HOUSTON — Space shuttle managers called for “radius block” modification to all 108 of the structural stringers on Discovery’s external tank (ET) on Jan. 10, the next step in the troubleshooting that has had the senior NASA orbiter’s 11-day mission to the International Space Station on hold since Nov. 5.
NASA also says it will aim for a liftoff as soon as Feb. 24 — if an investigation into the underlying cause and ongoing repairs instill confidence in managers that the appearance of small cracks in five of the 21-ft.-long aluminum lithium stringers has not increased flight risk.
Repairs to the last of the eight cracks was to be completed on Jan. 11.
At that point, technicians will start the installation of 32 radius blocks — structural doublers that fit over the upper tips of the stringers to withstand the higher launch stresses on the support beams on either side of two ET thrust panels.
The thrust panels face the shuttle’s two solid-rocket boosters and shoulder most of the fuel tank launch loads.
The installation of the remaining stringers will follow. Engineers believe the cracks may have formed in response to a combination of factors during a Nov. 5 launch scrub.
Those included a possible materials defect, a slight misassembly of the stringer section of the ET and the cold temperatures that accompanied the flow of chilled liquid oxygen and hydrogen propellants into the ET.
Discovery’s crew are scheduled to equip the station with a new stowage module and an external platform to hold spare parts.
On Jan. 6, shuttle managers called off efforts to make launch attempts during a Feb. 3-10 window, to allow more time for the repairs and investigation into the cause of damage.
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