Discovery formally set to launch next Wednesday
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: March 6, 2009;
Updated after news conference
Senior NASA managers met today at the Kennedy Space Center and officially cleared the shuttle Discovery for launch March 11 on a delayed space station assembly mission. The decision to proceed was based on a review of extensive testing and analysis, along with results from a new inspection technique, that gave engineers high confidence three hydrogen flow control valves installed aboard Discovery are crack free and can be safely launched as is.
"The mood is very, very upbeat compared to a couple or three weeks ago when we didn't know exactly where we were going to get with the launch date," Mike Leinbach, NASA's launch director, told reporters today after a flight readiness review. "Now we have one, and everybody feels really good. Team Discovery's ready to execute and I feel really good about the attempt next Wednesday night."
Shuttle commander Lee Archambault and his crewmates - pilot Dominic "Tony" Antonelli, John Phillips, Japanese astronaut Koichi Wakata and spacewalkers Steve Swanson, Richard Arnold and Joseph Acaba - plan to fly to the Florida spaceport Sunday for the 7 p.m. start of their countdown to launch.
If all goes well, Discovery will rocket away on the 125th shuttle mission at 9:20:10 p.m. Wednesday. Docking with the space station is targeted for 6:27 p.m. on Friday, March 13. Four spacewalks are planned for March 15, 17, 19 and 21 to connect a fourth and final set of solar arrays and to perform a variety of other tasks. Undocking is planned for 10:23 a.m. on March 23, with landing back at the Kennedy Space Center expected around 3:27 p.m. on March 25.
NASA is facing a deadline of sorts with DIscovery. The Russians plan to launch a Soyuz spacecraft carrying the next space station commander and flight engineer on March 26. The docked phase of the shuttle mission must be done by then to avoid a conflict. To get in a full-duration four-spacewalk mission, Discovery must take off by March 13.
By giving up one or two of the mission's planned spacewalks, along with off-duty time, Discovery could launch as late as March 16 or 17 in a worst-case scenario. After that, the flight would slip to April 7. But NASA managers are optimistic it won't come to that.
Archambault and his crewmates originally hoped to begin their mission Feb. 12. But the flight was repeatedly delayed because of concern about possible cracks in the three hydrogen flow control valves used to pressurize the hydrogen section of the ship's external tank during the climb to space.
During the most recent shuttle flight last November, a small piece of one valve poppet broke free. It was the first such incident in shuttle history and while it caused no problems, NASA managers ordered tests to assess the safety of the system.
As it turned out, the valve cracked and liberated debris because of high-cycle fatigue, the result of harmonics in the flow environment inside the pressurization line that engineers had not suspected. While analysis continued, three valves that passed an electron microscope inspection were installed aboard Discovery.
But testing continued and engineers discovered that surface roughness could mask small cracks, raising questions about the valves aboard Discovery. Those valves had flown about a dozen times each and they eventually were removed. Engineers planned to replace them with valves that had four, four and five flights respectively.
While all of that was going on, engineers carried out computer analysis to model the flow inside the hydrogen pressurization line and conducted impact testing to determine whether a piece of debris could cause damage if a fragment broke off in flight. Of special concern was a 90-degree bend in the line just five inches from each valve.
Against this backdrop, engineers came up with a new way to inspect the valves for cracks, adopting so-called eddy current analysis to look telltale defects indicative of cracks.
"It's a commercial way that they inspect bolt heads," said shuttle Program Manager John Shannon. "Basically, you put a magnetic field around the circumference of the bolt and then you measure the (induced) voltage you get through that magnetic field. Some of the really smart guys that we have ... adapted that to this problem and we ran several blind trials with (valve) poppets we knew had cracks. It performed so well that it found some cracks that we did not know we had, that we had not seen with the scanning electron microscope. So we had a lot of confidence in this inspection capability.
"So we took apart the valves that we had initially said we were going to put in with lower flight times and checked those out. Two of them were clean and one of them showed two cracks in it. That was a little bit of a surprise to us. So we screened the three valves that we had taken off of DIscovery that had 12 flights apiece and the first one we looked at had a crack in it. Then the next two did not have cracks in them. So we were able to put together, with a very high confidence level, a set of three valves and a flight spare that we could put in Discovery and have a lot of confidence that they did not have initiating cracks.
"Additionally, the teams did a lot of physics work assessing how this valve poppet fractures and they were able to significantly reduce (predictions of) the size of a potential fracture," Shannon said. "And that paired with the point that we were flying valves that didn't have initial cracks to give us a lot of comfort that if we did initiate a crack and it did liberate in one flight, which we've never seen and we don't expect, that the particle would be very small.
"We didn't stop there," he continued. "That was very strong rationale to just go fly but we worked on the consequence side as well with the impact testing and computer analysis that we did. And we showed that the likelihood if you did release a particle of it damaging anything in the orbiter or the external tank plumbing was extremely remote.
"And we didn't stop there. ... We went the extra step and said if we did damage the plumbing, what would that mean? We didn't take it on face value that if you punctured a line that that was an automatic bad day. We looked at all of the different consequences and we found out that the size hole we would need to cause an over-pressure in the aft or a significant flammable problem or not support the external tank structure with pressure, the holes required to do that were orders of magnitude bigger than what we could possibly do with this piece of poppet.
"So we really attacked this problem from all the different areas, we made sure we had good valves going in without cracks, we showed even if one or two start a crack and liberate, they would be small. We showed that if it got in the plumbing it's very unlikely to cause damage and then we showed that even if it does cause damage, that damage is not something that we needed to worry about."
The decision to proceed with launch was unanimous.
"The vehicle's in great shape and we're ready to pick up with our normal countdown," Shannon concluded.
For downstream flights, NASA managers are assessing possible redesigns or the feasibility of simply launching with new, verified crack-free valves, each flight.