The Fixit Stuff

Repair Time On The Fly

The shuttle crew are going to carry out some repairs on the shuttle .

Astronauts have never ventured beneath an orbiting shuttle before, and have never attempted repairs to the fragile thermal skin in space.

"No doubt about it, this is going to be a very delicate task, but as I say, a simple one," Robinson said Tuesday before the astronauts went to sleep.

They awoke late Tuesday to the inspirational song "Where My Heart Will Take Me" and were poised for the high-stakes repair job, which was set to begin about five hours later.

Mission Control told the astronauts the song from a Star Trek sound track reflects how much Deputy Shuttle Program Manager Wayne Hale — who picked the tune — believes in them.

Thirty-five years ago, a group of NASA engineers concocted a plan using plastic bags, cardboard and duct tape to save Apollo 13's astronauts after an oxygen tank exploded, crippling their spacecraft.

The latest repair plan was carefully worked out on the ground over the past four days. It called for maneuvering Robinson underneath Discovery — a no man's land, up to now — on the end of the linked international space station's 58-foot robot arm.

The waking up to the song bit kind of would irk me, if I were one of the astronauts. To be honest, the whole being-an-astronaut-aboard-the-shuttle-thing would irk me, but that aside, the choice of music is a bit... errr... tough.
Still, the NYT reports that the astronauts are very confident. Which is nice, but not nice enough.
So why are they doing this space-walk again?

NASA's concern about the cloth strips is that they will, in the parlance of aerothermodynamics, "trip the boundary layer" earlier than usual for a shuttle flight. The bottom of the shuttle has to be very smooth to extend as long as possible the part of entry known as laminar flow, when the plasma slides smoothly by with a protective boundary layer of air surrounding it.

After the shuttle has slowed considerably, the boundary layer breaks up into what is known as turbulent flow in what is known as boundary layer transition. The strips might trigger early boundary layer transition at much higher speeds than usual, which could cause heating of some 25 percent greater than it usually experiences. Insulating tiles that are usually exposed to temperatures of 2300 degrees could experience temperatures closer to 2900 degrees, and portions of the wing leading edge could undergo heating that, by some estimates, is slightly greater than the maximum temperature they are rated to resist.

Over three days of intense aerothermodynamic analysis, NASA mission managers struggled to balance the risks of trying a spacewalk - the first ever to the underside of the shuttle, and the first emergency on-orbit shuttle repair in the history of the program - and the risks of coming back with dangling gap fillers. Astronaut Charles Camarda said "in this particular case, it was a very close call."

The problem, he said, is that "We know very little about boundary layer transition at about Mach 21 to 24. Conservative estimates indicate that we'll be just at the limit of the hottest portion of the leading edge."

Because the operation is so straightforward, he said, "It's worth that risk."

Yeah. So it's spacewalk time.