Previous In-Flight Anomalies Evaluated for STS-129
As Atlantis’ launch date nears, engineers, Space Shuttle Program managers, and Agency personnel are continuing their analysis of the Main Engine Ignition acoustic environment on the Stinger pods of orbiter Atlantis – an analysis they are able to devote a great deal of time to thanks to an issue-free pad flow and excellent prior mission performances of orbiters Atlantis (STS-125) and Discovery (STS-128).
In-Flight Anomalies from Previous Vehicle Flight (STS-125):
Perhaps the most prominent In-Flight Anomaly experienced by Atlantis during her flagship mission to the Hubble Space Telescope in May was the failure of the ASA-1 (Aerosurface Servo-amplifier Assembly channel One) system.
“Immediately before launch, all aerosurface servo-amplifier assembly (ASA) channel one positions went off scale low,” notes the Orbiter Project Office’s (OPO’s) presentation to the SSP FRR in mid-October.
As a result, the Remote Power Controllers in Atlantis were tripped approximately 1-second after SRB (Solid Rocket Booster) ignition – which caused the annunciation of the Caution and Warning alarm on Atlantis’ Flight Deck.
Given the redundancy in the system, Atlantis’ crew was told to bypass the ASA-1 system. Flight Controllers monitored the remaining three ASA channels throughout Atlantis’ mission – as the failure of two ASA channels would have meant an early termination of the flight.
After Atlantis’ landing at the Dryden Flight Research Center, limited troubleshooting was performed – troubleshooting that did not reveal any shorts in the ASA system.
After Atlantis was ferried back to the Kennedy Space Center, the ASA-1 was Removed and Replaced (R&Red), with Hi-pot testing of the Right Hand (RH) inboard elevon’s primary delta pressure transducer circuit indicating a “short to ground.”
All other ASA-1 circuits with “400 Hz excitation voltage were Hi-pot tested.” No further anomalies were detected.
However, troubleshooting did not end there. “Further isolation and inspection found discrepant wire harness chafing against an adjacent Hi-Lok fastener in the actuator cavity region,” notes the OPO presentation – available for download on L2.
These wire harnesses were removed for failure analysis, which revealed the cause of the ASA-1 failure to be an “exposed conductor within the wire harness shorting to the Hi-Lok fastener.”
Furthermore, the Test, Teardown, and Evaluation (TT&E) and failure analysis at the NASA Shuttle Logistics Depot (NSLD) of ASA-1 verified that the short was external to the LRU – which caused the failure of the ASA-1 excitation card,” notes the OPO presentation.
Following the replacement of this card, ASA-1 was subjected to an ATP (Acceptance Test Procedure) vibration test. No shorts were monitored during this testing.
Overall, Flight Rationale for the ASA system has been documented and accepted by the Space Shuttle Program (SSP) and the Space Operations Mission Directorate (SOMD), which both cleared the issue for STS-129.
Given that the damaged wire harnesses were identified and replaced, that ASA wire redundancies were assessed and deemed to be well within safety standards, and that extensive, hands-on investigations of the redundancy systems were carried out, the ASA system is not expected to be an issue for Atlantis and STS-129.
Nevertheless, during these hands-on inspections, “two areas of redundancy routing violations were identified and corrected, remaining Right and Left Hand inboard/outboard elevon actuator harnesses were inspected for damage, and overbraid damages were repaired,” notes the OPO presentation.
Additionally, harness routing locations in the RH and LH elevons were re-documented and further baseline wire inspections were performed in the wings, vertical stabilizer, main landing gear wheel wells, and struts to “increase confidence in overall system integrity.”
In all, 27 Problem Report “conditions” were detected and repaired on OV-104 (Atlantis) during these wiring inspections. As such, Atlantis is cleared for flight on STS-129.
In addition to the ASA-1 issue, turnaround flow engineers faced another issue with Atlantis – in the form of a “quick show mount knob” that was found to be wedged between pressure pane #5 and the console dashboard close-out panel.
Engineers and technicians tried several times (using several different methods) to remove the wedged knob.
“After a number of methods were utilized to remove the knob without inducing further damage to the pressure pane, the knob was successfully removed without further damage to the pressure pane by chilling the knob while employing mechanical motion and pressurizing the crew compartment (2-3 psig).”
This pressurization of the crew compartment was attempted after careful consideration and analysis on the possible effects of pressurizing the crew compartment at sea level.
Originally, it was thought that pressurizing the crew compartment would not be possible because of over press concerns. However, the low pressurization rate was approved in order to provide more room between the pressure pane and the console dashboard close-out panel.
This procedure was approved as part of the effort to free the knob – which, based on engineering safety margins, had to be removed prior to the next flight of orbiter Atlantis.
Nevertheless, once the knob was removed, an extensive inspection and analysis of the damage induced to the pressure pane from the knob’s edge was undertaken to ensure that flight safety margins on the pressure pane had not been breached.
“Mold impressions were taken and the larger of the defects to the pressure pane measured 0.00035 inches deep,” notes the OPO presentation. “Per the MT0501-514 spec; scratches, dings, and impact with depths exceeding 0.0015 inches are reportable to Design Engineering.”
This proved to be extremely positive news as the defect was not large enough to mandate the replacement of pressure pane #5 – an endeavor that would have taken at least one year to accomplish and could have resulted in Atlantis’ permanent removal from flight status.
Based on mold impressions taken of the damage site, the Micro Inspection Team (MIT) was able to confirm the integrity of the pressure pane and thus clear this issue for flight.
In-Flight Anomalies from Previous Program Flight (STS-128):
Highlighting the remarkable job by ground processing engineers was the near complete lack of IFAs carried forward from the previous program flight – STS-128 and the Space Shuttle Discovery.
In fact, aside from the normal review of the Thermal Protection System (TPS), only two IFAs from STS-128 were discussed during the FRR season for STS-129.
The first of these IFAs was, in fact, a ground issue experienced by the Launch Team during the second STS-128 launch attempt.
“During the transition to reduced fast fill (85 percent), the LH2 (Liquid Hydrogen) inboard fill and drain valve (PV12) closed position indicator (PI) did not come on when the valve was commanded closed,” notes the OPO presentation.
This was a violation of Launch Commit Criteria (LCC) MPS-04 (Main Propulsion System 04). As such, a 48-hour scrub was called and an ambient cycling test of the PV-12 valve performed once ET (External Tank) boil-off was complete.
During the ambient test, the PV-12 valve functioned as designed, with no anomalies detected.
“Data obtained following ET drain, along with historical valve cycle timing and low pressure actuation test data concluded that the failure was most likely the valve position indicator and not a failure of the valve to cycle properly.”
As such, an LCC deviation was accepted by the Mission Management Team (MMT) for STS-128 – which would have allowed for the launch of STS-128 if the Launch Team could have verified the closure of the PV-12 valve even if the sensor issue reappeared.
However, the LCC deviation proved unnecessary as the PV-12 valve and associated sensors worked perfectly during the third and final launch attempt for STS-128.
Post-flight borescope inspections identified “contamination under the close micro-switch arm,” notes the OPO presentation. “Contamination dislodged during removal attempt; other retrieved material consisted of putty with embedded foam and metal shaving.”
Similar borescope inspections were performed on Atlantis, which revealed that OV-104′s PV-11 valve was worn where contact with the actuator cam occurs. There was also a minor “ding” noted on the PV-9 valve.
The PV-9 valve “ding” was accepted for flight, while the PV-11 valve was R&Red.
The second issue from STS-128 brought forward to the STS-129 FRR was that of the Forward Reaction Control System (FRCS) F5R thruster.
“During the initial usage of Vernier RCS (VRCS) on the evening of FD-1, the VRCS thruster F5R fuel and oxygen injector temperatures began to drop. The F5R thruster was deselected as Fail Leak when temperatures dropped below 130 deg F limit.”
In all, there was no indication of “safety of flight” issues and all flight rules were followed during STS-128.
Since the VRCS is not mandatory for ISS/Shuttle rendezvous and docking operations – and since no generic fault could be identified for the failure of the F5R thruster on Discovery – there is no constraint for STS-129 and Atlantis.
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.