Sub-Surface Inertial Wave Excitation in Liquid Propellants and Its Effects on Fuel Tanks of Spin-Stabilized Spacecraft

The unsteady motion of liquid propellant onboard a spinning spacecraft is of concern when considering the attitude stability of the entire vehicle. Spin-stabilization maneuvers performed by the spacecraft, while on-orbit, induce periodic liquid motions within the propellant tank known as fuel slosh. As the liquid propellant sloshes, sub-surface currents and viscous forces at the tank walls create small internal torques which force the spacecraft into an unsteady, oscillatory spin known as nutation. Resonant frequencies increase the nutation growth rate and can result in the spin of the spacecraft switching from a spin about the minor axis to a spin about the major axis. New slosh research has shown that a unique phenomenon occurs when a spherical fuel tank is filled to maximum capacity and placed within a spacecraft with a 0.9 effective inertia ratio. Sub-surface inertial waves are created inside the fuel tank, causing a rotational instability in the spacecraft. Therefore, to better understand sub-surface inertial waves and their attitude effects on spin-stabilized spacecraft, an experiment is performed onboard NASA’s parabolic, microgravity aircraft. Accelerometers attached to a mock spacecraft measure the rate of nutation growth as the spacecraft is spun in zero gravity. The data is analyzed and compared to a MATLAB SimMechanics simulation to validate mechanical models. The comparison of the two forms of data will be used to better understand sub-surface inertial waves and their effects on nutation growth.