spin stabilization

Spin-stabilized satellite

A spin-stabilized satellite is a satellite which has the motion of one axis held (relatively) fixed by spinning the satellite around that axis, using the gyroscopic effect.

The attitude of a satellite or any rigid body is its orientation in space. If such a body initially has a fixed orientation (relative inertial space) it will start to wobble because it will always be subject to small torques. The easiest form of attitude stabilization is to give the rigid body an initial spin around a principal axis. This is what is done to for example a rifle bullet. Because of the rotation the spin axis will be (essentially) fixed relative inertial space because of the gyroscopic effect.

This method to stabilize one axis is utilized also for many spacecraft, these spacecraft are said to be spin stabilized. For some types of spacecraft such a rotation is an advantage also for other aspects of the mission. If the spin axis makes a angle to the Sun direction that is approximately 90 deg the heating effects of the Sun will be rather uniformly distributed over the surface (the "barbecue effect"). Sometimes the rotation can also be useful for instruments that should scan the environment. But most modern spacecraft have a panel with solar cells that should be directed towards the Sun and antennas that should be directed towards the Earth. These spacecraft are then instead kept in a desired attitude with an active attitude control using attitude sensors to detect mispointing and attitude control actuators, mostly momentum wheels or thrusters, to compensate for the outer torques and to keep the spacecraft in the desired attitude. This type of attitude control is called "3-axis stablization".

The entire space vehicle itself can be spun up to stabilize the orientation of a single vehicle axis. This method is widely used to stabilize the final stage of a launch vehicle. The entire spacecraft and an attached solid rocket motor are spun up about the rocket's thrust axis, on a "spin table" oriented by the attitude control system of the lower stage on which the spin table is mounted. When final orbit is achieved, the satellite may be de-spun by various means, or left spinning. Spin stabilization of satellites is only applicable to those missions with a primary axis of orientation that need not change dramatically over the lifetime of the satellite and no need for extremely high precision pointing. It is also useful for missions with instruments that must scan the starfield or the Earth's surface or atmosphere.

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