Sloshing affects the motion of vehicles and can cause accidents in critical cases. In particular, if liquid constitutes a large amount of a system's mass, the interaction between the liquid and the system cannot be neglected. In aerospace applications, the propellants mass of a geostationary satellite is approximately 40% of its mass-at-launch. Typical active sloshing control studies focus on only attitude dynamics [1,2], attitude and translational dynamics without consideration of gravitational energy [3,4], or attitude and translational dynamics with constant gravity through the space domain [5]. These simplified dynamics are sufficient to illustrate the effectiveness of a proposed control algorithm to suppress propellant sloshing. However, they do not allow for the quantification of the influence of the sloshing perturbing force on orbit parameters. Most importantly, sloshing perturbing forces are also structurally coupled with the attitude dynamics of spacecraft. Sloshing-induced attitude errors can be especially costly during a mission stage of a geostationary satellite called the liquid apogee engine (LAE) phase. Only by adopting a more complex dynamics model capable of orbit propagation, can the control algorithm performance be assessed by the accuracy of the attained target orbit. In this work, we develop an orbit propagator, that allows for the inclusion of the sloshing perturbing force and its coupling with the spacecraft attitude dynamics. The sloshing of the fuel is modelled using the pendulum Equivalent Mechanical Model (EMM). The spacecraft’s model has 8 DOFs, which comprise of 6 DOFs of the spacecraft, and 2 DOFs of the EMM. We demonstrate the usefulness of the model for the LAE phase of a typical geostationary satellite. The presented dynamics equations can be modified to adopt different EMMs and thus can be used for other mission phases such as e.g. the docking phase.