June 8, 2006 -- -- Touting design changes and improvements -- such as a redesigned fuel tank and new sensors and windows -- NASA said today that Space Shuttle Discovery has been cleared to fly.
The agency warned, however, that there are still engineering questions to resolve and cautioned about the risks inherent in space flight.
"We are ready to go into what I think will be a rapid succession of flights in the next few months," said space shuttle program manager Wayne Hale.
The next shuttle flight is scheduled to launch in a window that opens July 1 and closes July 19.
In a series of press briefings today at the Johnson Space Center in Houston, NASA managers talked about the changes to the shuttle and noted that the next flight would test what Hale called "the largest aerodynamic change to this vehicle that has been made since we started flying 25 years ago."
That change involves foam structures on the large orange external fuel tank that on previous flights acted as windshields, protecting the cables and pressurized fuel lines that run along the outside of the tank.
Hale said that the foam windshields were added to the shuttle design shortly before the first flight in 1981 because engineers weren't sure at the time whether the external components could withstand the tremendous force and turbulence of a launch.
In 2003, a chunk of foam insulation fell off Columbia's fuel tank during launch, hit the ship's wing, and critically damaged its heat shield. Seven astronauts aboard the shuttle died when Columbia broke apart as it flew through the atmosphere for landing.
Large foam chunks also fell off during Discovery's liftoff last year but did not strike the orbiter.
Hale said today that unlike in 1981, engineers have now improved wind tunnel technology and provided better aerodynamic modeling that helped NASA conclude that the shuttle could fly without the foam windshields.
"We certainly believe we have sufficient engineering evidence to show that we will not lose large pieces of foam off the tank the next time we fly," he said.
"We are ready to go fly," said John Chapman, the space shuttle external tank manager.
At the same time, Hale cautioned that it would take real-life conditions that can only be experienced during a shuttle launch for engineers to "flight test" the changes and see if they are right.
"All those wind tunnels, and all that analysis and all those computer models are never quite as good as real life," Hale said. "You really have to go fly it and see if it works like you think it was going to."
Engineers say they are working on other issues, including a redesign of ice-frost ramps on the external tank that can also shed small amounts of foam. But for now, the ice- frost ramp will fly as is until engineers know the potential impact of redesigning them.
"You want to make sure that whenever you make any change at all, you understand it fully," said Chapman. He said a redesign of the ice-frost ramps is at the top of the list for improvements to the external tank.
Hale said it was "guaranteed" that small pieces of foam would still come off the orbiter but added "we meet the required factor of safety."
Mission Commander Steve Lindsey said he is confident that the shuttle is safe to fly even without a redesigned ice frost ramp.
"We've tested the entire vehicle," Lindsay said. "And we think it's time to go flight test."
The next shuttle flight will dock with the International Space Station. Over the next four years, the shuttle is expected to make 16 flights to help finish building the station before the shuttles are retired in 2010.
It's a busy flight schedule, but Hale said NASA would not be rushed.
"We take every mission in turn and make sure we feel comfortable with flying it before we do," said Hale. "You cannot let the schedule drive you to make stupid decisions."
Even with the design improvements, NASA managers and astronauts that there are no guarantees.
"We pound those risks as flat as we can but there are unknowns out there," Lindsay said.
"Are we at risk? Absolutely we're at risk," said Hale. "If you're not a little scared when you launch the shuttle you don't understand what's happening."
Reuters contributed to this report.