With just under one week to go before the opening launch attempt for Space Shuttle Endeavour and the STS-130 crew, the Flight Readiness Reviews (FRR) have thoroughly reviewed all open and nonconformance outstanding issues with the Solid Rocket Boosters (SRBs) and Reusable Solid Rocket Motors (RSRMs) ahead of the first shuttle mission of the year.
In all, three items were brought forward to the FRR meetings from the RSRM team.
The first two pertained to “significant hardware changes” to the RSRMs, including the removal of an inactive stiffener stub and the relocation of “porta-pull locations” to an area further aft on the motor.
For the removal of the stiffener stub, “Hardware Change – First effective on STS-128 (RSRM-107); also on STS-129 (RSRM-108) and subsequent,” notes the RSRM presentation to the SSP FRR, available for download on L2.
Post-flight inspections revealed no flight anomalies after this stiffener stub was removed. Also, all paint in the region remained intact.
For the porta-pull locations change, this adjustment was first made on the RSRM for the Ares I-X rocket and was subsequently flown on STS-129. No excavation or filling was necessary at the porta-pull locations following the implementation of this plan.
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During post-flight inspections, it was determined that all expectations in terms of the design of the relocation were met. Further, no anomalies were identified at the porta-pull location sites.
“Change Description: Replaced V1247 fluorocarbon O-rings with V1288 (improved-resiliency fluorocarbon) O-rings in nozzle joint no. 1,” notes the RSRM presentation.
The change in O-rings came after the discovery that the “obsolescence” of one of the ingredients in the V1247 O-rings either required the “requalification” of the V1247 or the creation and certification of a new O-ring material.
Originally, Shuttle Program managers intended to fly out the Shuttle manifest using “V1247 in fly-nozzle joints (static/slightly opening joints, so resiliency is not a key parameter).”
However, it was ultimately decided to switch to the V1288 O-rings because of their “improved ablation properties, ready availability, and simplified manufacturing process.”
The new V1288 O-rings – which hold the same dimensions as the V1247 O-rings – were verified to have material properties that were greater than or equal to the V1247 O-rings.
The V1288 O-rings have also successfully been flown on the BSMs of STS-122 and subsequent flights, the RSRM field joints of STS-125 and subsequent flights, and the RSRM nozzle-to-case joints on STS-128 and subsequent flights.
The V1288 O-rings also flew successfully on the Ares I-X RSRM. As such, the SSP considers the O-rings “safe for flight” in nozzle joint #1 for STS-130 and subsequent flights.
In all, only two issues with the SRBs were presented to the SSP FRR – one of which was an IFA (In-Flight Anomaly) from STS-129 in November 2009.
This IFA concerned a 30 to 32 inch crack in the aft skirt trailing edge foam on the Right Hand SRB. During the STS-129 launch, the crack in the aft skirt trailing edge foam was first seen 0.333-seconds after SRB ignition.
The crack can clearly be seen on long-range tracking camera footage through the first 66-seconds of flight – at which point the resolution of the footage begins to decline.
Further post-launch review of liftoff video revealed that portions of the foam “adhered” to a gaseous nitrogen purge probe immediately following liftoff.
“High resolution video review shows foam adhered to GN2 purge probe during liftoff,” notes the presentation – available for download on L2.
The aft skirt trailing edge foam is applied in a two-step process, the first occurring in the Rotation Processing and Surge Facility and the final step occurring at the launch pad.
“Final closeout around GN2 purge probe and Range Safety System (RSS) cable performed at Pad. Teflon shop aid and Tygon tubing utilized to prevent foam adhesion to purge probe and RSS (Range Safety System) cable respectively.”
An investigation into this IFA determined that the shop aide must have moved, allowing the foam to adhere to the gaseous nitrogen probe. The shop aide is usually taped to the gaseous nitrogen purge probe, thereby holding it in place.
The primary concern for a crack of this magnitude is its potential to liberate during flight and strike the SSV (Space Shuttle Vehicle) – potentially causing critical damage to the orbiter’s Thermal Protection System tiles.
In response to this IFA and subsequent findings, the review team developed a series of improvements for the application of the aft skirt trailing edge foam.
These improvements include: using a “position jack to ensure purge probe shop aid remains in place during foaming operations, (adding) tape reference marks below aft skirt interface as reference points during foaming and inspections, and [adding] petroleum jelly specifically to purge probe shop aid seam gaps and BTA to shop aid interface to mitigate foam intrusion.”
Further improvements are the addition of “vinyl tape to Tygon tubing slit to mitigate foam intrusion on RSS cable, the use of a new tool to physically verify clearance between purge probe/RSS cable and foam, and the addition of Quality verification to work paper.”
These improvements were implemented on STS-130 and will be followed through on the remaining Shuttle missions. As such, the IFA from STS-129 is not a safety risk to STS-130.
The second issue from the SRB team pertained to an open nonconformance with the Right Hand Rock Fuel Supply Module system.
“Nonconformance: RH Rock Fuel Supply Module (FSM) system pressure dropped from 123 to 24 psia,” notes the presentation.
The worst-case scenario/concern for this issue would be an external hydrazine leak leading to a Loss of Crew and Vehicle scenario.
Nonetheless, a more likely scenario would see a loss of FMS pad pressure. This would lead to a slow start of the RH SRB Hydraulic Power Unit and an abort of the launch countdown after the T-31 second mark.
The issue was worked at the launch pad and grease was added to the GN2 (gaseous nitrogen) purge and poppet valve – which was determined to be the cause of the leak.
Follow-up leaks checks revealed no further leaks from this system.
As such, flight managers are in the process of closing out this issue and clearing the RH Rock FSM system for flight.
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