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Orbiter: Discovery
Mission: STS-133
Payload: Leonardo
Launch: Feb. 24, 2011
Time: 4:53 p.m. EST
Site: Pad 39A, Kennedy Space Center
Landing: March 9 @
11:57 a.m. EST
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Discovery's final launch postponed until February
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 3, 2010;
Updated @ 7:25 p.m. with adding tanking test details


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KENNEDY SPACE CENTER, FL--Launch of the shuttle Discovery on a space station resupply mission will be delayed until at least Feb. 3, NASA managers announced Friday, to give engineers more time to carry out tests to help figure out what caused cracks in the ship's external tank and what, if any, modifications might be needed before the ship can be cleared for flight.


Credit: Justin Ray/Spaceflight Now
 
"It's time to pursue a different path, and that's to head out with some test data," Bill Gerstenmaier, chief of space operations at NASA headquarters, told reporters. "Basically what we're going to do with these tests is make sure we didn't overlook anything, we'll see if these tests can reveal any new information for us and it'll also help us sort out what the real problems are we need to be working on versus ones that we just think theoretically may be there."

Assuming the upcoming work gives NASA the confidence to proceed, a launch on Feb. 3 would be targeted for 1:34:28 a.m. EST, roughly the moment Earth's rotation carries launch pad 39A into the plane of the International Space Station's orbit. Landing back at the Kennedy Space Center would be expected the evening of Feb. 13.

If that schedule holds up, launch of the shuttle Endeavour on a mission to deliver a $2 billion physics experiment to the space station, which had been scheduled for Feb. 27, would slip to April 1 at 3:15:55 a.m. EDT. Launch of NASA's final planned shuttle mission, a station resupply flight with the shuttle Atlantis, would remain targeted for next summer.

But that schedule assumes engineers resolve major questions about the ability of Discovery's tank to withstand the rigors of fueling and launch.

Despite weeks of around-the-clock analysis, engineers have not been able to determine the root cause of four cracks in two adjacent structural ribs, or stringers, making up a compartment in the external tank that separates liquid oxygen and hydrogen tanks. The cracks were discovered after a Nov. 5 launch attempt was scrubbed because of a gaseous hydrogen leak where a vent line attaches to the side of the tank. That problem was traced to a quick-disconnect misalignment issue, repairs were made and NASA managers are confident the system will work properly the next time Discovery is fueled for launch.

Engineers also repaired the stringer cracks, splicing in replacement sections, attaching "doublers" to add strength and re-applying foam insulation. But figuring out what caused the cracks in the first place has turned out to be a major engineering challenge. Before Discovery can be cleared for launch, the community needs to understand the likelihood of new cracks forming that could lead to potentially dangerous foam debris during ascent or possibly even compromise the tank's structural integrity.

"We've hit a point where there is no obvious answer as to what occurred," said John Shannon, the shuttle program manager at the Johnson Space Center in Houston. "What that means is we have to take the next step, we have to look in greater detail to understand what types of stresses you could put in these stringers in the assembly process, see how they could line up to add stress to that stringer and we have to do that through a demonstration. Analysis is not going to get us there."

Repeating a well-known phrase attributed to Wernher von Braun, Shannon said "one good test is equal to a thousand expert opinions, right? So we're at the point where we need that test, we need that fine level of data to understand exactly how those assembly stresses could line up to give us a crack when we initially loaded."

The lightweight aluminum-lithium alloy used in shuttle external tanks is known to be more brittle than the heavier aluminum material used in earlier fuel tanks. Cracks in aluminum-lithium components are not unusual -- an average of one per tank is found and repaired during manufacturing -- but Discovery's are the first to be detected at the launch pad, the apparent result of thermal stresses.

External tank hardware contracts or shrinks slightly when exposed to ultra-low-temperature rocket fuel. The cracks in this case occurred near the top of the intertank where the stringers meet a flange that runs around the circumference of the lower section of the liquid oxygen tank. Thermal stresses tend to pull the tops of the stringers inward as the oxygen tank is chilled, but the hardware is designed to accommodate that.

Engineers suspect some sort of unexpected built-in load was present that, in combination with normal thermal stress, resulted in stress-relief fractures in two stringers when the tank was loaded with propellants Nov. 5. But the precise mechanism is unclear and as a result, engineers are unable to accurately model the tank's response to the forces it will experience during another fueling and, eventually, launch.

To get a better understanding of what might be going on, two major tests are planned. One team of engineers will build a high-fidelity mockup of the ribbed intertank components that cracked, including deliberate flaws. The idea is to replicate, if possible, the damage seen during the Nov. 5 fueling.

At the Kennedy Space Center, strain gauges and temperature sensors will be attached to Discovery's tank before a fueling test later this month to collect data on the actual thermal stresses experienced when liquid hydrogen and oxygen are pumped aboard. Along with the sensors, stereo cameras will be used to measure exactly how much the tank shrinks during fueling.

"We're hoping the optical piece, plus the strain gauge measurements, will give us a really great indication of what the stress level is in those stringers," Shannon said. "We believe the design is robust and should not fracture under that stress. We'll verify that and then we'll add the assembly issues you could potentially have to see if we can get to a root cause.

"We were hopeful early on that it would be some kind of very obvious kind of flaw. Didn't happen. Then we were hopeful that just a simple cryo tanking would cover us for any ascent loads. It's very close, but it's not quite there. So now we have to go to that next level ... to get the root cause and determine what our screening criteria is to fly that tank confidently."

Engineers hope to carry out the test by Dec. 16. That would allow enough time to haul Discovery back to the Vehicle Assembly Building for additional X-ray inspections, if necessary, and still get the ship back out to the launch pad in time to meet the Feb. 3 target date. As of this writing, there are no plans for a roll back. But if NASA managers ultimately decide to inspect stringers on the opposite side of the tank from the shuttle, a roll back would be required because of access problems at the pad.

Whenever it occurs, the fueling test will mimic an actual launch countdown.

"I want to do a test in a flight configuration," Shannon said. "What that means is that we would remove foam, that nice foam they just put on the tank (for repairs), we would go remove that, we would put our instrumentation on there and then we're going to foam it back up. Because I want the most accurate models I could possibly get of the stress and the temperatures in that area in a flight configuration. That takes time. That's what's kind of driven us out of the December window."

NASA originally hoped to launch Discovery Nov. 1, but the flight was delayed one and then two days because of work to replace quick-disconnect fittings in the shuttle's right-side orbital maneuvering system rocket pod. Launch then was delayed 24 hours to Nov. 4 because of an electrical glitch in the circuitry associated with a main engine controller. Troubleshooting showed the likely cause was transient contamination in a cockpit circuit breaker.

A launch attempt Nov. 4 was called off before the start of fueling because of stormy weather at the Kennedy Space Center. The launch attempt on Nov. 5 was cancelled because of the gaseous hydrogen leak. After the cracks were discovered, launch was delayed to no earlier than Nov. 30. Launch then slipped to no earlier than Dec. 3 and finally to NET Dec. 17. It became obvious after a PRCB meeting Thursday that more time was needed to resolve the problem and launch was delayed to at least Feb. 3.

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Spaceflight Now Plus
Additional coverage for subscribers:
VIDEO: SHUTTLE DISCOVERY'S LAUNCH DELAYED TO FEBRUARY PLAY

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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