EVA-2 was debated as conjunction threat to Discovery and ISS removed

by Chris Bergin

NASA Managers debated advancing or delaying EVA-2, due to a now-removed conjunction threat to Discovery and the ISS – relating to part of an expended Ariane 5 upper stage – before deciding the spacewalk would remain on schedule. Unique operations to maneuver the Shuttle/Station stack are already on the drawing board, due to the issue with Discovery’s vernier thrusters, as STS-128 continues Flight Day 6 operations, which are dedicated to transfers between the MPLM and Station.

STS-128 Latest:

Discovery has no additional technical issues reported over the past day, as she remains patiently docked to the International Space Station (ISS).

neptec“The STS-128 mission is progressing satisfactorily. The fuel cell temperatures and electrolyte (KOH) concentration levels have remained within acceptable limits while at low power levels during the Station-to-Shuttle Power Transfer System (SSPTS) operation,” noted the Mission Evaluation Room (MER) status on Discovery (L2).

“Fuel cell 2 condenser exit temperature has slowly ramped up from 154 to 159 F, while the KOH has gone down from an average of approximately 34 percent to approximately 32 percent for the inlet and 30 percent for the exit concentration.”

Flight Day 6 is mainly focused on the ongoing transfers between the Multi-Purpose Logistics Module “Leonardo” and the Station, which have been progressing well since the cargo container was berthed with the ISS.

“The mission is proceeding nominally and per the timeline. Rack and COLBERT treadmill transfers from Leonardo to the ISS,” noted current status early on Flight Day 6.

“Flight Day 6 Activities Planned: Rack and cargo transfers from Leonardo to the ISS. EVA-2 procedure review and campout in Quest airlock by Olivas and Fugelsang. MER Problems: No New. Funnies: No new.”

With STS-128 primarily being a logistics mission, transfers are scheduled to take a total of five days worth of crew hours on the timeline, with the major push of transferring the MPLM enjoying its first fully dedicated day on FD6.

“Transfer Status: Middeck: 64 percent complete (93 percent resupply complete),” added the latest Mission Management Team (MMT) presentation on L2. “MPLM: 3 percent complete (4 percent resupply complete). 118 hours in the timeline overall; on schedule to complete.”

The Damage Assessment Team (DAT) also officially cleared Discovery’s Thermal Protection System (TPS) for entry. Around 13 items remained under evaluation by the DAT engineers, following the confirmation no Focused Inspection (FI) was required on the vehicle.

“No Focused Inspection (FI) is required for STS-128/17A. The DAT officially cleared the orbiter tile and RCC following review of all remaining FD2 inspection and RPM imagery after review and concurrence by the OPO (Orbiter Project Office),” noted a communication sent to the MMT (L2).

D2The work that was outstanding related to a few minor TPS protrusions, including one on the Flexible Reusable Surface Insulation (FRSI) plug – which is not uncommon.

“The DAT has forward work associated with FRSI plug protrusion (i.e., debris transport and potential downstream impact analysis on Rudder/Speedbrake). The estimated completion date for this follow-on analysis is 9/1/09 – with final analysis QA (Quality Assurance) review at the OPO on Wednesday followed by Shuttle MMT disposition,” added the note.

By around 2pm Central time on Wednesday, the MMT officially cleared Discovery for entry.

The latest DAT presentation on L2 also claimed that a total of just eight tiles received damage – and just minor/cosmetic damage at that – on Discovery, which may be a record low.

Potential Conjunction:

On Tuesday night, engineers were informed of a potential conjunction threat that would be making its closest pass to the Shuttle/Station stack on Friday morning.

“Potential Conjunction – Ariane 5 debris (Obj 29274): TCA (Time of Closest Approach): 247/15:06 GMT, 6/11:06 MET (Friday 10:06 a.m. CDT). Overall miss distance 16.609 km. Tasking has been increased and valid PC expected Thursday morning,” noted MMT documentation on L2. However, that “miss distance” has reduced dramatically as tracking models are refined.

This in turn – pending how much a threat the object becomes – could lead to a Debris Avoidance Maneuver (DAM), which is not unusual, but would become more interesting given the issue related to Discovery’s vernier thrusters, which were supposed to be helping control the attitude of the Shuttle/Station stack.

D3“CHIT 7695 contains the options for mated debris avoidance maneuvers for the loss of verniers configuration,” added the MMT presentation on what has become one of the most-talked about subjects for managers – partly in thanks for the lack of any other issues with the orbiter.

The “loss of vernier configuration” is due to the problem relating to the F5R thruster that had leaked shortly after Discovery launched, resulting in the decision to close the manifold to both forward facing vernier thrusters – also know as VRCS (Vernier Reaction Control System), in order to contain the leak.

This in turn has impacted on Discovery’s role to provide attitude control for the Station, given the vernier jets – of which there are two located in the nose of the orbiter, and four located aft – provide the refined thrust required for aiding the Shuttle/Station stack’s orientation.

The job of controlling the ISS’ attitude is the primary role of the four Control Moment Gyroscopes (CMGs) and – if required – the thrusters on the Russian segment. However, the Station takes advantage of the docked orbiter to allow for the management of its own resources.

These ISS resources were used to rotate the Station 180 degrees – to place the Russian segment into the direction of travel and protect Discovery from heat exposure and MMOD – due to the inability for Discovery to aid the process via her now-disabled vernier thrusters.

However, the actual amount of propellant used by the Russian thrusters was a far greater than predicted.

“While the Russians were in attitude control of the mated stack after the mnvr to the mated TEA, they reported that prop usage was greater than expected and they need to swap prop tanks,” noted the MER.

Discovery could use her larger PRCS (Primary Reaction Control System) thrusters to provide the required maneuverability. However, the greater force produced by the PRCS could induce overloading on the Station and cause damage.

Work has been conducted on simulations that utilizes Discovery’s Digital Autopilot (DAP) to refine the firings of her PRCS to reduce the threat of overloading the stack, but these evaluations are mainly related to the maneuver of Discovery for undocking, with enough Russian prop onboard to deal with the attitude control role whilst Discovery is docked.

Plans are being worked on regardless, as managers look to manage propellant use between their available resources. Those expansive planning notes – available on L2 – go into great detail on the potential options, and pros and cons of each option, one of which includes the first time use of one of Discovery’s APUs (Auxiliary Power Units) whilst still docked to the Station.

“Contingency running of APU while Docked (for leak control): Due to adverse loading issues, ALT DAP control is not viable to allow running of an APU for leak control during docked ops. Would have to use either RS thruster control or USTO,” noted one element of the planning notes.

D4“To use Russian thruster control, GN&C (Guidance Navigation and Control) and loads analysis is required. Based on the time it takes to do the analysis runs, the analysis team needs until Friday to respond to this. Discussion continued on when will we know which of the two options are best (so can shut down the work on one of the tasks). A decision was not arrived at on how or when we could decide between the two options.”

A related item to address is a potential need for the stack to carry out the aforementioned DAM.

As of the MMT meeting on Wednesday afternoon, managers started to discuss the tradeoffs between a reboost or a deboost in order to avoid the debris – should it be required – which includes the potential to reschedule the timeline for EVA-2.

“Attitude Control/MNVR Plans: DAM – ALT to/from attitude and config 1 burn. Undocking – ALT for mnvr to undock, nominal flyaround. Running APU while docked – looking at USTO and RS Thruster modes,” noted raw notes from the MMT on L2.

“Debris Avoidance MNVR (DAM) – deboost option preferred for Soyuz. Reboost option would not impact EVA-2 – could occur prior to the EVA or after EVA2 complete. Would occur FD7 – using the +1 Day and sliding EVA2 to the right. Miss distance – 2.6 km with a shrinking covariance matrix. May need ATA thermal analysis for delaying EVA2 by a day. Deboost – 0.5 m/s.”

Questions are also noted on the minutes from the MMT meeting, with manager’s asking “is it possible to do deboost after EVA-2 if we get out the door early?” Which was responded by “Work impact crew sleep up to three hours and not work for downstream sleep phasing.

Astronaut Chief Steve Lindsey noted that there would be an “issue with crew sleep shifting for HTV, (and that they) would stay on this schedule if we add the +1 day.

This in turn was debated with MOD noting they “need to decide by the time you start the mnvr, crew is going to prebreathe tonight and call them off in the morning if DAM is required. Pc (refined data on the conjunction threat) becomes valid 30 hrs before TCA (4 am THUR) we might know by tonight a good estimate if we need to do this.”

According to the MMT notes, the “plan leaving here today is move forward for EVA-2 and call off if required,” while managers “understand the challenges and the trajectory specialist are going to work it and see if reboost is acceptable for Soyuz.”

So, at present, the STS-128 schedule remains the same, but with the option to move the EVA hatch opening to earlier on Flight Day 7, or delay the spacewalk by one day. This plan will be refined as more information on the potential need to carry out a DAM is known.

UPDATE: If a DAM is required, it will be carried out after EVA-2 – with the option for a deboost removed – thus the spacewalk will remain on its nominal timeline.

UPDATE 2: No DAM required following tracking showed the debris will pass by the ISS at the required safe distance.

L2 members: Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.

Related Articles