The key to shuttle safety is to keep the tank from shedding foam insulation during launch so it will not slam into either the thermal protection tiles that blanket the orbiter's underside, or the brittle reinforced carbon-carbon that protects the orbiter's nose and the leading edges of the wings from the extreme heat of reentry.
During the past two years, engineers have modified several of the tank's external fixtures, so the foam insulation can be sprayed on more uniformly without "voids" that can expand during launch, causing foam to pop off.
The new external fuel tank for the shuttle Discovery -- the product of an extensive redesign -- is wheeled out of the barge at Kennedy Space Center.
(Peter Cosgrove -- AP)
New heaters are designed to keep ice from forming on the tank's external surfaces.
Unfortunately, there has never been a way to peer inside the foam to see whether it has been correctly applied, and the foam in some of the critical areas must be applied manually with a sprayer. "It is the toughest part of working with this system," acknowledged Neil Otte, NASA's chief engineer for the external tank.
"We can't look inside the foam," Otte said, so "we have one sprayer spray while another watches; we have videotape and high-fidelity mock-ups to check the procedure; we have technique cards that tell them exactly what they have to do."
But "we will lose foam," Otte acknowledged. "We have never said we were going to a zero-debris tank."
Have they done enough?
Otte said engineers have run millions of computer simulations that fire all sizes of foam debris at tile and reinforced carbon-carbon, from all angles, and from all parts of the external tank to all parts of the shielding. The foam is designed to ensure that the largest piece to break away from the top of the tank will weigh 0.023 pounds. The fatal hole in Columbia was made by a 1.67-pound foam chunk.
The problem here, however, is that the "cannon" used by NASA for actual lab tests cannot fire foam "bullets" as small as 0.023 pounds, whereas bullets twice as big have penetrated the exterior coating of reinforced carbon-carbon panels to a degree that could eventually cause catastrophic damage.
Still, Hale pointed out, they "are trying to find the threshold. Better than three-quarters of the shots have not damaged the panels. We deliberately creep up on the energy until the panel breaks."
During launch and in orbit, new sensors, imagers and cameras will allow NASA to check Discovery's heat-shielding more thoroughly than any shuttle ground crew in history. Yet this, too, carries a burden.
"I worry that we haven't used some of these sensors before, and we'll be [trying] to decide whether it's a crack or a shadow," said former shuttle pilot Stephen S. Oswald, now vice president and shuttle program manager for Boeing. "There's always the question in the back of your mind -- are you being tricked by a sensor, or is it a problem?"
If it is a problem, the shuttle crew will try to repair it, but this is not easy, and Discovery astronauts will test only two methods in space: a caulk gun that extrudes a "crack repair material" to seal small breaks in reinforced carbon-carbon (the goop is smoothed in with a putty knife); and an "emittance wash" that acts like a heat reflector for damaged tiles (applied with a swab, like liquid shoe polish). The techniques will be tried on test panels in the open shuttle bay.
Inside the orbiter, astronauts will also experiment with plugs for reinforced carbon-carbon -- potato chip-like, flexible patches installed with "molly bolts" that screw down over a four- to six-inch hole and open beneath the insulation's inside surface. It is like anchoring a bathroom fixture in drywall.
The shuttle will carry two other repair kits, for use in an emergency. One is an "overlay" in which a tile gouge is stuffed with insulation and bolted down. The other is the tile "goo gun," an awkward backpack device that was discredited recently because it extrudes bubbles along with the goo.
All these methods have a cardinal virtue that pleased Oswald -- "I'm a big fan of 'keep it simple, stupid' " -- but the bow-and-arrow approach carries a risk that imperfectly applied goo or caulk will leave protruding beads that could cause the damaged area to overheat during reentry.
"If we had real 'no kidding' damage to the leading edge, you'd do what you had to do," Oswald said. "But we think we have a lot of margin on dinged tile, and there we have a potential chance of doing more harm than good."
With a bit less than three months before liftoff, however, NASA, its aerospace industry partners and, NASA hopes, ultimately the public, have come to realize that "at some point you have to go fly," said Scott Horowitz, another former shuttle pilot and director of space transportation and exploration for Alliant Techsystems Inc.
"You do everything you can, and then you have to make a decision that you are satisfied," Horowitz said. "It's risk-reward, and the risk will never be zero. At some point it becomes good enough."