STS-135: Engineers retest MFV – ET-138 stringer inspections completed

by Chris Bergin

Following its replacement on Atlantis’ SSME-3 (Space Shuttle Main Engine), the new Main Fuel Valve (MFV) underwent testing at Pad 39A – with a slight delay to its completion due to a hydraulic leak – aimed at providing confidence the hardware will perform as required during the launch countdown. Meanwhile, inspections on ET-138’s stringers have been completed with no obvious issues reported.

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Following the completion of the final S0017 Terminal Countdown Demonstration Test (TCDT) for the Space Shuttle Program (SSP), STS-135’s flow is now officially into preparations for the Launch Countdown (S0007 operations), as Atlantis closes in on her July 8 launch date target.

A sign the launch date is getting closer is usually marked with waves of tankers heading to the pad, with Friday no exception as two convoys of LH2 tankers replenished the Liquid Hydrogen tank at the pad complex.

With Atlantis safely tucked away inside the Rotating Service Structure (RSS), a final walkdown of her payload was also carried out on Friday, while the Sensor Package 1 (SP1) was re-installed and retested on the end of the Orbiter Boom Sensor System (OBSS) after showing signs of a problem last week.

At the top of the stack, technicians have wrapped up their Non Destructive Evaluations (NDE) on the circumference of the LO2 and LH2 flanges at the top and bottom of the Intertank.

The X-Ray and Backscatter inspections checked the support beams – known as Stringers – ensuring they remained crack-free, following the recent Tanking Test, which was called for after STS-133’s ET-137 suffered from cracked stringers ahead of Discovery’s first launch attempt last year.

No issues were expected, given ET-138 had already been modified with radius blocks, a proven mitigation procedure to strengthen the Stringers as they endure the stresses of cryo temperatures and pressure cycles during tanking.

“ET NDE operations: NDE X-rays on the LH2 flange were completed and are in review. This completes the ET NDE x-rays barring any required re-shoots,” noted the NASA Test Director (NTD) report (L2). “No problems have been detected on the x-rays that have been analyzed to date.”

ET/Stringer Specific Articles:

Work will continue over the weekend to remove the NDE equipment from the pad, pending the confirmation all the required data has been gathered.

No notable Interim Problem Reports (IPRs) have been charged to Atlantis of late, with the latest – IPR-50, relating to a SSME-2 GN2 heater issue – only requiring a micro-switch adjustment and solder joint repair.

STS-135 Specific Articles:

SSME-3 MFV Update:

The main IPR of interest – IPR-49, relating to the MFV on SSME-3 – is close to being resolved, as much as the real test will come on launch day when cryogenic propellant is once again rushed through the system. It was that environment during the Tanking Test which found the problem with the MFV, as outlined at the crew briefing provided by engineers during the TCDT.

“Problem: Engine 3 Main Fuel Valve leakage experienced during the ET tanking on 6/15/11. A contingency procedure was worked to isolate the fuel system (close all LH2 prevalves, LH2 recirculation iso valves and the LH2 hi-point bleed valve) that allowed us to continue with the tanking test,” noted the Crew Briefing presentation (available on L2).

The MFV is a ball valve with a 2.5-inch tubular flow passage and is flange-mounted between the high pressure fuel duct and nozzle diffuser. The valve controls the flow of fuel from the HPFTP (High Pressure Fuel Turbopump) to the coolant circuits and preburners.

Notably, a root cause of the leak is yet to be confirmed – something NASA engineers always prefer to have in the bag ahead of launch. It is hoped an obvious problem will be spotted by engineers at the MFV’s home in California, where the removed hardware was sent to this week, allowing for an overview to be provided at the L-1 Mission Management Team (MMT) meeting.

“Root Cause: Cause of valve leakage unknown until a failure analysis completed,” added the presentation. “Resolution: Valve removed, replaced and retested (but not under cryogenic conditions) before launch. An update on this issue will be given at the LCD L-1 briefing. Impact to this flight: None anticipated.”

The issue was also covered in depth via the STS-135 SSP Flight Readiness Review (FRR) presentation for the SSMEs, which covered how the issue was spotted during the Tanking Test, as it breached the Launch Commit Criteria (LCC) limitations. As a result, the issue would have scrubbed the launch day countdown, showing a bonus side-effect of finding the problem during the Tanking Test.

“Issue: STS-135, ME-3 (2045) Main Fuel Valve (MFV) skin temperatures indicated a MFV leak during the early stages of STS-135 tanking test. Temps violated minimum limit (LCC SSME-02). Tanking test continued with engines isolated from the fuel supply,” noted the FRR presentation.

The reference to the skin temperatures relates to sensors mounted to the outside wall of the downstream duct of the MFV to detect leakage during chill. Low temperatures are indicative of a MFV leak. The LCC limits are based on the vast flight experience of the Shuttle Program.

Although – as mentioned in the Crew Briefing presentation – no root cause has yet been found, a likely suspect failure path was presented to the SSP FRR.

“Most Probable Cause – MFV ball seal leak: Data appears real. Characteristics match previous ball seal leaks,” added the SSME FRR presentation for STS-135. “Scenarios assessed as unlikely: Liquid nitrogen dripping on sensors. Inconsistent with data characteristics and time in chill. Damaged instrumentation. Inconsistent with data characteristics and visual inspections.”

The FRR presentation also noted that there have been two other MFV leaks in the flight history of the Space Shuttle, listed as STS-2 – where metallic contamination found upon disassembly, and STS-73 – attributed to transient contamination (solid N2).

In addition to the above, 20 pre-start MFV leaks have observed during ground tests, half of which are attributed to solid N2 contamination, which is usually caused by an intermittent helium fuel system purge, allowing the solid N2 to form on the MFV.

By way of mitigation, a continuous purge was implemented into the procedures, meaning STS-135’s SSME-3 MFV issue is the first such leak since efforts were made to prevent sold N2 build up.

Almost immediately after the suspect leak was observed during the Tanking Test, engineers knew they would have to replace the MFV, resulting in a plan of action for work to take place at the pad.

“KSC Plan of Action: Ambient ball seal leak check completed – Zero leakage. Aft access Saturday, June 18. Detailed inspection in the area at KSC & Valve Room (on MFV and above MFV skin temp sensors). Valve removal midweek. Ship to Canoga, Californian for investigation,” added the FRR presentation.

“Borescope and visual inspection of fuel system. Replace with spare MFV. Perform standard system checkouts. Bubble soap leak checks. Valve and actuator functional tests. Helium signature and ball seal leak checks. FRTs Saturday, June 25. Helium Signature test Sunday, June 26.”

All work up to the weekend’s testing was completed on schedule, allowing for the Flight Readiness Test (FRT) to take place on Saturday, after the Dome Heat Shield around SSME-3 was re-installed.

“IPR 0049 SSME 3 Main Fuel Valve (MFV) R&R update: SSME 3 MFV electrical mates were completed. Heat shield installation is in work,” added the NTD report – with L2 updates noting the shield is now back in place. “MFV retest is scheduled for this weekend and will include SSME 3 FRT on Saturday and He Signature test on Sunday.”

UPDATE: Due to an unspecified hydraulic leak on the vehicle, the helium signature test has been delayed until at least 4am Monday. The issue is being tagged as IPR-53, pointing to two additional IPRs over the weekend. A full overview of the latest IPRs are expected via the NTD report on L2 around 7am Monday.

UPDATE 2: Leak was minor and SSME-3 MFV retests have been successfully completed.

These tests include the use of a mass spectrometer device, which will sniff for any helium leakage as the MFV is purged. Should no leak be detected, it is highly unlikely a leak will be suffered during launch day.

As such, the SSP FRR accepted the flight rationale for the engines to proceed to the Agency FRR at the Kennedy Space Center (KSC) on Tuesday, pending the successful conclusion of testing on the newly installed MFV.

SSME Specific Articles:

“Flight Rationale: LCC SSME-02 in place to protect for excessive MFV leakage. Safing procedures in place for hydrogen leak. SSME GN2 purge and helium fuel system purge dilute MFV leakage. Worst case impact is launch scrub. Pending results of valve and engine inspections. The Atlantis Main Engines are in a ready condition for STS-135.”

This article will be updated as information arrives on the weekend testing progress.

Fun Item:

Known by some of her engineers to be somewhat of a diva during some of her flows, with some of her dedicated workforce nicknaming her “Britney” (Spears) and others nicknaming her “the penguin”, both for seperate reasons which may become apparent when googling both names – Atlantis should now have a clear path for a managerial approval of the July 8 launch date.

She may also be on her best behaviour, after the Crew Briefing presentation noted Atlantis will be carrying an extra item into orbit next month, in the form of a man’s gold wedding band, which was listed as one of the items lost and unrecovered during her STS-135 flow.

Listed as one of five items on the “Lost, Not Found” pages of the Crew Briefing presentation, the gold wedding ring was noted as lost on the orbiter – in the Crew Module – back on March 7. Efforts to find the item have proven to be in vain and now will remain on board until Atlantis undergoes post-mission deservicing.

While the owner of the ring may have received an ear-bashing from his wife, the engineer can look forward to proudly announcing he was reunited with the space flown ring by Atlantis when she returns to her Orbiter Processing Facility (OPF)… providing she doesn’t take the opportunity to propose to another vehicle whilst on orbit.

(Images: Via L2 presentations and (KSC). Further articles on STS-135′s status in work, driven by L2′s fast expanding STS-135 Special Section which is already into the FRR content and live flow coverage, plus more. As with all recent missions, L2 is providing full exclusive level flow and mission coverage, available no where else on the internet. To join L2, click here:

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