Docking Loads Due to Low-Gravity Propellant Motion

Slosh dynamics can have a detrimental effect on spacecraft approach, pointing, and docking in a low-gravity (low-g) environment when there are significant amounts of both propellant mass and open tank volume. This report uses four methods to estimate chaser/target vehicle docking loads that consider the effect of the chaser vehicle propellant movement and assumes a 50% full liquid oxygen (LOX)-Methane (LCH4) dual tank configuration. All the models leveraged assumptions on the propellant slosh behavior that have not been validated since the dynamic behavior of fluid in a low-g environment is an area of active research with limited data for model validation.
The first two analysis methods were intended to provide rough order of magnitude (ROM) loads estimates — one based on fluid mechanics and one on impulse momentum. The third method, a multi-body dynamic simulation, was developed which incorporated single particle mechanical slosh models for LOX and LCH4. The slosh model consists of a free-moving particle within a constraint surface with penalty method-based contact dynamics and an inert particle that augments the mass properties of the spacecraft. The parameters of the slosh model are tuned with computational fluid dynamics (CFD) simulations of a representative scenario. The simulation framework allowed for the Monte Carlo analysis of the chaser vehicle approach in order to evaluate the associated slosh loading before, during, and after docking. The fourth method used the dynamic simulation states at first contact with the target vehicle as initial conditions for a high-fidelity Adams model that included the NASA Docking System Block 2 (NDSB2) mechanism. The inclusion of this fourth analytical method permits a more detailed evaluation of potential challenges with docking beyond basic loads calculations, including the impact of misalignments on docking, linear actuator stroke length limitations, and the potential for damaging contact during docking.
The impact of post-contact propellant motion on loads for the docking profile evaluated appeared to be minor compared to the impact of misalignments due to the assumed controller and actuator design. Only the high-fidelity Adams model that included the NDSB2 docking mechanism was able to identify missed capture cases, with and without propellant motion, and scenarios that would be potentially damaging to the NDSB2 docking mechanism.