Control moment gyroscope
Single-gimbal CMGs exchange angular momentum in a way that requires very little power, with the result that they can apply very large torques for minimal electrical input.
As an actuator, it is more versatile than a single-gimbal CMG because it is capable of pointing the rotor's angular momentum vector in any direction.
If the goal is simply to store angular momentum in a mass-efficient way, as in the case of the International Space Station, dual-gimbal CMGs are a good design choice.
However, if a spacecraft instead requires large output torque while consuming minimal power, single-gimbal CMGs are a better choice.
Most CMGs hold rotor speed constant using relatively small motors to offset changes due to dynamic coupling and non-conservative effects.
Variable-speed CMGs (VSCMGs) offer few practical advantages when considering actuation capability because the output torque from the rotor is typically much smaller than that caused by the gimbal motion.
However, no matter how many CMGs a spacecraft uses, gimbal motion can lead to relative orientations that produce no usable output torque along certain directions.
These orientations are known as singularities and are related to the kinematics of robotic systems that encounter limits on the end-effector velocities due to certain joint alignments.
When the transient torque ends, the control program will stop the gimbal movement, and the rotors will be left pointing more forward than before.
If these events are repeated, the angular momentum vectors of the individual rotors will bunch more and more closely together round the forward direction.
In the limiting case, they will all end up parallel, and the CMG cluster will now be saturated in that direction; it can hold no more angular momentum.
If however (for example) they were already holding a little angular momentum in the "up" (yaw left) direction, they will saturate (end up parallel) along an axis pointing forward and slightly up, and so on.
This contrasts with a single reaction wheel, which can absorb more and more angular momentum along its fixed axis by spinning faster, until it reaches saturation at its maximum design speed.
An unwanted left yaw can be dealt with by storing some "up" angular momentum, which is easily done by tilting both rotor spin axes slightly up by equal amounts.
Since their fore and aft components will still be equal and opposite, there is no change in fore-and-aft angular momentum (it will still be zero) and therefore no unwanted roll.
[c] In practice the CMG control program will continuously redistribute the total angular momentum to avoid the situation arising in the first place.
[9]: 11 Skylab carried three CMGs, mounted with their casings (and therefore their rotor axes when the gimbals were set to zero) facing in three mutually perpendicular directions.
The 45 degree zero offset then ensured that the three 'blind strips' of the outer gimbals would pass halfway between neighbouring 'polar blind spots' and at a maximum distance from each other.
[14] A third set of gyrodynes was installed in Kristall during Mir-18[15] The ISS employs a total of four CMGs, mounted on Z1 truss[16] as primary actuating devices during normal flight mode operation.
The objective of the CMG flight control system is to hold the space station at a fixed attitude relative to the surface of the Earth.
In the presence of these continual environmental disturbances CMGs absorb angular momentum in an attempt to maintain the space station at a desired attitude.
Some kind of angular momentum management scheme (MMS) is necessary to allow the CMGs to hold a desired attitude and at the same time prevent CMG saturation.
On 24 February 2015, the Scientific and Technical Council of Roscosmos announced that after decommissioning of the ISS (then planned for 2024) the newer Russian modules would be detached and form the nucleus of a small all-Russian space station to be called OPSEK.
