three CMGs never overlapped, and thus that at least two of the three rotor spins could be pointed in any given direction. The CMG control program was responsible for making sure that the gimbals never hit the stops, by redistributing angular momentum between the three rotors to bring large gimbal angles closer to zero. Since the total angular momentum to be stored had only three degrees of freedom, while the control program could change six independent variables (the three pairs of gimbal angles), the program had sufficient freedom of action to do this while still obeying other constraints such as avoiding anti-parallel alignments. One advantage of limited gimbal movement such as Skylab's is that singularities are less of a problem. If Skylab's inner gimbals had been able to reach 90 degrees or more away from zero, then the 'North and South poles' could have become singularities; the gimbal stops prevented this. More modern CMGs such as the four units installed on the ISS in 2000 have unlimited gimbal travel and therefore no 'blind areas'. Thus they do not have to be mounted facing along mutually perpendicular directions; the four units on the ISS all face the same way. The control program need not concern itself with gimbal stops, but on the other hand it must pay more attention to avoiding singularities. Applications Skylab Skylab, launched in May 1973, was the first manned spacecraft to be fitted with large CMGs for attitude control. Three dual-gimbal CMGs were mounted on the equipment rack of the Apollo Telescope Mount at the hub of the windmill-shaped array of solar panels on the side of the station. They were arranged so that the casings (and therefore the rotors when all gimbals were at their zero positions) pointed in three mutually perpendicular directions. Since the units were dual-gimballed, each one could produce a torque about any axis at right angles to its rotor axis, thus providing some redundancy; if any one of the three failed, the combination of the remaining two could in general still produce a torque around any desired axis. Gyrodynes on Salyut and Mir CMGs were used for attitude control on the Salyut and Mir space stations, where they were called gyrodynes (from the Russian гиродин girodin; this word is also sometimes used – especially by Russian crew – for the CMGs on the ISS). They were first tested on Salyut 3 in