Sellers, Fossum set for dramatic spacewalk
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 8, 2006
Astronauts Piers Sellers and Mike Fossum are gearing up for a dramatic six-and-a-half-hour spacewalk today to test Discovery's robot arm and a long inspection boom as a potential platform to make heat shield repairs down the road.
First Sellers and then both spacewalkers will secure themselves to the end of the 50-foot-long orbiter boom sensor system - OBSS - while the shuttle's 50-foot-robot arm moves them from point to point to test the stability of the arm/boom combination. At one point, the boom will be extended some six stories or more from the shuttle's cargo bay.
"The question we want to answer is, can you use the boom as a worksite, as a platform to repairing underneath the orbiter?" shuttle commander Steve Lindsey said in a NASA interview. " So, the scenario is you have a problem, you want to go repair a tile or a leading edge (panel) or something like that. Can you put (one or two spacewalkers) out on the end of this boom, maneuver them underneath the vehicle, and is the platform stable enough to allow them to do repairs?"
The shuttle's robot arm is 50 feet long as is the OBSS extension.
"If an EVA crewmember puts a load or pushes at the end of the boom, how much flex do you get in that boom, how much flex do you get in the arm?" Lindsey asked. "Do the joints on the arm back drive? How much do they back drive? And, is it stable enough to be able to do work, precise work, where you have to be careful in how you do it?
"We're going to put one and then two crewmembers on that boom, go to a couple of positions, and they're going to do some typical EVA-type maneuvers. For example, they'll lean back aggressively while they're standing at the end of the arm to see what kind of drive you get on the joint. They might reach back for a tool and just kind of simulate those kinds of motions.
"And then, one of the test points will go actually to structure, where they're pushing against a truss structure in the payload bay, and see what kind of flex you're doing. So, we're going to get two things out of that. We're going to get loads information; in other words, how much force you're putting into the arm, how much do the joints move? And we're going to get an operational assessment, if you will. The EVA crewmember will simulate doing a repair and say, 'Can I do a repair with the arm in this configuration?' So, the goal is to be able to figure out at what points underneath the Orbiter do we need a further way to stabilize the crew, like some, putting something to stabilize them on the bottom of the vehicle itself? Or, can we get away without doing that at all?"
The success of today's tests may play into a decision on what to do about a protruding gap filler seen poking up between heat shield tiles on the underside of Discovery near a propellant feedline access door. Other than undocking and repositioning the shuttle - a complex, time-consuming procedure - the only way for spacewalking astronauts to reach the site is to use the shuttle arm/OBSS combination. NASA managers have not yet decided whether the gap filler needs removal.
Today's spacewalk, the first of three by Sellers and Fossum, is scheduled to begin around 9:13 a.m. Here is an updated timeline of today's activities (in EDT and mission elapsed time):
EDT DD HH MM EVENT 03:08 AM 03 12 30 STS crew wakeup (flight day 5) 03:38 AM 03 13 00 ISS crew wakeup 04:23 AM 03 13 45 EVA-1: Preps with ISS oxygen 07:13 AM 03 16 35 EVA-1: Spacesuit purge 07:28 AM 03 16 50 MPLM cargo module transfers 07:28 AM 03 16 50 EVA-1: Spacesuit prebreathe 08:23 AM 03 17 45 Station robot arm (SSRMS) ungrapples lab module 08:28 AM 03 17 50 EVA-1: Crew lock depressurization 09:13 AM 03 18 35 EVA-1: Airlock egress; tool setup 09:33 AM 03 18 55 EVA-1: Zenith IUA replacement 10:03 AM 03 19 25 EVA-1: Tool setup 10:43 AM 03 20 05 EVA-1: OBSS setup (part 1) 10:48 AM 03 20 10 MELFI lab freezer transfer 11:33 AM 03 20 55 EVA-1: 1 spacewalker/position 1 evaluation 11:48 AM 03 21 10 Oxygen generation system transfer 12:13 PM 03 21 35 EVA-1: OBSS setup (part 2) 12:43 PM 03 22 05 EVA-1: 2 spacewalkers loads evaluation 01:18 PM 03 22 40 EVA-1: 2 spacewalkers push evaluation 01:48 PM 03 23 10 EVA-1: OBSS cleanup 02:38 PM 04 00 00 EVA-1: Tool cleanup 03:23 PM 04 00 45 MPLM transfer tagup 03:28 PM 04 00 50 EVA-1: Airlock ingress 03:43 PM 04 01 05 EVA-1: Airlock repressurization 05:00 PM 04 02 22 Mission status/MMT briefing on NASA TV 07:08 PM 04 04 30 STS/ISS crew sleep begins 08:00 PM 04 05 22 Daily video highlights reel on NASA TV
Sellers and Fossum will exit through the space station's Quest airlock module, first moving up to the lab's main solar array truss to install a device designed to prevent a cable cutter from inadvertantnly severing critical video and data cables leading to a stalled rail car used to move the station's robot arm. Repairing the stalled transporter is the primary objective of the crew's second spacewalk Monday.
Making their way down to the shuttle's payload bay, Sellers and Fossum will rig safety tethers and attach a foot restraint to the end of the OBSS along with sensors to record how much force is imparted to the boom when they move about. Sellers then will climb onto the OBSS, connected to the shuttle's robot arm by a long tether and to the sensor boom by another, shorter tether. Fossum said they refer to the latter as "our sissy tether."
Safely tethered, Sellers will be moved to a point just above the starboard sill of the cargo bay, about 14 feet from any other structure. This is considered a "strong" configuration for the boom. After moving about to impart various forces, he will be moved to a point high above the port wing for another round of "bouncing the boom" tests. Extended some 60 feet from the shuttle, the boom is expected to exhibit a slow back-and-forth sway as Sellers moves about.
"The idea is to see if you could do a repair while standing on the end of this," Sellers said in a NASA interview. "The problem is, of course, that it's a very long, bendy fishing pole with somebody standing on the end, so it's not a completely stable platform. It sways around a bit; we've tested it in virtual reality, we've tested it on an air-bearing floor. We know that it has some motion, some sway, associated with it."
In an interview with CBS News, he likened it to painting the side of a boat while standing in a raft bobbing up and down.
"It will have a long, slow sway, about four to eight seconds period," he said. "So I want you to imagine that you're standing in a little row boat that's going up and down in the swell, slowly, and you're trying to paint the side of a ship that's not moving, or the side of a dock next to you. You have to put quite a lot of compensation in, you have to be careful, but it's not impossible. You just have to think about what you're doing."
After the first two tests, the arm will be maneuvered back into the cargo bay and Fossum will climb aboard. The boom then will take both spacewalkers back out over the port wing and finally, to a cargo truss in the payload bay that Fossum will push against to mimic actual repair work.
"After we test it out with one person, we'll bring the boom back down to the payload bay and I'll get into the foot restraint," Fossum said. "Piers will be hanging on to the boom. This is to simulate the kind of situation where we'll actually (be) doing a repair, where I would be the one in the foot restraint, with both hands free so I can do the repair, and Piers will be moving around the outside.
"We'll have kind of a tool stanchion and some other support equipment set up, so he'll act as if he's getting tools out and handing them to me. We'll be able to evaluate the stability of the system with both of us moving around and putting force into the whole boom and arm just to make sure that it's stable enough. We'll be doing some basic maneuvers in free space.
"And then, we'll go down to some structure and I'll actually put force into the structure as if I'm tamping down, the repairs, like for a tile repair that requires a continuous tamping motion to push all of the repair material down into the patch site. We'll just do that to see if it holds us stable enough to accomplish those kinds of basic tasks."
The work could help clear the way for an eventual mission to service the Hubble Space Telescope. The observatory circles the globe in a different orbit from the space station and the crew of any proposed servicing mission would not be able to take advantage of the space station.
As a result, NASA needs credible ways to make repairs if any impact damage is seen and with the OBSS, the astronauts would have a potential way to work on the belly of the shuttle.
Ground testing indicates the robot arm/OBSS combination will be relatively sturdy and the astronauts said they had no concerns about the possibility the OBSS - and the astronauts - could somehow break free of the shuttle's robot arm.
"We've thought about that and all the different ways to get ourselves out of trouble," Sellers said. "We'll still be tethered, by the way, to the main shuttle arm so if we came off, that's our first recourse. So the first thing you've got to do is get off the boom, then you've got to stabilize yourself (and) use the tether to get back to the arm. So a multi-step process, but I think pretty safe."