STS-133: Discovery into Shuttle Interface Test – Review of sep bolt/nut issue

by Chris Bergin

Following an engineering solution to the left External Tank separation bolt nut and nut retaining fixture issue, Discovery is moving at a pace through her Vehicle Assembly Building (VAB) milestones. The stack is currently being put through its Shuttle Interface Test (S0008), ahead of next week’s rollout to Pad 39A.

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Discovery is enjoying her final visit to the VAB, given this is her final mission. She will return to her Orbiter Processing Facility (OPF) at the conclusion of STS-133 for a long period of deservicing, although she’ll never return to the VAB again, unless she is rolled back from 39A due to a large technical issue, or a severe weather threat.

Although she did suffer from an engineering issue during the early part of her mating process – see overview below – mating operations between the External Tank (ET-137) and the orbiter have since been completed.

“S0004 Orbiter/ET Mate operations required for vehicle power up are complete. ET Separation Camera is installed. LO2 and LH2 Stud Closeout (ET Side) complete. LO2 and LH2 Main Separation Closeout is complete. LO2 and LH2 Monoball installation and electrical mate complete. LO2 and LH2 Purge Curtain are installed,” outlined the NASA Test Director (NTD) report (L2).

One issue was reported ahead of the start of S0008 operations, relating to installation of the T-0 umbilical to the Tail Service Mask (TSM) – which links the orbiter to the Mobile Launch Platform (MLP).

“Problem Report (PR): LH (Left Hand) Lower T-0 Bolt damage: During T-0 umbilical installation the TSM carrier plate foot impacted the smaller of the two bolts on the lower LH attach point hard enough to drive a preliminary gauge pin inspection at the location,” added the NTD.

“The gauge pin could not be fully inserted and the bolt is believed to be bent. Engineering is discussing the ‘go forward’ plan. This PR condition is not constraining any other current work.

S0008 – the Shuttle Interface Test – began at midday local time on Tuesday, and will carry through to Wednesday afternoon.

“Picks up today with Call to Stations (CTS) is planned for noon EDT,” added the NTD report on Tuesday. “CTS may occur earlier if conditions warrant (monoballs complete, FR4 LPS line validation confirmed) and supporting systems are ready. S0008 is scheduled to proceed through tomorrow afternoon.”

Discovery is scheduled to rollout from the VAB on Monday, September 20, with official preparations for her departure as the STS-133 stack Рknown as A5214 operations Рalready in work.

The Intertank Access Kit was also removed from ET-137 on Monday, following the investigation into a flight washer that was found inside ET-138’s LH2/LOX Feedline. No issues were found on Discovery’s tank.

Over in the Launch Control Center (LCC), controllers and engineers were put through their paces with the Countdown Simulation (S0044) on Monday.

This test run of the business end of a launch countdown is used for certifying the launch team for STS-133.

S0044 took place in Firing Room 4 (FR4), with three runs made in the morning and two in the afternoon. Two of the “attempts” worked from the T-20 minute hold, while the other three started at different times ahead of coming out of the T-9 minute hold,” as per the STS-133 S0044 presentation, acquired by L2.

A “bonus round” was also included in the morning runs, which usually includes an additional number of glitches for the teams to work through.

Controllers on the integration console threw random issues at the teams during the runs, allowing them to practise making the calls to alert the loops and perform troubleshooting. A debrief was carried out after each run.

Reports of the runs speak of the final morning run counting down to a simulated “on pad engine ignition abort”, while the morning “bonus round” included issues such as a “discrepant ET door centerline latch indicator”.

One run simulated a serious issue on Discovery – an APU (Auxiliary Power Unit) tank leak – which involved controllers calling for the evacuation of the simulated crew down the Emergency Egress System (EES) slide wires.

Such simulations provide priceless training for controllers in the highly unlikely event of a serious issue during the real countdown.

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Review of IPR-39:

As reported, Discovery suffered a problem during mating operations between the orbiter and her External Tank (ET-137) last Friday, when an internal nut – prepositioned inside the aft compartment of the orbiter – became loose and moved out of position, negating the possibility of installing a flight structural bolt.

“IPR-39: Picked up Friday afternoon during S0004 aft soft mate when the left ET separation bolt nut and nut retaining fixture became dislodged from its proper position in the Orbiter aft fuselage,” noted the NTD, overviewing the issue.

Following an Engineering Review Board (ERB) meeting, an engineer successfully accessed the aft area of the orbiter to conduct a repair.

This was one of the main sticking points for the repair, given the need for waivers to allow access in the vertical prior to the completion of hard mate. The other option would have been to lift Discovery back over to the Transfer Aisle and roll the orbiter back to the OPF.

“An ERB was held to review the plan to gain access to the Orbiter aft in order to reposition the nut and retaining fixture. An additional ERB was held to review the plan to remove the 50-1 door with the Orbiter weight still on the lifting sling,” added the NTD. (Image left: As found by the tech after entering the aft).

“The ERBs concurred with both plans. The IPR was upgraded to a PR. Entry into the aft was achieved without incident. The nut and retaining fixture were repositioned, the left ET separation bolt was installed, GSE access platforms removed and the 50-1 door was reinstalled.”

This was the first time in the history of the Space Shuttle Program (SSP) that such an issue was found during mating operations, again showing off the skills of the engineers via their ability to fix the problem without anything more than a 12 hour delay to operations.

The fixture in question is held into place with the three pip pins that are tethered to the fixture. In the OPF the fixture with the nut is lowered into the flange on the vehicle and the three pip pins are inserted through the flange holes into the fixture holes to lock it into place.

The problem suffered by Discovery related to the fixture being higher in the flange, meaning the pip pins slid through the flange holes underneath the fixture – resulting in the fixture ending up resting atop of the pins.

When the technician entered the vehicle he found the two lower tethers were slack, with the 70 pound fixture hanging from the upper tether. This was pre-empted by the ERB, which noted it could be a problem if Discovery was moved back to the horizontal for rollback to the OPF, as the fixture could have fallen on to flight hardware inside Discovery. (Image left: Post repair).

With the repair carried out, engineers will install a blast can over the nut to prevent the nut from escaping once it has blown apart after Main Engine Cut Off (MECO).

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