STS-125: TPS health takes center stage for Hubble mission

TPS Inspections:

As with all post-Columbia missions, Orbiter Boom Sensor System (OBSS) inspections of Atlantis’ RCC panels and upper surface TPS will be carried out by the seven crew members on FD-2 (Flight Day-2).

However, given the unique nature of the Hubble mission – most notably the inability for the crew to reach the International Space Station (ISS) – Atlantis’ crew will also have to perform an inspection of their ship’s underbelly TPS, a critical procedure given the lack of the Rbar Pitch Maneuver (RPM) normally conducted on Flight Day 3.

‘Mandatory to determine health of Orbiter TPS as soon as possible so required follow on actions/activities can be initiated. FD-2 surveys consist of: LDRI/IDC (OBSS) WLE and Nose Cap Surveys, Crew Cabin Survey, ITVC Tile Acreage Survey, and Upper Crew Cabin,” notes a STS-125 Mission Operations Directorate (MOD) presentation.

The ITVC (Intensified Television Camera) Tile Acreage Survey relates to the OBSS’s ability to scan a wide portion of the Orbiter’s underbelly during a single pass.

‘ITVC Tile Acreage Survey replaces ‘RPM (Rendezvous Pitch Maneuver) Photography’ performed on ISS missions. Procedures (were) developed in support of RTF (Return To Flight STS-114) in case the RPM photos could not be accomplished,’ added the MOD mission planning document.

While this Tile Acreage Survey is critical in determining the health of Atlantis’ TPS post-launch, the exact time needed to execute this new procedure is not precisely known; therefore, the possibility remains that some portions of the Tile Acreage Survey will take place on FD-3 after rendezvous and capture operations with the HST are complete.

Additional TPS duties for the crew may include a survey of the Crew Cabin with the Shuttle Remote Manipulator System (SRMS), though this requirement has yet to be finalized.

Presently, there are no plans to change the Late-Inspection operations post-Hubble deploy; however, these inspections are not tradable for HST mission success and will be performed even at the expense STS-125’s HST mission objectives.

The reason for this round of Late-Inspections is to verify that Atlantis’ TPS has not suffered any critical Micro-Meteoroid Orbiting Debris (MMOD) impacts during her mission.

Focused Inspection and Possible Mission Impacts:

Following the FD-2 TPS inspections, analysis of the OBSS data will begin ‘as soon as the first piece of survey data reaches the Mission Control Center (MCC),’ notes the MOD presentation.

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The goal of the TPS Imagery Inspection Group is to have all data reviewed and Areas of Interest (AOI) for Focused Inspection (FI) consideration identified by the start of the Rendezvous timeline approximately one day 15 hours after lift-off.

‘Have initiated discussions with TPS Imagery Inspection Group on this accelerated review. High confidence this can be completed with the additional flight experience we will gain prior to STS-125,’ notes L2 documentation.

In the event that a FI is needed – a probability classed as ‘very low’ by the MOD presentation – flight planners have put together a FI schedule that will have as little impact as possible of the main goals of the HST servicing flight.

If FI activities are deemed necessary, the OBSS will be unberthed following HST capture and berthed following FI activities on FD-4 prior to the start of EVA-1.

Unlike clearance concerns with SRMS berth/unberth of the OBSS at the ISS, studies show that the standard OBSS unberth procedures will not violate clearance margins with the berthed HST.

Similarly, should the Tile Acreage Survey need be completed in the post-HST berth timeframe, there are no perceived clearance issues between the SRMS and Hubble.

However, in good fashion, HST mission planners are taking into account the accelerated nature of the OBSS imagery analysis as well as the possibility that all TPS inspections may not be completed on FD-2 – which would result in a delay of the review of this imagery. A delay in the review of this data would mean a delay to the FI until NET FD-5.

Despite mission planners’ best efforts to minimize the impacts of a FI on the EVA tasks of the crew, the number, location, and size of the FI site(s) will ultimately dictate the time necessary to successfully complete the FI.

While current mission timelines have allotted a two and a half hour period of time on FD-4, just prior to EVA-1, for FI operations, all EVA activities will be assessed in real time and, if necessary, differed or cancelled due to the need to obtain the FI images.

‘Goal is to minimize EVA impacts and perform FI in parallel with EVA prep activities. Possible that FI survey time requirements may not provide sufficient time to execute all/any of scheduled EVA-1 tasks (if FI on FD4) or scheduled EVA-2 tasks (if FI delayed to FD5),’ added the MOD presentation.

‘EVA execution and/or content is ‘TBD’ and is based on the time required to complete FI. Possibility that downstream EVA’s and planning will be affected.’

While continuing upgrades to the External Tank and other critical TPS systems on the Space Shuttle system make the possibility of a FI ‘very low,’ the extensive planning for this scenario continues to emphasize NASA’s commitment to mission safety in the post-Columbia era.

The Mission:

While the launch date of Atlantis’ flagship mission to the Hubble Space Telescope still depends on the production of two External Tanks (ET) at the Michaud Assembly Facility – necessary for Launch on Need rescue operations – the mission’s astronauts are well into their training for the 11+0+2 day mission.

Atlantis’ mission currently has five planned EVAs and one unplanned EVA in support of Hubble operations. With the final decision on the number of EVAs and their respective contents is dependent on TPS inspections, mission planners have fine-tuned the priority of the EVA tasks.

However, ‘history has shown that you should expect the unexpected during EVA operations,’ claimed the MOD presentation. ‘EVA times padded 20 percent to take into account Neutral Buoyancy Lab (NBL) vs. Zero G experience.

‘Minimal ‘Real Time’ response/workarounds for STS-125 tasks are limited. Most STS-125 tasks require 2+ hours to accomplish, and once started, must be completed, thus minimal workarounds.’

An example of these unexpected occurrences has been seen with the RSU (Rate Sensor Units) removal and replacement tasks on previous HST servicing missions. As a result, mission planners have added additional time to the RSU removal and replacement task on this mission – operating under the ‘lesson learned’ adage.

Another part of this task definition was the implementation of a possible contingency EVA on FD-9 should the crew encounter issues in the deployment of the HST.

Another MOD presentation – one of around 40 such presentations available on L2 – adds: ‘Program requirements limit ‘Unscheduled EVA’ opportunities. Only option available is EVA-6 on FD-9. Limits EVA-6 duration to protect crew day length and support HST release.’

Portions of the mission timelines present the possible reasons for the call-up of EVA-6 as issues pertaining to the HST/Orbiter berthing latches, End Effector (on the SRMS), and the Aperture Door (on HST).

If required, EVA-6 would be no longer than four and half hours in duration due to the need to limit of crew’s work day on FD-9. Additionally, if EVA-6 were required, all Late-Inspections would have to be performed on FD-10 instead of the spreading out the Late-Inspections over FD-9 (post-HST deploy) and FD-10.

Given all these considerations, the amount of time Atlantis can safely stay in orbit is governed by the cryogenic consumables for her electricity producing Fuel Cells. Under normal cryo margins, Atlantis will land on FD-12 with two contingency days reserved for landing weather delays.

STS-125 Processing Latest:

Atlantis’ extended processing flow, which includes modifications ahead of her debut mission to Hubble, remains relatively issue free inside her Orbiter Processing Facility (OPF).

‘APCU troubleshooting was worked last week. Measurements on APCU 2’s output converter confirmed the theory of a delaminated heater in the APCU 2 output circuit,’ noted processing information this week. ‘Engineering is evaluating a possible R&R of APCU 2 following HST testing. No decision has been made as yet.’

Work is continuing on Atlantis’ TPS, alongside work on her Main Landing Gear (MLG), which has received new seals following her last flight on STS-122.

‘MLG was disassembled yesterday. Seals were R&R’d and have been put back together. Have couple of days of leak checks to verify this system. This work will be completed in a couple of days. TPS: Have 63 TPS cavities; 116 are complete. There are 140 repairs.’

Meanwhile, STS-125’s boosters are continuing to undergo stacking in High Bay 1 of the Vehicle Assembly Building (VAB), with only the two required External Tanks, ET-127 for Atlantis, and ET-129 for Launch On Need (LON-400) dictating the launch date.

Currently, STS-125 will not be able to launch until October 8 at the earliest, with the threat of another slip being evaluated by shuttle management, based on the latest processing information coming out of the Michoud Assembly Facility (MAF).