STS-133: Discovery delayed ahead of tanking for 24 hours
The Mission Management Team (MMT) have concluded their evalations on Thursday morning by deciding to delay for 24 hours, due to unacceptable weather conditions. The decision came just prior to coming out of the hold for the loading of Discovery’s External Tank (ET-137).
STS-133 Launch Attempt 1 Scrub:
With the flight rationale now in place for the issues suffered by the redundant Main Engine Controller (MEC) on Main Engine 3 (ME-3), the weather was Discovery’s next challenge. Forecasts had placed the acceptable weather as bad as just one percent overnight, with at least 40 percent unacceptable weather conditions for tanking.
MMT Chair Mike Moses hosted the tanking meeting at the Kennedy Space Center (KSC), with the possibility already pre-empted that he could delay 24 hours if the weather showed no signs of giving Discovery a chance to launch later in the day. Options also remained to press ahead with loading the External Tank, prior to calling a scrub throughout the day, including just ahead of coming out of the T-9 minute hold.
For the interim, Discovery remains out in the open, following the retraction of her Rotating Service Structure (RSS) on Wednesday night, bathing in the powerful floodlights for what would be potentially the final time. However, by morning, she was receiving a soaking, as the rain started to pour down around 4am, pre-empting the challenging situation faced by the MMT.
S0007 operations at the pad continued to be on schedule, with only a loose strap requiring the attention of pad crews. Discovery’s Interim Problem Report (IPR) count has risen to 64, although the addtional four problems were all cleared during the morning.
Managers have decided to recyle to T-11 hours at 11:39pm on Thursday evening, with the next MMT meeting set for 5am, where it will be decided whether to tank for the Friday launch attempt or delay further. Friday’s tanking would begin at 5:38am – ahead of a target launch time of 3:03pm Eastern.
Friday’s weather is forecasting 40 percent unacceptable weather – far improved on Thursdays.
Further updates will follow. Refer to this article, the live launch day thread, and the L2 STS-133 Special for full coverage.
STS-133 Specific Articles: http://www.nasaspaceflight.com/tag/sts-133/
Explained Anomalies from Previous Vehicle Flight (STS-131):
Before lifting off on her 39th and final mission, preparations for Discovery’s capstone mission included the drive to close the books on the last few outstanding items from Discovery’s previous flight – STS-131.
In all, the Space Shuttle Program (SSP) identified three (3) unexplained anomalies from the STS-131/19A mission, Discovery’s previous flight (April 5 – 20, 2010). Among these unexplained items are a failure of the MDU MFD1 to successfully port mode to the secondary port, multiple communication errors on Data Bus OI-1, and a pump motor condition sensor voltage anomaly on Fuel Cell 2’s H2 pump motor condition sensor.
According to the FRR (Flight Readiness Review) presentation, “During STS-131 ground processing, MDU (Multifunction Display Unit) MFD1 was unable to communicate with IDP3 on its secondary port.”
While a root cause for the issue could not be identified, engineers suspect a temporary copper path irregularity from the coupler and the MDU as the most likely cause of the failure. In turn, the cause of this irregularity is thought to be “either a partially recessed socket or transient contamination in coupler connector 34P26.”
To combat this problem (prior to STS-131), engineers replaced the suspect 1553 data bus stub cable and processed an MR (Material Review) “due to inability to feed new cable through one clamp.” Spot ties were used to secure cable.
Additionally, the DBC (Data Bus Coupler) mating connection 34P26 was R&Red (Removed and Replaced). The DBC itself, however, was not replaced due to the low likelihood of the coupler itself being the cause of the problem and the extremely invasive nature of a DBC R&R.
Therefore, the root cause of the issue was never determined prior to STS-131. As such, the same rationale that was used to clear the issue for STS-131 (Flight 38) remains in effect for STS-133.
“Flight 38 rationale remains valid, with no increase in risk to safety of flight or mission objectives,” notes the FRR. “Problem only affected MFD1 secondary port bus stub – no other MEDS (Multifunction Electronic Display Subsystem) functionality was affected.”
The MDU MFD1 is a Crit1R3 system, but flexibility and work around options for the loss of MFD1 secondary port during flight are available thanks to the “robust” MEDS system which is “two fault tolerant” throughout flight.
Moreover, the “Program has previously accepted flying with one non-CRT forward station MDU down (ref. LCC DPS01a / BFS-10a).”
In the event of an MDU MFD1 secondary port failure during STS-133/ULF5, there would be no impact to the crew’s “operations, procedures, or situational awareness” since the MFD1 would only switch to the secondary port if the primary port or bus commander failed.
Communications Errors on Data Bus OI-1:
The second unexplained anomaly from Discovery’s 38th flight was in the form communications errors on Data Bus OI-1.
“GPC (General Purpose Computer) logged errors were response to BITE (Built-In Test Equipment) errors annunciated by other LRUs (Line Replaceable Units). Multiple LRUs indicated incoming data errors for 2 simultaneous major frames,” notes the FRR.
At the time of the errors, PCMMU 1 (Pulse Code Modulator Master Unit 1) BITE bit 10 was set and the MDM (Multiplexer-Demultiplexer) OF2, OF3, and OA1 also logged BITE bit 2 errors – including a second occurrence of data invalidity errors on PCMMU 1.
On this second occurrence of the PCMMU 1 data invalidity error there were no GPC I/O (Input/Output) errors or MDM BITE bit failures registered, and no vehicle commanding was occurring at the time.
According to documentation at the time of the first error on March 5, “the four GPCs running in the redundant set all logged an I/O error. Reviewed the data and saw that the PCMMU and several OI MDMs had logged data validity errors.”
Engineers initially believed a data bus of an MIA (Multiplex Interface Adapter) to be the cause of the data invalidity errors, but further investigation and troubleshooting failed to identify root cause.
Nonetheless, three probable causes for the issue were identified: a discrepant MIA (of which there are 8, seven OI MDMs and the PCMMU), a damaged data bus path resulting in a noisy OI data bus, and the internal circuitry of the PCMMU itself.
The decision was then made to launch Discovery on STS-131 in PCMMU 2 instead of PCMMU 1 since the PCMMU is a devise that routes orbiter data to the OPS recorders and the communications systems for downlink to the Mission Control Center. The launch/mission proceeded without further communication and data invalidity errors.
For STS-133 ground processing, further resolution of this issue was attempted. “The 1553 Test Box was installed on OI Data Bus 1 at the data bus coupler near Av Bay 4,” states the FRR. “No anomalies occurred during this monitoring [period], and review of the data revealed no unexpected OI Data Bus 1 failures.”
Likewise, the FRR notes that during testing, PCMMU 1 and OI Data Bus 1 were activated in conjunction with the BFS (Backup Flight Software) GPC 5 (General Purpose Computer 5) in an attempt to gain as much insight into the system as possible in the event that the anomalies reoccurred. They did not.
A recommendation was then put forward and adopted to launch STS-133/ULF5 in PCMMU 2 and fly “as is,” thus mirroring the strategy used for STS-131.
Fuel Cell 2 H2 Pump Motor Condition Sensor Voltage Anomaly:
As described by the STS-133 FRR, “During STS-131 S0007, countdown monitoring after Fuel Cell 2 (FC2) start: the FC2 H2 pump motor condition (PMC) voltage measurement was found to have a gradual rise from 0.62 VDC to 0.98 VDC over approximately 5 minutes without a corresponding change in FC2 current.”
The rise in voltage did not violate the LCC (Launch Commit Criteria) and the Fuel Cells were all cleared for flight. The IRP (In Process Report) was deferred following a Firing Room poll and the inability to identify the cause of the voltage rise.
The most probable cause of the voltage rise was determined to be “Unique AC2 bus loads” registered by FC2’s H2 PMC sensor. However, it was not possible to isolate the anomaly to a single system given the “insufficient Shuttle telemetry of the AC Bus loads.”
During ground processing of OV-103 for STS-133, inverters 4, 5, and 6 were R&Red and troubleshooting of FC2’s PMC revealed that the circuit is “‘hyper sensitive’ to any phase imbalance.”
A phase imbalance can occur due to any load on the associated Bus. As the FRR notes, “This puts the team in the unique situation for launch countdown (of STS-133) of potentially having to troubleshoot a recurrence from the perspective of examining all the associated loads on the Bus.”
(Lead Photograph: Brian Papke, NASASpaceflight.com and MaxQ Entertainment. Others: L2)