Discovery may skip APU hot fire as STS-120 TCDT closes in

by Chris Bergin

Shuttle Discovery is continuing to enjoy a smooth pad flow ahead of STS-120, with the next milestone event of the Terminal Countdown Demonstration Test (TCDT) due next week.

The postponed APU (Auxiliary Power Units) hot fire test was to take place shortly after the TCDT, although managers are now evaluating whether to skip the process and continue with the push towards the start of the launch countdown (S0007).

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Discovery remains on track for the October 23 launch date target, with the payload now installed into the cargo bay of the orbiter. The APU hot fire test was originally set to take place shortly after Discovery arrived at Pad 39A over the weekend, before high winds cancelled the operation.

‘PLBD (Payload Bay Door) opening was completed yesterday and payload installation is scheduled for today,’ noted processing information on Wednesday morning. ‘There are currently no constraints to picking up with the transfer.’

‘Due to weather, the planned APU hot fire test was postponed until next week, after the TCDT. Winds were too high Sunday to pull plugs for hot fire. (However) a decision is pending on whether to perform the APU hot fire.’

Not carrying out the APU hot fire is not in itself unusual, and has been the case before. Most recently, the APU ‘confidence run’ wasn’t carried out during the pad flow for Discovery’s STS-114 Return To Flight mission.

Skipping the process – which isn’t vital – would further protect the one remaining day of contingency – now less than a full day – ahead of the October 23 target. A final decision may be taken on Thursday, with an APU hot fire pre-brief meeting called for later in the day. Should the hot fire proceed, it will be carried out next Thursday.

(UPDATE: Decision is to proceed with APU hot fire on Thursday).

Back out at the pad, processing of the Discovery stack continues at a pace, with only minor issues being reported.

‘Hydraulics picked up new IPR (Interim Problem Report): During (process) operations, the brake iso valve number 1 did not go closed when commanded at 400 psi,’ noted one example in processing information on Wednesday. ‘Troubleshooting validated that a new motorized GSE (Ground Support Equipment) by-pass valve did not provide the required pressure.

‘The brake iso valve was successfully cycled at 400 psi using the manual GSE by-pass valve. The IPR will close as an explained condition.’

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Meanwhile, the HPU (Hydraulic Power Units) carts have been transferred from Pad 39B to Pad 39A. These ground service carts for the APU and HPU are filled with Monopropellant-grade Hydrazine (MPH or N2H4) hypergolics from a small tanker called a Generic Propellant Transfer Unit (GPTU). Hypergolic loading on Discovery will be carried out this weekend.

With the Flight Readiness Review meetings fast approaching, the mission itself continues to be tweaked. Two items, relating to the gloves that will be used by the spacewalkers, and the T-Rad tile repair demonstration, continue to be aligned.

‘Wrapped up second round of crew evaluations for overglove design for STS-120,’ noted the latest Stand-up report, which was referring to a special protective layer on the glove, following the recent events with EVA members suffering from cuts to their gloves during STS-116 and STS-118 (notes and presentations on the overglove are on L2).

‘Crew consensus was redesign is acceptable. Will fly glove as is with tweaks, and leave in locker unless circumstances necessitate using it.’

‘On track with other work. Working T-RAD DTO paperwork and procedures. Did third cycle VLA on STS-120 because of addition of Tool Stowage Assembly. Heard from Node 2 that they cleared structural analysis; still need to hear from two sidewalls (MBSU and SASA) for return. Thought would complete the work post-FRR, but appears may be able to converge by Friday.’

All mission related items will go to the FRR for a full overview and approval by engineers and shuttle management. Timelines for the FRRs show the Shuttle Program Flight Readiness Review on October 9th and 10th, the MMT (Mission Management Team) Prebriefing on October 12th, the Level 1 SOMD Flight Readiness Review on October 16th and L-2 Prelaunch MMT on October 21st.

STS-120 ISS-related preps

Also related to STS-120 are the recent events at Discovery’s destination later this month – the International Space Station (ISS). The Expedition 15 crewmembers successfully relocated the Russian Soyuz last week, along with the retraction of two solar arrays on the Russian FGB.

‘Had successful Soyuz relocation Thursday. In anticipation of STS-120, had FGB solar arrays successfully retracted over weekend; this was required to install Node 2,’ added the Stand-up. ‘Getting ready for STS-120.’

Preparations also included the Station Operations Readiness Review (SORR), which was used to clear the station for the arrival of the next Soyuz, and the following arrival of Discovery.

‘The ISS Program conducted their SORR for 10A/15S,’ noted information in the 8th Floor update (a NASA HQ memo). ‘The marathon meeting concluded with all parties being GO to proceed with the launch of the 15S (Soyuz) and GO to proceed with 10A, with the 120 launch on October 23 at 10:41 AM CDT.

‘On orbit, Soyuz relocation occurred. The crew and ground team did extensive work to configure the onboard systems in preparation for the relocation, which proceeded nominally. Also, after weeks of negotiations with our Russian colleagues, the FGB Solar Arrays were retracted,’ before adding a note which has yet to be clarified by sources.

‘The only issue that will be forward work is to understand why the Russians did not follow an agreed to flight rule, which required 2 ARCUs (American-to-Russian Converter Unit) to be online.’

Regardless, operations proceeded without issue, as confirmed in an ISS On Orbit Status report.

‘Both solar array wings of the FGB ‘Zarya’ module are in nominal configuration, i.e., properly folded in accordion-wise,’ noted the report. ‘Without additional heat rejection capability, the US segment (USOS) cannot support the ESA/’Columbus’ and JAXA/’Kibo’ modules, when attached.

‘To provide this capability, the ISS starboard PVR will be deployed during Shuttle mission STS-120/10A, followed by the port PVR shortly after 10A.

‘These activities require that the FGB SAWs must be completely retracted to avoid physical contact. With its SAWs no longer producing electricity, the FGB depends solely upon USOS power feeds (PMA-1 and PMA-2).

‘In the event of some USOS electrical failures, a contingency electrical jumper would be available to quickly reconnect USOS power to the FGB’s ARCU (American-to-Russian Converter Unit), or its backup, for converting from American 124V to Russian 28V. Currently, the six FGB batteries are fully charged.’

Providing there are no hitches with the arrival of the next Soyuz, everything should be in place for the third shuttle launch of 2007 in less than three weeks time.

‘We have a busy fall,’ summarized shuttle manager Wayne Hale to the workforce. ‘It’s great to be flying; keep up the good work.’

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