The technology for maintaining the orientation of the rotation axis of the artificial earth satellite in space includes single-rotation stabilization and dual-rotation stabilization.
Single Turn Stabilization” is abbreviated as Turn Stabilization, which is a type of passive attitude stabilization (see Spacecraft Attitude Control). Most of Earth’s early artificial satellites were stable in rotation (rotating at a constant speed around a major axis of inertia).
When the angular momentum of a star’s rotation is large enough, the direction of the angular momentum changes very slowly under the action of the ambient perturbation torque. This feature is called the fixed axis of the gyroscope.
When a satellite rotates at a constant speed, the direction of the axis of rotation is the same as the direction of the angular momentum. Rigid body dynamics proves that when a rigid body rotates about the axis of maximum principal inertia or the axis of minimum principal inertia, it has the property of a gyro-fixed axis. But the real satellite is not a rigid body.
The satellite contains elastic components and is equipped with liquid fuel needed for attitude control and orbit control. For a satellite whose nominal axis of rotation is the minimum principal axis of inertia, when the axis of rotation does not coincide with the total angular momentum vector and nutation occurs, the axis of rotation rotates around the total angular momentum vector.
This will cause the satellite’s elastic components to vibrate and liquid fuel to spill into the fuel tank. These motions will consume the rotational kinetic energy of the satellite, and the satellite’s motion will eventually tend towards the state of minimum kinetic energy.
In the case of conservation of angular momentum, the state of minimum kinetic energy is the state of the satellite rotating around the axis of maximum principal inertia. In other words, when the satellite rotates around the axis of minimum main inertia and there is internal mechanical energy dissipation, the nominal axis of rotation will rotate in space.
Therefore, the axis of rotation of the satellite must be the axis of maximum principal inertia, which is the criterion of maximum principal inertia that must be followed when designing a rotation satellite (see Spacecraft Attitude Dynamics). In maneuvering artificial satellites and correcting the intermediate orbit of interplanetary spacecraft, spin stabilization can also be used to maintain thrust direction stability.
Dual Turn Stabilization is a type of semi-active attitude control. Mainly used for communication satellites. The dual-turn stabilized satellite consists of a rotor and a rotating platform, which are connected by a shaft and a bearing.
Most of the satellite’s auxiliary systems are placed on the rotor, and the mass of the rotor is much greater than that of the platform. The rotor rotates at a constant speed to keep the attitude of the satellite’s axis of rotation stable. The rotor mounted motor rotates the platform in the opposite direction. When the rotation speed of the platform relative to the rotor is equal to the rotation speed of the rotor, the platform performs rotation.
At that time, the payload on the platform (such as detection instruments, communication antennas, etc.) will be continuously oriented towards the ground. With the development of satellite application technology, satellites need to get more solar energy, so it is necessary to expand the surface area of the cylindrical rotor equipped with solar cells.
The rotor diameter is limited by the outer dimensions of the launch vehicle, so the height of the cylinder can only be increased to make the rotor appear slender. At that moment, the rotor rotation axis becomes the axis of lowest main inertia and no longer has the fixed-gyro axis. In this case, the simplest and most effective way to keep the axis of rotation stable is to install a high-efficiency nutation damper on the de-rotation platform.
When the satellite is cruising, the movement of the moving working medium (working medium) inside the damper produces a reaction torque on the satellite. As the platform does not follow the rotation of the rotor, this reaction torque can eliminate the lateral angular rate of the satellite, so that the whole platform can stabilize the orientation of the spin axis of the double spin satellite.
Nutation damping: Eliminating the nutation of a rotating satellite is to keep the satellite in a pure state of rotation, that is, the axis of rotation coincides with the vector of total angular momentum. As the energy (electrical or chemical) carried by the satellite is consumed, nutation damping is divided into two passive and active types:
① Nutation Passive Damping: The damper contains movable damper working fluid. Nutation causes the centrifugal force of various points in the stellar body to change periodically, which stimulates the damping working fluid to produce relative motion and dissipates the kinetic energy of nutation to reduce the nutation angle. There are many types of passive nutation dampers, such as pendulum type, tube ball type and liquid ring type.
The main difference between them is the type of damping working medium (solid or liquid), the method of supporting the damping working medium (bearing suspension or closed container), the damping method (viscous fluid or magnetic eddy current ) and the nature of the restoring force (centripetal force or mechanical spring force).
② Active nutation damping: The active nutation damping device is composed of nutation sensors, control circuits and actuators.
The nutation phase is measured by the nutation sensor. Through the control line, the lateral moment of the actuator acting on the satellite is opposite to the direction of the lateral angular velocity of the satellite, eliminating nutation. There are two types of implementing agencies.
One is the jet actuator. Due to limited fuel on the satellite, jet nutation damping can only be used for a short time; the other is the anti-rotation motor, which makes the axial velocity of the anti-double-spin satellite, the rotating platform changes slightly. Its own product of inertia produces a lateral coupling moment.
When the external interference torque causes a large change in the angular momentum of the rotating satellite, it can be adjusted by active control methods (such as jets).
If you want to maneuver the rotation axis from the starting direction to a certain target direction, pulse jet control can be used. The satellite rotates once and the nozzle sprays air once. The resulting lateral momentum precesses the satellite’s angular momentum once.
As the jet thrust is very small, the axis of rotation is basically the same as the angular momentum vector and precesses with the angular momentum. The main task of controlling maneuvers in the direction of the axis of rotation is to determine the phase of the jet pulse and ensure that the axis of rotation can be maneuvered in the direction of the objective. The isometric control method is generally used to solve this problem.