Re: ISS Gyro Problems

From: Mgr. CSc. Antonín Vítek (AVitek@seznam.cz)
Date: Sun Dec 07 2003 - 03:10:08 EST

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    ================= Původní zpráva =================
    Od (From): "Michael DiMuzio" <mikedimuzio001@sbcglobal.net>
    Komu (To): "SeeSat-L" <SeeSat-L@satobs.org>
    Kopie (Cc): 
    Předmět (Subject): ISS Gyro Problems
    Datum (Date): 7. 12. 2003 8:06
    ==================================================
    
    > They are getting current spikes and vibration in one of the three
    > remaining gyros.
    > 
    > http://www.cnn.com/2003/TECH/space/12/05/spacestation.thursters.ap/index.html
    > 
    
    This info (about unexpected telemetry pattern received from CMG-3 on 2003-11-08) is not new for those, who regularly read NASA's ISS On-Orbit Status Reports (Those reports can be found either on http://www.spaceref.com or on a mirror at my pages http://www.lib.cas.cz/knav/space.40/MRPO/INDEX.HTM ).
    
    Quoting:
    
    ISS On-Orbit Status 15 November 2003
    
    On 11/8, when a manual CMG (control moment gyroscope) desaturation was commanded after transitioning from LVLH to XPOP attitude, the ground observed a previously unseen vibration signature on CMG-3. The signature lasted for approximately 30 minutes; upon returning from a 12-min LOS, the vibration status had returned to normal. No impact on current operations. 
    
    ISS On-Orbit Status 26 November 2003
    
    At 10:56am, the ISS performed its attitude transition maneuver from sun-pointing XPOP (x-axis perpendicular to orbit plane) to earth-oriented LVLH (local vertical/local horizontal). Planning of the maneuver has emphasized conservatism to minimize the gimbal rates on the control moment gyros (CMGs). [Recap: On 11/8, CMG-3 exhibited a previously
    unobserved vibration. Further analysis revealed other abnormal vibration responses on CMG-3, all associated with CMG desaturations (by Russian thruster firings). While the direct cause of the vibrations is not understood at this time, it is clear that limiting the gimbal rates associated with desats is less stressful on the CMG bearings and may aid in mitigating the vibration-causing mechanism.] 
    
    ISS On-Orbit Status 2 December 2003
    
    CMG Update: For continued monitoring of the U.S. CMGs (control moment gyroscopes), MCC-H will initiate tomorrow an on-board data "dump" to capture high-rate data for CMG-2, -3 and -4 (CMG-1 is failed). [Downlink of the data dump is to be performed if vibration on any CMG reaches the value of 0.08 g (eight hundredth of a G), even for a single pulse
    (spike). If there is a LOS period (loss-of-signal, i.e., no-comm) within three hours of a spike exceeding 0.08g, the data recorded for that period on the ZOE (zone-of-exclusion) recorder are dumped. These procedures will remain in force until 12/9. The continued monitoring is attempting to characterize the 11/8 event of a previously unseen vibration signature on
    CMG-3.] 
    
    ISS On-Orbit Status 4 December 2003
    
    CMG Update: Status of and plans for the three U.S. control moment gyros (CMGs) were discussed with Russia yesterday at a joint operations planning (JOP) meeting. Both sides agreed that ISS attitude hold and all attitude maneuvers will be performed on Russian thrusters only between now and mid-January. The CMGs will be available for attitude hold if needed, but their use for this will be kept at a minimum. Meanwhile, specialist teams will evaluate CMG data and provide new recommendations for CMG constraints by mid-January. Propellant use for this ops mode still needs to be assessed by Moscow but is anticipated to be small ("less than 25 kg"), a minor impact in light of the large propellant amounts currently stored in the FGB, SM and Progress (~ 3700 kg). 
    
    End quote
    
    I think, that this source of information (NASA's ISS On-Orbit Reports) is more relevant ("ad fontes"), than news reports, filtered through journalists' heads. 
    
    BTW, for full attitude contral and stabilization of ISS only two CMGs are needed.
    
    
    
    Mgr. Antonin Vitek, CSc.
    
    Office: Main Library, Academy of Sciences of the Czech Republic
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