Spacewalkers to replace failed station component
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: August 13, 2007
Amid ongoing work in Houston to assess the potential impact of heat shield tile damage, the Endeavour astronauts are gearing up for a planned six-and-a-half-hour spacewalk today to replace one of the international space station's stabilizing gyroscopes, one of four needed to maintain the lab's orientation, or attitude, in orbit.
Astronauts Rick Mastracchio and Dave Williams spent the night in the station's Quest airlock module to purge nitrogen from their bloodstreams in preparation for their second spacewalk in three days. The excursion will be the 90th devoted to station assembly and maintenance since construction began in 1998 and the 13th so far this year. Going into today's spacewalk, 72 men and women from the United States, Russia, Canada, Japan, Germany, France and Sweden had logged 551 hours and one minute of EVA time building and maintaining the lab complex.
The Endeavour astronauts staged the first of four planned outings on Saturday. A third spacewalk, with Mastracchio and station astronaut Clay Anderson, is on tap Wednesday and the fourth, with Anderson and Williams, is planned for Friday.
The space station uses four massive control moement gyroscopes, or CMGs, to control the lab's orientation without burning hard-to-replace rocket fuel. One of them, CMG No. 3, acted up last year and was taken off line on Oct. 10, 2006. The refurbished unit being installed during Endeavour's mission will restore full redundancy to the critical orientation system.
The replacement CMG rode into space mounted on a pallet in the shuttle's cargo bay. Mastracchio and Williams first will remove the suspect CMG-3 from its place in the Z1 truss atop the station's central Unity module and temporarily mount it on a handrail fitting. Then they'll move down to the shuttle's cargo bay, remove the new CMG and a carrier platform and move them up to External Stowage Platform No. 2 near the Quest airlock. The new CMG then will be removed from its carrier and installed in Z1. CMG-3 will be mounted on the ESP-2 carrier and returned to Earth later this year.
"Our second spacewalk, I think, is almost going to be as exciting as the first," Williams said in a NASA interview. "Our job is to replace one of the gyros that we use to stabilize the position of the space station. There are four gyros that we currently have on board the space station. The gyro essentially is a spinning disk that conserves momentum. It's used to stabilize the station. One of them isn't functioning properly, so we have to replace it.
"To do that, Rick and I will go up to the Z1 area, take out the old gyro that's not working properly, temporarily stow it, then Rick is going to go down to the payload bay of the space shuttle, I'm going to get onto the robotic arm and I will go down to the payload bay of the space shuttle on the robotic arm. We're going to remove the new CMG, the new gyro, from the payload bay of the shuttle. It weighs 1,200 pounds. I'm going to be holding on to it on each side, with my heels turned outward holding me in place in the foot restraint on the end of the robotic arm.
"And then we're going to come back up to the stowage platform, ESP-2, right by the airlock, install the stowage assembly for the new gyro, then take the new gyro off, bring it back to the work site where we had the old gyro, swap the two and bring the old one back to the stowage platform. A lot of choreography there. It gets kind of confusing about which is going where and things. So we've worked very hard on the choreography."
Here is an updated timeline of today's activity (in EDT and mission elapsed time):
DATE/EDT.......HH...MM...SS...EVENT 08/13/07 05:00 AM...04...10...24...Flight director update on NASA TV 06:37 AM...04...12...00...STS/ISS crew wakeup 07:17 AM...04...12...40...EVA-2: Airlock repress to 14.7 psi 07:30 AM...04...12...54...Video file on NASA TV 07:37 AM...04...13...00...EVA-2: Hygiene break 08:07 AM...04...13...30...EVA-2: Airlock depress to 10.2 psi 08:27 AM...04...13...50...EVA-2: Campout EVA prep 09:37 AM...04...15...00...Logistics transfers resume 09:57 AM...04...15...20...EVA-2: Spacesuit purge 10:12 AM...04...15...35...EVA-2: Spacesuit pre-breathe 11:02 AM...04...16...25...EVA-2: Crew lock depressurization 11:31 AM...04...16...55...EVA-2: Spacesuits to battery power; EVA-2 begins 11:37 AM...04...17...00...EVA-2: Airlock egress 11:52 AM...04...17...15...EVA-2: Tethers/tools set up 12:22 PM...04...17...45...EVA-2: Remove failed control moment gyro 12:52 PM...04...18...15...EVA-2: Transfer new CMG to ESP-2 01:37 PM...04...19...00...Crew meals begin 02:22 PM...04...19...45...EVA-2: Remove new CMG from ESP-2 03:12 PM...04...20...35...EVA-2: Install new CMG on Z1 truss 04:07 PM...04...21...30...EVA-2: Install failed CMG on ESP-2 05:17 PM...04...22...40...EVA-2: Payload bay cleanup 05:42 PM...04...23...05...EVA-2: Airlock ingress 06:02 PM...04...23...25...EVA-2: Airlock repressurization 06:17 PM...04...23...40...Spacesuit servicing 06:22 PM...04...23...45...Station arm (SSRMS) walk off lab to PDGF-2 07:30 PM...05...00...54...Mission status briefing on NASA TV 07:37 PM...05...01...00...Transfer tagup 09:37 PM...05...03...00...ISS crew sleep begins 10:07 PM...05...03...30...STS crew sleep beginsBack at the Johnson Space Center in Houston, engineers assessing data collected Sunday on the depth and extent of a gouge in the shuttle's heat shield tile planned to subject a mockup to the extreme heat of re-entry in a furnace that can mimic the sort of environment Endeavour will experience during its return to Earth.
During launch Wednesday, a softball-size piece of foam insulation broke away from a propellant feedline support bracket, hit a strut near the base of the shuttle's external tank and broke apart. One piece then ricocheted off the strut and hit the belly of the shuttle, gouging a deep pit in two heat shield tiles. The tiles are only about 1.12 inches thick in that area and the inspection Sunday revealed the impact blasted out a pit that almost reached the aluminum skin of the orbiter in a small 1-inch by .2-inch area.
Based on pictures snapped by the space station's crew during Endeavour's final approach Friday, mission managers had already decided the shuttle could safely re-enter as is if some other emergency forced a speedy return to Earth. The close-up pictures taken Sunday did not change that judgment and John Shannon, chairman of NASA's Mission Management Team, said the orbiter's heat shield was healthy enough to handle re-entry as is if another emergency of some sort forced a speedy return to Earth.
That means engineers do not believe the gouged tile is a Columbia-class problem.
"I did poll the team and it was still unanimous that there was no change in the thought process," Shannon said. "If we were in a significant emergency case we would feel comfortable deorbiting this vehicle. However, not being in an emergency case, we're going to proceed very methodically, understand exactly what we have and go get the vehicle in the best configuration we can for re-entry."
Data from a laser scanner used to map out the damage site Sunday was expected to provide the three-dimensional data needed to carve an exact duplicate of the damage site using tiles identical to those on Endeavour. How that mockup fares in NASA's arc jet facility will play a major role in deciding whether to order an emergency spacewalk repair job or whether Endeavour can safely return to Earth as is. A decision is expected by Tuesday.
NASA managers have plenty of time to mount a repair effort if one is needed. On Monday, the Mission Management Team extended Endeavour's mission by three days based on the near-flawless performance of a new station-to-shuttle power transfer system that lets Endeavour tap into the station's solar power grid. Drawing up to 6 kilowatts from the station, the shuttle's on-board supplies of liquid oxygen and hydrogen, which are combined to produce electricity in the orbiter's fuel cells, will last longer, permitting the mission extension.
Under the extended mission scenario, four spacewalks are planned. The final excursion, however, is devoted primarily to get-ahead activities that are not considered critical to the next station assembly mission. If NASA managers decide to order a tile repair, it likely would be carried out Friday, during the slot originally booked for the mission's fourth and final spacewalk.
Endeavour's crew is trained and equipped to make three different types of heat shield repairs.
A spacewalking astronaut anchored to the same boom used to inspect the tiles Sunday could apply a black, paint-like "emittance wash" to the exposed silica fibers of the damaged tiles to improve their ability to reject heat. The astronauts also could fill the gouge with a putty like material known as STA-54. They even have carbon composite panels that can be screwed into the tile to cover large areas of damage.
While it's too soon to say whether a repair might be needed, "I have a lot of confidence based on the testing we've done that if a repair is required, that we can go execute it," Shannon said Sunday. "As to which way we'll go, we'll see through the analysis over the next 24 to 48 hours."
Even before Sunday's inspection, engineers assumed a deep pit for purposes of worst-case modeling of re-entry heat loads. As luck would have it, the damage site is right above an internal rib in the right wing called a stringer. Even if the tile was gouged out all the way to its base, Shannon said Saturday, any unusual heat during re-entry that made it to the underlying aluminum skin would spread out in the structure and not result in a localized hot spot. In addition, there is no wiring or any other systems on the interior of the right wing in that area.
Shannon said Sunday the actual damage closely mirrored the presumed worst-case scenario.
"Surprisingly, it's almost exactly what we thought we had after the 2D images where you potentially have a very small piece of the filler bar material showing," he said. "We have some flight history and now that we know exactly what we have, we'll go and make sure we understand the differences between flight history and what we have here today and we will test the exact condition. I can't stress enough how incredibly valuable the laser data is and the optical data is. Now I have the opportunity to go model exactly what I have and put it in an arc jet (furnace), put it through a re-entry environment. That's really valuable.
"This is something we would rather not deal with, but we have really prepared for exactly this case," Shannon said. "So I feel very comfortable that whatever is required we con go do and do successfully."
A larger question is how NASA will handle what apparently is an ongoing problem with the external fuel tank: foam shedding from the feedline support brackets. NASA and tank builder Lockheed Martin already were implementing a design change, but it won't be ready for flight until next year. The next three missions will rely on tanks similar to Endeavour's and NASA managers now face the prospect of trying to develop a flight rationale or implementing some sort of a fix in the near term.
The next shuttle launch is scheduled for Oct. 23, followed by a high-profile Dec. 6 flight to attach a European lab module to the station.