Spin Control for Earth Satellites

Abstract

One of the simplest and most frequently used attitude control systems for earth satellites consists of spinning the vehicle about an axis of maximum moment of inertia. Two difficulties that occur with this attitude control system are that magnetic drag will decrease the satellite spin rate and the spin axis will lose its initial orientation due to a variety of perturbing torques. A magnetic torqueing system has been developed at the Applied Physics Laboratory for controlling spin rate and spin axis orientation. The spin rate control system consists of two vector magnetometers mounted perpendicular to the satellite’s spin axis whose output is amplified and fed into the coil of an electromagnet. As the satellite spins, this system provides a constant amplitude magnetic dipole moment that is always perpendicular to the component of the earth’s magnetic field in the X-Y plane of the satellite. The resulting torque can be used to cause the satellite to spin faster or slower, depending on whether the resulting dipole moment leads or lags the component of the earth’s magnetic field in the X-Y plane. Changing from lead to lag can easily be accomplished by a reversing switch operated by radio command; thereby allowing the satellite to have a completely adjustable spin rate. Typical weight for this system is three pounds; typical power consumption, when activated, is less than 2 watts. Control of spin axis orientation is accomplished by preces-sing the spin axis using a large magnetic dipole mounted along the satellite’s Z-axis. This dipole can be magnetized to any state by discharging a condenser through a winding wrapped around a permanent magnet core. The magnet strength and polarity can be adjusted by radio command from a ground station. Typical weight for the spin axis orientation control system is three pounds and there is no steady drain of electrical power. An electronic digital computer has been programmed to calculate the times at which the magnet should be turned on and off to precess the spin axis to any desired orientation.