This paper focusses on the design of a sliding-mode observer to estimate the states of the slosh-mass position and velocity. These states are subsequently used as feedback signals for the attitude controller. A simple proportional-derivative (PD) rigid-body controller cannot safely execute attitude commands without encountering stability problems, and even in a steady state situation a non-zero slosh-mass positions leads to jittering in the swivel angle. With slosh-state feedback, even a non-optimal PD controller exhibits a smooth response with limited actuator activity. However, a slight uncertainty in the slosh-model parameters of the observer will negatively affect the controller performance, and, as a consequence, increase the amplitude and oscillatory behaviour of the liquids. An additional observer will be required to estimate the correct parameter values. A more robust incremental non-linear dynamic inversion controller improves the controller response, but further development will be required, for instance, by including notch filters in the feedback loops.