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updated 12/19/2010 3:16:35 PM ET 2010-12-19T20:16:35

HOUSTON NASA pumped hundreds of thousands of gallons of super-cold propellants into space shuttle Discovery's external fuel tank Friday morning to test why it had developed cracks along two support beams during a launch attempt last month.

The so-called " tanking test " got underway at 7 a.m. EST (1200 GMT), when the three-hour fueling process began. It is scheduled to continue until 2:25 p.m. EST (1925 GMT) when the test's simulated launch countdown ends after a five-minute hold at the T-minus 31 second mark.

Sensors installed on the tank will continue to collect data as the tank is emptied this afternoon and then continue to record the tank's performance through Saturday as it warms to ambient temperature.

The test, which also serves to verify the repairs technicians made to the 154-foot tall tank, was proceeding without any noticeable issues. Results from the test were not expected immediately. Managers and engineers will review the data generated by the exercise before determining the next course of action.

"It looks like we're getting good data," said Mike Moses, NASA's space shuttle launch integration manager at the Kennedy Space Center in Cape Canaveral, Fla. "Like anything with a hazardous operation, it is nice and boring today, which is a good thing. Everything is looking like it is supposed to."

Discovery will launch no earlier than Feb. 3, 2011 on its final flight to space before being retired. But first, the shuttle will be rolled off its launch pad and returned to the 52-story tall Vehicle Assembly Building (VAB) to undergo further tank inspections.

Cracked stringers

Friday's tanking test will help shuttle engineers determine what caused two 21-foot-long, U-shaped aluminum brackets, called stringers, to crack during the lead-up to a Nov. 5 attempt to launch Discovery on its STS-133 mission. That launch attempt was called off not because of the cracks but rather due to an unrelated gaseous hydrogen leak that emanated from a cable connecting the tank to the pad.

The hydrogen leak was subsequently traced back to a hardware alignment issue and was corrected. During Friday's test, no hydrogen was detected escaping from the umbilical.

During the Nov. 5 launch attempt, the cracked stringers caused their overlying foam insulation to crack, which is how NASA initially discovered the damage in the hours following the foiled countdown. In addition to the cracked beams posing a potential structural concern, cracks in the foam could cause the insulation to fall off during the shuttle's climb to orbit, creating a risk it might impact and damage the orbiter's heat shield. [ Photo of cracks on shuttle Discovery's fuel tank ]

In February 2003, falling foam from the fuel tank hit the space shuttle Columbia's left wing, damaging its leading edge. The impact ultimately led to the loss of the vehicle and its seven-person crew when Columbia re-entered Earth's atmosphere at the conclusion of its STS-107 mission.

Though NASA has encountered cracks in a tank's stringers before, they have always shown up during its assembly. This was the first time that cracks were found while the space shuttle was on the launch pad, leading engineers to wonder if they missed something during the beams' fabrication or installation.

"Either the stringer was the cause of the problem, that there was some assembly tolerance, some build up of stress in that part, or a crack that went through the system and we didn't see, that caused this stringer to break and so it is an isolated event it's this stringer and that's the problem or the... stringer is a victim here and the design has a problem, the tank itself has a bigger issue to it, the loading had a problem and that there is something wrong generically that is causing stress to be concentrated in this area that's never been there before," explained Moses.

Because engineers have not yet discovered the root cause for the damage, managers called for Friday's tanking test to gather more data.

Instrumented tank

Shuttle technicians outfitted the tank's ribbed midsection, or intertank region, with nearly 90 instruments, including strain gauges to precisely record movement and temperatures as the tank chilled and warmed again during the test's fuel loading and emptying process.

The orange-brown tank holds more than 500,000 gallons of super-cold liquid oxygen at minus 297 degrees Fahrenheit and liquid hydrogen at minus 423 degrees F. Earlier measurements have shown that these cryogenic propellants cause the tank to shrink by about half an inch.

The test will provide more information about how the tank performs during fueling, as well as verify the repairs made to the cracked stringers. Technicians installed new sections of metal, called "doublers" because they are twice as thick as the original stringer metal, to replace the tops of the two cracked stringers. [ INFOGRAPHIC: NASA's Space Shuttle from Top to Bottom ]

"The point of the [test] is to gather performance data on the tank," Moses said. "We have stringers instrumented in the repair area so we can look and see how the repair performs under a cryo load. We have stringers instrumented next to that repair that aren't damaged, so we can see how they perform... and then on the opposite side of the tank, we've instrumented stringers to kind of go to a control-type theory, to say over here in a completely different area, here's how the tank performs."

The big picture moving forward

According to Moses, the test will generate more than five or six terabytes 5,000 to 6,000 gigabytes of data.

"All of that is going to be post-processed and shipped to the Michoud Assembly Facility [in Louisiana] and Marshall Space Flight Center [in Alabama] where the analysis will be done," said Moses. "That data will be there tonight. The expectation is by the weekend, we'll have reviewed all of that."

The data needs to be verified, though not analyzed, before Discovery can be rolled back to its assembly hangar for further analysis and to be prepared for launch.

"We've decided to roll back to the VAB and so one of the go-no go's for that rollback is a report from that instrumentation team that they are happy with the data. That doesn't mean we've analyzed it and the results of the data are good but that we've captured good data," Moses explained.

"We can then cut the wiring and de-configure the vehicle and be ready to rollback," he said.

Discovery is tentatively scheduled to begin its 3.4 mile (5.5 km) journey back to the VAB just after midnight on Tuesday, Dec. 21. Once inside the building, the fuel tank will undergo X-ray scans to examine the stringers along its backside. The tanking test instrumentation will also be removed, and along with the installation of any modifications called for by the test data the tank will be readied again for launch.

Meanwhile, shuttle managers will begin seeing the results from the test.

"Our big picture plan is that on Monday, Tuesday, Wednesday next week we'll start to get the results of that data in," said Moses. "It is going to take a while before we get all the fine details, but the big picture... all that's going to happen next week."

If all proceeds smoothly, Discovery will be returned to the launch pad in mid-January.

"We're going to have folks working around the country right through the Christmas holiday here to be ready to potentially fly again in this February launch window when it comes around," said Moses.

Discovery's next opportunity to embark on its 39th and final mission is scheduled for no earlier than 1:37 a.m. EST (0637 GMT) on Feb. 3. The shuttle's planned 11-day trip will deliver a storage room and Robonaut 2, a humanoid robot, to the International Space Station. Two spacewalks are also planned.

Editor's note: This article was updated to correct the number of gallons of fuel that were loaded into Discovery's tank.

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