STS-131 enjoys smooth SSP FRR – Discovery into hypergolic loading

by Chris Bergin

As expected, STS-131 has passed its latest milestone, with the approval to push forward to the Agency level Flight Readiness Review on March 26 – following the conclusion of the SSP (Space Shuttle Program) FRR this week. No issues of note are being worked on Discovery out at Pad 39A, as engineers work through hypergolic loading on the stack.

STS-131 Processing Latest:

Hypergolic loading into various systems – such as the Solid Rocket Boosters (SRBs) and Reaction Control Systems (RCS) – is a key moment in the pad flow, involving strict safety measures, due to the extremely hazardous nature of the chemicals.

Engineers at the pad don SCAPE (Self-Contained Atmospheric Protection Ensemble) suits to protect themselves from the deadly hydrazine, which when mixed with oxidizer in the RCS thrusters, causes a reaction that allows the orbiter to maneuver on orbit.

“OV-103 / ET-135 / SRB BI-142 / RSRM 110 (Pad A): Blank off plate removals were completed. GUCP (Ground Umbilical Carrier Plate) QD (Quick Disconnect) – on the External Tank leak – checks were completed,” noted the NASA Test Director (NTD) on L2-acquired processing information, which noted an issue with one of the right hand (RH) RCS QDs.

“S0024 Hypergolic Propellant Servicing is in-work. Hypergolic loading: RJD (Reaction Jet Driver) driver test is complete. SRB HPU (Hydraulic Power Unit) hydrazine loading is in work.

“RH RCS QD 322 issue: Bore Scope inspection of the QD was inconclusive. After HPU servicing, all QDs on the RH OMS Pod will be de-matted, the QD rack will be adjusted, QDs re-matted and leak checked in an attempt to resolve this issue. OMS (Orbital Maneuvering System)/RCS oxidizer servicing in work.”


The Shuttle Program Flight Readiness Review was held on Wednesday – and as per STS-130, managers only required one day of evaluations to move the process forward. Usually, SSP FRRs take up to two days to conclude at the Johnson Space Center.

The next meeting is scheduled to be the JSC Center Director Pre-FRR, which will be on March 23rd, followed by the Agency FRR on March 26th – which will set the launch date, currently tracking April 5.

STS-131 Specific Articles:

As expected, the observation of intertank foam liberations during a number of recent ascents provided the main talking point at the SSP FRR – although it should be stressed none of the losses have resulted in damage to the orbiter’s TPS (Thermal Protection System).

Regardless, SSP managers take all foam loss events extremely seriously, to the point predictions are made to pre-empt the threat level, should a liberation impact the orbiter.

“There was significant discussion on the intertank foam losses experienced on STS-130. It is still believed that contamination prior to TPS application is causing poor adhesion and in-flight foam losses,” noted one of the main post-FRR overview memos (L2).

“Flight rationale is based on expected conservatism in mass estimates (wind tunnel tests and imagery suggest debris will break-up upon release), stress analysis indicates losses in critical areas less likely, damage model assumes worst case orientations for debris impactors, and there have been no debris observed coincident with bond adhesion test locations.”

It is hoped that a change of cleaning procedures at the tank’s manufacturing base – the Michoud Assembly Facility (MAF) – may have resolved the issue of contamination remaining on the tank’s structure prior to foam application, resulting in stronger adhesive properties between the intertank stringer foam and structure.

“ET-135 (STS-131) had additional cleaning/inspections due to spray abort of initial TPS application (~lower half of intertank). Qualitatively, the likelihood of catastrophic damage is probably remote but due to uncertainties SE&I (Systems Engineering and Integration) continues to carry it as infrequent catastrophic hazard,” the memo continued.

As noted ahead of the SSP FRR, engineers have been working on Probabilistic Risk Assessments (PRA) on the threat levels associated with intertank foam losses, although this is a routine process and mainly carried out from a documentation standpoint to ensure flight rationale remains in place by the time the Agency FRR meets.

As with all risk assessments, the numbers generated are mainly based on flight history. However, due to only a few flights suffering from the intertank liberations, not enough data is at hand to create an accurate PRA number.

This in turn led to some debate on the strength of the flight rationale – although the post-Columbia inspection techniques and Launch On Need (LON) ability automatically means the crew would not be in any danger, even in the highly unlikely scenario of serious damage to the TPS via a foam liberation from this area of the tank.

“There was a lot of discussion on whether there should be a quantitative PRA to characterize the risk. SE&I says they do not have enough confidence in the absolute number that would come out of a PRA and didn’t see it as value added to determine whether we should fly.

“The value of the PRA was to identify the highest contributors to risk and then go after those areas in terms of making changes or performing tests on the hardware. There was general agreement that with or without the PRA, the foam losses are of concern but a better understanding of the problem likely won’t be gained until we fly and collect more data.

“Some considered the flight rationale as weak without inspection and repair/LON capability but all agreed because we have those real-time mitigations in case we are wrong about the risk assessment, it is acceptable to fly.”

Four other issues – relating to IPRs (Interim Problem Reports) during Discovery’s STS-131 flow – were also discussed, such as the recent General Purpose Computer (GPC) issue.

All were cleared as no issue for gaining flight rationale, although it was noted the GPC Input/Output (I/O) issue may require the removal and replacement of a PCMMU (Pulse Code Modulation Master Unit) or an MDM (Multiplexer Demultiplexer) at the pad – pending the results of troubleshooting.

“The GPC I/O error that occurred last week was a correct response to annunciated BITE errors on PCMMU 1, MDM OF2, OF3, and OA1,” added the notes. All affected LRUs are on the OI data bus and the problem appears to be associated with data bus or a discrepant MIA.

“The plan is to install break out boxes on the OI data buses and go back to PCMMU1 to collect further data for troubleshooting.”

As seen with the intertank foam liberation discussions, IFAs (In Flight Anomalies) from previous flights make up a large part of the next mission’s FRR process. Although it wasn’t technically an IFA from STS-130, engineers discussed the protruding ceramic window insert that had been observed on orbit by the DAT (Damage Assessment Team).

Again, the level of evaluation that goes into even the smallest issue was exemplified by follow-on notes relating to Discovery’s inserts – even though Endeavour’s protrusion remained in place throughout entry and landing on STS-130.

“Due to the protruding ceramic insert on STS-130, the ceramic inserts on OV-103 (Discovery) with potential debris transport were visually inspected and replaced as required prior to OPF (Orbiter Processing Facility) rollout,” the FRR overview noted.

Thanks to the due diligence of not only evaluating potential issues relating to the insert on orbit during STS-130, but also via the inspections on the fleet, a problem was found with the inserts, which may result in a late change of plug on Discovery.

“A subsequent problem was identified during the OV-105 (Endeavour)/STS-130 post-flight inspections, which revealed an interference condition between the plug and the fastener head,” the notes continued. “The concern is that this interference could produce a load resulting in debris liberation.

“A design tolerance stack-up was performed for select locations and some were found to have negative clearance. The plan is for potential negative clearance locations, to perform direct clearance measurements and replace with a shorter plug if necessary.”

Only two other items of interest made the overview summary, one in reference to EVA support during the mission, and another noting a potential change to the mission’s landing opportunities.

“The EVA team is still working the Metox regenerator error. A combination of LiOH and available Metox will support the planned EVAs and Shuttle contingency EVAs but not a docked contingency EVA. They are looking at options such as flying additional LiOH, exercise prebreathe protocol, and R&Ring the Metox regenerator (spare onboard). Recommendation is expected in next few days,” added the summary.

“Discussion on changing to descending landing opportunities, cryo offload, and possible use or deletion of one of the +2 days (recall STS-131 is 13+2+2 to cover DDO (Dual Docked Operations) which is no longer a requirement) will be discussed (end of week).”

A second post-FRR overview memo – which will be summarized into upcoming articles, along with notes from the FRR’s 31 presentations (available on L2) – also noted that Discovery will be precluded from launching in-plane on the 9th and 11th, based on current state vector.

Between now and the Agency FRR, managers will also work on several items of open work, although none hold a threat to the launch date target at this time.

“Significant Open work: Descending landing opportunity/mission duration discussion. Metox regenerator recommendations. RSRM (Reusable Solid Rocket Motor) Random vibration exceedances. SRB RGA failure analysis. PCMMU BITE bit 10 errors. Ceramic insert inspection and corrective action for carrier panel fastener interference. STS-130 nozzle leak check.”

L2 members : Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4500 gbs in size

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