NASA’s Program Requirements Control Board (PRCB) have ordered a modification – within existing certification – to the Solid Rocket Boosters (SRB), following a debris strike during STS-116’s launch last December.
The debris, which was insulation flying off the aft of the left SRB, impacting on Discovery, was “greater than defined allowable (0.0002 lbm)” – and work is being carried out in the Vehicle Assembly Building (VAB) to trim the area which shed the material on Atlantis’ boosters ahead of STS-117.
**The most comprehensive collection of STS-117 onwards related presentions and mission documentation are available to download on L2 **
Presentations from the SRB Anomaly Resolution Team (ART) – aquired and downloadable on L2 – confirmed that NASA Imagery Reporting Database (NIRD) identified debris at SRB separation (T+125.2 sec) impacting Discovery during the December 9 launch. ‘Impact potentially correlated to damage noted during orbiter tile inspection,’ noted the presentation.
‘SRB Anomaly Resolution Team (ART) formed to identify liberated material and investigate cause and corrective action. Team assessed potential sources to identify maximum expected debris size for risk assessment, as needed Booster Separation Motor (BSM) aft exit cone Booster Trowelable Ablative (BTA) liberation primary focus.’
While NASA’s work on reducing the risks from External Tank (ET) foam liberation hitting the orbiter’s Thermal Protection System (TPS) has been a huge success based on recent flights, the threat from the boosters has been less publicized.
The strike on Discovery did not cause anything more than cosmetic damage, but NASA aren’t taking any chances, carrying out the trimming of the material – which started last week – on the aft of the booster, which is proving to be a tricky task.
‘SRB AFT BSM TPS trimming will be continuing today; there are 2 issues that the SRB element is working to resolve: On the right stack, dimension in one area on the top of one nozzle (measured from the outer edge of the nozzle inward) is 0.800 (approx.) should be 0.815 in,’ noted the February 8 Expansive Launch Operations report.
‘Difficulty getting measurements of BTA thickness over the retaining ring. The surface of the nozzle is not as flat as other areas, which is causing the micrometer to rock. TPS Ops and Quality Assurance personnel were evaluating issues and how to address them yesterday.’
The actual process of trimming the BTA doesn’t require anything more than utilizing the standard hand tools, used on previous work carried out on BSM aft exit cones.
The debris hit Discovery in three areas, one on her body flap, and in two locations around the left ET Umbilical Door – the latter being visible via images taken from the International Space Station (ISS) during Discovery’s RPM.
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How NASA confirmed what material hit Discovery is fascinating, as they employed a technique called Scanning Electron Microscopy (SEM), which revealed ‘glass ecosphere material in removed Orbiter tiles, constituent of BTA. No evidence of other foreign material.’
Another interesting finding is related to how material could come off an area of the Shuttle that was flying behind the orbiter, yet still managed to fly up the stack and hit the orbiter.
That was evaluated by United Space Alliance – the design agency and integrator of the SRBs – who found that the BTA debris ‘ejected’ towards the vehicle, as opposed to flying off harmlessly behind.
‘Failure mechanism shows potential BTA debris ejection vector in direction of flight,’ added the presentation. ‘Peel load created by skewed aft heat seal ejection hinging about potential failure plane. ATK qualification test motor firings indicate debris ejection outside of BSM plume.’
While Atlantis’ boosters are being trimmed, future boosters will have a modification implemented to mitigate future BTA debris concerns – or to the point that future ejections of material are at least understood and accurately predicted.
‘ART downselected aft exit cone L-cut closeout design modification,’ the presentation continued. ‘Minimizes potential debris and provides predictable failure location. Optimum angle of 48 -69 degrees determined by analysis. Minimizes effect of plume impingement on TPS. Analysis shows underlying structures maintain structural integrity under design ascent environments.
‘Design modification within existing certification with no Interface Control Document (ICD) changes.’
Demonstration tests are planned for modified design prior to STS-117’s launch, with Air Pressure Testing at KSC/Launch Environments Test Facility.
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