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Tanking Test Evaluates Stringers, GUCP
Wires and sensors on Discovery's tank.Image above: A detail of the wires and sensors placed on Discoery's external fuel tank for the tanking test. Photo credit: NASA TV
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A daylong test of space shuttle Discovery's external fuel tank took place Dec. 17 at NASA's Kennedy Space Center in Florida to help engineers evaluate support beams called stringers that make up the ribbed portion of the tank. The tops of two stringers cracked during fueling operations on Nov. 5. That launch attempt was scrubbed after an attaching point between a ventilation pipe and the external tank, known as the ground umbilical carrier plate, or GUCP, developed a hydrogen leak.

The “tanking test”, as it’s called, is a critical step in preparing Discovery for launch on its STS-133 mission to the International Space Station. The STS-133 launch window opens February 3.

Several hours into the test, Mike Moses, Space Shuttle Program Launch Integration manager, said the results looked good although it would take a few weeks to fully analyze them.

Technicians placed 39 strain gauges and 50 thermal sensors in the stringer areas of the tank to precisely measure the pressure on the stringers as the tank chills with the addition of the propellants. Typically, the tank's diameter shrinks by about an inch as it cools down.

The sensors were basically focused on two areas of the tank, one where the pair of cracked stringers were repaired and one in a similar section of the tank where there was no stringers damage. But the preparation did not stop there. Workers also had to run 162 wires, each 200 feet long, from the recording gear at the fixed service structure along an access arm to the shuttle and finally attaching to the individual sensors. And it was done during a cold snap at the Florida spaceport.

"The biggest challenge was the weather," said Alicia Mendoza, NASA's External Tank and Solid Rocket Booster vehicle manager at Kennedy.

The sensors are expected to gather at least 6 terabytes of data.

Moses explained that the data will first be compared to computer models and then, over the course of a couple weeks, be used to back up flight rationale about the stringers. Other aspects of the data will not be completed for months as they are used for long-term projections, but that information is not needed for Discovery's upcoming mission.

"It's definitely getting us back to launch posture," Moses said.

The GUCP, which was replaced, went through the countdown without leaking, verifying that component is ready for its launch day duties. The GUCP, pronounced "gup," connects to the external tank's hydrogen vent valve to funnel excess gaseous hydrogen from the tank to a flare stack near the launch pad.

"It's certainly good to get that behind us," said Charlie Blackwell-Thompson, NASA test director.

The tanking test filled Discovery's 15-story-tall external tank with more than 535,000 gallons of super-cold liquid oxygen and liquid hydrogen, the same propellants used to power the shuttle's three main engines. The liquid oxygen is at minus 297 degrees F and the liquid hydrogen is at minus 423 degrees F.

The test stuck closely to the course of a regular launch day, with some obvious exceptions, most notably the absence of the astronauts. Some of the software that runs the ground launch sequencer was changed to remove steps the computer would typically make, Shuttle Launch Director Mike Leinbach said.

For instance, programmers removed the step in the process calling for the orbiter access arm to be retracted late in the countdown.

"We're looking forward to the real thing," Leinbach added.

Teams plan to roll Discovery back into Kennedy’s Vehicle Assembly Building, or VAB, so technicians can take x-ray type scans the stringers on the back side of the external tank, the ones not facing the shuttle. Foam also will be reapplied after the sensors are removed from the tank.

Depending on how operations go inside the VAB and what additional work may be done on the tank, Discovery could be rolled back out to Launch Pad 39A in mid-January.

Steven Siceloff
NASA's John F. Kennedy Space Center