloshing problem, as well as to benchmark and validate sloshing models and tools uncoupled or coupled to the spacecraft dynamics and its AOCS, DLR and Airbus DS teamed-up to build the MICROSLOSH experimental platform, whose flight on the ISS is foreseen for 2018 and whose design is herein outlined.