CMGs; as long as some rotors are aligned parallel along a particular axis, and all the others point in exactly the opposite direction, there is no saturation but still no roll control about that axis. With three or more CMGs the situation can be immediately rectified simply by redistributing the existing total angular momentum among the rotors (even if that total is zero). In practice the CMG control program will continuously redistribute the total angular momentum to avoid the situation arising in the first place. If there are only two CMGs in the cluster, as in our first example, then anti-parallel alignment will inevitably occur if the total stored angular momentum reaches zero. The remedy is to keep it away from zero, possibly by using RCS firings. This is not very satisfactory, and in practice all spacecraft using CMGs are fitted with at least three. However it sometimes happens that after malfunctions a cluster is left with only two working CMGs, and the control program must be able to deal with this situation. Hitting the gimbal stops Older CMG models like the ones launched with Skylab in 1973 had limited gimbal travel between fixed mechanical stops. On the Skylab CMGs the limits were plus or minus 80 degrees from zero for the inner gimbals, and from plus 220 degrees to minus 130 degrees for the outer ones (so zero was offset by 45 degrees from the centre of travel). Visualising the inner angle as 'latitude' and the outer as 'longitude', it can be seen that for an individual CMG there were 'blind spots' with radius 10 degrees of latitude at the 'North and South poles', and an additional 'blind strip' of width 10 degrees of 'longitude' running from pole to pole, centred on the line of 'longitude' at plus 135 degrees. These 'blind areas' represented directions in which the rotor's spin axis could never be pointed. 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. This ensured that the six 'polar blind spots' were spaced 90 degrees apart from each other. 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. The whole arrangement ensured that the 'blind areas' of the