1974, and introduced as standard components from Salyut 6 onwards. The completed Mir station had 18 gyrodynes altogether, starting with six in the pressurised interior of the Kvant-1 module. These were later supplemented by another six on the unpressurised outside of Kvant-2. According to NPO Energia, putting them outside turned out to be a mistake, as it made gyrodyne replacement much more difficult. A third set of gyrodynes was installed in Kristall during Mir-18 International Space Station The ISS employs a total of four CMGs, mounted on Z1 truss 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 addition, it seeks a torque equilibrium attitude (TEA), in which the combined torque contribution of gravity gradient, atmospheric drag, solar pressure, and geomagnetic interactions are minimized. In the presence of these continual environmental disturbances CMGs absorb angular momentum in an attempt to maintain the space station at a desired attitude. The CMGs will eventually saturate (accumulating angular momentum to the point where they can accumulate no more), resulting in loss of effectiveness of the CMG array for control. 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. Since in the absence of an external torque the CMGs can only exchange angular momentum between themselves without changing the total, external control torques must be used to desaturate the CMGs, that is, bring the angular momentum back to nominal value. Some methods for unloading CMG angular momentum include the use of magnetic torques, reaction thrusters, and gravity gradient torque. For the space station, the gravity gradient torque approach is preferred because it requires no consumables or external hardware and because the gravity-gradient torque on the ISS can be very high. CMG saturation has been observed during spacewalks, requiring propellant to be used to maintain desired attitude. In 2006 and 2007, CMG-based experiments demonstrated the viability of zero-propellant maneuvers to adjust attitude of the ISS 90° and 180°. By 2016, four Soyuz undockings had been done using CMG-based attitude adjustment, resulting in considerable propellant savings. Tiangong station Tiangong has a total of 6 CMGs, mounted on the Tianhe core module, with round parts visible on the side. Proposed As of 2016,