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An Overview of the Recent HST Safings Ken Sembach TIPS Presentation September 20, 2007

An Overview of the Recent HST Safings Ken Sembach TIPS Presentation September 20, 2007. ZGSP Safe Mode Entry. HST entered zero-gyro sun-point safe mode at ~7 PM EDT on August 31, 2007 Timeline of event 243/22:43:00 LOS 243/22:51:59 Enter orbit night

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An Overview of the Recent HST Safings Ken Sembach TIPS Presentation September 20, 2007

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  1. An Overview of the Recent HST Safings Ken Sembach TIPS PresentationSeptember 20, 2007

  2. ZGSP Safe Mode Entry • HST entered zero-gyro sun-point safe mode at ~7 PM EDT on August 31, 2007 • Timeline of event • 243/22:43:00 LOS • 243/22:51:59 Enter orbit night • 243/22:53:33 Two-Gyro transition to M2G mode • 243/22:53:33 Type 2 vehicle slew (13 min 33 sec duration) • Gyro Bad Disparity count exceeded limit of 1500 counts • 243/23:01:54 Two-Gyro configuration failed (Zero-Gyro Drift Mode – AD commanded closed) • 243/23:27:06 Enter orbit day - Sun Bright Object Protection failed • Begin Zero-Gyro control • 243/23:31 Expected AOS • 243/23:39 Acquired telemetry • 244/03:51 Engineering data dumped from recorder • 244/05:52 OR# 18123-1 Power on Gyro 6 Sembach/TIPS

  3. Gyro #2 Failure • HST was preemptively placed in two-gyro mode in August 2005 to conserve gyro lifetime • Gyro #2 failure • HST responded as expected • Eventual failure of Gyro #2 was anticipated • HST had reached approximate date of 50% probability of 4 working gyros • Most likely cause: flex lead failure • Corrosion of thin electrical wire (“traditional” flex lead, no silver plating) • Corrosion a function of current, diffusion rates, wire inhomgeneities • Had improved fluid and fluid fill process (pressurized nitrogen vs. air) • Restart of Gyro #2 was deemed infeasible (failure permanent) • Gyro #6 was powered on at ~2 AM EDT on 1-Sep-2007 • Gyro #6 had been turned off early in its lifetime • Gyro #6 shows some bias drift (noise) • Gyro #6 bias is settling slowly (1-2 months expected) Sembach/TIPS

  4. Gas-Bearing Gyroscope Sembach/TIPS

  5. Gyro Status • Full complement of gyros (6) was replaced during SM3A in December 1999 • Gyro run times (31-Aug-2007) • G1 45578 hours - operational, powered • G2 57315 hours - failed, 31-Aug-2007, flex lead • G3 33197 hours - failed 29-Apr-2003, rotor restriction • G4 53505 hours - operational, turned off 28-Aug-2005 • G5 16126 hours - failed 28-Apr-2001, rotor restriction • G6 12547 hours - operational, powered • All 6 gyros (3 rate sensing units - RSUs) are slated to be replaced during SM4 Sembach/TIPS

  6. SM4 Jul-2007 Feb-2011 Jan-2009 May-2008 Gyro Lifetime Estimates • Chart below shows approximate gyro failure probabilities assuming a gyro failure in July 2007 (close to time of Gyro #2 failure) Predictions from Helen Wong (Aerospace Corp) as communicated to Art Whipple (HSTP) Sembach/TIPS

  7. Impact to Science • Safing occurred near end of week (late Friday) • ~25 orbits of science deferred (see Adler talk) • Health and safety SMS loaded 1-Sep-2007 (Saturday) • Science SMS loaded 2-Sep-2007 (Sunday) • All WFPC2 • Time critical WFPC2 orbits from previous week rescheduled • No NICMOS orbits because of unrelated NICMOS safing event • Congratulations to entire team here and at GSFC on speedy recovery! Sembach/TIPS

  8. One-Gyro Preparations • One-gyro mode is in an advanced stage of preparation, work begun in mid-2005 • To be used only if necessary • Target availability should be similar to two-gyro mode • Jitter should be slightly larger than in two-gyro mode, but still very good • Initial PCS estimates indicate <10 mas persistent disturbance jitter (60-sec, RMS) • Power on Gyro #4 if either Gyro #1 or Gyro #6 fails prior to SM4 • Gyro #2 failure has not significantly altered one-gyro work schedule • On-orbit test planned during week of 28-Jan-2008 • External observations with WFPC2 and FGS • PSF width and jitter impact (Sirianni: 11077) • FGS astrometry (Nelan: 11078) • Pointing and control maneuvers (Reinhart: 11345) • Checkout similar to two-gyro mode checkout Sembach/TIPS

  9. NICMOS Safing • NICMOS entered safe mode at ~11 PM EDT on 1-Sep-2007 • NICMOS was in SAA/Operate mode as a result of previous HST safing • No NICMOS activities at the time of event • HST was not in SAA at time of safing • HST power profile nominal at time of safing • Expected PDU3 current drop seen in response safing • NICMOS telemetry and NED telemetry prior to safing appeared normal • No out-of-family issues with ACS or WFPC2 at time of safing • Error: A/D FIFO empty prior to reading all expected data • Appropriate response is to safe instrument • Buffer read every 0.5 seconds • Many (tens of thousands) normal reads between HST safing event or two-gyro recovery and NICMOS safing => unrelated events Sembach/TIPS

  10. NICMOS Safing (continued) • Cause: Single event upset (SEU) affecting the engineering data buffer, a CPU register, or a memory location • “Verified” on VSTIF ops bench • Recommended action was to recover from safe mode • NICMOS was brought up to SAA/Operate mode • Telemetry nominal • Normal engineering data process functioned as expected • Buffer box temperatures were raised to nominal values • Had been running cold due to NICMOS inactivity • Brief transition to Operate and back to SAA/Operate to refresh buffer box telemetry • NICMOS science resumed with SMS loaded 9-Sep-2007 Sembach/TIPS

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