Shuttle managers have created a waiver that allows Endeavour to proceed towards S0007 (Launch Countdown) operations, following a full review of the amount of debris found inside the orbiter after the failure of a filter she was hooked up to inside OPF-2.
The problem with the HEPA filter contamination had threatened her original rollover date, before engineers were satisfied they could clear out most of the remaining debris prior to the targeted March 11 launch date.
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The HEPA (High Efficiency Particulate Air) filters are used in the Orbiter Processing Facility to remove the possibility of airborne particles entering into the orbiter’s critical hardware. They are also used at the pad to eliminate microscopic particles from the cargo bay prior to launch.
However, the HEPA filter in question failed, due to a build-up of contamination on the filter from the outside air, over-stressing it rather like a blocked vacuum cleaner, leading to parts of the failed filter itself being blown into Endeavour via her ‘purge circuit one’ ducting.
The material that was blown inside Endeavour consisted of borosilicate micro-fiberglass cloth and wafer thin aluminum sheeting.
Once the issue was spotted, borescope inspections of Endeavour were ordered. This process included checks on all of the duct system in the aft compartment, Payload Bay, Forward Reaction Control System cavity ducting and four feet of OMS Pod main ducting – among other areas.
Parts of the failed filter were found, which included pieces of foil and fiber material that makes up the barrier in the filter. Some of the material is located in areas that means it is impossible to remove. In fact, the filter was in such a bad condition, it literally fell apart during its removal.
This immediately caused concern with some departments of the Orbiter community, with one e-mail to managers classing Endeavour’s situation as grim – claiming: ‘OV-105 (Endeavour) has had a problem and will not be cleared for launch as the rescue flight,’ just hours prior to Atlantis’ launch on STS-122.
The plan would have consisted of Discovery taking up station as the rescue orbiter, should Atlantis have suffered a serious problem on orbit.
However, a ‘positive pressure’ test was conducted to see if any of the debris would cause a problem for the orbiter, which gained good results, as noted in the waiver that will go to the Flight Readiness Review (FRR) later this week for approval of flight rationale.
‘The OPF-2 ECS (Environmental Control System) Room FWD circuit HEPA slowly degraded over time when the pressure across the filter (Delta P) exceeded the structural capability of the filter,’ noted an associated presentation, available on L2, that was reviewed by a daily PRCB (Program Requirements Control Board) meeting on Monday afternoon.
‘Analysis of the filter media showed normal levels of carbon and higher levels of sulphur. This indicates that outside air, not the upstream carbon filter, was the primary cause of filter blockage.
‘This filter was installed in 1983, which indicates that normal build-up of contamination could cause significant blockage or the filter media could degrade due to age and cause the filter to fail below its rated delta P. The upstream side of the failed filter media is black instead of white which confirms contamination build-up.
‘The filters are visually inspected every three years, and the purge air from this circuit is sampled and verified to meet cleanliness requirements each time an Orbiter rolls into the (OPF High) Bay.’
The waiver also covered a non compliance to set rules on how long an orbiter is allowed to have a purge outage via a failed filter. However, this is not deemed to be a problem.
‘The eight hour purge outage has a very low risk assessment: The Orbiter was in the controlled environment of the OPF. The time frame was limited, less than 0.2 percent of the total purge time in the OPF and less than 5 percent of total purge down time in the OPF. Only residual Hypers in the RCS (Reaction Control Systems).’
The associated flight rationale that is being presented is based on findings that shows contamination inside the orbiter only carries a very small risk for Endeavour during her mission to the International Space Station.
This includes the main area of interest, the Star Tracker system on Endeavour – which was actually found to be cleaner than the same hardware on her older sister Discovery, following swab samples.
‘Contamination due to lack of HEPA filtration has a very low risk assessment for flight. The bag filter and three pre-filters will prevent visible contamination build up in the Orbiter for a short duration. The two GSE (Ground Support Equipment) FWD (Forward) circuit pre-filters and the carbon filter were still intact and operational,’ added the presentation.
‘Contamination of the Star Tracker Cavity due to blockage of the Star Tracker purge duct filter has a very low risk for flight. The Star Tracker compartment pressure was measured during the last OPF positive pressure test and found to be greater than the forward plenum pressure.
‘Swab samples from the OV-105 Star Tracker cavity were cleaner than the baseline samples taken from the OV-103 Star Tracker cavity. This data indicates that the 15 micron Star Tracker cavity purge filter was not blocked by FOD (Foreign Object Debris).’
Most of the debris is understood to have been removed during Endeavour’s pre-rollover OPF flow, and subsequent mating operations inside the Vehicle Assembly Building (VAB) earlier this month. Some of the debris remains, but it is only believed to be small fibers, which do not hold a concern for safe flight.
‘Damage to Orbiter components from the remaining FOD in the orbiter has a very low risk assessment for flight. L/H OMS pod samples only contained microscopic fibers from the filter media. The nose landing gear filter was removed and no contamination was found,’ added the flight rationale.
‘Other sub-system assessments for potential FOD in the FRCS, OMS Pods, Drag Chute compartment and Vertical Stabilizer. This was based on material found and the maximum size of FOD that could get through a duct orifice. No negative assessments were generated.’
Evaluations also showed that the amount of debris that was blown into Endeavour was only a very small percentage of the actual filter itself. Only 0.53 grams worth of debris was removed from Endeavour.
‘The recovered filter pieces weighed one percent more than a nominal filter. This shows that the amount of material missing from the damaged filter is smaller than can be conclusively determined by weight,’ added the expansive presentation.
‘Total weight of FOD removed from orbiter ducts was 0.53 grams. This affirms that a very small amount of filter material was transported to orbiter. (Total weight of filter is 10,187 grams).
‘The filter material that was found in the Orbiter duct system was in small pieces, including a piece of fiberglass (2” x 2.5”). Two Positive Pressure tests were run after duct system inspection. The results were close to previous results and well within limits. This means that any un-discovered blockage is minimal.’
This event is highly unlikely to occur again, with engineers replacing two HEPA filters inside the now-vacated OPF-2. They have also checked out and cleared the other filters.
Potential design changes are being looked into, in addition to updating maintenance procedures and filter changeout intervals.
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