Atlantis into pressurization tasks – numerous SSME items discussed at FRR

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Engineers are heading into a weekend of critical operations for STS-132, as Atlantis undergoes flight pressurization operations on the systems that will aid her journey to the International Space Station (ISS) in a week’s time. Wednesday’s Agency Flight Readiness Review (FRR) focused on one of those systems – the Space Shuttle Main Engines (SSMEs) – providing reviews on several items of interest.

STS-132 Processing Latest:

All continues to proceed to plan out at Pad 39A with one week to launch. Engineers have completed aft closeouts on the vehicle ahead of flight, while Atlantis’ Russian payload passed several checks on its status inside the Payload Bay.

“OV-104 / ET-136 / SRB BI-143 / RSRM 111 (Pad A): Orbiter Aft closeout is complete. The 50-1 & 2 doors have been installed for flight,” noted the NASA Test Director (NTD) via processing information on L2. “The payload MRM1 initial state/environmental checks (S07132) were successfully completed Thursday night.

“Payload Bay Doors will be cycled open then closed to support payload operations and to inspect the bulb seal repair completed earlier this week. LH2 storage tank replenishment and GO2 recharge operations are complete.”

The timeline continues to support the start of Launch Countdown (S0007) operations next week, while engineers still hold the one day of contingency – if required – in the flow. Last week’s Interim Problem Report (IPR 29) is still undergoing closure, although it isn’t causing any interruptions to operations.

“S0007 Launch Countdown preps are in work and will continue through the week. IPR 0029 Partial Pressure Oxygen (PPO2) sensor C failure to calibrate) update: Calibration will be performed today (Friday),” added the NTD.

A new IPR was added to status on Friday – only the 38th in the entire STS-132 flow – which will require a waiver, while pressurization tasks are conducted over the weekend.

“NEW IPR 0038. While securing from S1006, Space Shuttle Main Engine (SSME) pneumatics system was secured prior to engine controller securing, causing the engines to blow down with engine bleed valves only, violating an OMRS (Operations and Maintenance Requirements and Specifications) requirement. Will seek a waiver of the requirement,” added the report.

STS-132 Specific Articles: http://www.nasaspaceflight.com/tag/sts-132/

“Weekend Work OMS/RCS OMS (Orbital Maneuvering System)/RCS (Reaction Control System), SSME and MPS (Main Propulsion System) helium and nitrogen tank pressurization to flight mass (S0071) on Sunday.”

SSME FRR:

Atlantis will fly with Main Engines 2052, 2051 and 2047, aiding Atlantis’ ascent during first stage, and responsible for the completion of the eight and a half minute ride into initial orbit during second stage flight.

The performance of the engines has been superb during the vast majority of the shuttle program, with recent FRRs showing hardly any items of interest for managers to discuss at the meetings. However, STS-132′s FRR for the SSMEs was relatively packed with items.

The first issue listed came via an observation at the pad with Atlantis’ engines.

“Description: Walkdown Inspections of E2051, ME-2, nozzle 5007 identified two small nicks on nozzle tubes ~8 inches from aft manifold. Nicks occurred during repair of tube 1036 in June 2005,” noted the Agency FRR presentation for the SSMEs, one of 50 FRR presentations available on L2.

Walkdowns of the vehicle at several stages of the flow is a required procedure for the engineers tasked with ensuring the condition of the hardware remains perfect during processing and the pad flows.

Following the discovery of the small nicks in the nozzle, a decision was made to either repair, or fly as-is with flight rationale.

“Analysis: Detailed mold impression taken to determine depth. Structural analysis completed – CEI requirements maintained. Digital microscope inspection confirmed no cracks. Use-as-is disposition approval ECD (Engineering Control Board) – 5/5/10,” added the presentation.

“Flight Rationale Structural assessment shows CEI requirements maintained. Since the repair of tube 1036, the nozzle has undergone: A 5 cycle proof pressure test @ 8675 psi. 3 Ground Tests (including one unrestrained test). 2 Flights (STS-119 and STS-128). No leakage or anomalies have been noted in this area since the nicks occurred.”

As with all FRRs, the previous flight’s IFAs (In Flight Anomalies) make up a large section of the next flight’s FRR. Interestingly, STS-132′s SSME FRR reviewed several flight’s worth of IFA-level content, including previously unreported notes from STS-130.

“Issue: Areas of discoloration, melted screen on Nozzle 5014 (Engine 2061). Concern is potential jacket annealing,” added the FRR presentation, which referenced a potential concern the discoloration may have been caused by the engines experiencing higher than normal re-entry temperatures – an undesirable situation.

“Three observations: Small melted area of thermal protection system (TPS) screen. Similar occurrences on 5 previous flights. Screen melts at ~2600F. Discolored paint stencil on jacket in one flight. Normally occurs over multiple flights. Discolors in minutes at 400F, and seconds at 1900F. Hotwall surface discoloration/bluing near hat bands 5 and 6. No record of similar hotwall bluing.

“Investigation: Identify source of heat discoloration/damage. Evaluate effects on nozzle.”

Lab tests were utilized to aid the investigation results, which found that outgassing – rather than higher heat exposure – is most probable cause of the bluing or discoloration.

“Coolant tube bluing reproduced in lab tests: Initial testing with heat only. Temperature to blue >1200 F. Normal reentry temperatures 600 to 850 F. Manifold coloration did not indicate reentry temperatures above normal. Individual areas of bluing inconsistent with global heating of jacket,” the investigation notes added.

“Conclusion: Tube bluing during flight due to heat alone is unlikely ‘out-gassing’ – Nozzle 5014 gaps within spec, on high side of family. Each blued area surrounds a tube gap. Conclusion: outgassing is most probable cause of bluing on N5014.

“Nozzle structural integrity assessment: Melted screen. Minor localized damage, repair per standard process. Stencil – No significant damage. No tube damage. No tube leaks post flight. No annealing or softening of nozzle jacket. Confirmed by oxalic acid etch testing. No jacket wrinkles or distortion. No effect on nozzle structural integrity.”

In order to gain flight rationale for STS-132, Atlantis’ SSME nozzles were checked and found to be in excellent condition.

“Flight Rationale: Post flight inspections confirm Nozzle 5014 did not incur structural damage. Inspections and testing indicate tube discoloration occurred post-MECO (Main Engine Cut Off). No credible scenario to discolor hotwall while hydrogen coolant is flowing through tubes. Nozzles on STS-132 inspected and meet all requirements.”

The checks on STS-132′s nozzles are of a greater interest than usual, given Engine 2051′s nozzle is flying after 60 areas of repair were carried out, following a run of flights where corrosion at the pad resulted in numerous pinhole leaks – though none had any effect on performance or safety.

The root cause was soon found, pointing back to the the lining of the nozzles becoming contaminated with chlorides on an application sponge during the flow, resulting in corrosion-related leaks.

“Issue: First flight of Nozzle repaired for out of family number of leaks. Cause identified and validated. Nozzles 2031, 4027, 5007 and 2035 contaminated with chlorides resulting in corrosion leaks,” confirmed the FRR presentation.

“Chloride source identified as sponge applicator used during post flight processing (3x more chlorides than sea water). Chlorides combined with humid environment to accelerate corrosion in braze shrinkage cracks. Resulted in out of family number of microscopic corrosion leaks.”

The largest number of pinholes in a nozzle was recorded on STS-127, with 340 leaks noted on nozzle 2031. A large number of leaks was noted on nozzles for several flights, prior to a change of procedure with the application process – the removal of the use of a sponge, and extra protection out at the pad – solved the issue.

Although it was feared some nozzles would have to be scrapped, all four nozzles that suffered from leaks have been repaired. The first repaired nozzle flying with STS-132 is number 5007, which suffered from 63 leaks during its role with STS-128.

“All four subject nozzles successfully repaired per standard repair process: Standard Tungsten Inert Gas (TIG) overlay. Sectioning of samples validated repair technique. Size and spacing of repairs within previous experience. No indication of dense areas of corrosion,” the presentation continued.

“Integrity of repairs validated using Nozzle 2031 with operational pressure test to 109 percent power level. No repair leaks or failures. Corrective actions to mitigate future corrosion. Higher concentration corrosion inhibitor applied to extended area using compatible materials. Nozzle conditioned air purge in VAB and on Pad implemented to reduce environmental moisture.”

As noted, the leaks are a repair timeline and manifest issue, as opposed to a safety issue. The debut of a repaired nozzle will be in combination with two nozzles that have not suffered from the same problem.

“Corrosion leakage levels experienced are low and have large margin to engine performance effects. Worst case leakage on Nozzle 2031 (340 leaks) maintained large margin to redline limits.

“STS-132 nozzle leakage history: ME-2 E2051 – Nozzle 5007 – Exposed to chloride contamination. All 63 corrosion leaks successfully repaired. ME-1 E2052 – Nozzle 5004 – No indication of chloride contamination. Leakage history low. ME-3 E2047 – Nozzle 2030 – No corrosion leakage detected, potentially exposed to chlorides.”

Flight rationale was reasonable easy to achieve, due to the lack of risk involved with flying a nozzle that has previously leaked, even if further leaks are observed during flight.

“Flight Rationale: STS-132 units and all four repaired nozzles meet flight leakage criteria. Nozzles repaired using standard repair process. Corrosion inhibitor applied. Conditioned air purge minimizes environmental moisture. Overall leakage rate on corrosion nozzles remains low with respect to redline limits.

“Leakage well within limits safely demonstrated in ground test. Large margin to engine performance effect. No impact on measurable data in prior flights. Experience and analysis demonstrate large changes in leakage due to corrosion are not credible.”

Another issue was listed from STS-131, when one SSME’s Advanced Health Monitoring System (AMHS) noted a problem at T+445 seconds. However, according to the FRR presentation, flight data noted no problems were caused on the engine itself.

“Issue: Disqualification of High Pressure Fuel Turbopump (HPFTP) 21 degree Accelerometer during STS-131 ascent. Background: Controller reporting indicated disqualification due to high noise floor. Disqualification requires fixed and synch noise floor levels above 3 Gs for two consecutive 50 msec cycles. Monitor disqualifies one accelerometer and switches from 3-of-3 to 2-of-2 voting logic,” noted the FRR.

“If monitor disqualifies 2nd accelerometer on a turbopump, redline would be deactivated for that turbopump. Real time data indicated instrumentation spike and not turbopump anomaly. Momentary increase in synch and return to normal levels. Spike observed on only one of three accelerometers. Multiple spikes observed during chill – not unusual.”

With HPFTP accel noise spikes deemed as common, and a worse case scenario resulting in LOV/C (Loss Of Vehicle/Crew) as unlikely as 1 in 763, flight rationale was once again accepted, fully clearing Atlantis’ SSMEs as ready for flight.

“Analysis: Flight data analysis confirms nominal HPFTP performance. Post ascent AHMS memory dump data used to assess noise level. High noise floor confirmed as cause of disqualification. Measurement spiking not unusual. Algorithm designed to discriminate between instrumentation and turbopump anomalies.

“Predicted probability (2002) of disqualification of 1 sensor in 9 flights. Predicted probability (2002) of loss of redline 1 in 763 flights. AHMS functioned as designed.

“The Atlantis Main Engines are in a ready condition for STS-132.”

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