Wayne Hale told reporters during an all-day briefing last Thursday that NASA engineers have completed several important steps in the process of making the three remaining shuttles - all now at least two decades old and still using some devices containing 1970s-era electronics - as safe as possible.

These steps included installing new thermal-protection systems for the external cryogenic fuel tanks, replacing the windows and about one-third of the 15,000 gap fillers between each vehicle's insulating tiles, and improving the resolving power of video camera carried by the shuttle's onboard robotic arm.

"In all areas," Hale said, "we meet the required factor of safety - we are safe to fly."

Regarding perhaps the most controversial item in the agency's pre-launch adjustments - removal of the Protuberance Air Load ramp on the external tank - he said the decision was made after engineers at Lockheed Martin's tank assembly facility in Michoud, La., conducted a design-certification review of the change.

The PAL ramp, which covers pressurization lines and wiring along the tank's body, lost a piece of insulating foam weighing about one pound (about 0.45 kilogram) during shuttle Discovery's launch in August 2005.

A piece weighing about 1.5 pounds (0.7 kilograms) fell off during Columbia's launch in January 2003. Caught in the shuttle's slipstream and traveling at about 500 miles an hour, it punched a large hole in the leading edge of the vehicle's left wing, causing Columbia's eventual disintegration during reentry.

The Michoud engineers removed the PAL ramp from a shuttle tank, conducted computational fluid dynamics tests of the tank without it, and concluded that Discovery could fly safely if the component was eliminated, Hale said.

He explained that about 100 parts on the tank are affected by the PAL ramp removal. He said engineers have calculated the maximum stress on every part, and applied what he called NASA's standard "1.4 factor" of safety, meaning each part can withstand 40 percent more stress than the maximum expected during a shuttle launch.

"We are structurally sound to fly," Hale said, but added that NASA still has "a lot of work to do" on the science of the foam itself, 4,000 pounds (1,800 kilograms) of which are used to insulate the shuttle's main tank. The shuttle is the only spacecraft in history that requires flying polyurethane foam at supersonic speeds.

"We are still learning about the physics of foam," he said. Toward that end, Discovery will be carrying new cameras on its solid-fuel rocket boosters to track any foam losses. Cameras aboard Discovery itself photographed the foam piece missing from the PAL ramp during the launch last August.

One problem in studying the shuttle's external tank, Hale noted, was its non-reusability. "I really wish we could get it back," he said, referring to the fact that the tank falls into the ocean and is not recovered. The shuttle's twin booster rockets, in contrast, parachute back to Earth and are used in subsequent launches.

Another issue that has worried NASA engineers for months - also related to the insulating foam - is the current design of the ice frost ramps. The ramps cover vital connections between the shuttle and the tank and are supposed to channel away condensation during liftoff. The ramps are another source of foam loss, but engineers have not yet been able to devise a better design, so mission controllers have decided to launch without changes to the components.

"The point is not that there are concerns," Hale said. For example, engineers fully expect more foam to break off - but in smaller pieces and, in aggregate, probably less than a half pound in weight, he noted.

He assured reporters that he and other NASA officials are confident in the current safety margin - that enough engineering tests have been completed to give a reasonable assurance of success. "If you stood down until you fixed all of your problems," Hale said, "you'd never fly."

Among other questions, Hale was asked how realistic NASA was being by planning at least 16 more flights to the International Space Station between now and 2010, when the shuttle fleet is supposed to be retired. He responded that because NASA did at certain points manage up to nine flights a year, scheduling four per year is not considered unreasonable.

Meanwhile, he announced that the external fuel tank for Atlantis, currently scheduled for liftoff Aug. 28 - assuming the Discovery mission to the International Space Station goes off without a hitch - was due to arrive at Kennedy Space Center over the weekend.

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