Discovery docks with ISS following successful Ku Band workaround

by Chris Gebhardt

Discovery and her seven member crew have arrived at the International Space Station (ISS) following a visually stunning R-bar Pitch Maneuver (RPM) 600 feet below the orbiting science laboratory. Following pressurization and leak checks of the ISS’s PMA-2 (Pressurized Mating Adaptor-2) and Discovery’s ODS (Orbiter Docking System), hatches between Discovery and the ISS will be opened at ~5:44a.m. EDT.

Flight Day 3:

Also on the docket for Flight Day 3 is the SSMRS (Space Station Remote Manipulator System) grapple of the OBSS (Orbiter Boom Sensor System) at ~6:26a.m. The SSRMS will then handoff the OBSS to the SRSM (Shuttle Remote Manipulator System) to remove that arm’s clearance constraint with FD-4’s (Flight Day 4’s) scheduled unberth of the MPLM Leonardo from Discovery’s payload bay.

Discovery’s crew will cap their day by heading to bed at 12:21p.m. for some much deserved rest.

Due to the lack of Ku-Band communications, all of the Discovery crew’s FD-2 OBSS scans of their vehicle’s TPS (Thermal Protection System), as well as the RPM photography of Discovery’s underbelly TPS tiles, will be downlinked to the ground via the ISS’s Ku-Band antenna for analysis by the DAT (Damage Assessment Team).

Nonetheless, in terms of mission performance, Discovery is enjoying a relatively issue-free flight, save the loss of her primary communications antenna.  In fact, as of this publication, there are only three confirmed items on the MER (Mission Evaluation Room) board that have been noted by STS-131 ground teams.

These items, including the Fuel Cell #2 H2 Pump Motor condition (MER-01), the Main Propulsion System (MPS) Engine 3 LH2 Inlet Pressure 3 Off-Scale Low indication (MER-02), and the Ku-Band Forward and Return Links Failed (MER-03), are not deemed a concern in terms of Discovery’s Flight Crew’s ability to carry out the STS-131/19A mission.

In fact, with the Ku-Band issue becoming – for all intents a purposes – null and void once Discovery docks with the ISS, the only remaining item on the flight timeline that may require a significant readjustment is the late-inspection of Discovery’s WLE (Wing Leading Edge) RCC (Reinforced Carbon-Carbon) panels and nose cap RCC.

STS-131 Specific Articles:

Currently, it is unknown whether this late-inspection will be moved up into the docked portion of Discovery’s mission (thereby assuring Ku-Band downlink of late-inspection imagery through the ISS’s Ku antenna) or kept in its normal timeframe following Discovery’s undocking from the ISS, with data downlink to the ground coming through special workaround procedures in-place for just such an orbiter Ku-Band failure.

MER:01 – Fuel Cell #2 Pre-launch Issue:

The first issue of Discovery’s STS-131 mission – as recorded by the MER is the voltage spike seen on Fuel Cell #2 in Discovery just prior to External Tank fueling operations on Sunday evening.

“At 094:22:30:44 GMT (4 April 2010 18:30:44 EDT) Fuel Cell 2, s/n 104, Pump Motor Condition (PMC) spiked up from 0.62 Volts to 0.74 Volts and then trended up to a maximum value 0.98 Volts over the next 5 minutes,” notes a MER issue presentation, available for download L2.

Approximately 5-minutes later, the PMC dropped quickly back to a nominal reading of 0.62 Volts.

The problem report notes that technicians at Launch Pad 39A were in Discovery’s Crew Cabin performing “lighting adjustments” around the time of the Voltage spike.

“Observed AC 2 bus current changes correlated to the time of the Pump Motor Condition changes but the reading appeared to be increasing over time instead of the more familiar step change response.”

Following this initial spike, launch personnel did record an IPR (Interim Problem Report), IPR-46, for the STS-131 vehicle; however, since the PMC returned to a nominal voltage and remained stable for the duration of the launch countdown, the issue was not a constraint to the launch of Discovery and did not represent an impending failure of the Fuel Cell.

The PMC problem report notes that the mostly likely cause of the Voltage spike: “Fuel Cell 2 H2 motor status was most likely responding to small phase imbalances on the AC bus.”

The report further notes that any additional upward trend in Voltage from the PMC on Fuel Cell 2 “would have indicated an impending failure of the Fuel Cell 2.”

Currently, Fuel Cell 2’s H2 pump motor and sensor are “steady and fully operational.”

However, should the H2 pump motor fail during flight, the failure would render Fuel Cell 2 lost.

“Failure of pump motor would result in loss of the affected fuel cell. Loss of H2 Pump Motor  = Loss of Fuel Cell 2- MDF (Minimum Duration Flight) per flight rule = NEOM (Nominal End Of Mission). Loss of 2nd Fuel Cell- NPLS (Next Primary Land Site).”

If the H2 pump motor sensor were to fail during the mission, further information could be gained as the flooding of Fuel Cell #2 via the KOH (potassium hydroxide) levels, CPM, and other avenues.

However, the failure of the pump motor would force the shutdown of Fuel Cell 2 and immediate implementation of a Minimum Duration Flight – which, for STS-131, is only one day shorter than the nominal mission timeline.

However, if a second Fuel Cell were to fail after that, Flight Rules mandate the immediate termination of STS-131 and an immediate landing at the Next Primary Landing Site – one of the three U.S. landing sites: KSC, Edwards, or White Sands.

As part of the review into the Fuel Cell 2 PMC issue, a history of this issue on Discovery (known internally as OV-103) revealed two prior occurrences.

“Fuel Cell 2 Pump Motor Condition indications above the Launch Commit Criteria of 0.75 Volts were observed on 2 previous OV-103 missions: STS-42 and STS-63,” notes the MER document.

However, both of those occurrences were seen as changes to the AC bus phase currents instead of an upward trend like this event.

Currently, the IPR taken on this issue has been deferred until after the STS-131 mission is complete.  “This IPR was deferred during the prelaunch timeframe and potential post-flight actions are currently being evaluated.”

MER-02: MPS LH2 Inlet Pressure Off Scale Low:

The second issue to make the MER board (though, as with the Fuel Cell issue, is not under evaluation at this time) is the MPS Engine 3 LH2 (Liquid Hydrogen) Inlet Pressure 3 Off-Scale Low indication.

According to the STS-131 ascent summary, “Just prior to SSME (Space Shuttle Main Engine) start, the MPS LH2 inlet pressure measurement began behaving erratically at 095/10:21:15 GMT (06:21:15 EDT – 4.4-seconds before SSME start).”

The reading eventually went Off-Scale Low.

Nonetheless, the loss of this data circuit is of no concern for the remainder of Discovery’s mission as the MPS is only used during ascent.

MER-03: Ku Band Antenna Failure:

The primary issue for STS-131’s on-orbit mission is the loss of Ku-Band communications antenna high speed uplink and downlink capability.

“During Post-Insertion while the crew was working through Ku-Band Activation, at approximately GMT 2010/095:12:15 (08:15 EDT), the Ku-Band failed the self test,” notes the MER issue report.

The MER write up indicates that during the first self-test, the signal strength was low (0.56 V). Normally, the signal strength would be greater than 0.9 V. Furthermore, “range during test sequence 7.3 Radar Passive stayed 0 ft, but should have been 10,240 +/- 500 ft.”

Additionally, all “detect and track” indications were missing and the self-test registered “failed” due to Electronics Assembly 2 (EA-2).

A repeat of the self-test was performed shortly thereafter and all indications were positive, but the signal strength was still low. Again, the self-test registered “failed” due to EA-2.

Following these two failed self-tests, the Ku-Band was configured to COMM mode with “GPC ACQ antenna steering mode.”

At this setting, the forward link signal was low at ~-102 dBm and all detect, track, and frame sync indications were not present.  Furthermore, the return link receiver did “lock on the Ku-Band signal, but . could not acquire bit and frame sync. TV modulation could be seen but it was scrolling, snowy, and unusable.”

The presentation also notes that several troubleshooting attempts were made, including “commanding to GPC DESIG antenna steering mode, power cycling via command, power cycling via the MNB R14 KU ELEC cb, trying PM mode (48 Mbps on Ku-Band channel 3; default is FM mode with analog TV), and commanding the Ku-Band forward link from 216 Kbps mode to 72 Kbps mode.”

After the power cycle troubleshooting, the signal strength fell below the limit to be seen in the telemetry.

After this, a third self-test was performed and also failed.

Then, at 11:59a.m. EDT, the Ku-Band signal strength increased to ~-103dBm. Nonetheless, forward and return links were still non-functional. After that, signal strength fluctuated between levels that were and were not evident in the data stream.

The Ku-Band was officially declared “failed” in-between FD-1 and FD-2. An additional self-test just prior to rendezvous operations with the ISS on FD-3 also failed and the Ku-Band is non-operational in the COMM and RADAR modes – meaning that no “operational command, voice, or data” can be transmitted through the Ku-Band.

The most probable cause of this failure is a Deployed Assembly (DA)/Deployed Electronics Assembly (DEA) or Electronic Assembly 1 (EA-1) component or internal connection issue.

Communications with Discovery are being handled through the S-Band systems which provide limited redundancy for TV and low data rate transmissions.

“S-Band FM is available for video and recorder playbacks when over a supporting ground site,” notes the MER issue report. “S-Band PM is available for operational command, voice, and data.”

Additionally, while no confirmation has been heard from Mission Managers regarding when the late-inspection of Discovery’s WLE RCC panels and nose cap RCC will take place, Flight Rule A11-55 states that late-inspection is to be performed during ISS docked ops in the event of an orbiter Ku-Band failure.

“Flight rule A11-55 calls for late inspection to be performed while docked to ISS to allow downlink via ISS Ku-Band,” notes the MER write-up.

As with the Fuel Cell issue, a review of Ku-Band history shows one previous occurrence of a similar Ku-Band failure. That failure occurred on STS-92 back in 2000.

“During STS-92, at approximately 286:14:07 GMT (000:14:50 MET), the Ku-Band system failed to transmit or receive in the communications mode,” notes the MER document.

During that mission, the Ku-Band was power cycled and self-tested, with all self-tests failing and the power cycling not recovering transmit and receive capabilities.

The signal strength during those self-tests was lower than expected (~-105dBm) and an EA-2 failure was reported.

Similarly to STS-131, the Ku-Band did not work in the RADAR mode during STS-92.

Post-flight inspections of the system following the mission showed that “the reference 156 MHz frequency was good coming out of EA-1; however, the local oscillator frequency was not nominal coming out of the DA test cables.”

The problem was later indicated to be with the DEA. “The results of the troubleshooting indicated a DA (S/N 102) removal and replacement was required.”

The problem was eventually isolated to a failed transistor Q5 on the “Harmonic Phase Comparator (HPC) circuit card, which is part of the Exciter located in the DEA.”

The exciter itself is a common circuit associated with the forward and return link functions as well as the self-test of the Ku-Band system.

Turnaround options for the Ku-Band system once Discovery is back at KSC will consist of troubleshooting efforts and removal and replacement of all non-functioning systems.

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