STS-119 gains waiver for age life issue with SRB aft cone LSCs

by Chris Bergin

A waiver to be issued for STS-119 on the RSRB (Reusable Solid Rocket Motor) Aft exit cone linear shaped charge (LSC) – which is designed to sever exit cone from the booster shortly before impact with the ocean.

Each booster has four of the charges attached to the aft of the booster, and four of the eight charges are deemed to be outside of age life requirements. The LSCs are placed at 90 degree points around circumference of the aft cone.

The concern with the age of the charges has only recently been discovered via shelf life extension testing last month, which resulted in one charge failing to function as designed. A second failure was observed on the same system during an Ares I-X test by ATK on January 29.

The batch lot of the problematic LSCs matches that of four of STS-119’s LSCs, leading to the issue being raised at the Agency level Flight Readiness Review (FRR) for STS-119, and last week’s PRCB (Program Requirements Control Board) meeting.

“Aft exit cone LSC: Aft exit cone linear shaped charge (LSC) designed to sever exit cone to hit the water with small funnel instead of large funnel to reduce impact loads,” outlined notes from the Agency FRR on L2.

“During seven year shelf life extension testing, third unit failed to function as designed in that detonation did not propagate down length of LSC. STS-119 includes LSCs from the lot that experienced failures and from a different lot. Ares 1-X had a similar failure during their third test that was performed January 29th.”

The Ares I-X test objectives included “the demonstration that the linear shaped charge used to separate the forward skirt extension created a clean severance, and measurement of the shock created by that charge,” though it is not yet known if a changeout of LSCs will be required on the test vehicle that is taking shape inside the Vehicle Assembly Building (VAB).

The main concern – which has been removed, allowing for the waiver – is the risk of premature ignition of the LSCs during ascent.

“No indication of premature ignition risk so the risk is to reusability of the hardware,” added the STS-119 Agency FRR notes. “Waiver in work for STS-119, PRCB review on 2/5/09.”

The PRCB wavier presentation from ATK further outlined the issue, which again confirmed the problem appears to relate to a specific lot/batch of LSCs.

“Reason for Waiver: Aft exit cone (AEC) LSC shelf life extension testing (SLET) for LOT AAM initiated early January 2009 in response to new interpretation of requirements,” noted the PRCB presentation on L2. “NASA Standard Detonator (NSD) fired, severing LSC at detonator location only. Detonation did not propagate down length of LSC.

“First time occurrence. Similar result on (same) Lot (of) LSC on Ares pathfinder test (29 January). Four of eight LSCs on STS-119 are outside age life requirements. Following two nominal tests, third lot LSC test article failed to function as designed (26 January) – testing halted.”

The flight rationale was approved as written, due to the fact that the only viable risk is based on increased damage to the booster when hitting the ocean ahead of recovery, with confidence added via issue-free flight history.

“Flight Rationale: Failure to function (e.g. observed test anomaly in flight) would be Crit. 3 – increased hardware attrition potential only,” the PRCB presentation noted. “Demonstrated flight performance of all other lots excellent. LSCs functioned as designed on STS-120, -122, -123, -124, -126

“Initial acceptance includes RDX and copper certification, core loading density, dimensional, velocity of detonation, x-ray and N-ray, end cap leak check, temperature cycling, vibration, plate penetration, 4 and 7 year SLET includes temperature cycling, vibration, dimensional, plate penetration

“For (this) lot: 10 Acceptance detonations, 5 four-year SLET detonations, 2 seven-year SLET detonations (+ 1 seven-year SLET failure). No observed age degradation for RDX/LSCs. 7.5 year old Lot LSCs well within flight age family. Severed STS-123 exit cone when the LSCs were ten years, one month old

“Previous LSC test failures (primarily low penetration depth and segment to segment transfer) were attributed to setup and manufacturing discrepancy and not age.”

Age life discussions are a natural requirement, due to the production date of flight hardware in relation to delays in the flight schedule. A similar debate took place for the Reusable Solid Rocket Motors following the delay caused by the need to make an additional redesign on the External Tanks after STS-114.

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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