re: Mir orbital control

John Locker (Jan.Vansteelandt@agr.kuleuven.ac.be)
Thu, 21 Aug 97 10:47:13 CET

Willie Koorts <wpk@saao.ac.za> Wrote:

|..... made me wonder just how much efford 
| is required to keep
| the station under control? 
| but  it seems like the station requires a lot of power and/or 
| computing power to
| keep it from spinning out of control. Why is this? Why are 
| thrusters needed
| to get it under control again? What is the main source of 
| "orbital control"
| for a spacecraft of this size? 

I' am not a space expert, but I'll give it a try....

As far as I know, the main source of spinning-control for Mir exists in 
inertion wheels. 
Power and computer are required to keep this wheels spinning.
When the wheels are spinning, they cause a large circular momentum. 
As a consequence, large forces are required to change the direction of that 
circular momentum. In other words: the ship is spin and tumble stabilized.
When power interups and wheels are slowing down, one can imagine
that Mir starts to spin and tumble. Solar pannels are then not longer pointed 
to the sun. To correct that behaviour, thrusters are to be used. 
This is also what happened on july 17th, when the power cable in Mir was 
interupted. 
Solar panels were than held in position by igniting the thrusters of the 
docked 
Soyuz spaceship for several seconds. This can not be done for to long, since 
Soyuz
needs a minimum amount of fuel for an evacuation back to earth. On july 18th 
at 06h30, the
inertion wheels worked again. Soyuz had spare fuel to maintain the correction 
burst till 13h the
same day.... 
So the cosmonauts only had 8.5 hours left. Otherwise, they had to return to 
earth with