STS-133: S-Band Antenna Cable Conundrum for Discovery

by Chris Bergin

Discovery’s engineers are attempting to evaluate options for a suitable coax cable on her S-Band antenna system, following a decision to changeout the cable after the orbiter returned from STS-131. The first replacement cable failed bench tests, leading to the request for a second, third and fourth¬†replacement. However, these latest cables are all classed as “too short”, despite being “in spec”.

STS-133 Processing Latest:

Discovery flow has begun to pick up again, as she prepares to hit the milestone of rollover to the VAB for stacking in the second week of September. The veteran orbiter currently being processed through a number of system checkouts.

“Forward ET (External Tank) Sep yoke was installed Monday; Re-torque being performed Wednesday. RH (Right Hand) OMS (Orbital Maneuvering System) pod isolation valve verification; RRCS (Right Reaction Control System) thruster checks and heater tests are were completed Monday,” noted the latest NASA Test Director (NTD) processing flow report (L2).

“Landing Gear Functional was successfully completed. DPS (Data Processing System) computer complex checkout is complete. OMS Flight Control checkout is complete and good. ECL rotary equipment checkout was successfully completed Tuesday.

“SSME (Space Shuttle Main Engine) leak checks and MPS (Main Propulsion System) VJ line checks are complete and good. Brake/Anti-skid checkout will be performed by end of Wednesday. OMS Fuel/Ox Crossfeed Connections will be performed on Thursday.”

STS-133 Specific Articles:

The latest – and 31st – Interim Problem Report (IPR) to be charged to Discovery’s flow since returning from STS-131 related to audio communication issues during the checkout of the centerline camera – which views the International Space Station (ISS) through the Orbiter Docking Ring (ODS) on approach to the orbital outpost. This issue has since been resolved and closed.

“New IPR-0031: During Centerline Camera Adjust, no communication was detected between the airlock and the pilot station using ICOM (Internal Communications) B,” added the NTD report.

“Engineering performed real-time troubleshooting to verify the test setup and discovered it was a procedural error due to the new revision of the document leaving out the required jumper installation to the test set. The test set was reconfigured correctly, and the audio communication was restored.”

The issue with the coax cable stretches back to earlier in Discovery’s flow inside Orbiter Processing Facility (OPF-3), when engineers began S-band troubleshooting (May 7), due the observation of erratic measurements on the upper left antenna. While no dropouts were experienced during initial evaluations, engineers decided to call for the replacement of the cable.

The damaged cable was removed from Discovery last Friday, in preparation for the installation of its replacement on Monday. However, the installation was postponed, after the replacement was deemed to be unsuitable.

“IPR-0005 Coax R&R update: Bench tests of the replacement coax showed it will be unusable as a replacement. Engineering has ordered another cable, and will test it prior to installation,” noted the NTD report earlier this week, before Wednesday’s processing update noted the latest cable was half an inch too short.

“S-band antenna coax R&R update: The replacement coax cable passed the bench tests, however when it was attempted to be installed, it was about 1/2” too short. Engineering will evaluate.”

UPDATE: Thursday morning’s NTD report notes that two additional cables were tested¬†within the last 24 hours, both “built to spec” – yet strangely, both of those cables are also classed as “too short”. Engineering evaluations are continuing.

This system operates in the S-band portion of the RF spectrum of 1,700 to 2,300 MHz. It was put additional workloads during Discovery’s STS-131 mission, following the failure of the orbiter’s Ku Band system.

The root cause of that issue was tracked back to a fault with the Deployed Assembly (DA) unit, which has since been removed and replaced.

Meanwhile, over in the VAB, work is picking up on repairing the chip noticed on the Fairing Support Plate of the mated ET-137 and twin Solid Rocket Boosters (SRB) stack in High Bay 3 (HB-3).

“SRB BI-144/RSRM 112/ET-137 (VAB HB-3): PR (Problem Report) ET-137-TS-0023 Damaged SLA On EB-2 Fairing Support Plate. Repair area preps and hand pack are in work, DC 1200 silicone primer application and cart preps are complete. Type 1 hand pack application is ready to work Wednesday,” added the NTD latest.

“L/R (Left/Right) Forward Crossover; RH ET Fairing cover next after damaged SLA PR repair.”

Discovery remains well within the timeline to make her current launch date of November 1, which is currently scheduled to be her final mission.

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