Is the ISS actually stable?

[chris_in_space]

I've heard recently that the ISS wasn't stable, which means that due to it's layout it gradually starts spinning around itself. Which means that the attitude thrusters have to come into action very often to correct this. I've heard that they have to come into action every 20 seconds or so. It also would mean that every 20 seconds the microgravity environement is being disturbed.<br /><br />So does anyone have some details one this. Is the ISS really that unstable? And does anyone know how often the attitude thrusters get fired for corrective measures?<br /><br />Thanks.

 

[najab]

The thrusters get used very rarely now that the control moment gyro system is back up and fully functional.

 

[tap_sa]

How about adding some controllable 'sails' to the structure, control the drag and use to maintain attitude. Even CMGs get saturated and need thrusters help to bleed momentum off. Btw does ISS always have the same side facing the Earth, if so then gravity boom to control that. Few modifications more and it begins to look like Cloud City <img src="/images/icons/wink.gif" />

 

[CalliArcale]

"Every 20 seconds" seems improbable to me, since ISS doesn't need absolutely perfect orientation. It needs correct orientation, but it doesn't need the kind of precision used on space telescopes and things like that. It's also worth pointing out that it is put into a "free drift" mode before any docking (by Shuttle, Progress, or Soyuz), shutting off its attitude control system (whether gyros or thrusters) to allow the docking craft to be in control of the approach and not worry about the target suddenly shifting, possibly causing a collision. It stays that way for minutes without noticably yawing/pitching/rolling, so I don't think it ordinarily has to fire every 20 seconds. There may be special circumstances where it needs to, however.<br /><br />The ISS's orbit is unstable, though -- mainly due to the effects of the extremely thin ionosphere. IT has to be reboosted periodically to counter the cumulative effects of drag. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>