STS-128 landing moves to Friday – Vernier Thrusters recovery options outlined

by Chris Gebhardt

After 13-days on orbit and a spectacularly successful logistics mission to boost the science capability of the International Space Station (ISS), Discovery and her seven member crew will spend an extra day on orbit, as unacceptable weather conditions over the Kennedy Space Center (KSC) failed to clear in time for both of the landing opportunities on Thursday.

STS-128 Status:

As Discovery entered what was expected to be her final day on orbit, Mission Controllers continued to revel in the extremely clean nature of Discovery and her various systems.

neptec“The mission is proceeding nominally and per the timeline. Landing is scheduled for today at 1905L at KSC,” notes this morning NTD report on L2.

TPS (Thermal Protection System) late-inspections are complete and there are no concerns for reentry.

All Flight Control System (FCS) and Reaction Control System (RCS) checkouts were completed without issue on Wednesday and pre-landing cabin configurations on FD-13 (Flight Day 13) are complete.

Furthermore, the Wing Leading Edge sensor system – the WLE IDS (Impact Detection System) was deactivated for landing on Wednesday and the Ku-Band communications antenna was successfully stowed as well.

Thursday operations saw the crew carry out the required preps for landing, as controllers left it to the last moment in the hope the weather would clear.

Discovery’s Payload Bay doors were closed for entry at 3:19P.M. EDT and Mission Control gave a “GO” for the transition to Ops 3 software at 3:29P.M. Commander CJ Sturckow, and Pilot Kevin Ford, will strap into their seats on the Flight Deck just before 5P.M. and will perform an OMS engine gimbal check at 5:16P.M.

However, due to poor weather over KSC, the first opportunity was waved off.

The second – and final – opportunity would have resulted in a Deorbit Burn at 7:39P.M. for a landing on Runway 15 at 8:41P.M. However, despite leaving the call to just 10 minutes head of the burn, the forecast failed to improve enough – with the threat of a wet runway just one of the concerns.

The same weather concerns are expected for Friday as well at KSC; however, Edwards Air Force Base in California is expected to be available as well.

Also of note on Thursday’s NTD report is the upgrade of the Hydraulic Right Hand Outboard Main Landing Gear Brake Line temperature transient to a Mission Evaluation Room Problem – it had been a “funny” up until now.

There were also two new “funnies” on the latest report – the lack of HUD (Heads Up Display) video downlink and the F1D thruster which is nearing its temperature operating limit.

Options for Recovering the Vernier Thrusters on Discovery:

Following the failure of the F5R Vernier thruster on FD-1, and the shutdown of the Vernier Reaction Control System (VRCS) on FD-2, Mission Controllers developed various plans for recovering the VRCS during Discovery’s mission if the need arose.

The recovery options presentation – available for download on L2 – outlines four options for recovering the system.

D2“Option 1: Opening F5 (directly pressurize from tank),” which would have been the simplest option to implement as the procedure was performed during qualification testing.

“Proof pressure level exceeded but no damage or leakage noted. Max surge pressure of 1,541 psia measured on Ox side; Fuel side remained below 806 psia. Exceeds thruster valve proof of 1500 psia and feedline/manifold iso valve proof of 1130 psia. Burst margin of safety still positive,” added the presentation.

As such, this would be acceptable for flight contingencies.

The Cons for this option were the exceedance of proof pressure by ~36 percent and the possibility of creating a worse leak in the system.

The remaining three options all deal with staged repressurization of the system.

“Option 2: Crew valve cycling only. Close Prop Tank Isos and/or Manifold Isos; Open F5 manifold valve.”

This option would have put the lower propellant volumes on line to minimize surge levels and could have been performed by the Flight Crew via cockpit switch throws.

Nevertheless, the exact procedures for this operation still need to be developed. Furthermore, there was “No direct insight into manifold 5 to know when it is fully repressed.”

“Option 3: ‘Pulse’ F5 manifold valves. Close tank isos. Then send both open/close F5 manifold valve commands in rapid succession.” This option would theoretically limit the incoming volume of propellant to “minimize surge pressures.”

This would give controllers the option to check for leaks after the original F5 closure on FD-2. That way, if the leak were to re-occur, the closed valve would limit any contamination issues. However, the crew’s cycling of the F5 manifold may have been no different than Options 1 and 2 with no surging limiting.

Option 3 would have required “Flight Software command to be faster and may not have reduce surges over option 2,” notes the presentation. Further, it would have required valve cycles in 10 ms when the fastest available is only 40 ms.

“Option 4: Use a primary thruster firing to further reduce manifold pressure,” had the potential to reduce surge pressures and could be monitored via instrumentation.

Nevertheless, this option would have transmitted loads to the Space Station’s structure that could have been above limits and the configuration for any mated burn would have been “challenging.”

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.

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