amount of forward angular momentum has been drained out of the CMG rotors; it is transformed into the moment of momentum of the moving matter in the RCS thruster exhausts and carried away from the spacecraft. It is worth noting that "saturation" can only apply to a cluster of two or more CMGs, since it means that their rotor spins have become parallel. It is meaningless to say that a single constant-speed CMG can become saturated; in a sense it is "permanently saturated" in whatever direction the rotor happens to be pointing. 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. Anti-parallel alignment There are other undesirable rotor axis configurations apart from saturation, notably anti-parallel alignments. For example, if a spacecraft with two dual-gimbal CMGs gets into a state in which one rotor spin axis is facing directly forward, while the other rotor spin is facing directly aft (i.e. anti-parallel to the first), then all roll control will be lost. This happens for the same reason as for saturation; the rotors can only produce gyroscopic torques at right angles to their spin axes, and here these torques will have no fore-and-aft components and so no influence on roll. However, in this case the CMGs are not saturated at all; their angular momenta are equal and opposite, so the total stored angular momentum adds up to zero. Just as for saturation, however, and for exactly the same reasons, roll control will become increasingly difficult if the CMGs even approach anti-parallel alignment. In the anti-parallel configuration, although roll control is lost, control about other axes still works well (in contrast to the situation with saturation). 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. In fact the situation will be improved, because the rotor axes are no longer quite anti-parallel and some roll control will be restored. Anti-parallel alignment is therefore not quite as serious as saturation but must still be avoided. It is theoretically possible with any number of