NASA, the United Space Alliance and Lockheed Martin have presented their flight rationale for the External Tank that will fly with Endeavour next month (STS-126), following an extensive investigation into a suspect noise from within the tank, heard shortly after its arrival inside the Vehicle Assembly Building.
ET-129 was being transitioned into the vertical position – following its arrival from the Michoud Assembly Facility – for transition into its checkout cell, when a handful of engineers heard a metallic noise coming from inside the tank.
Even though most of the engineers present didn’t hear the noise, managers immediately called for a full investigation – one which would reach as far as the Flight Readiness Review (FRR) and the Space Shuttle Program (SSP).
“Investigation Approach/Status: KSC-led integrated team was formed to investigate the source of the noise,” noted a SSP level investigation overview presentation (available on L2). “Fault tree approach used to identify potential source candidates.
“ET inspections (visual and NDE – Non Destructive Evaluation) were given top priority to provide SSP with additional data prior to ET/SRB (Solid Rocket Booster) mate milestone.”
Those inspections went on for several days, before approval was given to proceed to mating the tank with its SRBs, as Endeavour’s stack pressed forward with becoming the STS-400 LON (Launch On Need) shuttle in support of Atlantis’ now-postponed STS-125 mission.
Noise in the Tank:
A total of 15 engineers were present in the VAB when ET-129 was being lifted for its placement in the checkout cell. Interviews with all of the workers found that only five of them at ground level actually heard the noise as it was being tilted from a horizontal to vertical position.
“Description of Event: Suspect noise was heard by technicians when the tank was between 30 degrees and 45 degrees from horizontal, possibly originating from +Y Intertank region, with light metallic descending sound lasting around two seconds,” added the presentation.
Tanks are known to make noises, especially during their barge transit from New Orleans. Ground Support Equipment – such as lanyards and the crane – can also be blamed for such noises – though in this situation, all such equipment was cleared from the root cause.
Thus the concern related to the potential of FOD (Foreign Object Debris) being loose inside the tank, which would become a serious threat during ascent. Clearing the tank from FOD became the centralized approach for the investigation, regardless of how unlikely FOD would have been found.
“ET Internal Assessment Approach: ET internal FOD is unlikely based on strength of controls at MAF. Stringent in-process and back-out inspection process controls in place to ensure critical FOD/contamination is mitigated,” the overview continued.
“Quality verify visual clean of all ET internal volumes prior to assembly and following operations. MPS (Main Propulsion System) hardware cleaned (Type 1) and remains covered/protected prior to assembly. Multiple visual inspections performed.
“Inspections performed at KSC to provide additional flight rationale for critical hardware consistent with described scenario. ET FOD Prevention Program greatly enhanced post-Columbia. Stringent controls makes internal ET FOD unlikely.”
Even with the lack of FOD found during the inspections, the structure of the tank’s inner workings did not support the presence of debris, adding confidence to flight rationale.
“Inspections focused on hardware features where falling FOD could be retained. Inspections of internal LO2 and LH2 tank volumes not top priority based on scenario description and hardware configuration. Unlikely for sound to be heard from large insulated volumes and to be non-symmetrical.
“LH2 tank internal hardware does not support ‘sliding’ nature of sound (i.e. orthogrid, frames). Origin of sound heard aft of LO2 tank. Acceptable flight risk based on assumed FOD condition with additional mitigation (contained or additional controls).
“All planned inspections complete: No evidence of FOD identified. Investigation complete.”
Threats and Clearance:
FOD inside the tank holds three main threats to safety during ascent, all of which have been deemed as low risks, post-inspections.
1) Ingestion into the orbiter’s MPS: where FOD failed to be caught by one of the ET screens, and caught inside by a pre-valve screen in the MPS, but to the point it restricts the flow of propellant.
“Tank FOD is unlikely based on cleaning/inspection processes used at MAF (Type 1 clean maintained post wash),” noted Flight Rationale. “Critical blockage unlikely based on description of sound (i.e. light metallic) and strength of controls used at MAF.”
2) Ingestion of FOD into a SSME (Space Shuttle Main Engine) – caused by the FOD smashing through one or more screens – potentially causing an engine out scenario.
“Loss of screens is unlikely due to robust design and configuration,” reassured Flight Rationale. “Hardware configuration prevent high energy impact to screens.”
3) Potential of high energy impact of FOD on flight hardware causing a spark that leads to flammability/explosion.
“Assumed FOD in LO2 tank likely to have very low potential energy (i.e. low mass and located at tank bottom). Engineering assessments show other potential contributors are improbable and low risk based on hardware configuration, process controls, and built-in design controls.”
It is unlikely the source of the noise will ever be pinpointed – unless one of the downstream tanks suffers a similar incident that shows a potentially identical root cause, but thanks to built-in safeguards, and the additional inspections carried out in the VAB, ET-129 is clear to fly with Endeavour next month.
“ET FOD in Critical Systems is Improbable cause of suspect noise,” the presentation noted in closing. “Rigorous controls ensure cleanliness requirements and acceptable performance. KSC Inspections provide High Confidence for Critical Systems.”
L2 members: All documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, updated live.