STS-125: Planning continues with scheduled EVA-6 option

by Chris Bergin

With unique Thermal Protection System (TPS) inspection techniques already installed into Atlantis’ flagship STS-125 mission this October, focus has now moved on to refining the Extra-Vehicular Activities (EVAs) that are scheduled to be performed as part of the complex ballet to increase the HST’s longevity well into the next decade.

Planning documentation shows the possibility of record sixth back-to-back “scheduled” EVA.

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With the mission limited to a duration of 11+2 flight days – due to cryo tank margins – mission planners have already baselined five scheduled EVAs and one unscheduled EVA – all of which come in under seven hours in duration.

However, planning documentation shows the mission now has an option of six EVAs, plus one unscheduled EVA, which would be ‘breaking new ground’.

As per original baseline, once Atlantis’ crew has captured the HST on Flight Day 3 (FD-3), the crew will perform five back-to-back EVAs.

‘Five scheduled EVAs: EVA-1 (RSU, Battery/Bay 3) 6hrs 45mins; EVA-2 (Cosmic Origins Spectrograph, Battery/Bay 2) 6hrs 50mins; EVA-3 (Wide Field Camera III, NOBL 5, 7, and 8) 6hrs 10mins,’ notes the mission planning Mission Operations Directorate (MOD) presentation available for download on L2.

‘EVA-4 (Space Telescope Imaging Spectrograph, STIKER) 6hrs 25mins;  EVA-5 (FGS 3 (possibly #2), OVP, OCE-EK (not required if FGS 2) 6hrs 30mins.’

Nevertheless, while the completion of these EVAs are necessary for HST life extension, they are not critical to a successful landing of Atlantis at the end of her mission.

Therefore, all EVA activities are considered secondary to TPS concerns relating to the inspections of the vehicle’s heat shield performed by the crew on FD-2.

If a Focused Inspection (FI) is deemed necessary based on FD-2 images, the FI would be performed at the beginning of FD-4.
‘EVA-1 execution and/or content ‘TBD’ and is based on the time required to complete FI which is dependent on the number of Areas of Interest that require inspection, their location, and number of views. Possibility that downstream EVAs and planning will be affected,’ notes the MOD presentation.

Meanwhile, to ensure there are no complications for Atlantis from the HST during deorbit, entry, and landing, a sixth, unscheduled EVA is being protected for in the mission timeline.

This sixth EVA would be performed only if complications arise during the deployment of the HST at the end of the mission.

‘Plan is to prepare for a ‘Rapid Response’ EVA in parallel with HST release activities on FD-9 since only one ‘unscheduled’ EVA available,’ added the flight planning document.

For this possibility, mission planners have identified a few issues that may lead to an unscheduled EVA to aid the HST’s deployment from Atlantis.

These issues include the failed deployment of the HST’s Hi Gain antenna, incomplete detachment of umbilicals from HST and the FSS in Atlantis’ Payload Bay, and the failure of HST’s aperture door.

To accomplish all five back-to-back EVAs with minimal stress on the crew, mission planners have identified the need for four Extra-vehicular Mobility Units (EMUs), one for each spacewalker. In the past, HST repair missions have only carried three EMUs for their spacewalkers.

Possible Sixth ‘Scheduled’ EVA:

During the initial planning phase of the mission, the MOD conducted a trade-off study to determine if a sixth scheduled EVA could be added to the STS-125 timeline.

‘Replace ‘Unscheduled’ EVA with ‘Scheduled’ EVA-6. Results in six ‘Back to Back’ EVAs. Breaking new ground. Will have crew fatigue concerns,’ notes the MOD presentation.

Crew fatigue concerns would become an issue because all of the Late Inspections would have to occur on FD-10. This would mean the elimination of the crew’s off-duty time that would normally be scheduled for FD-10, following the completion of Late Inspections, that would normally be spread over FD-9 and FD-10 of a five EVA mission.

The elimination of the crew’s off-duty time creates a violation of Shuttle Crew Scheduling Constraints (SCSC).

If the Space Shuttle Program (SSP) were to proceed with a six planned EVA mission, the request would have to come from the HST program and a review of the Mission Safety Critical Requirements would have to be conducted by the SSP.

‘SCSC developed to ‘ensure safety and well being of crewmembers who perform a wide variety of tasks associated with the vast challenges of a Shuttle mission ensuring a safe mission first and successful mission second,” added the MOD presentation.

‘Understand HST Program’s desire to optimize HST capability with SM4 being the last servicing mission but do not want to compromise safety or incur any additional risks.’

Nevertheless, exchanging the unscheduled EVA with a scheduled EVA limits the possibility of a contingency EVA should some component of the HST fail during the first deployment opportunity.

To accomplish a potential sixth planned EVA while still protecting for an unscheduled EVA for HST deployment, the MOD is investigating three additional options aside from simply replacing the unscheduled EVA with a scheduled EVA. 

These remaining three options include rendezvousing with HST on FD-2 instead of FD-3 (thereby deferring all TPS inspections to FD-9 and FD-10), eliminating one of the two End of Mission (EOM) contingency days, and eliminating/modifying the Late Inspection requirement.

The first option would see Atlantis rendezvousing with the HST on FD-2 instead of FD-3.

‘For specific Launch Phase angles the capability exists to support a FD-2 rendezvous. Thought is that rendezvous on FD-2 will buy back additional time for EVA activities,’ notes the mission planning document.

There are, however, many impediments to this option, the first of which being the elimination of TPS inspections until after HST deployment at the end of the mission.

‘FD-2 TPS inspections provide expedited assessment of TPS after ascent. Expedited assessment provides early response and kickoff of any activities required to verify/provide integrity to TPS for Entry support,’ the MOD presentation adds.

‘Provides maximum ‘on-orbit’ time to evaluate and plan repair activities as required (such as) Focused Inspections/EVA repair. Delay in acquiring this data delays working/executing ‘corrective’ actions.’

Other concerns for a FD-2 rendezvous include the time necessary to configure the orbiter for HST capture and berthing.  These concerns include Remote Manipulator System checkout, photo TV setup, FSS preparations for HST berthing, EMU checkout, and EVA-1 preparations to name a few.

The second option would see the elimination of the EOM+2 flight day. This would allow for the normal FD-2 TPS inspections and FD-3 rendezvous with HST while adding an additional FD to accommodate six scheduled EVAs and one unscheduled EVA.

The two EOM contingency days are a required part of every Space Shuttle mission to ensure protection against weather and systems wave-offs. However, there is a flight history to support the elimination of EOM+2 in support of mission objectives.

‘For STS-116, EOM+2 was traded to support EVA-4 (ISS critical safety item) and retain Late Inspection. Both items defined as Safety of Flight/Mission Critical. Loss of EOM+2 required landing the vehicle on nominal EOM at any of the available CONUS sites.’

If this strategy were employed on STS-125, it would protect for all mission safety and HST success operations while translating to a mandatory landing on FD-13 at any of the three United States landing sites, unless weather or systems failure occurs.

The third option would see the elimination of the Late Inspection requirement. While this option will most likely be decided against in the best interest of crew and vehicle safety, it is note worthy simply because it is the first time since the employment of Late Inspection on STS-121 that its possible elimination has been mentioned in mission planning documents.

‘Loss of TPS Wing Leading Edge (WLE) integrity checks could result in a catastrophic failure. Late Inspection buys down odds of this risk from 1/180 to 1/240. Risks are managed within capabilities,’ the MOD presentation notes.

Another Late Inspection discussion in the MOD flight planning document is the possibility of partial Late Inspections. This option would see the flight crew inspect a portion of the vehicle’s WLE; however, ‘the integrity of the (WLE) that (is) not surveyed will not be established.’

If a sixth scheduled EVA were to be added to the STS-125 mission timeline, it would more than likely include the repair of the Advanced Camera for Surveys which recently failed on the orbiting telescope.

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