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
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
2
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
•
•
Most likely cause: flex lead failure
•
•
•
•
HST had reached approximate date of 50% probability of 4 working gyros
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)
20-Sep-2007
Sembach/TIPS
3
Gas-Bearing Gyroscope
20-Sep-2007
Sembach/TIPS
4
Gyro Status
• Full complement of gyros (6) was replaced during
SM3A in December 1999
• Gyro run times (31-Aug-2007)
•
•
•
•
•
•
G1 45578 hours
G2 57315 hours
G3 33197 hours
G4 53505 hours
G5 16126 hours
G6 12547 hours
- operational, powered
- failed, 31-Aug-2007, flex lead
- failed 29-Apr-2003, rotor restriction
- operational, turned off 28-Aug-2005
- failed 28-Apr-2001, rotor restriction
- operational, powered
• All 6 gyros (3 rate sensing units - RSUs) are slated to
be replaced during SM4
20-Sep-2007
Sembach/TIPS
5
Gyro Lifetime Estimates
• Chart below shows approximate gyro failure probabilities assuming a
gyro failure in July 2007 (close to time of Gyro #2 failure)
1
0.9
SM4
0.8
Jul-2007
Availability
0.7
0.6
Feb-2011
Jan-2009
0.5
0.4
0.3
May-2008
0.2
0.1
0
Feb-2007
Feb-2008
Feb-2009
Feb-2010
Feb-2011
Feb-2012
Feb-2013
Feb-2014
Feb-2015
Feb-2016
Feb-2017
Tim e (calendar date)
1 or more operating gyros
2 or more operating gyros
3 gyros (2 operating, 1 in-reserve)
Predictions from Helen Wong (Aerospace Corp)
as communicated to Art Whipple (HSTP)
20-Sep-2007
Sembach/TIPS
6
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!
20-Sep-2007
Sembach/TIPS
7
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)
•
•
20-Sep-2007
Pointing and control maneuvers (Reinhart: 11345)
Checkout similar to two-gyro mode checkout
Sembach/TIPS
8
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
•
20-Sep-2007
Many (tens of thousands) normal reads between HST safing event or two-gyro recovery
and NICMOS safing => unrelated events
Sembach/TIPS
9
NICMOS Safing (continued)
•
Cause: Single event upset (SEU) affecting the engineering data buffer, a
CPU register, or a memory location
•
•
•
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
•
•
•
“Verified” on VSTIF ops bench
Recommended action was to recover from safe mode
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
20-Sep-2007
Sembach/TIPS
10