After eight full days of docked operations, Discovery has undocked from the International Space Station, bringing the joint operation phase of the 30th construction/utilization flight of the Space Shuttle to the orbiting outpost to a close. Later, Discovery’s crew will perform the customary OBSS (Orbiter Boom Sensor System) late inspection of their ship’s Wing Leading Edge RCC panels and nose cap ahead of a scheduled landing on Thursday evening at 7:05P.M. EDT.
As has been documented throughout STS-128, Discovery continues to perform exceptionally well on orbit as she nears the end of her 37th voyage to space.
“The mission is proceeding nominally and per the timeline,” notes this morning’s NTD status report on L2. “Landing is scheduled for Thursday, September 10th at 1905L at KSC.”
All FD-8 through FD-11 activities were completed over the holiday weekend – with MPLM Leonardo safely berthed in Discovery’s payload for the return trip to Earth.
Furthermore, the NTD report notes that “EVA3 by Olivas and Fuglesang to route cables for Tranquility Node 3 installation, replace the Rate Gyro Assembly on the S0 truss, and install two GPS antennas on the S0 truss was completed on Friday” without major complications.
Additionally, the Mission Evaluation Room is tracking only one additional IFA (In-Flight Anomaly) at this time: the TCS (Trajectory Control System) bearing calculation error.
There are also four new funnies since FD-7. An “unacceptable” Comm Carrier Assembly, the Right Outboard Main Landing Gear unexplained Brake Line temperature drop and recovery, the unavailability on the ground of the digital TV Downlink, and the Hydrogen flowmeter transient drops in Fuel Cell #1 (FC-1) were all noted by the Mission Evaluation Room.
None of these funnies are a constraint to the continued flight of Discovery and are not a constraint to landing later this week.
Moreover, the FC-1 issue was noted to be instrumentation only and the digital TV downlink error will undergo troubleshooting tomorrow to determine if the failure is a Orbiter hardware, ground hardware, and/or configuration issue.
Also, a separate presentation pertaining to the Right Outboard Main Landing Gear brake Line Temperature issue — available for download on L2 — shows that all temperature margins have been recovered.
While the exact cause of the temperature drop has not yet been ascertained, the presentation does note that brake line heaters are installed on the “full length of lines and flexhoses” in the landing gear assembly.
Furthermore, each hydraulic line has three, redundant, “spiral wrapped heaters” to prevent the complete loss of heating capability.
Thus far, this issue has only been classed as a “funny” and will not be a concern for landing.
Additionally, the MMT (Mission Management Team) meeting yesterday discussed the FC-2 KOH transient level anomaly — which was observed on not only this flight, but also STS-127 in July.
“Have observed intermittent (instantaneous) drops in the FC-2 inlet KOH concentration below the 29% concentration rule,” notes the MMT presentation.
These transients have been seen multiple times since the SSPTS (Station to Shuttle Power Transfer System) was activated. Furthermore, these readings “correspond to transients in shuttle vehicle power level often associated with cryo tank O2 heater cycles,” notes the presentation.
The current flight rule, which protects for the safe operation of a Fuel Cell with a 29% KOH limit, assumes a 5% protection for possible instrumentation errors — thus providing for Fuel Cell operation with a 24% KOH level.
“Error measured during ATP (Acceptance Test Plan/Procedure) for this fuel cell was less than 1% on all parameters used in computing cell KOH concentration.”
As a result, transients below the 29% level are not a concern for the continuation of Discovery’s mission per the nominal timeline.
As such, a CHIT was written to update the KOH limit to “24% average” — which would be consistent with the rationale used in July during STS-127.
However, if the KOH concentration approaches 24%, “EGIL (Electrical, General Instrumentation, and Lighting Engineer) will increase FC-2 load in an attempt to raise the KOH concentration,” notes the MMT presentation.
Lastly, Mission managers and ISS managers discussed the possibility of shutting down the SSPTS system on FD-10 in order to ensure the transfer of three “technical water CWCs (Contingency Waste Collections)” prior to undocking.
This option was ultimately decided against as it would have negated the possibility of preserving the +1 docked day for Discovery.
As such, one CWC was transferred with the remaining two eliminated from the timeline to preserve the +1 docked day.
Undocking and Post-Undocking Considerations:
Given the failure of Discovery’s Vernier Reaction Control System (VRCS) on FD-2, the mission plan — in terms of undocking — was altered accordingly.
Maneuvering of the Shuttle/Station complex into the proper undocking orientation was accomplished using Discovery’s Primary RCS in the ALT DAP mode.
“Nominal Undock Plan (VRCS failed) – PRCS: No Low Z from undock to 75′, then select Low Z >75′,” note the MMT FD-11 presentation.
ISS loads are acceptable if No Low Z is required for Orbiter control out to 75 feet.
Further, the Solar Array orientations were updated to maintain loads below 50%. The Solar Alpha Rotary Joint angles were not be changed, but six of eight (instead of four of eight) Beta Gimble Assemblies were “feathered” during undocking
Moreover, as noted on yesterday’s MMT report, flight controllers are currently tracking a possible “repeater” conjunction with Fengyun debris.
Currently, it is estimated that the Time of Closest Approach (TCA) will be approximately 13-hours after undocking (or, around 4:30A.M. EDT on Wednesday).
The miss distance of this debris from Discovery and the ISS is approximately 57.4 km for the first TCA and 25.03 km for the second TCA.
“- TCAs: 252/08:37 (Wed, Sep 9, 04:37 am EDT) and one orbit later at 252/10:08 GMT (06:08 am EDT),” notes the MMT presentation.
As of yesterday, the miss distances looked to be improving, however, “very large uncertainties exist due to the decaying features of the debris each revolution.”
However, no further word has been issued as of this publication as to whether or not a Debris Avoidance Maneuver will be required for Shuttle and/or Station.
Lastly, Discovery’s SSPTS has provided mission controllers and Discovery’s Flight Crew with an extra contingency day post-undocking thanks to the cryo (fueling) saving system.
The new mission duration plan shows a 13+1+3 day mission (an increase from the pre-launch 13+1+2 day mission), giving Mission Control an additional landing day should weather prove an issue on Thursday, Friday, and Saturday.
Right now, it appears that Discovery’s onboard cryo will be the limiting factor for exactly how long she can stay on orbit after her nominal End Of Mission day — which is Thursday. Currently, cryo margins above the 13+1+3 day baseline are less than one 24-hour cycle.
L2 members: Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.