Shuttle Engineers Expect Smooth Tank Swap for Discovery

The space shuttle Discovery and its booster-external tank stack are backlit by the open VAB doors as they rollout to the launch pad for the first time under NASA's STS-114 mission. CREDIT: NASA/KSC.

Space shuttle engineers have no worries over the planned swap out of the Discovery orbiter's external tank this month, a major step forward in NASA's first return to flight mission.

"I have every confidence that the process will go smoothly," said Tim Riley, integration operations chief for United Space Alliance (USA), NASA's prime contractor for its shuttle fleet.

Riley's team and a host of other engineers and technicians are preparing to exchange Discovery's fuel tank with a replacement equipped with an extra heater to limit ice debris hazards to the orbiter during launch.

Discovery's STS-114 mission, commanded by veteran astronaut Eileen Collins, is slated launch spaceward from Pad 39B no earlier than July 13 of this year. The orbiter is NASA's first shuttle to fly since the loss of Columbia and its seven-astronaut crew during reentry on Feb. 1, 2003.

NASA officials pushed back Discovery's launch from a May 22 target to allow time for additional ice debris tests and the tank change out.

Switching tanks

It is actually the orbiter that moves between external tanks, not vice versa, Riley told SPACE.com.

During the process, technicians will detach Discovery from its perch alongside its current 154-foot-tall (46-meter-tall) tank, and hoist it over the launch stack towards a different work bay. There, the external tank for Discovery's follow-up mission STS-121 aboard Atlantis, is awaiting its new spacecraft.

"Once we get the [Discovery] back into the bay, it's roughly a five-day process," Riley said. "It's pretty straightforward then."

Discovery and its launch stack are expected to cross the 4.2 miles (6.7 kilometers) separating the launch pad and 52-story VAB next week, NASA officials said Wednesday. The move will follow a May 20 fueling test of the current tank, they added.

The tank swap will mark the eighth time a NASA space shuttle launch stack has been rolled back into the VAB for a partial or complete disassembly. Technicians most recently adjusted a shuttle launch stack in 2003, when they destacked the launch system for the Atlantis orbiter, Riley said. At that time, Atlantis was expected to carry the STS-114 crew into orbit in March 2003.

Increasing safety

The driving force behind Discovery external fuel tank swap is launch and crew safety.

In 2003, investigators traced Columbia's destruction back to wing damage caused by a suitcase-sized chunk of foam insulation that separated from the tank's bipod ramp, where an orbiter connects to a fuel tank, and struck the shuttle during launch. NASA has since redesigned shuttle external tanks to replace bipod ramp foam with a heater at the.

But last month, shuttle managers announced that ice forming on an external tank's bellows unit, which expands and contracts in response to supercold fuel, could pose a similar danger to Discovery if it strikes the orbiter during the high speeds of launch.

The new heater, combined with a drip lip modification to the bellows unit, should limit that risk, NASA officials said. Heater kits arrived on May 5 for both the STS-121 launch - which will feed Discovery's main engines in July - and for ET-120 currently attached to the STS-114 launch stack, they added.

"The heater is here and they're doing the work on it," said USA spokesperson Tracy Yates.

Despite Discovery's launch delay, NASA's push to return its shuttle fleet to flight status is a step forward for the U.S. space program, USA officials said.

"The return to safe flight brings me great satisfaction in that we faced the adversity of [Columbia's] tragedy and recovery as a team, and together carried out the rigorous and responsible plan for getting back into the business of spaceflight," USA president and CEO Michael McCulley said in an e-mail interview. "We can now focus our energy and talents on safe, successful missions."

Shuttle integration crews may also benefit from the upcoming tank change operation, since it calls for additional hands-on work with flight hardware, which keeps technician skills fresh.

"It certainly has values in that respect," Riley said. "The guys jump at any chance, whether we're mating or de-mating hardware, to work with an orbiter."

First things first

Before Riley and his team can carry out Discovery's external tank swap, they must first wait for NASA to put the orbiter's current tank through its second fueling test.

Launch managers hope the May 20 tanking test, which will fill Discovery's external tank with more than 500,000 gallons (1.9 million liters) of liquid oxygen and liquid hydrogen fuel, will shed light on sensor and valve questions raised in the initial April 14 test.

In the meantime, Riley and other shuttle technicians will prepare for Discovery's rollback by conducting maintenance on the orbiter lifting swing that hoists shuttles about the VAB, as well as other equipment used to support the vehicle during transfer operations.

"I am extremely eager to see it fly," Riley said of Discovery. "I am even more eager to see it land [back] here."

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