STS-133: Discovery through PRSD load tasks – Stalwart SSMEs reviewed

Discovery is enjoying an issue-free S0007 (Launch Countdown), as engineers complete the loading of the orbiter’s Power Reactant Storage and Distribution (PRSD) system. The veteran orbiter remains on track for an opening launch attempt on Wednesday, ahead of the ride uphill – to be aided by her three Space Shuttle Main Engines (SSMEs), which received a full overview at the Flight Readiness Reviews (FRRs).

STS-133 Pad Flow Latest:

The loading of Discovery’s PRSD systems allows the orbiter to store the reactants (cryogenic hydrogen and oxygen) which will be used to supply the three Fuel Cells (FCs) – which in turn provide all the electrical power for the vehicle on orbit.

Loading of the reactants is always scheduled early into S0007 operations, along with any required offload, which in turn holds relation to the fine-tuning of the Ascent Performance Margin (APM).

“OV-103 / SRB BI-144 / RSRM 112 / ET-137 (Pad-A): The STS-133 S0007 Launch Countdown is underway and proceeding nominally along the timeline. Call to stations was completed on Sunday at 1330 hrs and the countdown clock picked up and began counting at 1400 hrs,” noted the latest NASA Test Director (NTD) processing report (L2).

“Avionics configuration and checkout completed on Sunday. Preparations for PRSD load completed this morning (Monday) at 0500L. The vehicle and pad were configured for remote operations in preparation for orbiter, SRB (Solid Rocket Booster) PIC tests and PRSD load which is scheduled to begin shortly.”

PRSD loading began at 11:30 local on Monday, continuing through the first shift with the pad re-opening at approximately 17:30 -  followed by the planned PRSD LH2 offload. Preparations were also carried out on the External Tank, ahead of Wednesday morning’s tanking operations.

The NTD also officially cleared two of the recent Interim Problem Reports (IPRs) from Discovery’s list of issues, following the successful resolution of troublesome Quick Disconnects (QDs), in order to allow the completion of the pressurization tasks on Discovery’s Right OMS (Orbital Maneuvering System).

“IPR-52 RH OMS GN2 QD 525 update: All work is complete and the problem was resolved over the weekend. Flight cap installation and door closeouts were worked on Sunday. Troubleshooting over the weekend removed and replaced the QD on Saturday with additional troubleshooting performed when the GN2 tank continued to vent. This issue was isolated to a GSE valve. The tank is now pressurized for flight,” added the NTD.

“IPR-53 RH OMS GHe QD 515 update: This issue was also resolved with all troubleshooting and corrective measures completing prior to S0007 CTS (Call To Stations). The QD was successfully removed and replaced on Saturday with flight pressurization finishing early on Sunday morning. Mass specs were also worked on Sunday morning with no discrepancies.”

Two additional IPRs have been listed by the NTD – both minor and hold no impact to the flow – the latter of which was noted later on Monday, relating to a “primary panel dome and handrail” which will require changing out late on Monday. IPR-54, meanwhile, related to a display issue on an event timer, which was in the process of being cleared at the time of Monday’s NTD report. 

STS-133 Specific Articles: http://www.nasaspaceflight.com/tag/sts-133/

“IPR-54: Picked up IPR against Forward Event Timer, 10th digit top left is out. T/S (Troubleshooting) plan is to look at Fault Light documentation. Constraint is S0007.200 15-507. T/S reveals Event Timer operating nominally with one segment out. (Engineering) recommends going MR (Material Review) to fly as-is and the test team is in concurrence. Will verify the Commander has no issues with flying as-is when the crew wakes up.”

SSMEs Ready To Fly:

On Tuesday, the Space Shuttle Main Engines (SSMEs) will undergo their final preparations for launch – a processing task set to begin once the count is released from the 8 hour Built In Hold (BIH) at T-19 hours.

UPDATE: (Task was on hold due to IPR-58 relating to Main Engine Controller (MEC) issue. Engineering Review Board (ERB) discussing – and cleared after recycling – before once again showing problems. New article pending.)

As per usual, the three workhorses on Discovery’s aft are expected to perform as advertised during ascent, with Flight Readiness Review documentation (over 50 presentations, available on L2) overviewing their status.

All three of Discovery’s SSMEs last flew with Atlantis during STS-129, although in different positions – after they required removing and re-installing in different positions, in order to allow a changeout of ME-1′s Low Pressure Oxidizer Turbo Pump (LPOTP) early in the flow.

For STS-133, Main Engine 1 (ME-1) is serial number 2044, ME-2 is 2048 and ME-3 is 2058. All their related hardware is the same as that which flew with Atlantis, bar a couple of elements, such as a new nozzle for ME-1.

As is usual for the FRR process, previous flight experience and previous In Flight Anomalies (IFAs) are brought to the table for discussion.

These included a one page review regarding IFAs as far back as STS-130, such as “Nozzle 5014 Thermal Discoloration & Damage,” and an “Accelerometer Disqualification” issue on STS-131, all of which were minor and classed as “closed”.

The last flight – STS-132 – was the main subject of the FRR’s deliberations, which concentrated on one of Atlantis’ IFAs, relating to Nozzle 5007′s Flow Recirculation Inhibitor (FRI), which was seen to be frayed in an “area of higher gap and negative protrusion” at the top of the nozzle.

“Background: FRI is used to reduce hot-gas flow and seal bluing in area of MCC/nozzle G15 joint. Bellows seal prevents leakage to aft compartment. Fraying identified during standard post flight inspections,” noted the Agency FRR presentation for the SSMEs.

“Nozzle demated from MCC (Main Combustion Chamber). No bellows seal discoloration. Materials evaluation of FRI showed broken fibers, no indication of overheating or melting.”

The documentation continued by noting the history of this specific area of the engine, along with how improvements have continually been made over the near-30 years they have flown with the Space Shuttle.

“Original joint configuration had very small gap between MCC aft tip and nozzle tubes. No seal discoloration in the first 25 flights (no FRI used),” added the presentation. “Re-occurring issues with MCC and nozzle tube contact/damage (led to) gap between MCC and nozzle increased to mitigate MCC damage.

“After STS-26R, an overheated and cracked bellows seal was found (no FRI used). Analysis and testing defined influence of cavity entrance geometry on hot-gas recirculation into bellows seal cavity. Crown leakage also identified as a contributor to recirculation. FRI introduced to inhibit recirculation flow.

“First flight of FRI was STS-33R. FRI appeared to reduce, but not eliminate recirculation. Thickness of FRI later increased to improve effectiveness.”

Also referenced was the complex testing and analysis via Computational Fluid Dynamics (CFD) models, in order to simulate circumferential recirculation in bellows seal cavity, with the results noting the Nozzle/MCC gap and nozzle tube protrusion cause hot gas recirculation, leading to the introduction of the FRI material.

“FRI developed to reduce hot gas flow to the G15 seal. (Made from) Ceramic fiber rope (which) withstands high temperatures.

“(It is) pliable (and) provides a flow resistance to reduce circumferential circulation in the G15 cavity. Installed in 4 sections (2 long and 2 short). Long sections clocked in high protrusion areas,” the presentation continued.

“Standard FRI (was used between) 1988 to 1995. Seal bluing on units with large gaps (was noticed, leading to) enhanced FRI (being) developed. Contains approximately 5 times more batting than standard FRI. Blued seals occurred less frequently. Always associated with high gap and protrusion and/or leakage.”

The FRR also looked at recent flight history, such as the numerous – yet tiny – nozzle leaks suffered on STS-128′s launch (originally seen during STS-127), cited as a contributor to a weakened/shifted FRI, although this caused “no damage/effect on (the G15) seal,” thus aiding the subsequent flight rationale for both STS-133 and STS-134.

“Rationale for flight: Frayed FRI did not affect G15 seal during STS-132. Significant hot fire experience without FRI. Dominant influences in seal bluing are gap/protrusion/leakage. STS-133 and STS-134 units have gap and protrusion within acceptable limits per hot-fire experience.

“STS-133 and STS-134 have no significant leakage near G15 (seal). STS-133 and STS-134 nozzles have no history of seal bluing.”

The relevance to STS-134 is noted due to Endeavour holding the role of Discovery’s LON (Launch On Need) vehicle. Managers always aim to be confident of clearing any potential issues on all future flights, in order to avoid the potential nightmare scenario of a problematic launch during a rescue mission – as much as requiring a LON mission is highly unlikely.

“Launch on Need Assessment: STS-134 Engines: ME-1 2059 ME-2 2061 ME-3 2057. Engines installed on Endeavour 27 – 29 July 2010,” added the presentation. “Engines reviewed and ready to support contingency if required. Spare Engine set: ME-1 2045 ME-2 2060 ME-3 2047. Post-flight processing complete.

“All current flight and ground test anomalies have been evaluated with respect to impact on Flight Engines.”

Thanks to the historical and current confidence gained via the SSME FRR evaluations – which will prove to be priceless if, as currently expected, the SSMEs live on via the Heavy Lift Launch Vehicle (HLV) or Space Launch System (SLS) – managers polled to give their approval for the SSMEs to help Discovery make her final ascent into space.

“SSME Mission Readiness Assessment: The Discovery Main Engines are in a ready condition for STS-133.”

(Lead Image via HD QuVIS Video of SSME Start (L2). Graphics via L2. SSME installation image via NASA.gov)

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