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NASA zeroes in on root cause of shuttle tank cracks
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: January 11, 2011


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Engineers believe they have zeroed in on the root cause of cracks in the shuttle Discovery's external tank, NASA officials said Tuesday. Installation of a relatively simple modification to the tops of the structural ribs, or stringers, where the ship's liquid oxygen tank is supported by a massive flange should resolve the problem once and for all, setting the stage for another launch attempt Feb. 24.


Engineers successfully duplicated a crack in a stringer using a test rig that subjected the U-shaped stringer to the forces experienced during fueling. Credit: NASA
 
"We're on the road to bringing this tank to a hundred percent," said shuttle Program Manager John Shannon. "It's been two months of very vigorous activity, it took a lot of investigation to piece the different components of how this could be happening together, but I'm very confident we've finally got this figured out and we have a fix that's easy to implement and will be 100 percent effective."

Discovery's late February launch window opens the day after the European Space Agency's unmanned Automated Transfer Vehicle, loaded with supplies and equipment, reaches the International Space Station. The ATV is scheduled for launch from French Guiana on Feb. 15. Mission managers initially planned a docking on Feb. 26, but ESA, NASA and Russian managers have agreed to aim for a docking Feb. 23 instead. Assuming the ATV takes off and docks on time, Discovery would be set for another launch attempt at 4:50:13 p.m. EST on Feb. 24.

If all goes well, NASA hopes to have the shuttle Endeavour ready for launch on its final mission by around April 18. But planning for that flight is somewhat uncertain. Commander Mark Kelly's wife, Rep. Gabrielle Giffords, was shot in the head Saturday in Tucson by a gunman who killed six and wounded 13 others. Giffords remains in critical condition.

At a news briefing Tuesday to discuss the external tank investigation, Bill Gerstenmaier, director of space operations at NASA headquarters in Washington, declined to discuss Kelly's status, saying "out of respect to the family, we really are not ready to answer those questions today."

"We're going to let Mark decide really kind of what he needs to do," Gerstenmaier said. "We'll let his personal situation take focus on his activities and his thoughts and later when we know more about where (the Endeavour mission) sits and what our options are for Mark and his activities, we'll come talk to you at that time. ... Our hearts and prayers go out to the family and we're really thinking about Mark in everything we do."


A drawing illustrates the components of the intertank region on a shuttle's external tank. The top few inches of the vertical stringers, which are pulled inward during fueling by low-temperature contraction, can experience cracks where fasteners attach them to the tank. Credit: NASA
 
The external tank crack problem first surfaced during fueling for a Nov. 5 launch attempt. Discovery's countdown was called off because of a gaseous hydrogen leak, a problem that was later resolved. But engineers also noticed a large crack in the external tank's foam insulation that developed during fueling. When the foam was removed later, four cracks were found in the "feet" of two adjacent ribs, or stringers, in a compartment that separates the tank's liquid oxygen and hydrogen sections.

The cracks presumably developed when the liquid oxygen tank was loaded with super-cold propellant, causing the flange that supports it to contract, pulling the top few inches of the intertank stringers inward. Tank components are designed to cope with that stress without breaking but for some reason, cracks developed near the tops of two stringers in Discovery's tank.

Engineers were immediately concerned, citing two potentially serious issues. Cracked stringers could result in foam insulation falling off during flight that could pose an impact threat to the shuttle's fragile heat shield. And if enough stringers failed, the structural integrity of the tank could be in question.

NASA launched a wide-ranging investigation to figure out the root cause of the cracks and what might be needed to correct the problem. A full-scale instrumented fueling test was carried out Dec. 17 to precisely map out the stresses acting on the tank during fueling. Discovery then was moved off the pad and back to the Vehicle Assembly Building for additional inspections.


Cracks found on the tank's backside Panel 6. Credit: NASA
 
Engineers then found four more cracks on the back side of the tank near the tops of three stringers, which could not be inspected at the launch pad. Like the first set of cracks, these were repaired by splicing in fresh stringer segments and attaching doublers for additional strength.

Believing built-in stresses from assembly likely caused the cracks, engineers proposed installing doublers on stringers to either side of massive solid rocket booster attachment panels that experience the most stress during flight.

But as it turned out, the issue was more complicated than first suspected. Engineers reviewing manufacturing records discovered that the aluminum-lithium alloy used in 78 of the 108 stringers in the intertank of Discovery's external tank came from a batch that had a mottled appearance, possibly the result of heat treatments during the alloy's formulation. Testing showed the material used in those stringers had just 65 percent of the expected fracture strength.

On New Year's Eve, Shannon said, engineers successfully re-created a crack using the same mottled aluminum-lithium alloy in a test rig that subjected it to the same sorts of stress the tank experiences during fueling.


Normal aluminum-lithium alloy, using in external tank stringers, is shown in the top sample. Mottled material in the lower sample was used in Discovery's external tank. It has a lower fracture strength. Credit: NASA
 
"We absolutely have root cause and we have been able to show that through test," he said. "It's a combination of two factors. It's the low fracture toughness of the material, we have about 65 percent of the expected fracture toughness, combined with some assembly stresses that were built up in different areas.

"We know that because when we loaded the tank (with propellant), we put high stress on all of those stringers that had that low fracture toughness and we only had cracks in five of what we believe were 78 stringers. In looking at those stringers that cracked, we're seeing the evidence of those assembly stresses that combined with that low fracture toughness to cause the problem. So we have root cause and we have a fix that we're completely confident will eliminate those root causes."

The fix is installation of radius block plates all the way around the circumference of the upper intertank to beef up the under-strength stringers. The repair calls for engineers to remove the top few rivets holding the sides of each stringer in place. An aluminum plate -- a radius block -- is then placed over the stringer attachment "feet" and riveted into place. The idea is to lock the stringer flat against the side of the tank and make it less susceptible to the stresses that could cause fractures to develop.


NASA's fix calls for bolting so called "radius blocks" over stringer attachment fittings to ensure a flat, tight fit, making them less susceptible to cracking. Credit: NASA
 
Shannon said engineers are carrying out tests and analyses to make sure the repairs do not introduce any unintended consequences.

"We're proving that out through tests, we're looking at the strain, we're looking at the stiffness and verifying in the tests that material that does not have a radius block does not behave any different than one that does have a radius block," he said. "We're going through and modeling that through the entire tank and verifying we're not going to do any harm. Right now, it looks really good."

The repair work is expected to be completed by around Jan. 23. After that, engineers will move Discovery back to pad 39A for work to ready the ship for launch.

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Spaceflight Now Plus
Additional coverage for subscribers:
VIDEO: ROOT CAUSE OF SHUTTLE TANK CRACKS FOUND PLAY

VIDEO: WORK UNDERWAY TO REMOVE FOAM AND SENSORS PLAY | HI-DEF
VIDEO: SETTING UP ACCESS PLATFORMS AROUND TANK PLAY | HI-DEF

VIDEO: SHUTTLE ROLLED BACK TO ASSEMBLY BUILDING PLAY | HI-DEF
VIDEO: DISCOVERY DEPARTS LAUNCH PAD 39A PLAY | HI-DEF
VIDEO: LUNAR ECLIPSE OVER DISCOVERY PLAY | HI-DEF

VIDEO: HIGHLIGHTS FROM THE FUELING TEST PLAY | HI-DEF
VIDEO: RECAP OF THE FUELING EXERCISE PLAY
VIDEO: FUELING TEST COUNTDOWN UPDATE PLAY
VIDEO: COMMENTS FROM SHUTTLE LAUNCH DIRECTOR PLAY
VIDEO: GROUND UMBILICAL CARRIER PLATE CHECKED PLAY
VIDEO: INFO ON EXTERNAL TANK INSTRUMENTATION PLAY
VIDEO: DETAILED INFORMATION ON SPECIAL FUELING TEST PLAY

VIDEO: SHUTTLE DISCOVERY'S LAUNCH DELAYED TO FEBRUARY PLAY

VIDEO: FINAL PREPS FOR INSTRUMENTED TEST PLAY | HI-DEF
VIDEO: STRAIN GAUGES ATTACHED TO TANK PLAY | HI-DEF
VIDEO: BACKSCATTER INSPECTIONS OF TANK MIDSECTION PLAY | HI-DEF

VIDEO: INSTALLING DOUBLERS OVER STRINGERS PLAY | HI-DEF
VIDEO: GASEOUS HYDROGEN VENT ARM RETURNED PLAY | HI-DEF
VIDEO: DOUBLERS TO BEEF UP CRACKED BEAMS PLAY | HI-DEF
VIDEO: HYDROGEN FITTING IS REINSTALLED PLAY | HI-DEF
VIDEO: OPENING UP TANK'S ENTRANCE DOOR PLAY | HI-DEF
VIDEO: GUCP DETACHED AND SEALS REMOVED PLAY | HI-DEF
VIDEO: BROKEN FOAM REMOVED FROM THE TANK PLAY | HI-DEF
VIDEO: VENT ARM LETS GO FROM LEAKY GUCP PLAY | HI-DEF
VIDEO: DISCOVERY AWAITS REPAIRS TO TANK PROBLEMS PLAY | HI-DEF

VIDEO: INFORMATIVE DESCRIPTION OF TANK CRACK PROBLEM PLAY
VIDEO: DISCOVERY'S LAUNCH DELAYED TO MID-DECEMBER PLAY

VIDEO: GASEOUS HYDROGEN VENT ARM DETACHED FROM SHUTTLE PLAY

VIDEO: STS-133 MISSION PREVIEW MOVIE PLAY | HI-DEF
VIDEO: SHUTTLE DISCOVERY CREW BIOGRAPHIES PLAY | HI-DEF
VIDEO: ISS EXPEDITION 25-26 PREVIEW MOVIE PLAY | HI-DEF
VIDEO: SPACE STATION CREW BIOGRAPHIES PLAY | HI-DEF

VIDEO: POST-SCRUB NEWS BRIEFING ON LEAK PLAY
VIDEO: GASEOUS HYDROGEN LEAK SCRUBS LAUNCH PLAY | HI-DEF
VIDEO: BAD WEATHER DELAYS DISCOVERY LAUNCH PLAY
VIDEO: ELECTRICAL ISSUE NO LONGER A CONCERN FOR DISCOVERY PLAY
VIDEO: LAUNCH POSTPONED BY ELECTRICAL CONCERN PLAY
VIDEO: TUESDAY MORNING'S COUNTDOWN STATUS CHECK PLAY
VIDEO: DISCOVERY'S PRE-LAUNCH NEWS CONFERENCE PLAY
VIDEO: COUNTDOWN PREVIEW BRIEFING AND WEATHER FORECAST PLAY
VIDEO: ANOTHER 24-HOUR DELAY ORDERED TO FINISH REPAIRS PLAY
VIDEO: LAUNCH DELAYED 24 HOURS BY LEAK REPAIRS PLAY

VIDEO: ASTRONAUTS ARRIVE FOR LAUNCH PLAY | HI-DEF

VIDEO: FLIGHT READINESS REVIEW SETS LAUNCH DATE PLAY

VIDEO: SHUTTLE AND STATION PROGRAM BRIEFING PLAY
VIDEO: THE STS-133 MISSION OVERVIEW PRESENTATIONS PLAY
VIDEO: PREVIEW BRIEFING ON MISSION'S SPACEWALKS PLAY
VIDEO: IN-DEPTH BACKGROUND ON ROBONAUT 2 HUMANOID PLAY
VIDEO: THE ASTRONAUTS' PRE-FLIGHT NEWS BRIEFING PLAY

VIDEO: PAYLOAD BAY DOORS CLOSED FOR LAUNCH PLAY | HI-DEF
VIDEO: CREW LEAVES KSC FOR TRIP TO HOUSTON PLAY | HI-DEF
VIDEO: ASTRONAUTS INSPECT THE PAYLOAD BAY PLAY | HI-DEF
VIDEO: SHUTTLE EVACUATION PRACTICE PLAY | HI-DEF
VIDEO: ASTRONAUTS EGRESS SHUTTLE AS SEEN LIVE PLAY
VIDEO: ASTRONAUTS BOARD DISCOVERY PLAY | HI-DEF
VIDEO: SUN RISES ON LAUNCH PAD 39A PLAY | HI-DEF
VIDEO: LAUNCH DAY REHEARSAL BEGINS PLAY | HI-DEF
VIDEO: COMMEMORATIVE WALL SIGNING IN VAB PLAY | HI-DEF
VIDEO: CREW BRIEFED ON EMERGENCY PROCEDURES PLAY | HI-DEF
VIDEO: IN SHUTTLE TRAINING AIRCRAFT'S COCKPIT PLAY | HI-DEF
VIDEO: TEST-DRIVING AN EMERGENCY ARMORED TANK PLAY | HI-DEF

VIDEO: PRE-LAUNCH INTERVIEW WITH STEVE LINDSEY PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH ERIC BOE PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH AL DREW PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH TIM KOPRA PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH MIKE BARRATT PLAY
VIDEO: PRE-LAUNCH INTERVIEW WITH NICOLE STOTT PLAY

VIDEO: PAYLOADS INSTALLED INTO DISCOVERY PLAY | HI-DEF
VIDEO: MISSION PAYLOADS ARRIVE AT LAUNCH PAD PLAY | HI-DEF
VIDEO: CANISTER HAULING PAYLOADS TURNED UPRIGHT PLAY | HI-DEF
VIDEO: MODULE HOISTED INTO SHIPPING CANISTER PLAY | HI-DEF
VIDEO: WEIGHING NEW SPACE STATION MODULE PLAY | HI-DEF

VIDEO: GANTRY PLACED AROUND DISCOVERY PLAY | HI-DEF
VIDEO: SHUTTLE ATLANTIS REACHES PAD 39A PLAY | HI-DEF
VIDEO: CROWDS WATCH DISCOVERY'S FINAL ROLLOUT PLAY | HI-DEF

VIDEO: SHUTTLE HOISTED FOR ATTACHMENT TO TANK PLAY | HI-DEF
VIDEO: CRANE ROTATES THE ORBITER VERTICALLY PLAY | HI-DEF
VIDEO: DISCOVERY DEPARTS ITS HANGAR PLAY | HI-DEF

VIDEO: TIME-LAPSE SHOWS DISCOVERY ASCENDING IN VAB PLAY
VIDEO: TIME-LAPSE SHOWS THE MOVE TO ASSEMBLY BUILDING PLAY

VIDEO: DISCOVERY'S MAIDEN FLIGHT: FIRST TRIP TO VAB PLAY
VIDEO: DISCOVERY'S MAIDEN FLIGHT: ROLLOUT TO PAD 39A PLAY
VIDEO: DISCOVERY'S MAIDEN FLIGHT: TEST-FIRING ENGINES PLAY
VIDEO: DISCOVERY'S MAIDEN FLIGHT: ASSORTED VIEWS OF FRF PLAY

VIDEO: THE HISTORY OF SHUTTLE DISCOVERY PLAY
VIDEO: THE HISTORY OF SHUTTLE ENDEAVOUR PLAY
VIDEO: THE HISTORY OF SHUTTLE ATLANTIS PLAY

VIDEO: INSPECTION OF THE MISSION PAYLOADS PLAY | HI-DEF
VIDEO: ROBONAUT ARRIVES AT KENNEDY SPACE CENTER PLAY | HI-DEF
VIDEO: SPACE STATION'S SPARE THERMAL RADIATOR PLAY | HI-DEF
VIDEO: BLANKETING LEONARDO WITH INSULATION PLAY | HI-DEF
VIDEO: RACK INSERTED INTO LEONARDO FOR LAUNCH PLAY | HI-DEF
VIDEO: LEONARDO RETURNS FROM ITS PREVIOUS FLIGHT PLAY | HI-DEF
VIDEO: STATION'S SPARE PARTS DEPOT ARRIVES PLAY | HI-DEF

VIDEO: ORBITER'S PAYLOAD BAY CLOSED FOR ROLLOUT PLAY | HI-DEF
VIDEO: ASTRONAUTS VISIT THEIR SPACECRAFT PLAY | HI-DEF
VIDEO: CREW INSPECTS LEONARDO MODULE PLAY | HI-DEF
VIDEO: DISCOVERY RECEIVES ITS MAIN ENGINES PLAY | HI-DEF
VIDEO: FUEL TANK MATED TO SOLID ROCKET BOOSTERS PLAY | HI-DEF
VIDEO: HOISTING FUEL TANK INTO CHECKOUT BAY PLAY | HI-DEF
VIDEO: EXTERNAL FUEL TANK UNLOADED FROM BARGE PLAY | HI-DEF
VIDEO: MISSION'S FUEL TANK ARRIVES AT SPACEPORT PLAY | HI-DEF

VIDEO: POST-FLIGHT DESERVICING: OMS POD PLAY | HI-DEF
VIDEO: POST-FLIGHT DESERVICING: OBSS BOOM PLAY | HI-DEF
VIDEO: POST-FLIGHT DESERVICING: ENGINES PLAY | HI-DEF
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