An Overview An Overview of the Recent HST Safings of the Recent HST Safings

Ken Sembach

TIPS PresentationSeptember 20, 2007

20-Sep-2007 Sembach/TIPS 2

ZGSP Safe Mode EntryZGSP 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

20-Sep-2007 Sembach/TIPS 3

Gyro #2 FailureGyro #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 gyrosHST 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 of thin electrical wire (“traditional” flex lead, no silver plating)• Corrosion a function of current, diffusion rates, wire inhomgeneitiesCorrosion a function of current, diffusion rates, wire inhomgeneities• Had improved fluid and fluid fill process (pressurized nitrogen vs. air)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)

20-Sep-2007 Sembach/TIPS 4

Gas-Bearing GyroscopeGas-Bearing Gyroscope

20-Sep-2007 Sembach/TIPS 5

Gyro StatusGyro 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

20-Sep-2007 Sembach/TIPS 6

Gyro Lifetime EstimatesGyro Lifetime Estimates

Predictions from Helen Wong (Aerospace Corp)as communicated to Art Whipple (HSTP)

• Chart below shows approximate gyro failure probabilities assuming a gyro failure in July 2007 (close to time of Gyro #2 failure)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Feb-2007 Feb-2008 Feb-2009 Feb-2010 Feb-2011 Feb-2012 Feb-2013 Feb-2014 Feb-2015 Feb-2016 Feb-2017

Time (calendar date)

Availability

1 or more operating gyros 2 or more operating gyros 3 gyros (2 operating, 1 in-reserve)

Jul-2007

May-2008

Jan-2009 Feb-2011

SM4

20-Sep-2007 Sembach/TIPS 7

Impact to ScienceImpact 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 rescheduledTime 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!

20-Sep-2007 Sembach/TIPS 8

One-Gyro PreparationsOne-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 modeTarget availability should be similar to two-gyro mode• Jitter should be slightly larger than in two-gyro mode, but still very goodJitter 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 SM4Power 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 FGSExternal observations with WFPC2 and FGS

• PSF width and jitter impact (Sirianni: 11077) • FGS astrometry (Nelan: 11078)

• Pointing and control maneuvers (Reinhart: 11345)Pointing and control maneuvers (Reinhart: 11345)• Checkout similar to two-gyro mode checkoutCheckout similar to two-gyro mode checkout

20-Sep-2007 Sembach/TIPS 9

NICMOS SafingNICMOS 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 safingExpected 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 Many (tens of thousands) normal reads between HST safing event or two-gyro recovery and NICMOS safing => unrelated eventsand NICMOS safing => unrelated events

20-Sep-2007 Sembach/TIPS 10

NICMOS Safing (continued)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