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What do two Space Shuttle Solid Rocket Boosters plus one External Tank equal? Enough thrust to power Space Shuttle Discovery on its Return to Flight mission to the International Space Station. Recently at NASA's Kennedy Space Center in Florida, the redesigned External Tank was joined to the twin Solid Rocket Boosters -- marking a major step in assembling the Space Shuttle stack for its safe return to flight. During the process, the left and right boosters are bolted to the tank at the top and tail ends. At liftoff, each booster produces 2.65 million pounds of thrust and burns about 4.5 tons of fuel per second. At approximately two minutes after liftoff, these boosters separate when pyrotechnical devices fire to break the 25-inch, 62-pound steel bolts. The External Tank is the largest element of the Shuttle system. It holds a combined volume of seventy-three thousand cubic feet of liquid hydrogen and liquid oxygen, used to power the Space Shuttle's three main engines. The next Return to Flight milestone is scheduled to take place in mid-March. That's when the orbiter Discovery will move from the Orbiter Processing Facility to the Vehicle Assembly Building. There it will be joined to the tank and boosters before being moved to the launch pad. Discovery is scheduled for launch on mission STS-114 during a window that extends from May 15 to June 3.