Electric Power and Attitude Control on Station Mir
During the summer of 1997, the Russian space station Mir suffered a number of problems, resulting in the total loss of attitude control. A specific inertial attitude is essential for the Mir to keep its solar arrays illuminated by the sun. A rapid energy shortfall, and total loss of power to the station, can result if the arrays are not in a suitable orientation. The usual system of attitude control utilizes sets of momentum wheels, called gyrodynes, spun at high speeds, to provide reactive torques to keep the Mir inertially stable. When an attitude control failure occurs, the angular momentum stored in the gyrodynes is passed into the Mir body as a whole, as the gyrodynes slowly brake their rotation. So even if the Mir loses attitude control in a good orientation with no initial rotation rates, rates build up over the course of hours in all Mir axes, as the gyrodynes spin down.
The immediate problem presented to the crew of Mir was the establishment of a free body rotation of the Mir to provide sufficient solar illumination of the arrays. With some electrical energy restored, contact with the ground controllers can be established, and workarounds developed to replace the attitude control system box that had failed. The solution developed using the Soyuz transport spacecraft was very crude, but effective. The Soyuz docked to the Mir has a power system independent of the Mir, enabling it to be powered even when the station is not. The Soyuz reaction control jets were manually fired to first arrest the resultant rotation caused by the gyrodynes, and then subsequently to reorient Mir, finally followed by a spin of Mir. However, because of the tiny roll moment arm of the Soyuz jets about the Mir long axis, rotation about this axis could not be influenced, complicating the resultant rotation dynamics of Mir.
Much time was spent by the crew considering the effects of initial rotations on the resultant rotation dynamics of Mir. This paper presents the equations and visualization model that permit the correct rotation impulse strategy to be applied.
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