Spacecraft Thruster Efficiency Optimization with Respect to Coupled Solid-Liquid Dynamics

Abstract

Liquid dynamics in microgravity is one of the most challenging factors in spacecraft design. Performance and safety especially for a manned spacecraft considered for the US Constellation Program contribute to a big extend to the exact knowledge of the mass properties (centre of gravity and inertia) at any time during the mission. Due to fuel consumption, the spacecraft mass changes. In addition to that, the physical characteristics of the residual fuel become an important driver for the dynamics of the spacecraft. The fueltank interaction influences the spacecraft’s trajectory mainly in three ways. First, during maneuvers the physical properties rapidly change in dependency of the thruster action. Second, fuel impact on the tank walls is a limiting factor for the tank structure design as well as for the guidance accuracy of the spacecraft. Third, the thruster system has only limited capacity in terms of fuel resources and thrust for guidance and attitude control. In the same time, it is used to correct the trajectory of the spacecraft if deviations due to fuel sloshing occur. The target of this paper is to highlight an innovative scheme that allows the optimization of thruster control laws with respect to liquid dynamics in microgravity. The analysis is done by numerical simulation using a three-dimensional time accurate particlecluster method that has been successfully validated against experimental findings from test cases defined for the analysis of liquid sloshing. Here, a partially filled cylindrical tank with two semi-spheres at the extremities has been submitted to forced accelerations and rotations, simulating spacecraft instantaneous thruster ignition and spin maneuvers. Thrust profiles of the experimental satellite Sloshsat FLEVO have been simulated with the particle-cluster approach. Sloshing effects and impact forces have been found very close to those obtained from the experimental satellite. Finally, typical thrust commands for translation maneuvers have been optimized in terms of fuel consumption and thruster efficiency. An outlook is provided showing the potential of this method for even more complex maneuvers involving subsequent thruster action.