Search

When reaction wheels of a satellite drift toward saturation caused by the accumulated effect of external torque, it is common to employ thrusters or magnetorquers to actively unload extra momentum of the wheels. This paper presents several optimal approaches to manage the three-axis reaction wheel momentum of Earth-pointing satellites actuated by three-axis magnetorquers and/or thrusters. The optimalmomentumdumping using magnetorquers only behaves fairly slowly, and there are difŽ culties in solving a two-point boundary problem for onboard applications.Comparatively, thrusters can achieve relatively fastmomentumdumpingat the cost of consuming expendable fuel. Based upon relatively simple optimal algorithmsusing thrusters only, the newly proposed combined algorithmseffectively separate the required torques formagnetorquersand thrusters commandingsimultaneously. They simply employ on-line geomagneticmeasurements. The key beneŽ t of these combined approaches is that they can save a large amount of thruster propellant because of the assistance of the magnetorquers. The realistic simulations showed that the optimal combined method can economize at least 20% thruster fuel within an unloading time of 1 10 of an orbit (about 10 min) for a low-Earth-orbit mini-satellite. The combined controllers are readily applicable to real-time practical momentum dumping.