Shuttle Endeavour heads for Tuesday landing in Florida
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: August 20, 2007
NASA managers today formally cleared the shuttle Endeavour for re-entry and landing Tuesday, weather permitting, to close out an action-packed space station assembly flight. Program managers say a small-but-deep gouge in the shuttle's heat shield poses no threat to Endeavour or its crew. But the jury is still out on what sort of near-term fix might be needed to keep shuttles flying until the external tank problem that caused the damage can be eliminated.
"We will expect there will be some readjustment to our schedule as we work through those options," said Program Manager Wayne Hale. "However, I believe that based on the discussions we've had, that our impacts to the next flight, in terms of the actual launch date of Oct. 23, will be small."
With Hurricane Dean no longer threatening mission control in Houston, NASA is no longer under pressure to bring Endeavour down Tuesday, one way or the other, in Florida or backup landing sites in California or New Mexico. Instead, the astronauts will only attempt to land at the Kennedy Space Center and if the local weather doesn't cooperate, they will stay in orbit an additional day and try again Wednesday. NASA no longer plans to staff White Sands Space Harbor, N.M.. But if Endeavour doesn't make it home to Florida Tuesday, Edwards will be available for use as needed Wednesday.
Endeavour has enough on-board supplies to stay in orbit until Thursday, or even Friday in a worst-case scenario. But flight controllers are hopeful it won't come to that. Forecasters are predicting slightly out-of-limits crosswinds in Florida Tuesday, with just a slight chance of showers near the shuttle's runway. The forecast for Edwards calls for high headwinds, but conditions are expected to be within flight rule guidelines Tuesday through Thursday.
"We're proceeding with our nominal entry plans," Hale said. "Tomorrow, we will primarily look at the Kennedy Space Center and if weather conditions are not conducive to getting back to Florida with Endeavour, we most likely will wave off and attempt Kennedy on a subsequent day."
Commander Scott Kelly, pilot Charles Hobaugh, Tracy Caldwell, Rick Mastracchio, Dave Williams, educator-astronaut Barbara Morgan and Al Drew have two opportunities on successive orbits to land at Kennedy. The first opportunity calls for the shuttle's braking rockets to fire on orbit 201 at 11:25 a.m., setting up a touchdown on runway 15 at 12:32 p.m. The second deorbit opportunity comes at 1 p.m., for a landing at 2:06 p.m.
The gouge in Endeavour's belly was discovered after launch Aug. 8. Two heat-shield tiles were damaged when a chunk of foam debris, possibly including ice, slammed into the orbiter 58 seconds after liftoff. The impact gouged out an irregular pit crossing the boundary between two tiles, measuring roughly two inches by three inches across and nearly penetrating the full 1.12-inch thickness of the tile. A small, 1-inch by 0.2-inch gash at the bottom of the pit exposed an underlying support pad just above the shuttle's aluminum skin.
The shuttle Columbia was destroyed Feb. 1, 2003, when it re-entered the atmosphere with a gaping 4- to 6-inch hole in the leading edge of its left wing. The wing melted from the inside out, the shuttle broke apart above Texas and all seven crew members were killed. NASA managers do not view Endeavour's gouge as a Columbia-class problem. Mission Management Team Chairman John Shannon said the issue was simply whether re-entry heating might cause damage to the shuttle's aluminum skin in the immediate area that would require time-consuming post-landing repairs.
Testing and computer models predict the underlying aluminum skin of the shuttle's right wing will never get hotter than 350 degrees Fahrenheit, NASA's safety limit. At worst, Shannon said, a few downstream tiles could be damaged, along with the two that were gouged out by the impact during launch.
In the end, after nearly a week of around-the-clock testing and analysis, the Mission Management Team cleared Endeavour for re-entry as is, deciding that a spacewalk repair was not necessary and posed more risk to the shuttle than the gouge itself.
The damage was caused by three-hundredths of a pound of foam that fell away from a liquid oxygen feedline support bracket. In an unlucky break, the foam struck an aft strut that helps hold the shuttle to the external tank. Most of the debris survived that impact intact and ricocheted into the shuttle. NASA already was designing a fix for the brackets, but the upgrade will not be ready until the fourth flight from now.
Hale said today engineers are evaluating five near-term modifications to the feed line brackets, adding he is optimistic the next shuttle flight, scheduled for launch Oct. 23, can stay roughly on schedule. It will be more difficult to keep the flight after that on track, a high-priority mission to launch a European research module. Currently scheduled for launch Dec. 6, the launch window closes about one week later.
Today, Hale provided a snapshot of where the foam issue stands, how it might play out and why the debate over Endeavour's heat shield gouge is very different from the limited analysis that preceded Columbia's ill-fated re-entry.
But Hale would not predict where the foam will fit in NASA's probabilistic risk matrix and how that might play into the agency's planning for the next shuttle flight. Here are Hale's opening remarks at a news briefing Monday:
"When I was sitting in (entry flight director) Steve Stich's chair as entry flight director for 28 shuttle landings, we never knew what the condition of the entry heat shield was," Hale said. "It was always assumed to be good, we did no inspections, we did no analysis, we just trusted that it was a good thing and we would be safe to enter. Now, we're smarter. And over the past four or five years we have put together a great plan to inspect and make sure the heat shield is in good shape and if there are any concerns, we have put together the equipment, the procedures, the planning and the entire process to evaluate whether or not the heat shield is safe for re-entry and if it's not, to provide the capability to repair.
"However, repairs are in and of themselves somethings hazardous to execute. So like so many things in our business, it becomes a risk trade. At the final analysis, having allowed over 200 engineers and specialists in the areas of thermal protection systems, aerothermal heating, to work hundreds of hours, including 4,000-plus hours of super computer simulation (time) using well-anchored, test-validated models, a series of extremely accurate and extensive tests in the re-entry simulators both here at Johnson and at other facilities around the NASA team, and having been peer reviewed by specialists in supersonic flight, hypersonic thermal environments and so forth, we can say with a high degree of confidence that Endeavour is safe to come home without needing a repair.
"That does not mean that we're entirely happy with the situation and in fact, I had a discussion this morning with the team about what our future steps are to make sure that the next flight and those following it are safe to fly. Again, this was a problem that originally came from debris coming off the external tank. After Columbia, we had a long discussion about how we could best eliminate the hazards of debris coming off the external tank and it becomes very obvious in a very short order as you look at the system, it's an inherent design situation with the space shuttle that we will never be able to completely eliminate the potential of debris coming of the external tank, or some of the other elements of the launch vehicle stack. And so there will never be a launch where we have zero risk.
"We will continue to improve as we go forward. What we did, however, do for return to flight and are continuing to do today, is to take a hard look at those areas where debris may be shed to try to categorize them in order of their risk factor, in other words, which are the most serious risks, and to methodically whittle away and reduce those risks by eliminating hazards one at a time. As you know, we got quite a lot of risk reduction and hazard improvement with the first return to flight, STS-114. We spent another period of time where we wanted to make another significant improvement and we whittled that risk down significantly more by the time we flew the second return to flight (mission), STS-121. We knew we were not finished at that point and in fact, set out on future redesigns. And we have a number of improvements that are coming forward on a tank that we will fly about four flights from now.
"We have three flights, however, with tanks that have not been modified. And in particular, one of the areas on that external tank that we have already got a design in work for is off these liquid oxygen feedline brackets that was the cause of the incident on Endeavour's launch. So we know that four flights from now we have a good fix that will eliminate that hazard. The discussions I've been having with the team in the last few days are what will we do with the next three tanks, and the next three flights, until we get that final design into the fleet to, again, mitigate and reduce the hazard.
"Today we had about a two hour telecon with the tank designers and the folks that are working on five different options to improve the situation on the next tank. We will expect there will be some readjustment to our schedule as we work through those options. However, I believe that based on the discussions we've had, that our impacts to the next flight in terms of the actual launch date of Oct. 23 will be small, we think we have plenty of time to evaluate some changes and in fact implement them if we feel that they are well justified.
"We are also looking at the further implications to the next flight because as we take time to prepare this tank it does impact the schedule on the subsequent flight and we would really like to hold that flight to the December launch window if we can do it safely. And I think that's the clue. As we look to the manifest ahead of us, we can expect there will be challenges, whether they're from hurricanes or from equipment that is causing us a problem on board the shuttle. However, we do have an amount of time ahead of us, a skilled work force and a umber of options that will allow us to fly the manifest as it's been laid out and complete the international space station with a significant margin of time before the date that the president and the congress have directed us to retire the space shuttle vehicle."
Going into Endeavour's flight, Hale said engineers viewed the risk posed by foam falling off oxygen feedline brackets as "rather low."
"In the analysis, it was assumed that if the aerodynamics carried this foam down to the attach bracket of the external tank and struck it, that the foam would come apart and that would be the end of the hazard. And in fact what we saw on the video ... we saw this piece of foam strike this strut which is angled at a 30- to 45-degree angle to the flight path and ricochet off and hit the bottom of the orbiter, it's clear as day.
"We didn't think that could happen before. So we would have told you that was a non-credible, non-possible thing to happen. Therefore, no risk, at least in a rebound case. Clearly, we're smarter now than we were a couple of weeks ago and we're looking very hard at those particular locations and those particular aerodynamic paths that might lead this foam or foam and ice combination to hit the strut and bounce off and strike the underside of the orbiter. So we have a more sophisticated risk analysis in work. That's why I think the risk is probably higher than we had characterized before."
But Hale said he does not believe the problem represents a "probable-catastrophic" threat to the shuttle, a level of risk that is not generally considered acceptable for flight.
"It is currently not categorized as probable-catastrophic," Hale said. "It is my belief that it will not be classified as probable-catastrophic. The risk assessment people, however, are off doing their work and we do not have the final numbers back from that. And we will be informed by that analysis when it comes in. So, it's under review."