Attitude dynamics of spacecraft with time-varying inertia during on-orbit refueling

On-orbit refueling technologies provide a new way to extend the lifespan of satellites that run out of propellant. During propellant transfer, the equivalent mass redistribution between the servicer and the client inevitably leads to internal disturbance torque in the system. To investigate the attitude motions of the servicer–client combination, the attitude dynamics of time-varying inertia are developed. These address variations of inertia tensors throughout refueling, and refueling disturbance torque is finally represented in terms of the fuel transfer rate between the two spacecraft. The effect of gravity gradient torque on the attitude motion is also investigated. Because fuel transfer rate significantly depends on refueling pressure, a mathematical model describing the relationship between refueling pressure and fuel transfer rate is also established. The Reynolds number, distance loss, and local loss are also taken into account by using the Navier–Stokes equation. Finally, different simulations are performed to address the effects from both fuel transfer and gravity gradient on the attitude; the effectiveness of the proposed attitude dynamics is numerically verified.