CAPE CANAVERAL - Kennedy Space Center technicians will take on a lengthy and unusual job today, one aimed at getting inside a shuttle external tank to swap out suspect fuel sensors.
"It's not a trivial process," said John Chapman, manager of NASA's external tank project at Marshall Space Flight Center in Alabama.
"There are certain risks involved," NASA shuttle program manager Wayne Hale added. "You could damage the tank."
Delivered to KSC earlier this month, the 154-foot-tall (47-meter) tank is hanging vertically inside the 52-story Vehicle Assembly Building.
A March 14 decision to replace four fuel-depletion sensors in the bottom of the tank prompted NASA to delay the planned launch of its second post-Columbia shuttle mission from May to July.
The sensors serve the same purpose as automobile fuel gauges and also provide a backup means of making sure the shuttle's three liquid-fueled main engines shut down properly in flight. A malfunction could prompt a premature shutdown, which could lead to a risky and unprecedented emergency-landing attempt.
A failure also could allow the engines to run dry, causing powerful turbopumps to seize up, break apart and possibly prompt a catastrophe.
Technicians have built scaffolding that will provide a work platform beneath the dome-shaped bottom of the tank. A makeshift clean room also was put in place to make certain no dirt or other contaminants get inside the tank.
The replacement work will begin today as technicians start to remove foam insulation from a 3-foot-wide (about one-meter) manhole cover bolted on the bottom of the tank. Later this week, the cover will be taken off and a technician will climb inside.
The four sensors are on a bracket attached to an internal shock mount that crisscrosses the bottom of the tank. The technician will remove the bracket, replace it and then exit the tank.
After the manhole cover is bolted back in place, the tank will be hoisted out of the checkout cell and onto a horizontal transporter in the center aisle of the assembly building.
The move will enable technicians to reapply foam insulation on the bottom of the tank, a lengthy process that involves spraying several layers of liquid foam and allowing each to harden.
All in all, the sensor swap is expected to take about 22 days.
"It takes time," Chapman said. "But it's a straight-forward process."
The change-out work will be followed by the mating of the tank to a pair of 149-foot (45-meter) tall solid rocket boosters that already are stacked on a mobile launcher platform in the assembly building. The orbiter Discovery will be connected to the tank late next month.
NASA hopes to launch the shuttle about July 1.
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