STS-131: ET/SRB mating scheduled for next Monday – MAF plan for IFRs refined

by Chris Bergin

With Endeavour enjoying a smooth flow on Pad 39A, preparations for Discovery’s STS-131 mission in March are picking up the pace – with the milestone of External Tank (ET-135) mating with the twin Solid Rocket Boosters (SRBs) set to take place next Monday. Meanwhile, the Michoud Assembly Facility (MAF) in New Orleans has been handed a forward plan to check all of the LO2 Ice Frost Ramps (IFRs) prior to shipping.

STS-131 Processing Latest:

Discovery’s major processing milestones take place around the time her younger sister carries out STS-130, with rollout to Pad 39A set to take place on February 16, following rollover from OPF-1 (Orbiter Processing Facility) on February 9 – currently just two days after Endeavour launches.

These milestones are tracking a March 18 launch for the mission which will deliver supplies to the International Space Station (ISS) via the Multi-Purpose Logistics Module (MPLM) Raffaello and the Light-weight Multipurpose Experiment Support Structure Carrier (LMC).

Discovery’s Payload Bay Doors (PLBD) were closed for the final time ahead of rollover to the Vehicle Assembly Building (VAB), as leak checks – set to be completed on Thursday – are carried out on the orbiter.

“Final PLBD closure for rollover to the VAB; Stongbacks are scheduled to be removed Thursday,” noted processing information on L2. “ECLSS (Environmental Control and Life Support System) GN2 72-hr decay test began Monday and will complete Thursday.”

Only two relatively minor issues are in work at this time, relating to the Data Processing System (DPS) and “challenges” with the LH (Left Hand) ET Door.

STS-131 Specific Articles:

A3“New IPR 0022 to DPS – taken when MFD1 (Multifunction Display) failed to switch to secondary data port when requested,” noted the NASA Test Director (NTD). “Several re-tries resulted in same indications as well as a retry after a power cycle; all other results were nominal. Engineering evaluation is in-work; additional troubleshooting to follow.

“Work continues on the path to ET door functional. Challenges with the LH inboard thermal barrier and other conflicting work has caused the functional test to be rescheduled for early next week.”

Otherwise, all is going to plan, with the flow continuing to track preparations for Discovery’s arrival next month via the mating of ET-135 and the SRBs in the VAB’s High Bay 3 (HB3) – pending a mate review that will take place on Friday.

“S0003 ET/SRB mate preps are continuing. Shims, FWD Sep Bolt and Strut Bolt preps are complete,” added NTD status on the build-up of the boosters in HB3. “Left Forward segment mate is complete. Right and Left Forward Assemblies are mated, FWD Skirt Closeouts are in work.”

A2Over in the Checkout Cell (HB-4E), ET-135 is near completion with several routine “shakedown” inspections, in preparation with its lift over to the awaiting boosters for mating. This process also includes the pull tests that take place on the tanks, following a couple of incidents with foam loss from the intertank region during ascent.

“ET-135: Intertank Foam Adhesion Testing; Preps and foam application are complete. Trim is in work, “continued the NTD processing flow report.

“Shakedown and Receiving Inspections; Discrepancies are continuing to be worked noninterference – basis to processing. No major issues to report. AFT Hard-point closeout; Foam application is complete. Trim operations will in work.

“Helium Inject Box closeout; Conathane/foam application is complete. Trim is in work. LO2/LH2 transfer preps in work.”

MAF Procedures For IFR Checks:

A6Foam loss – albeit minor by way of threat to the orbiter’s Thermal Protection System (TPS) – from the intertank is one of two recent areas of evaluation, with incidents of liberations from the LO2 Ice Frost Ramps (IFRs) also recorded during STS-120, STS-125 (see left) and STS-127.

While foam loss from the LH2 IFRs further down the tank have been all-but successfully mitigated via modifications, the LO2 IFRs liberations kick-started new checks via an array of techniques that include X-ray, Terahertz scans and even the new observational tool of having a minicam ride up front on the flight deck in order to film liberations as they occur. This camera debuted on STS-129, but did not capture any events of interest.

The likely cause of the liberations on recent flights is believed to be associated with voids in the foam, which are affected by the dynamic pressure, heating and flight environment during ascent, which can ultimately lead to minor liberations. Finding voids ahead of launch is the key mitigation approach undertaken by ET engineers.

Known as Non Destructive Evaluations (NDE), the Program Requirements Control Board (PRCB) outlined increased testing to take place even before the tank arrives at the Kennedy Space Center (KSC), allowing for a full scan of the ramps at MAF – thus allowing for any repair work to take place ahead of shipping.

A4ET/MAF expanded their NDE Process to include prioritized LO2 IFR Ramps. Added capability in Bldg 420 (at MAF) includes Backscatter and Terahertz techniques,” noted the PRCB presentation, available on L2. “NDE is performed prior to ET ship with minimal impact to process or schedule.”

In the unlikely event a repair is required on one of the ramps, the option remains to ship the tank on schedule and carry out the repairs at KSC – with the focus on the initial goal of finding the voids and evaluating if a repair is required.

“Some risk that if repair is required, it may have to be performed at KSC. Flight history suggest a Low likelihood of having a large void in the foam,” added the presentation. “Probability of performing a repair for predicted divot near 0.01lbm is low.”

MAF already had the capability to check for voids on the LH2 ramps and some LO2 ramps. The increased testing on the entire set of LO2 ramps was initially restricted by a lack of access due to their locations on the tank.

“ET has done an outstanding job in implementing NDE for all critical IFRs. ET was already doing NDE on the Hydrogen (LH2) ramps and performing NDE on the more readily accessible LO2 IFRs. In their history of NDE they have never found void indications of the size believed necessary to produce suspect divots as single (foam) losses. But that does not mean it could not happen.”

A5Upon finding a void, evaluations will take place on its size – which will be the trigger for either informing the Space Shuttle Program (SSP) management for direction, or carrying out a repair.

“ET performs NDE on all IFRs: Any NDE voids with a predicted divot of 0.02lbm or larger – will be automatically repaired by ET (ET will advise is repair is not recommended),” the presentation noted. “For any NDE voids with a predicted divot size of 0.01lbm or larger: ET will notify SSP for Review, will review with ET and make recommendation to (management boards) as required.

“Decision on whether to repair voids above the screening mass will depend on multiple factors: Actual Location and Size of indication (ET is conservatively combining smaller adjacent voids together and reporting as a single large void). Reliability of indication and history of predictions. Repair option and access to perform the particular repair.”

The LO2 IFRs and the Intertank evaluations mark what is likely to be the final two areas of the tank that require this additional mitigation work, as the Shuttle Program continue to enjoy what has been a run of extremely “clean” tanks following ascent.

The amount of work that has been carried out on reducing foam liberations from the tanks since STS-107 has been nothing short of staggering, but it has reaped its rewards via the improved safety of the overall vehicle – even if bodies such as the Aerospace Safety Advisory Panel (ASAP) continue to fail in properly acknowledging those huge strides.

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

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