Tim Furniss/LONDON
NASA HAS POSTPONED the launch of the Space Shuttle mission STS 69/Endeavour until late this month, to complete a review of hardware associated with O rings in the nozzle joints of the solid-rocket boosters (SRBs).
Examination of the two boosters used for the STS71 and 70 launches in June and July respectively showed that, on one of each, hot gas from the interior of each nozzle singed the primary O rings at the top of the nozzle, where it joins to the booster.
The STS71's left-hand-booster nozzle joint had four singe marks and had lost a small amount of material, while the STS70's right-hand-booster nozzle joint displayed three singe marks.
The marks on the 50mm-diameter O rings were the size of a tip of a pencil. The secondary seals were undamaged. Exhaust had also burned through to the rings, but not singed them, on 11 previous missions.
While there are "no design issues", says NASA, "and the crews of the missions were in 'no added danger'", a team is studying a procedure, which could allow ultra-sound inspection and minor adjustments, to the injection of an insulation putty, which "may reduce" the possibility of a gas path reaching the primary O ring. It appears that tiny air pockets in this putty allowed the 2,760°C exhaust to penetrate it.
A leak in a larger field joint between two SRB segments was the primary cause of the STS 51L/Challenger accident in January 1986. The SRB joints were redesigned - along with several other Shuttle components - after the accident. Since then, the Shuttle has had 45 successful missions with no failures.
Source: Flight International
