Adhesive tape caused STS-127 SSME nozzle leaks – STS-130 SSME milestone
Pratt & Whitney Rocketdyne engineers have found the application of “tacky tape” – used to seal the throat plugs on the Space Shuttle Main Engines (SSME) since the 1990s – was the initial root cause of 340 leaks on Endeavour’s number one engine during STS-127’s ascent. Meanwhile, three SSMEs are scheduled to be installed on Endeavour next month, ahead of her STS-130 mission.
The ever-reliable SSMEs are recycled and reused for numerous missions, with engine 2047 – for example – set for a 14th flight with Atlantis on STS-132, following its successful flight with Discovery on the recent STS-128 mission.
With the SSMEs are already installed on Atlantis for the STS-129 mission in November, the upcoming engine milestone relates to STS-130 with Endeavour – which will actually include a first flight engine (SSME 2061), after it became the final engine to be delivered to the Kennedy Space Center (KSC) at the end of last year.
The engine has already had an eventful life, after testing found the Fuel Preburner FG1a elliptical ring was observed to be cracking, following an acceptance test. The FG1a elliptical ring provides access to fuel preburner (FPB) and HPFTP (High Pressure Fuel Turbopump) turbine for post hot fire borescope inspections.
“Cracking of elliptical ring, weld and/or liner is not uncommon,” explained the documentation presented to a recent Flight Readiness Review (FRR). “Borescope inspection of ring and weld required post every hot fire.
“Rules limit life of FPB liner and ring to 24,224 seconds with pump out visual inspection of ring at 6,056 second intervals. Erosion, pitting and/or cracking is unacceptable and results in liner and ring replacement.
“Concern is loss of piece greater than critical impact mass (0.142 g). No significant loss of mass experienced in program history. Multiple cracks required to liberate a particle of significant size.”
Also adding confidence was the overall test result for engine 2061, which was deemed to be ‘in family’ for its requirements. The HPFTP was replaced to ensure the engine is ready to fly.
“Engine 2061 Assessment: Detailed post test inspections noted no other hardware anomalies. Engine performance (temps, pressures, flow rates, speeds, etc.) and measured dynamics all as expected and ‘in family’,” confirmed the FRR presentation.
The reason cracks warrant an extensive evaluation is due to the threat of FOD (Foreign Object Debris) which would be a serious problem, should a crack liberate FOD into the super fast spinning turbine blades. However, such is the safety record of the SSMEs during flight and testing, a probability of that threat being realized is a staggering 1 in every 150,000 flights.
This engine will become engine number 2 on Endeavour for STS-130, joining SSMEs 2059 and 2057 – which are flying on their fourth and fifth flights respectively – although Thermal Protection System (TPS) work on engine 2 is being conducted ahead of its installation.
“SSME (Pratt & Whitney Rocketdyne/KSC): The next major milestone is engine installation on October 19, STS-130. Have allocated engines 2057, 2059, and 2061,” noted information on the latest Shuttle Standup/Integration report on L2.
“In the engine shop, 2057 and 2059 are in walkdowns so they are through all their major milestones. Engine 2061 is through the major milestones, but need time to put the nozzle TPS on since that was a brand new nozzle and was hot-fired without any TPS.
“It will take 3+ weeks to finish the TPS on 2061, but are targeting to have that engine ready for October 19.”
STS-127 SSME 1 Nozzle Update:
The failure investigation team – set up to evaluate why STS-127’s SSME number 1’s nozzle was found to have hundreds of small leaks post-flight – have revealed the leading candidate root cause.
“During the post-flight work of STS-127/2JA, while performing nozzle tube leak checks, Engine 1 (SN 2045) had 340 tube leaks,” noted an update on the latest Shuttle Standup/Integration report on L2.
“The total result for H2 loss is fairly small, as these are pinhole leaks. Four areas in the nozzle were sectioned and sent to Canoga Park for evaluation. The mechanism driving the corrosion is understood.”
The problem was caused by the sulfur contained in the adhesive tape that is applied to the throat plug area of the engine nozzle. The sulfur has been found to interact with the nozzle’s nickel finish under the extreme temperatures endured during their ascent role, causing embrittlement and then tiny cracks and corrosion of the A286 iron which the SSME nozzle tubes are made from.
Corrosion was already noted on the engines ahead of Endeavour’s flight, although engineers thought they had found the nozzle to be in good shape after the rust-like substance was cleaned from the hardware.
“The problem stems from the application of Tacky Tape to seal the throat plug during the 1990’s. The tape adhesive contained sulfur, and when this interacted with the nickel finish of the nozzle at temperatures in excess of 1000 degrees Fahrenheit, it embrittled the nickel, causing cracks,” added the Standup report.
“These cracks, in turn, allowed moisture to enter and attack the A286, resulting in the corrosion and pitting.
“In 1999, cylohexane was used to clean the sulfur residue, but the nickel finish was already compromised. To combat this problem, a corrosion inhibitor and clear-coat finish was applied. The Tacky Tape was originally applied 3-6” below G15, with the corrosion inhibitor and clear-coat applied 1-8” below G15 to fully cover the area.
“The 340 leaks that were discovered appear to have the same mechanism (crack through the nickel caused by embrittlement, followed by pinhole corrosion pits that caused the leak).”
The 340 leaks on SSME 1 was not a critical issue, as the combined leakage from the pinhole cracks was well below that of – for example – STS-93’s eventful ascent.
Endeavour suffered no ill effects from the leakage during her ascent, while Discovery is also believed to have suffered from leaks during STS-128’s ascent – though far less than STS-127 – and again the engine performance has been deemed as nominal.
A forward plan is being worked by engineers, which may include pumping dry air into the nozzles in order to protect the hardware from the Florida humidity.
“The Failure Investigation Team will meet to discuss actions that could be taken to protect the nozzle hotwall surface when it is outside of a controlled environment,” added the Standup report. “This could include plumbing dry air or N2 into the nozzle cavity, using some additional close-out covers like those used during rollout to protect from the warm, humid Florida air that attacks the A286.”
The damaged nozzle has now been removed from the SSME and will be substituted by a flight spare. The nozzle will be sent back to Canoga Park for either a repair or scrapping.
“From a total asset point, there are 16 nozzles at KSC – but 12 have been exposed to the Tacky Tape. These were treated with the corrosion inhibitor and clear coat. On engine SN2045 with nozzle unit 2031, the nozzle has been separated from the power head and the spare nozzle unit 4026 will be used to return the engine to the processing cycle.
“Nozzle 2031 will be shipped from KSC, because the amount of work required to repair the 340 leaks exceeds what the shop is capable of doing.
“Will cut a few more tube samples from the nozzle from STS-127 that had the 340 leaks, and get those back into the lab. Once get good lab data from those samples, the plan is to ship that nozzle back to Canoga Park to start either additional investigation or repair of those tube leaks.”
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.