Root cause of shuttle external tank cracks still not clear

CBS News

KENNEDY SPACE CENTER, FL--NASA managers and engineers met Thursday to discuss the potential root cause of cracks in the shuttle Discovery's external tank, what additional tests might be needed and what, if any, modifications might be required before another launch attempt can be made.

The cracks were discovered near the top of the ribbed intertank flange area at the base of the external tank's liquid oxygen section. (Photo: Spaceflight Now/Justin Ray)
Despite around-the-clock work to assess the problem, engineers still do not understand what caused cracks in structural ribs, or stringers, during fueling for a Nov. 5 launch attempt, a critical element in the development of an acceptable flight rationale, or engineering justification, for making another launch attempt.

"The one thing they don't have is root cause," said one NASA official. "They're just not there yet."

NASA is still holding open the possibility of a launch between Dec. 17 and 20. But the amount of work required to get there does not appear to fit in the available time. The schedule will be compressed even more by a decision late Thursday to carry out a fueling test to collect more data on how the tank's structural components respond to ultra-low temperatures.

It is not yet known when such a test could be carried out. In one option, sensors would be attached to the skin of the tank prior to fueling to precisely measure the stresses in the structure.

But installing such instrumentation and cabling at the launch pad, which would require cutting away more foam insulation, is a challenge in itself. Some engineers favor carrying out a fueling test without any additional sensors, loading the tank and pressurizing it for launch. Any cracks in the tank's foam insulation would be seen and X-ray analysis could find any cracks in the underlying metal.

Sources said Wednesday the requirements of an instrumented tanking test likely would rule out any attempt to launch this month. But they held open the possibility of a near-term launch if an un-instrumented test shows no problems and the on-going engineering analysis shows the tank has enough safety margin to withstand any credible failures.

During a four-hour program requirements control board meeting Thursday, engineers did not even get to a discussion of fueling test options or potential launch dates. The analysis will continue and in the meantime, the "no-earlier-than" Dec. 17 launch date placeholder is "still in play," an official said.

NASA originally hoped to launch Discovery on its 39th and final mission Nov. 1, but a series of technical problems caused repeated delays. After a gaseous hydrogen leak in an external tank vent arm derailed a launch attempt Nov. 5, engineers discovered cracks in two adjacent stringers making up the ribbed intertank compartment that separates the tank's liquid oxygen and hydrogen sections.

The vent line has since been repaired, but the crack issue has proven much more difficult to resolve. Engineers believe normal shrinkage due to ultra-low-temperature propellants, coupled with stresses built into the tank, caused the two stringers to crack near the top of the intertank.

What might have caused the presumed built-in stress, or pre load, is not yet known. But before Discovery can be cleared for another launch attempt, engineers must show the tank can withstand the rigors of fueling and launch without developing similar, or worse, cracks that could lead to the loss of foam insulation or compromise the structural integrity of the tank.

As of this writing, sources say the most likely scenario is an instrumented fueling test followed by a roll back to the Vehicle Assembly Building where platforms can be put in place to give engineers better access to the entire circumference of the intertank. Using sophisticated scanners, technicians then could closely inspect all of the stringers and their connections to other structural components to make sure no new cracks formed during exposure to low temperatures.

At the same time, structural braces called doublers are being prepared that could be attached to beef up any suspect or damaged stringers. But a roll back to the VAB would delay launch to the next window in February and NASA managers have not yet officially taken that step.