STS-123: Additional booster improvements to fly with Endeavour

by Chris Bergin

Another safety improvement to the Solid Rocket Boosters has been revealed in safety documentation, drawn up to approve the changes ahead of the flight of Endeavour on STS-123 – which has seen its launch date move up to March 11.

A redesign to RSRM (Reusable Solid Rocket Motor) Nozzle Joints 2 and 5 – the latter with an additional bolt enhancement – follows up the new Nozzle-to-Case J-leg Joint insulation configuration that debuts on STS-122’s motors.

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Booster Improvements:

The improvements to the boosters – which provide the majority of the thrust to the stack for the two minutes of first stage flight – have been years in development.

STS-122’s improvements to its RSRM-99 set was revealed in an expansive FRR (Flight Readiness Review) presentation, whereas additional improvements to STS-123’s RSRM-101 set are noted in a PRCB (Program Requirements Control Board) presentation, utilized to rubber stamp the ‘increase in safety’ by the Shuttle Program’s Safety and Mission Assurance Office.

Both improvements center around the decrease in risk of potential gas paths in the motor’s seals (O-ring to Case), with the implementation of a carbon-fiber rope (CFR).

While the improvements have been previously certified ahead of the production of the flight sets, NASA managers collate the paperwork surrounding any changes, for safety overviews ahead of flight.

‘Objective: Obtain PRCB approval of Hazard Report updates related to CFR. Background – Change Description: WAS: Nozzle Joint 2 backfilled with RTV as a thermal barrier. IS: Nozzle Joint 2 will have a CFR as a thermal barrier,’ noted the presentation, available on L2. ‘WAS: Nozzle Joint 5 buttered with RTV as a thermal barrier. IS: Nozzle Joint 5 will have a CFR as a thermal barrier.’

Observations that gas paths have formed on a fraction of previous boosters called for the modification. There wasn’t a safety issue or risk associated with the findings, given the numerous layers of protection before the gas paths cause any primary O-ring erosion. However, the addition of a CFR mitigates the risk of a gas path forming. 

‘Carbon Fiber Rope (CFR) cools the volume filling combustion gasses prior to pressurization of the o-rings. CFR dissipates heat and blocks particulates in combustion gasses,’ added the presentation.

‘Basis of Certification: Structural analysis shows CFR in joint has no effect on margins of safety; No compatibility concerns. Aero-thermal analysis using worst-case thermal conditions and flawed CFR, predicts adequate joint protection (no O-ring erosion).’

Interestingly, the presentation also shows the vast amount of testing and analysis that takes place ahead of any changes or additions to the motors, including numerous tests on motor firings at ATK’s test range in Utah. It also notes one test on a five segment motor, possibly showing this implementation will carry heritage through to Ares I and Ares V.

‘Successful testing in sub-scale motors and full scale motors: Four sub-scale MNASA motors. Nine full-scale motors (FSM-9, -10, -11, -12, -13, -14; ETM-2, ETM-3; and PRM-1). ETM-2 flawed CFR in flight configuration. ETM-3 margin test (5-segment motor).’

The tests showed that the use of the rope mitigated gas paths through the first of nine ‘barriers of safety’ – built in since the loss of Challenger in 1986, and improved on since. 

The presentation notes that all nine safety barriers would need to be breached before the ‘catastrophic’ risk of blow-by through the secondary O-ring could be observed.

An additional change is seen via a new bolt design in joint 5 of the motor nozzle. The shorter bolt eliminates bolt corrosion – as previously observed – in addition to a number of improvements.

‘Redesigned bolted joint incorporates a new bolt, new special washer, and shim. Improved ability to analyze bolted assembly structure by reducing ‘plastic behavior’ of the joint. Test results showed that joint skip and bolt bending are significantly reduced. Slight increase in phenolic bondline stresses,’ added the presentation.

‘All margins remain positive. Slight increase in primary seal gap opening. 2X O-ring tracking still met. New bolt eliminates bolt corrosion potential experienced in the past. High hand torque issues resolved with redesigned joint processing.’

This improvement also has been a long time in development, originally passing on from ATK to MSFC (Marshall Space Flight Center) for approval in 2004, before finally becoming flight hardware for STS-123 and beyond.

‘STS-123 (RSRM-101) and subsequent are safe to fly. Risk Assessment: Decrease in Risk. No Minority and/or Dissenting Opinions. No additional risk mitigations,’ rounded up the presentation. ‘Implementation of Joint 2 and 5 CFR and Joint 5 Bolt Enhancements have been assessed as: A Decrease in Risk.’

Another overview of these modifications is expected to be presented by ATK as part of the STS-123 Flight Readiness Review (FRR) in just over a month’s time.

Meanwhile, the STS-123 stack is taking shape inside the Vehicle Assembly Building (VAB), in preparation for Endeavour’s launch date – a date that has moved up by two days, to March 11, in order to aid ‘margin’ for the Russian Soyuz launch in April.

Most of the STS-123 work is concentrated on ET-126, with parallel LH2 Feed-through connector re-work, following the go-ahead to mirror the forward plan being utilized on ET-125 (STS-122) out at Pad 39A.

‘Build up of the modified feed through connector for this ET is in progress. The work on the ET, as well as the short pad turn-around time after the STS-122 launch are competing to be considered the critical path to meet the 3/13 launch date,’ noted processing information.

‘Additionally the shuttle program has challenged KSC to attempt to meet an earlier launch date in an effort to provide more margin for downstream activities like the April Soyuz launch. As for now, March 13th is the date we’re using for planning purposes.’ (NASA has now confirmed March 11 as the new launch NET (No Earlier Than).

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