Discovery hauled back to VAB; tank inspections, possible modifications on tap

CBS News

KENNEDY SPACE CENTER, FL--The shuttle Discovery was hauled off its launch pad and back to the Vehicle Assembly Building overnight Tuesday for additional inspections and possible external tank modifications in the wake of a fueling test last week to help pin down what caused cracks found in the wake of a Nov. 5 launch attempt.

Running a day late because of problems with NASA's shuttle transporter, the 3.2-mile trip from pad 39A to the VAB began at 10:48 a.m. EST (GMT-5) Tuesday and was declared complete at 7:01 a.m. Wednesday.

The shuttle Discovery returns to the Vehicle Assembly Building early Wednesday for additional external tank inspections and possible modifications. (Photo: NASA)
Launch remains on hold pending additional X-ray inspections and analysis, including possible modifications to beef up the structural ribs, or stringers, that experience the most stress during fueling and launch.

But based on an initial look at data collected during a fueling test last week, engineers are increasingly optimistic Discovery will be ready for another launch attempt during a seven-day window that opens at 1:37:36 a.m. on Feb. 3.

NASA managers had hoped to launch Discovery on a space station resupply mission -- the orbiter's 39th and final flight -- on Nov. 1, but the launching was delayed by technical problems and finally, on Nov. 5, by a gaseous hydrogen leak in a vent line quick-disconnect fitting on the side of the external tank.

During work to drain the tank, engineers spotted cracks in its foam insulation near the top of the intertank compartment that separates the tank's hydrogen and oxygen sections. When the damaged foam was removed, four cracks were found in two adjacent stringers near one of the tank's two booster attachment thrust panels.

Since then, troubleshooters have been trying to identify a root cause that could explain why the cracks formed in the first place to make sure the tank is structurally sound and able to withstand the rigors of fueling and launch.

Engineers have not found a "smoking gun" explanation. Instead, it appears the cracks were the result of manufacturing issues that resulted in a build up of stress in the two stringers in question that reached the breaking point when the tank was loaded with supercold propellants Nov. 5.

Last week, after installing dozens of strain gauges and temperature sensors, engineers carried out a full-scale tanking test, pumping more than a half-million gallons of liquid oxygen and hydrogen rocket fuel back into the huge tank to characterize its performance.

The sensor readings show the stresses and strains experienced by the tank during fueling matched up well with computer models as did the measured shrinkage of critical components. No sudden changes were noted in the stress data that might indicate additional stress-relief cracks.

To make the test as realistic as possible, the countdown was carried to the T-minus 31-second mark and the hydrogen and oxygen tanks were pressurized as required for a real launch. But pressurization had no major effect on the strain gauge data.

Overall, officials said, the data indicate the tank's design is robust and that the structure is as strong or stronger than initially believed.

With Discovery back in the Vehicle Assembly Building, engineers plan to carry out X-ray inspections all the way around the circumference of the tank where the stringers attach to a flange supporting the tank's liquid oxygen section. The sophisticated instruments can detect cracks smaller than those that could be reasonably expected from fueling and NASA managers hope to complete the work by Dec. 30.

At the same time, engineers will cut away foam insulation to remove the strain gauges and thermocouples that were installed for the tanking test. That foam will have to be reapplied.

An engineering review is planned at the end of the month to assess the work to date and to discuss proposed modifications to beef up stringers near the thrust panels where the shuttle's twin solid-fuel boosters are attached to the tank.

The nine stringers to either side of both thrust panels -- 36 in all -- experience the most stress during fueling and launch and the proposed modifications would provide additional strength and improve the tank's overall safety margin to protect against worst-case failures. That work, if approved, would begin Jan. 3.

Planners believe the modifications can be completed in time to support a move back to the launch pad around Jan. 13 and launch as early as Feb. 3. But no decisions are expected until the end of the month.