Cape Kennedy, Florida -- NASA technicians removed cracked foam insulation from space shuttle Discovery's external tank 10 Nov, after the defect was revealed during tank-filling operations for Discovery's aborted launch attempt on 5 Nov.
The foam removal revealed two 9-in long cracks at the top of one of the stringers on a section of the tank's metal exterior. The stringer is the aluminum strip that forms the intertank section between the Liquid Oxygen and Liquid Hydrogen tanks. Stringers provide structural integrity to the intertank, said an 11 Nov NASA statement.
The crack was revealed as the tank was being filled Friday 5 Nov, just before a leak in the hydrogen tank was also detected. Engineers have removed space shuttle Discovery's ground umbilical carrier assembly, or GUCA, which includes the flight seal, the 7-inch quick disconnect, or QD and the carrier plate, known as the GUCP. The parts are being examined, but a replacement GUCA is likely to be fitted.
In assessing the foam crack, NASA says it was caused by the cracks in the stringer, which moved the metal outward to dislocate the foam. Engineers are reviewing images of the cracks to determine the best possible repair method, which would be done at Launch Pad 39A.
The stringers are made using a lighter weight, stronger Aluminium Lithium material, which was initially incorporated with the Super Light Weight Tank on ET-96 and first flown on STS-91. While the material is stronger and lighter in weight, it is more brittle and has resulted in increased cases of cracked stringers in the areas noted, said NASA.
NASA seems confident that the stringer crack was the root cause of the foam crack.
NASA crews have repaired similar cracks during external tank production, by installing a second piece of aluminium for reinforcement on top of the cracked area, and then replacing the foam insulation. This piece is called a "doubler." Its current repair plan consists of cutting out the cracked section and replacing it with an available piece that matches its size. A doubler will be inserted before the mounting rivets to complete the repair.
Foam will be sprayed over the newly repaired stringer and will need about four days to cure.
Currently, NASA's team is evaluating schedules and repair options.
The next launch window for Discovery is 30 Nov to 5 Dec.
PIC: A technician examining the location of the external tank's insulation crack.