STS-122’s FRR – Atlantis flight to debut newly improved boosters

by Chris Bergin

NASA successfully completed STS-122’s Shuttle level Flight Readiness Review (FRR) on Tuesday, moving the pre-launch process forward to the final Agency level FRR at the end of this month.’s review of the FRR’s vast array of documentation begins with an overview of the newly improved Solid Rocket Boosters, with two major modifications making their debut on the ride uphill with Atlantis next month.

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The Shuttle FRR is by far the most fascinating element of the multi-leg review process, with a vast amount of overview material produced for engineers and managers to pore over.

Every key element of the mission is presented to the finest detail, via graphical and data rich presentations that are major testament to the thousands of engineers and technicians that allow for the safe flight of the world’s most complex flying machine.

Over 17 elements are covered by the Shuttle level FRR, with one of those presentations – relating to ATK’s Reusable Solid Rocket Motor (RSRM) – reviewed below.

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RSRM-99 is the set that will fly with STS-122, although – as with STS-120’s RSRM-98 – segment changes are in effect. This relates to the accident that saw a bridge collapse under the train carrying the segments, causing it to derail.

‘Original RSRM-99 forward segments were reassigned to RSRM-98 (STS-120) after trestle/bridge collapse (May 2007),’ noted the presentation. ‘Former RSRM-100 forward segments replaced the reassigned forward segments on STS-122. Original RSRM-99 center segments (not on train) swapped sides to reduce predicted thrust imbalance.

‘Original RSRM-99 aft segments were involved in the trestle collapse and were returned to Utah. Former RSRM-100 aft segments replaced the returned aft segments on STS-122. Original RSRM-99 AECs remain on STS-122. Involved in trestle collapse; however, car remained upright on undamaged portion of track.’

New ‘Nozzle-to-Case J-leg Joint’ safety improvement:

A large section of the 44 page presentation is dedicated to a safety improvement that has been under development until now, namely the new Nozzle-to-Case J-leg Joint insulation configuration.

As the name suggests, the improvement relates to seals between the nozzle of the motor and the surrounding case, utilizing a pressure-assisted J-leg, carbon-fiber rope (CFR) and a leak check barrier O-ring, in order to eliminate the potential gas paths, observed on a fraction of previous flights.

‘RSRM nozzle-to-case joint experienced gas paths through the polysulfide to the wiper O-ring in approximately 1 out of 8 motors,’ noted the presentation’s reason for change. ’17 total gas paths to the wiper O-ring in RSRM experience (151 motors) – no occurrences of primary O-ring erosion.

‘Intermediate fix – ‘polysulfide bump’ – first flown on STS-104 (RSRM-80) with no gas paths to wiper O-ring.’

Despite the large scale redundancy associated with the seals on the motors and boosters as a whole – brought in after the Challenger disaster – and the lack of any major concerns associated with the gas paths from flight experience, ATK are debuting this improvement to completely remove the gas path at the source.

‘J-leg insulation ensures motor combustion gasses do not penetrate joint,’ added the presentation. ‘J-leg is manufactured with 0.5” radically inward displacement. Nozzle insertion deflects the ASNBR J-leg outboard, resulting in a tight interference (stretch) fit.

‘Reduced gaps between fixed housing and insulation aft of CFR minimize free volume. Compression against fixed housing on ignition, further reduces potential to heat effect the leak-check barrier O-ring. Thin PSA to CCP bondline – eliminates voids and potential hot gas volume fill. CFR thermal barrier cools any volume filling hot gas to below 400 degrees F.’

The CFR (Carbon-Fiber Rope) is – as the name also suggests – a rope that ingeniously acts as a heat barrier, that not only protects heat from escaping, but is capable of exchanging 5,000 degree F hot combustion gas into less than 400 degree F cool gas.

‘Carbon fiber Rope (CFR) employed as joint redundant thermal barrier. In the event hot chamber gas reaches the CFR, CFR allows downstream volume to pressurize in a controlled fashion while dissipating heat and filtering out particulates,’ claim ATK.

‘Braided rope is very permeable: high heat capacity of carbon fibers combined with large surface area allows efficient removal of heat from combustion gas. CFR certified as part of Nozzle-Case Joint on RSRM via robust development and qualification program.’

The combination of the joint design improvements ultimately mean the area of the motor is now fault tolerant, even in a simulated worst case scenarios – as certified via a rigorous testing process, and review by the Marshall Space Flight Center (MSFC).

‘Testing, structural and thermal analyses verified joint will perform as-designed. Demonstrated on seven aft dome articles including nozzle joint-assembly-disassembly (NJAD) and nozzle joint environment simulation (NJES) tests. Evaluated on MNASA-9 and MNASA-10 motor tests. Inspection verifies molded joint configuration meets all dimensional requirements.

‘Processes demonstrated to be repeatable (7 articles, 8 static tests, 10 flight assemblies). Remaining accepted risk causes evaluated and accepted. Demonstrated on FSM-9 through FSM-13, FSM-14 CTR joint, ETM-3 and PRM-1.

‘CFR in-place and no evidence of combustion gas penetration beyond jointbondline. STS-122 and subsequent are safe to fly.’

New ATK Booster Separation Motors:

Also making their debut on STS-122 are a new set of ATK built Booster Separation Motors (BSMs), a process that started back in 2003, and will eventually see new aft BSMs flying with Atlantis on her flight to service the Hubble Space Telescope on STS-125.

‘STS-122 will be the first flight utilizing ATK-produced BSMs. Placed in the forward positions only. During contract transition, select changes to heritage BSM design were pursued. Heritage motor has proven to be reliable and effective,’ listed ATK’s design comparison to the previous BSMs.

‘Utilized on flight throughout the life of the program. Minimize risk through the use of standardized ATK processes. Improve margins. ATK-produced BSMs pose no significant change in risk for the Space Shuttle Program.’

The new BSMs sport a number of modifications, ranging from a redesigned ignitor, interchangeable metal components (eliminates matched sets), decreased O-Ring Groove Depths (increased O-Ring squeeze), along with new adhesives and paints, among other items.

‘Certification complete for forward configuration BSM. ATK BSMs are qualified for flight having undergone a rigorous certification process based on analysis, testing, inspection and similarity. Compliance to all requirements documented in BSM Master Verification Matrix.

‘Full-scale motor (42 full scale tests) and component level testing. Full-scale qualification motor testing includes a 17-motor matrix with varying parameters. 25 full-scale motors test fired. Flight of ATK BSMs: No Significant Change in Risk to the SSP (Space Shuttle Program).’

Further articles will follow, reviewing other key elements of the FRR – which include another note of concern relating to the RCC panels by NESC – though this time STS-120’s flight rationale counters any potential delay to Atlantis’ launch, and new findings on the SARJ metallic samples brought back on STS-120, major EVA notes, among several other notes of interest.

All the presentations can be downloaded from L2.

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

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