NASA managers order 'radius block' stiffeners for all external tank stringers

Editor's note...
  • Posted at 07:02 PM, 01/10/11: NASA managers order 'radius block' stiffeners for all external tank stringers
  • Corrected at 7:42 PM, 01/10/11: Fixing Endeavour launch target
CBS News

KENNEDY SPACE CENTER, FL--Senior NASA managers Monday agreed to install stiffeners all the way around the shuttle Discovery's external tank to beef up structural ribs, or stringers, that are susceptible to cracks when exposed to ultra-low-temperature propellant. Engineers say the modifications can be completed in time to support at launch as early as Feb. 24, assuming the work goes smoothly no other major problems develop.

The so-called radius block plates will be attached to the top few inches of all the stringers making up the ribbed intertank that separates the external tank's liquid oxygen and hydrogen sections. Eight known cracks in five of the 108 stringers already have been repaired by splicing in fresh stringer segments and installing doublers for additional strength.

Technicians work to install radius block stiffeners on structural ribs, or stringers, making up the intertank section of the shuttle Discovery's external tank. The modifications are expected to make the stringers less susceptible to cracks. (Photo: NASA)
The cracks occurred near the top of the intertank where the stringers meet a large flange that supports the bottom of the liquid oxygen tank. Ultra-low-temperature liquid oxygen causes the tank to contract during fueling, which tends to pull the upper sections of the stringers inward.

The tank was designed to cope with that contraction, but it appears a specific lot of aluminum alloy used in this tank's construction provided less strength than expected. The tanks scheduled for use with the shuttle Endeavour in April and the shuttle Atlantis in June or July were built with alloys that are believed to have the proper strength.

To strengthen the rest of the stringers in Discovery's tank, radius block plates will be riveted into place over the anchors that hold the existing stringers in place. Testing indicates the modification will ensure the tank is up to the rigors of fueling and launch, giving it the safety margin needed to clear Discovery for flight.

But additional analysis will be needed to make sure the modifications do not introduce any unintended consequences. Sources said shuttle Program Manager John Shannon decided to press ahead with the repairs in parallel with work to flight certify the modifications. While that approach could lead to additional delays if the certification runs into problems, a senior manager said the tank could not be flown "as is" and that it made sense to proceed with repairs in parallel based on the results of testing to date.

In any case, engineers are proceeding cautiously in the wake of an incident in which a technician, working to drill out a stringer fastener so a cracked segment could be removed, inadvertently drilled into the underlying skin of the liquid oxygen tank. The damage was minimal and technicians were expected to simply buff out the blemish. But as a result, NASA managers have decided to leave each stringer's top-most fastener in place and to install radius blocks over fasteners two through seven instead.

"It's tricky working in the area when they're that close to the tank," an official said. "So they decided not to worry about fastener No. 1 on any of these stringers."

Shuttle engineers briefed senior management Monday on the status of the repairs and Bill Gerstenmaier, director of space operations at NASA headquarters, planned to brief John Holdren, director of the Obama administration's Office of Science and Technology Policy, later in the day. Gerstenmaier, shuttle Program Manager John Shannon and space station Program Manager Mike Suffredini were scheduled to hold a news conference Tuesday to discuss the status of the tank repair work and analysis.

Discovery originally was scheduled for launch Nov. 1 on a space station resupply mission. But the flight was repeatedly delayed by relatively minor technical problems and finally, on Nov. 5, by a gaseous hydrogen leak in a 7-inch 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 the tank's left-side booster attachment thrust panel, which helps carry the load during launch.

After a fueling test Dec. 17 to collect additional data, Discovery was moved back to the Vehicle Assembly Building for additional inspections. Four more small cracks were found in three stringers on the back side of the tank, which could not be inspected at the launch pad.

Analysts have been unable to identify a single root cause explaining why the cracks formed in the first place, a key element in developing the engineering justification for proceeding with flight. But the radius block modification should provide the confidence needed to press ahead if no unintended consequences are identified.

Two launch windows were available in February, one opening on Feb. 3 and closing one week later and another opening Feb. 27 and closing March 6. NASA managers decided last week to pass up the first window to give engineers more time to assess the crack issue and to make a decision on whether to install the radius block stiffeners all the way around the tank.

The Feb. 27 opening of the second launch window was based on the planned Feb. 15 launch of a European Space Agency cargo spacecraft that was scheduled to dock at the space station on Feb. 26. But it appears the rendezvous timeline can be compressed, allowing NASA to launch Discovery as early as Feb. 24 if ESA's Automated Transfer Vehicle takes off on time and has no problems in orbit.

NASA and contractor managers and engineers are expected to set a new target launch date during a program requirements control board meeting Thursday. It's not yet clear how Discovery's launch delays will affect the following launch, a mission by the shuttle Endeavour to deliver a $2 billion physics experiment to the International Space Station.

It appears Endeavour will be delayed to mid April, but no decisions have been announced.