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Re-reading Linenger's Off the Planet and encountered a question from long ago that I never had found an answer to.

During Mir's troubled old age, the attitude control computer would fail frequently. The station then immediately started rotating and the solar panels stopped tracking the sun. This resulted in a long, multi-orbit process of slowly recharging the batteries and essentially rebooting the station.

But...why did Mir start to rotate immediately when the computer failed? The lack of control in and of itself wouldn't cause the station to rotate. There must have been some external torque that the system was constantly compensating for, but what?

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    $\begingroup$ Interesting question, I'm thinking something to do with gyrodynes disengaging and not being able to maintain gravity gradient or star-fixed attitude mode, whichever it was in before the failure. If some were designed to also lock and desaturate with power failure, that would immediately transfer all their stored momentum onto the station and spin it. $\endgroup$ – TildalWave Dec 31 '15 at 13:13
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    $\begingroup$ Personally, I believe it was a "wind turbine" effect. MIR was in a rather low orbit with a significant air drag. Uneven air drag would quickly cause a spin. $\endgroup$ – SF. Dec 31 '15 at 16:51
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    $\begingroup$ I can see two things: first, friction on the gyro will not be compensated anymore and result in immediate and continues spin increase. Second is that the rotating might be toward the earth plan of reference and not Mir. To keep nadir always towards the earth the station must rotate all the time. If the rotation stop, Mir will stop roaring relative to its Center but rotate relative to the earth. $\endgroup$ – Antzi Dec 31 '15 at 16:52
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    $\begingroup$ PearsonArtPhoto's answer is correct, no more power to reaction wheels means frictions transfers the angular momentum to the spacecraft. $\endgroup$ – Brian Lynch Dec 31 '15 at 17:25
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    $\begingroup$ @Sean if memory serves it acted like it lost power. If I knew that it send out a continuous attitude rate command, the answer to this question would be self-evident. $\endgroup$ – Organic Marble May 20 at 4:13
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Most such spacecraft, including Skylab and the ISS, have their attitude maintained by reaction wheels. These wheel essentially convert the rotational energy of the entire spacecraft into a smaller reaction wheel. If the attitude failed while these wheels were spun up, then they would eventually slow down (In a timeframe of minutes). As the wheels slowed down, the station would begin rotating.

MIR did have reaction wheels, as can be seen from this site.

It is very common for these wheels to be spun up, as the attitude of the station must be properly maintained for thermal and power reasons, and varies depending on where in the orbit it is found.

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    $\begingroup$ I will need to look at it again, but ISTR that the tumble started when the computer failed, not when the power failed. (The sequence was more computer fail => tumble => power off) It's not clear to me why the computer failing would cause the gyrodynes to spin down immediately. $\endgroup$ – Organic Marble Dec 31 '15 at 20:00
  • $\begingroup$ It wouldn't surprise me that if the computer spins down it forces the gyros to spindown. $\endgroup$ – PearsonArtPhoto Dec 31 '15 at 20:06
  • $\begingroup$ Agreed, that would be bad design. Not that bad design doesn't happen. $\endgroup$ – Organic Marble Dec 31 '15 at 20:42
  • $\begingroup$ Not necessarily. It could be required to calibrate the system, for instance. $\endgroup$ – PearsonArtPhoto Dec 31 '15 at 22:19
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    $\begingroup$ I think the way events played out demonstrated that it was a poor design. Anyway, I'm fairly sure your answer is correct, but I will wait a bit to see if anyone can provide a reference showing that a computer failure resulted in an immediate effect to the gyrodynes, while they were still powered. $\endgroup$ – Organic Marble Dec 31 '15 at 23:12
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It doesn't have to be an external torque, consider for example the crew changing position inside the space station, although the maximal torque provided by moving the entire crew to the Kvant-1 is only going to change the rotation rate by 0.15%.

Note: This is not violating the conservation of momentum, as they are simply changing the centre of mass in an already rotating station. (A space station does usually have one side always facing in the orbital direction.)

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    $\begingroup$ What would happen, if the attitude control failed and the crew didn't move and there was no external torque? Would the station rotate even then? $\endgroup$ – DP_ Dec 31 '15 at 16:52
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    $\begingroup$ This does violate conservation of momentum! There is no way a couple of astronauts will change the inertia enough to cause a significant spin-up or spin-down a la figure skater analogy. Any "velocity change" has to cancel out when the astronaut stops moving from one spot to another anyway. $\endgroup$ – Brian Lynch Dec 31 '15 at 17:29

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