Hardware in the loop sloshing using a testbed platform

Abstract

Sloshing in liquid-filled tanks remains a critical challenge for spacecraft attitude control due to its strong coupling with rotational dynamics. As modern missions demand increased agility, larger propellant volumes, longer lifetimes, and compliance with ESA debris mitigation requirements through guaranteed deorbiting, sloshing-induced forces and torques become increasingly significant. These disturbances can degrade pointing performance, increase actuator usage, and, in extreme cases, lead to mission failure. Both historical anomalies and more recent incidents demonstrate that sloshing continues to pose a tangible risk to guidance and control systems.
This thesis investigates the extent to which a ground-based experimental platform can replicate and characterize sloshing behavior relevant to spacecraft applications, with particular emphasis on the validity limits of linear sloshing models. While sloshing is inherently nonlinear, linear representations are widely used for control-oriented analysis and stability assessment, making identification of their applicable excitation range essential.
The experimental campaign was conducted at GMV Portugal using the TRACTOR platform, a rotational testbed providing near-free motion about a single vertical axis. A rigidly mounted sloshing tank was excited using a reaction wheel, and inertial sensors recorded the dynamic response. Sloshing dynamics were identified using frequency domain system identification, and experimental results were compared against synthetic responses generated from a strictly linear model.
The results reveal clear indicators of nonlinear sloshing beyond a threshold excitation amplitude, including amplitude dependent frequency shifts and reduced coherence. The achieved Bond number regime is representative of operational spacecraft tanks, demonstrating the platform’s capability to investigate space relevant sloshing phenomena in a laboratory environment.