STS-120 – Discovery’s boosters in focus

The documents, from last week’s two-day Shuttle FRR, rounded up the lengthy pre-flight evaluation process for each major flight element, revealing many interesting elements of the mission, not normally given media exposure.

‘Had pre-brief last week; well done by those who briefed. Got idea of how complicated STS-120 mission is,’ noted Mission Management Team (MMT) chairman John Shannon. ‘Must to stay focused on the extremely challenging on-orbit work and not be distracted by the pre-flight issues.’

Those pre-flight issues mainly related to the NESC recommendation to rollback Discovery, in order to replace three RCC panels. A review article on the highly interesting Joint SSP/ISS FRR Charts that argued flight rationale relating to this dissent will be published soon (all October 16 FRR presentations on L2 now).

This is part 3 of this site’s FRR review – Click here for **Part 1** and **Part 2**

STS-120 RSRM (Reusable Solid Rocket Motor) FRR (ATK)

ATK’s overview of the RSRM element of the boosters discussed flight history, booster age limits, and their decision to replace some of the segments – following the train accident in May.

The subject of a number of PRCB (Program Requirements Control Board) presentations, ATK again noted their flight rationale, after spotting a Joint 2 Gas Path/Penetration issue.

This relates to a back-filled RTV seal on the joint, spotted on post-flight evaluations of the used booster from STS-118. The issue is no cause for concern, although ATK are working on a solution in time for STS-123.

‘Gas Penetrations Through LH/RH Nozzle Joint No. 2 RTV. Cause is understood,’ noted ATK’s presentation to the FRR. ‘Good technical understanding of nozzle joint no. 2. Thermal analyses show joint no. 2 is fault tolerant. Condition of STS-118 is within experience.’

This experience is gained from looking back over the history of the program, looking at heating effects noted on boosters over 30-40 previous flights into the past.

‘Leak path potential studied extensively. O-ring protected by large metal flanges and tight interfaces. No adverse performance trends. Risk of O-ring erosion is very low. Carbon fiber rope will eliminate RTV at RSRM-101 (STS-123).’

Obviously, the performance of seals are extremely important, not least because of the mention that serious failures – or damaging effects on the seals – leads on to the mention of effects on the O-rings.

As reassurance, ATK provided a failure scenario scale, showing that it would take nine events on the chain to lead to blowby on the Secondary O-ring (boosters have three O-ring seals, as protection, since Challenger). The event observed on STS-118 does not breach the first of the nine events.

‘1. Gas Path (No Heat Effects). 2 – Heat Affected Virgin CCP. 3 – Heat Affected GCP/SCP. 4 – Heat Affected Paint/Adhesive. 5 – Heat Affected Metal. 6 – Heat Affected/Eroded Primary O-ring. 7 – Primary O-ring Failure (Blowby). 8 – Heat Affected/Eroded Secondary O-ring. 9 – Secondary O-ring Failure (Blowby).’

This list was also presented alongside a chart that showed the depth of heat effects on this area of the seal, showing that none of the previous 30-40 flights had ranked ‘5’ – which is listed as a ‘challenge to flight rationale’ – with STS-118’s event only ranking ‘1’ and ‘3’.

‘PRCB Briefing 6 Sep 2007 – No Issues. RSRB Project Approved – 13 Sep 2007. Final closure – 14 Sep 2007. Carbon-fiber rope effective STS-123. Joint No. 2 Performed as Expected – Consistent with Flight History. Established Flight Rationale is not Challenged. No Constraints to Launch of STS-120.’

RSRM-98 is the set that will fly with STS-120, although two forward segments have been replaced by those previously set to fly on STS-122 – namely LH (Left Hand) and RH (Right Hand) forward segments RSRM-99. This relates to the accident that saw a bridge collapse under the train carrying the segments, causing it to derail.

‘Original RSRM-98 forward segments returned to Utah after trestle collapse (May 2007). Former RSRM-99 forward segments reassigned to RSRM-98,’ ATK noted. ‘Reassigned hardware meets requirements. Center segments on aft (unaffected) end of the train are acceptable. Engineering Data Recorder data within requirements and history. Detailed physical inspection at KSC.’

It is not fully known if both forward segments were deemed as unusable, given if a single segment is damaged, then the same segment from the other booster would be removed and replaced with its ‘twin’, due to the potential issue of a thrust imbalance. With both forward segments replaced, this issue is mitigated.

In regards to the RSRM segments that were affected by the train accident, even though they may have appeared to be undamaged, ATK are not taking any chances, and have removed them from ever flying with the shuttle. Affected elements involved in the accident could be used on an Ares 1-X type demo flight, and certain parts of the hardware could be recycled (washed out and recovered).

Because of the delay in the shuttle program in the wake of the Columbia disaster, many of the segments now being prepared to fly are pushing the previous age limit of five years. However, extensive testing and evaluations – which took place ahead of this year’s STS-117 – extended the lifetime to allow boosters with segments over five years old to fly.

‘Flight Specific Age-Life: 5-year age-life certification is good through 23 Oct 2007 (TIGA bonds). All hardware will be within age-life certification requirements at time of launch on 23 Oct 2007 – no margin. STS-120 flight-specific age-life extension to 5.5 years has been approved by the RSRB Project (Safety Issue Briefing to PRCB 13 Sep 2007 – no issues).

‘Aging data, flight-specific witness panel testing, propellant loaf samples, and potential interactions with changes and non-conformances reviewed – no issues. Age-life extension to 5.5 years is good through 23 Apr 2008. Limiting requirement is now 180-day pad stay. Initial rollout 30 Sep 2007 – good through 28 Mar 2008. One-year stack-life good through 18 Jul 2008.’

United Space Alliance’s presentation on the RSRB element of the boosters (which they are responsible for) gave an insight into the continued push to cover all the bases – and them some – on being able to track any debris liberation events in high detail during ascent.

A new addition to the booster is a Modified RH Forward Skirt Aft Pointing (FSAP) Camera, which will make this flight only, as part of a request to take a closer look at the performance of the Ice Frost Ramps on the External Tank (ET).

‘Modified RH Forward Skirt Aft Pointing (FSAP) camera for one-flight only,’ noted the presentation. ‘Background: External Tank (ET) requested imagery of redesigned Liquid Hydrogen (LH2) Ice Frost Ramps (IFR) to assess performance. Required camera rotation and field of view expansion.

‘Includes: Modified wedge plate, rotated camera 5.5 degrees and changed camera lens from 12 mm to 6 mm. Incorporated new settings to brighten camera image into permanent engineering. Modified window cover plate to remove small section causing image inclusion.’

USA’s presentation also included a risk matrix, which notably was the ‘greenest’ of all risk matrix’s presented to the FRR, even though this was a slight increase on the previous flight – STS-118.

‘1×3 (Improbable/Catastrophic – Green): 1725 increased to 1759. 34 Causes for SRB Structural Failure (documenting baseline). No increase in accepted risk,’ which despite it’s wording, is highly acceptable flight rationale for such a complex vehicle.

‘Pending completion of open work, there are no constraints to continue launch processing for STS-120,’ USA concluded.

L2 members: All documentation and quotes – from which the above article has quoted snippets – is available in full in the related L2 sections, updated live.

**STARLIGHT: NEW STS-117 MISSION REVIEW MUSIC VIDEO** – STS-118 Mission review video also available. 

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