Optimal combined reaction-wheel momentum management for earth-pointing satellites

When reaction wheels of a satellite drift toward saturation caused by the a ccumulated effect of external torque, it is common to employ thrusters or m agnetorquers to actively unload extra momentum of the wheels. This paper pr esents several optimal approaches to manage the three-axis reaction wheel m omentum of Earth-pointing satellites actuated by three-axis magnetorquers a nd/or thrusters. The optimal momentum dumping using magnetorquers only beha ves fairly slowly, and there are difficulties in solving a two-point bounda ry problem for onboard applications. Comparatively, thrusters can achieve r elatively fast momentum dumping at the cost of consuming expendable fuel. B ased upon relatively simple optimal algorithms using thrusters only, the ne wly proposed combined algorithms effectively separate the required torques for magnetorquers and thrusters commanding simultaneously. They simply empl oy on-line geomagnetic measurements. The key benefit of these combined appr oaches is that they can save a large amount of thruster propellant because of the assistance of the magnetorquers. The realistic simulations showed th at the optimal combined method can economize at least 20% thruster fuel wit hin an unloading time of 1/10 of an orbit (about 10 min) for a low-Earth-or bit mini-satellite. The combined controllers are readily applicable to real -time practical momentum dumping.