Extra shuttle inspection to answer tile damage concern
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: August 11, 2007
Digital video from cameras mounted on the shuttle Endeavour's solid-fuel boosters shows debris that hit the underside of the orbiter during launch Wednesday was a chunk of foam, not ice, that first hit a support strut, broke apart and ricocheted into the ship's belly. A worst-case engineering analysis indicates the spacecraft could safely return to Earth "as is" if some other emergency forced a quick re-entry. But John Shannon, chairman of NASA's Mission Management Team, told reporters late Saturday that a focused inspection Sunday using a laser scanner and a high-resolution camera is needed to fully assess the extent of the damage and what, if any, impact it might have during re-entry.
In the same briefing, space station flight director Joel Montalbano said the unexpected shutdown of a command-and-control computer aboard the lab complex today apparently was caused by a software glitch and not a hardware failure. Responding to the shutdown, a backup computer immediately took over and a third machine switched from standby to backup as expected. Both of those computers are operating normally. Montalbano said experts are studying telemetry from the computer that locked up to find out what process or potential software conflict might have caused it to freeze. But he expects the problem to be resolved in fairly short order.
"We think once we ahead and get all the information, do all the data dumps of what happened, we'll power cycle this computer (turn it off and back on) and we expect it to be fully operational," Montalbano said.
Shannon said the Mission Management Team spent most of its time today discussing the heat shield impact that caught the team's attention after launch. The damage site is located on the bottom of the right wing near the shuttle's fuselage between the right main landing gear door and a smaller panel that covers the main liquid oxygen feedline inlet.
Looking at imagery from ground cameras, the engineering community initially thought the debris in question was ice. Being more dense than foam, ice could potentially cause more damage to the shuttle's fragile heat-shield tiles. But today, digital video from cameras mounted on Endeavour's twin solid-fuel booster rockets was recovered and the imagery showed the debris was, in fact, a baseball-size chunk of foam insulation from a propellant feedline bracket on the external fuel tank that broke away 58 seconds after launch. The foam then slammed into one of the struts that holds the back end of the shuttle to the external tank. It broke apart and a smaller piece bounced up into the belly of the shuttle.
Endeavour was roughly 6.5 miles up at that point and moving at a relative velocity of nearly 1,000 mph. Engineers do not yet have enough data to assess the mass of the initial debris.
Shannon said engineers are in the process of redesigning the bracket in question because of past incidents in which pre-launch ice formed, causing foam in that area to break away during launch.
Photos shot by the space station crew during Endeavour's approach to the lab complex Friday show a small area of damage measuring 3.48 by 2.31 inches where the debris in question struck. The tiles in that area are about 1.12 inches thick. The photos do not show how deep the gouge might be and the depth of the damage is critical for the thermal analysis of its impact during re-entry.
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, 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 engineers do not believe the tile in question was gouged out all the way to its base. But for a preliminary thermal analysis, that's what they assumed.
"We assumed if we had lost all of the tile down to the felt area (at its base), we modeled that, and because it was along that rib assembly, we didn't have loss of structural margin," Shannon said. "It was OK, basically is what you would say. That was our best judgment. We looked at the model and made our best judgment on what we thought the potential thermal impact would be. It looked like it was OK. Again, we don't exactly know what we have until we do the focused inspection.
"Based on the flight history we saw, based on this location and based on that thermal analysis, it was a unanimous consensus in the MIssion Management Team that if we had an emergency kind of case we'd be OK to deorbit with the vehicle as is. ... I think once we have the focused inspection tomorrow we'll know exactly what we're dealing with and those decisions will be much easier to make. But it was the engineering judgment of the team that if we were in an emergency we could come home as is."
Shannon said engineers collecting data on the incident know much more now than they did Thursday and Friday.
"We kind of have a chain of evidence here," he said. "We looked at the pre-launch photos and saw some ice buildup in that bracket, we have a history of cracking that foam from that ice buildup, the SRB (booster) photos show that foam loss from that bracket area and the flight path it took. We got a little unlucky that it hit the ET (external tank) attach strut, but that can happen. The ground cameras showed the puff where it hit, the radar showed the ice so maybe we got some ice liberation behind it. The (rendezvous) pitch maneuver showed us the damage site and now we're going to take the next step and do a focused inspection tomorrow. I expect we'll have a final resolution on Monday once we've run through the thermal analysis and understand what we have.
"There was some discussion about the depth (of the damage). I think the consensus was, in the MMT, that the flight history we have, the location of this (damage), all are very good signs this will not be something we will have to worry about. But again, we're going to just march through the procedures we had set up preflight and execute those and see where we come out on Monday."
He said "if we have even half the tile left we won't have any issues at all."
Asked if it was possible that Endeavour's crew might have to make repairs, Shannon said based on "the data that we pulled together today on exactly where the location was, what is under that location and the flight history data would say that that is much more doubtful than it was yesterday."
The focused inspection is scheduled to begin just after noon Sunday. Astronaut Tracy Caldwell, assisted by teacher-turned-astronaut Barbara Morgan, will use Endeavour's robot arm to position a laser scanner and high-resolution camera on the end of the 50-foot-long boom as close to the damage site as possible. The laser scanner will collect data allowing engineers to make a three-dimensional model of the damage site. Caldwell also will use the system to inspect a few other areas of interest. The entire procedure is expected to take about three hours.
Because of clearance issues when the shuttle is docked at the station, the space station's robot arm will be used to pull the 50-foot orbiter boom sensor system - OBSS - heat-shield inspection boom from its place in the shuttle's cargo bay and hand it off to Caldwell, operating the shuttle arm. Here is an updated timeline of Sunday's crew schedule (in EDT and mission elapsed time):
EDT........DD...HH...MM...EVENT 08/12/07 07:07 AM...03...12...30...STS/ISS crew wakeup 09:22 AM...03...14...45...ISS: Russian BOK-3 computer repair work (5 hours) 09:37 AM...03...15...00...Shuttle arm (SRMS) moves to inspection boom (OBSS) pre-grapple position 09:47 AM...03...15...10...Station arm (SSRMS) grapples/unberths OBSS 10:32 AM...03...15...55...SSRMS moves OBSS to handoff position 11:12 AM...03...16...35...SRMS grapples OBSS 11:22 AM...03...16...45...SSRMS ungrapples OBSS 11:32 AM...03...16...55...EVA-2: Tools configured 12:07 PM...03...17...30...Heat shield inspection begins 12:07 PM...03...17...30...Logistics transfer operations 12:42 PM...03...18...05...Spacesuit battery charging 02:37 PM...03...20...00...Shuttle crew meals begin 03:37 PM...03...21...00...SSRMS grapples OBSS 03:37 PM...03...21...00...Equipment lock preps for EVA-2 04:07 PM...03...21...30...SRMS ungrapples OBSS 04:07 PM...03...21...30...SAFER backpack checkout 04:22 PM...03...21...45...SSRMS berths OBSS 04:52 PM...03...22...15...EVA-2: Tools configured 05:07 PM...03...22...30...SSRMS releases OBSS 05:27 PM...03...22...50...EPO educational experiment transfer and video 06:32 PM...03...23...55...Logistics transfer tagup 06:47 PM...04...00...10...EVA-2: Procedures review 07:47 PM...04...01...10...Station arm walkoff to lab 09:02 PM...04...02...25...EVA-2: Mask pre-breathe 09:47 PM...04...03...10...EVA-2: Airlock depress to 10.2 psi 10:07 PM...04...03...30...ISS crew sleep begins 10:37 PM...04...04...00...STS crew sleep begins