Shuttle Discovery's launch delayed at least one week
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: February 3, 2009;
Updated after news conference
Launch of shuttle Discovery on a mission to deliver a final set of solar arrays to the international space station has been delayed at least one week, to no earlier than Feb. 19, to give engineers additional time to assess the health of critical hydrogen flow control valves that keep the ship's external tank pressurized during the climb to space.
Launch had been targeted for 7:32:11 a.m. on Feb. 12, but senior managers at an executive-level flight readiness review held at the Kennedy Space Center today decided not to set an official launch date and to meet again next Tuesday, on Feb. 10, to assess the results of ongoing engineering analyses.
If the valve issue is resolved, a launch on Feb. 19 would occur at 4:41:47 a.m. Docking with the space station would be targeted for around 1:06 a.m. on Feb. 21 with the first of four spacewalks starting around 10:11 p.m. on Feb. 22. Additional spacewalks would be planned for Feb. 24, 26 and 28, beginning between 9:41 p.m. and 8:11 p.m. Undocking would be expected around 6:39 p.m. on March 2, with landing back at the Kennedy Space Center a few minutes before 11 p.m. on March 4.
Going into today's FRR, shuttle and space station managers faced a variety of issues. On the shuttle front, engineers were still discussing recent problems with flow control valves used to pressurize the ship's hydrogen fuel tank during the climb to space.
Each shuttle features three such valves, one associated with each main engine, that operate like lawn sprinklers, popping up as required to route hydrogen gas to the external tank to maintain the internal pressure needed to feed propellant to the main engines.
During the most recent launch last November, one of the three flow control valves aboard the shuttle Endeavour began allowing more hydrogen to pass through than expected. The other two valves reduced flow to maintain the proper pressure and the shuttle's climb to space was uneventful.
After landing, engineers discovered a small crack and missing material on the lip of the valve in question. NASA managers ordered all the valves in Endeavour, Discovery and Atlantis removed for detailed inspections. One other valve was found to have a crack. Based on electron microscope inspections, engineers concluded the defect was the result of high-cycle fatigue. A fresh set of vales that flew previously on Discovery and were known to be in pristine condition was installed for launch.
But engineers were surprised that fatigue played a role.
"It's just like a pop-up lawn sprinkler," said shuttle Program Manager John Shannon. "When you want to flow gaseous hydrogen from the main engines to the external tank to pressurize that tank, it pops up a little bit and allows that flow. What happened on STS-126 (in November) is that a small little piece of it, about the size of the tip end of your thumbnail, liberated, it broke off.
"That was a big surprise to us, because it's really not in an environment where we would expect to have that kind of fatigue. It only cycles about 15 times during the flight. It slowly goes up and down. So it was a surprise that it broke off."
Engineers at NASA's Marshall Space Flight Center in Huntsville, Ala., used computational fluid dynamics to model the hydrogen flow environment in the system. What they found explained what happened.
"The way the flow goes across the top of that valve, just like the harmonic resonance you get when you blow over the top of a Coke bottle, was making it vibrate," Shannon said. "That vibration, if it lined up just with the right structural mode, could end up stressing that piece and causing high cycle fatigue. That was a great bit of investigative work by both the Marshall team and the Johnson Space Center engineering team."
So far so good. Engineers thought they understood the problem and given the pristine valves now aboard Discovery, mission managers had a good rationale for pressing ahead with launch.
But Shannon said today past experience showed "we had a failure of imagination in past design issues. We didn't take it a step further. The guys took it a step further and said OK, the piece is small and it was fine on STS-126 but what if that piece came off at maybe a different time or maybe had a different angle going down the pipe? Could it rupture the tube or could it cause any damage to the tube so we would lose some of the pressurization in the external tank?"
The valves are critical to safe shuttle operation. Even though the tanks are equipped with pressure relief valves to prevent over-pressurization, such venting could expose the shuttle to hydrogen in its immediate environment. At the other extreme, a loss of pressure could lead to premature engine shutdown. And even if the tank maintained the proper pressure, metallic debris possibly could rupture a pressurization line with catastrophic results.
Launch was delayed to give engineers time to finish a series of tests, firing small bits of metallic debris into targets similar to the pressurization lines to find out what might happen in a truly worst-case scenario.
"We don't expect there to be an issue," Shannon said. "We looked at the witness marks of where the little poppet hit on 126 and there were no issues at all. We could see little gouges or scratches where we think that poppet came off and hit. We don't expect there to be a problem, but we don't have the proof in hand. And we want to go have that proof in hand before we commit to go fly."
Said Bill Gerstenmaier, director of space operations at NASA Headquarters: "We want to make sure we've got this right. This has important consequence to us, so we think standing down a little bit of time and letting the folks do a little more work is a good thing."
On the station front, U.S. and Russian managers agreed earlier today not to carry out a planned reboost of the lab complex Wednesday.
During a reboost maneuver Jan. 14 using two rocket engines in the Russian Zvezda command module, the station crew and engineers monitoring telemetry noted unexpected vibrations and oscillations in the station structure as the thruster firing progressed. The issue later was attributed to human error on the ground, and Russian engineers believe there are no technical issues that would prevent a normal reboost using thrusters in the Russian command module or in a docked Progress supply ship.
But station managers today agreed to defer additional reboost maneuvers pending additional analysis to make sure the vibrations posed no structural threat to the station and that future firings go smoothly.
"There were a number of areas of concern, where initial indications were we had violated design limits for mechanical loads," said Mike Suffredini, space station program manager at the Johnson Space Center.
Areas susceptible to damage from such loads include the aft command module, the connection between the station's huge solar power truss and the top of the U .S. lab module, the docking port where the lab connects to the Unity module and the rotary joint on the left side of the station that repositions two outboard solar arrays.
From a purely structural standpoint, the station's design loads were exceeded by 150 percent during the January reboost maneuver. But when factored in with thermal stresses the station experiences in orbit, engineers concluded no critical safety margins were exceeded.
"The teams worked over the last week and a half or so and at the end of this last weekend cleared the structure both for strength and for the 15-year life," Suffredini said. "So we have found that even though we did excite the stack and created a wonderful video for everyone to watch, we didn't, in fact, shorten the design life of ISS. We did, of course, put more cycles on it than we had planned. So we did shorten its life whatever it is, but it certainly didn't affect our design life and we don't think it really affected significantly the overall life we might otherwise get out of the structure."
The Russians do not need any additional reboost for launch of a new Progress supply ship Feb. 10. Additional rocket firings likely will be carried out in March before launch of a Soyuz ferry craft carrying the next station crew. As of this writing, that launch is expected to slip one day, from March 25 to March 26.
Other issues facing NASA managers today included ongoing problems with the space station's life support system that need to be ironed out to support the planned expansion of the lab's crew from three to six in May.
Urine recycling gear installed late last year, part of a complex system to recycle water aboard the station, ran into problems right off the bat with a distillation assembly centrifuge. The station astronauts eventually coaxed it into operation, but it suffered additional problems later and currently is out of action. A replacement is being launched aboard Discovery.
Other nagging problems facing the station include a high bacteria count in the new potable water system and trouble with U.S. carbon dioxide removal hardware, which is experiencing elevated internal pressure. Discovery will carry up chemicals to flush the potable water system and Suffredini said spare CO2 removal system components are scheduled for launch in June.
He said the planned crew increase from three to six can take place as planned in May despite those problems, given the amount of fresh water already on board and the planned delivery of fresh water from visiting space shuttles.
But getting the life support system up and running smoothly is critical to the station's long-term health and to prevent shortages in the event of any extended shuttle delays down the road. Suffredini said engineers hope to have the system fully operational within the next several months.
"As long as the shuttle is flying pretty much on the schedule we've planned, we will be able to support six-person crew," he said. "We'll have to get more water from the shuttle than we might otherwise require, but at least we can support six-person crew. So we do have a little time to sort through those anomalies. Ultimately, we'd like to get the distillation assembly working and process all the water we can."
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