SRB Holddown posts undergoing redesign evaluation ahead of STS-119

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Engineers will meet in the middle of January to push forward with a redesign of the Holddown Post (HDP) Debris Containment System (DCS), in time for the launch of STS-119 – following an investigation into a debris incident during STS-126′s launch.

The “Holddown Post (HDP) #3 Debris Containment System (DCS) Failure,” received several expansive presentations (available on L2) as part of the Solid Rocket Booster IFA (In Flight Anomaly) review presentation.

Four holddown posts (studs) surround each booster, with each stud 28 inches long and 3.5 inches in diameter.

The stud has a frangible nut at the top end, which contains two NASA Standard Detonators (NSD), which are fired at the time of SRB ignition, breaking the nut and releasing the connection between the vehicle and the post.

Debris from that release is captured in the DCS, which avoids the debris potentially impacting the vehicle.

“SRB Blast Container (BC) Debris Containment System (DCS): Purpose to contain frangible nut debris. Stud travels aft with plunger after frangible nut fractures,” explained the opening remarks of one of several presentations on the anomaly.

“Plunger seats on washer and seals blast container. Stud continues into Mobile Launcher Platform (MLP) HDP.”

However, the DCS suffered a failure during STS-126′s launch, where the post’s plunger, spring and blast container debris liberated, which is unacceptable due to the risk such an event has for the vehicle.

Photography of the liberated debris included a large spring from the holddown post, which was found by engineers in the north flame trench at Pad 39A.

Due to the debris risk, NASA set up an Anomaly Resolution Team, which comprised of engineers from United Space Alliance Solid Rocket Booster (USA SRB), MSFC (Marshall Space Flight Center), NESC (NASA Engineering Safety Center), KSC, Integration, and Ground Ops, as an Integrated IFA was declared.

“Initial postflight inspection of MLP hardware and blast container hardware complete for HDP #3: No anomalies with aft skirt holddown post bore location. No evidence of stud hangup,” summarized the findings by ground teams that arrived for inspection of Pad 39A.

“Witness marks on BC lead from NSD contact and on stud from booster cartridge firing confirmed correct assembly. Plunger properly clocked relative to NASA Standard Detonators (NSD)/ Heavy flame erosion noted on forward end of Inconel holddown stud (length reduced by ~0.50 inch).

“Blast container debris assessment determined spring, plunger, frangible nut major web (1 of 4), both NSD and adapters missing. NSD with adapter and portion of spring found during postflight pad walk down inspection. Inspections and material evaluations unable to identify plunger remnants.

“Dimensional analysis shows plunger assembly with one of two shoulders missing contained within blast container. Impact loading with washer not included. Integrated fault tree established. Includes SRB hardware fault, KSC ground processing, and environmental affects. Fault tree blocks assigned and closures in work.”

While on-going work is taking place as to what level of debris threats such liberations may have on the vehicle, engineers have already put forward a redesign option, which is expected to be taken in time for the modification to debut on STS-119′s boosters.

“Modifying DCS to incorporate secondary retention feature. Adding two tapered stop blocks to guide housing 120 degrees apart,” noted the resolution presentation. “Prevents plunger travel following plunger shoulder contact with spherical washer. Only engages in event of total shoulder failure.

“Design minimizes impact to DC components and installation. No changes to plunger. Utilizes existing tapered rib. Allows implementation at guide housing assembly level.”

These stainless steel stop blocks will replace the current anti-rotation device, which would result in the plunger travelling half an inch in into washer bore, if stop block engages due to total plunger failure.

Successful pull tests have already been carried out on the stop blocks, although this is part of an ongoing test plan that will confirm design certification and allow for implementation on STS-119′s boosters.

“Ten drop tests without load carrying contribution from plunger shoulder to verify plunger/spring retention and frangible link fracture by stop blocks,” added the presentation.

“Static pull test to demonstrate ultimate capability. Two full assembly firings to verify no impact to holddown stud release: One flight configuration plunger, One with plunger shoulder removed to ensure secondary retention device activation.”

The effort of testing and certification is all heading towards two decision dates in January – the first relating to an engineering summit on moving forward with the design for STS-119, and the need date relating to the implementation deadline on STS-119′s stack at the pad.

“Hold-Down Post Blast Containment IFA from STS-126: Closed in on a preferred design concept,” noted the 8th Floor News MOD memo on L2.

“It will be discussed at SSP PRCB (Space Shuttle Program, Program Requirements Control Board). The final decision will be made January 13 as to how STS-119 stack will be handled.”

“Currently manufacturing stop blocks for developmental and certification testing at MSFC. Next flight modification planned,” added the resolution presentation.

“Coordinated with Ground Operations (GO) to begin installation on 1-27-09 given successful certification (for STS-119). Creating full assembly mock-up for GO technicians to assess installation.

“Recommendation: Proceed with design and certification as proposed for DCS stop block modification.”

Such incidents with the holddown posts are rare, with the only previous recorded anomaly that mirrors the STS-126 event occurring back in 1993 with STS-56.

“Plunger and spring exited through aft skirt bore but remained attached to stud and intact,” referenced a historical review of STS-56′s incident. “Team determined frangible nut halves contacted plunger assembly before sealing bore of blast container.

“Contact caused plunger shoulder to fail and exit blast container. Evidence of low skew.”

Stud hang ups on the holddown posts was the other issue that was recently addressed, which resulted in the debuting of a modification on STS-126 – initially set to debut on STS-125, before the Hubble mission slipped to May, 2009. It is not referenced as a potential cause of the anomaly with the DCS.

L2 members: All 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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