Astronauts confident about spacewalk fix-it mission
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: August 2, 2005
The Discovery astronauts said today they were initially concerned about the safety of a proposed spacewalk repair job Wednesday to remove two protruding "gap fillers" sticking out from heat-shield tiles on the shuttle's belly. But they are now convinced it's not only safe, but a relatively simple task that will eliminate any lingering concern about unwanted re-entry heating.
After discussing the spacewalk details with her crew mates today, Discovery commander Eileen Collins told flight controllers they had agreed to add the repair work to an already-planned spacewalk Wednesday, the crew's third and final planned outing.
"The crew has talked about the EVA (spacewalk repair) and we are go for EVA on flight day nine," Collins called down. "Go for EVA tomorrow."
"Eileen, we copy. Go for EVA on flight day nine," replied Canadian astronaut Julie Payette in mission control. "This will be exciting."
In a morning news conference, astronaut Andrew Thomas told reporters that when the crew first heard about the planned repair work, "I think a number of us did have misgivings. We were concerned about it, we were concerned about the implications of it and what was motivating it."
"The ground has been very good in sending us up a lot of information about it," he said. "We understand some of the physics behind what would happen if (the gap fillers) weren't there. We know if it were to remain there, even under a worst-case scenario, it wouldn't present a threat to the orbiter. There might be some minor structural damage that might require post-flight attention, but it wouldn't be a threat to us personally.
"However, it's a lot better for a number of reasons, performance reasons, if you can remove this material and that justifies doing it, considering that every indication is that the removal of the material should be straight forward and pretty easy."
The protruding gap fillers, two of thousands in use across the underside of the shuttle, are not associated with foam debris shed by Discovery's external fuel tank during launch. The stiff, ceramic cloth material is inserted between heat shield tiles to ensure smooth air flow and to prevent adjacent tiles from rubbing as the shuttle's aluminum skin flexes during launch or due to temperature swings.
During Discovery's approach to the international space station last week, commander Eileen Collins piloted the ship through a 360-degree pitch maneuver that allowed the station crew to photograph the ship's normally unseen belly. Imagine analysts quickly spotted two gap fillers that had been dislodged. According to documentation of a similar incident during an earlier mission, engineers believe gap fillers typically dislodge as the shuttle goes supersonic during launch.
One gap filler seen on Discovery protrudes a full inch above the surface of the surrounding tiles while the other extends 0.6 inches.
The concern is that one or both gap fillers in question could "trip the boundary layer" during re-entry, that is, disrupt the smooth, laminar flow of supersonic air across the belly of the shuttle and create eddies of turbulence that, in turn, result in higher downstream heating.
The issue is when that transition to turbulent flow might occur. It occurs normally when the shuttle's velocity has dropped to around eight to 10 times the speed of sound, starting toward the back of the heat shield and moving forward. But a protruding gap filler in a 1995 shuttle mission tripped the boundary layer at Mach 18, causing significant tile damage during entry.
A gap filler also can trigger an asymmetric boundary layer transition, changing the aerodynamics and causing the shuttle's flight computers to compensate by firing rocket thruysters or adjusting the ship's elevons.
That's not thought to be an issue with Discovery. Rather, engineers were concerned that the protruding gap fillers could disrupt the boundary layer at speeds higher than Mach 20, thus exposing downstream tiles to higher heating for an even longer period. In the end, mission managers decided not to risk any such damage to the shuttle and asked the astronauts to remove the offending gap fillers during the crew's third spacewalk.
The primary objective of the spacewalk is to attach a large tool caddy and spare parts kit called an external stowage platform to the side of the station's Quest airlock module. As soon as it's locked in place, the station's robot arm will disengage and lock its free end onto a mobile transporter mounted on rails along one side of the station's main solar array truss. Moving like slow-moving inchworm, the other end of the arm will then release a grapple fixture on the Destiny laboratory module, freeing it to grasp an astronaut foot restraint.
From its new perch on the truss, the robot arm will be able to move astronaut Stephen Robinson down the starboard side of the space shuttle, giving him access to the work sites on the far side of the orbiter. Television cameras on the station arm and the shuttle's own robot arm will let flight controllers monitor his position to help ensure he stays well away from any inadvertent contact with Discovery's heat shield.
The most significant concern is any such inadvertent contact that might damage one or more tiles.
"Like all kinds of repairs, it's conceptually very simple but it has to be done very, very carefully," Robinson said today. "The tiles as we all know are fragile and an EVA (spacewalk) crew member out there is a pretty large mass. I'll have to be very, very careful, but the task is extremely simple and we predict it won't be too complex."
Arm operators Wendy Lawrence and pilot James Kelly will take extra care moving Robinson into position.
"We're ready to do this in a very careful manner," Robinson said. "And besides, it's not just me. We'll also have a camera ... looking at me and trying to look at the clearance between me and the orbiter's belly. So we'll have lots of ways to be very, very conservative. It's going to be like watching grass grow. Nothing's going to happen fast."
The two gap fillers are located relatively close to the shuttle's nose. Robinson first will try to simply pull the spacers free.
"There won't be any yanking going on at all," he said. "It will be a gentle pull with my hands and if that doesn't work, I have some forceps, I'll give a slightly more than gentle pull and if that doesn't work, I'll saw it off with a hacksaw. No yanking."
Robinson, identified during spacewalks as EV-2, will be joined by Japanese astronaut Soichi Noguchi, EV-1. Here is an updated timeline of Wednesday's spacewalk activities (in EDT and elapsed time):
EDT........HH...MM...EVENT 04:14 AM...00...00...Airlock egress 04:44 AM...00...30...EV1/EV2: External stowage platform (ESP 2) installation 05:29 AM...01...15...EV1: MISSE 5 experiment package installation 05:29 AM...01...15...EV2: Grapple fixture removal and stowage 05:44 AM...01...30...Station arm (SSRMS): ESP 2 ungrapple 05:49 AM...01...35...SSRMS: walk off to S0 truss mobile transporter 06:14 AM...02...00...Shuttle arm (SRMS): TPS sample box scan 06:29 AM...02...15...EV1: Rotary joint motor coupler removal 06:29 AM...02...15...EV2: Gap filler tool prep 07:29 AM...03...15...EV1: Attach foot restraint to SSRMS 07:29 AM...03...15...EV2: Attach tethers, ingress SSRMS 07:44 AM...03...30...EV1: Close TPS sample box; stand by 07:44 AM...03...30...EV2: gap filler removal process begins 07:54 AM...03...40...SSRMS: move EV2 to work site 1 08:34 AM...04...20...SSRMS: move EV2 to work site 2 09:14 AM...05...00...SSRMS cleanup/egress 09:44 AM...05...30...IV egress for shuttle EVA 10:29 AM...06...15...EV1/EV2: Payload bay cleanup and tool stow 11:14 AM...07...00...Airlock repressurization
The space station's robotic arm will move Robinson down the right side of Discovery and then under its nose. He will not be visible from the shuttle's crew cabin or the space station. But he will be in view of the robot arm cameras and in constant UHF radio contact with his crew mates and flight controllers in Houston through the station's communications system.
Robinson said his major concern will be making sure his helmet doesn't bump into the shuttle's fragile tiles as he nears the orbiter. As for the actual repair work, "I've got several old airplanes at home that I've had for many, many years, so I'm pretty comfortable with using tools very carefully."
"But no doubt about it, this is going to be a very delicate task. But as I say, a simple one. The hacksaw is really a contingency device. The idea is to just pull out this thin gap filler, either by hand or with a pair of forceps, and we'll use the hacksaw only if necessary. But I think it's a great solution ... and it should be pretty safe."
Earlier today, President Bush called the astronauts from the White House.
"Thank you for taking my phone call," he said. "I just wanted to tell you all how proud the American people are of our astronauts. I want to thank you for being risk takers for the sake of exploration. I want to welcome our Japanese, Australian and Russian friends and wish you God speed in your mission. I know you've got very important work to do ahead of you and we look forward to seeing a successful completion of this mission. Obviously, as you prepare to come back, a lot of Americans will be praying for a safe return. So it's great talking to you. Thanks for being such great examples of courage for a lot of our fellow citizens."
"Well thank you very much, Mr. President," Collins replied. "We want to tell you we really enjoy what we're doing, we really believe in our mission and we believe in space exploration and getting people off the planet and seeing what's out there. The steps that we're taking right now are really worth it, we want everybody to know that. Thank you very much for taking the time out of your busy schedule to talk to us."
"Well listen," Bush replied, "I want to thank you, commander, and thank your fellow astronauts there. I agree with you, I think what you're doing is really important. You've got a strong supporter for your mission here in the White House. ... We're with you and wish you all the very best. Thanks for taking my phone call. Now get back to work!"
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