NASA targets March 12 for shuttle Discovery launch
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: February 25, 2009
NASA managers decided today enough progress had been made testing and evaluating suspect hydrogen flow control valves to tentatively set a March 12 target launch date for the shuttle Discovery's delayed mission to attach a final set of solar arrays to the international space station.
Another program-level review is scheduled for March 4 and if no major problems develop, senior managers will hold an executive-level flight readiness review - Discovery's third - on March 6 to make an official decision on whether to proceed with launch.
"NASA's space shuttle program has established a plan that could support shuttle Discovery's launch to the international space station, tentatively targeted for March 12," NASA said in a statement late today. "An exact target launch date will be determined as work progresses with the shuttle's three gaseous hydrogen flow control valves."
If the proposed schedule holds up, Discovery's countdown would begin around 7:30 p.m. EDT on March 9, setting up a launch attempt at 8:54:25 p.m. on March 12. Docking with the international space station would follow on March 14 with four spacewalks planned for March 16, 18, 20 and 22. Undocking would be expected on March 24 with landing back at the Kennedy Space Center on March 26.
The Russian space agency plans to launch a Soyuz spacecraft on Discovery's landing day - March 26 - to ferry the space station's next commander and flight engineer to the lab complex along with a wealthy space tourist. Discovery must be gone by then to avoid a conflict and as a result, the shuttle must take off by March 13 to get a full-duration four-spacewalk mission. A launch on March 14 or even 15 is possible, but the crew would have to give up one or two spacewalks.
If the shuttle does not get off before the Soyuz cutout, the flight will be delayed to April 7, the same day the outgoing station crew returns to Earth.
A shuttle liftoff in March would permit NASA to proceed as planned with the May 12 launch of shuttle Atlantis on a mission to service and upgrade the Hubble Space Telescope. If Discovery is delayed to April, the Hubble flight would slip to around June 2.
Discovery originally was scheduled for launch Feb. 12. But the flight was delayed, first to no earlier than Feb. 19, then to Feb. 22 and eventually to no earlier than Feb. 27 because of concern about the integrity of the shuttle's three hydrogen flow control valves. Last Friday, Feb. 27 was ruled out to permit more time for troubleshooting and analysis.
When launch was targeted for Feb. 12, NASA built a flight plan that called for a so-called ascending node landing, one that resulted in a southwest-to-northeast approach to Florida. That's a standard post-Columbia approach because it minimizes overflight of populated areas during the shuttle's descent from orbit.
But as Discovery encountered repeated delays, flight planners switched to a descending node entry trajectory, changing the timing of major mission events. This was done to avoid forcing the station crew to endure severe sleep shifting after the shuttle's undocking to put them in synch with the arriving Soyuz crew.
Now that the flight has slipped to no earlier than March 12, NASA is going back to the original ascending node approach trajectory. A new flight plan reflecting changes required by the new launch target has not yet been generated and the current timeline posted here is out of date. An update will be posted as soon as possible.
Concern about the hydrogen flow control valves, used to keep the hydrogen section of the shuttle's external tank pressurized during ascent, cropped up after the most recent shuttle launch last November.
NASA flow control valve background here.
During the climb to space, 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 in the external tank 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. At least 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 were inspected and installed for launch.
But engineers were surprised that fatigue played a role. Using computational fluid dynamics, they found that the environment inside the system can allow a harmonic resonance to set up that could cause the sort of fatigue seen in Endeavour's valve. Additional tests also found that surface roughness on the valve poppets could mask cracks.
While NASA plans to modify the valves for downstream flights, shuttle managers put Discovery's flight on hold for an exhaustive series of tests to determine if the shuttle could be safely launched as is, focusing on the effects of debris impacts inside the pressurization line.
A decision was made Tuesday to replace Discovery's flow control valves with a set that has flown fewer missions, presumably making them less susceptible to cracking if repeated exposure to stress plays a role. Impact testing using a mockup of the pressurization line shows it can handle impacts from debris that's larger than the piece liberated during Endeavour's flight.
"Technicians have started removing Discovery's three valves, two of which will undergo detailed inspection," NASA said in a statement. "Approximately 4,000 images of each valve will be reviewed for evidence of cracks. Valves that have flown fewer times will be installed in Discovery.
"Engineering teams also will complete analysis and testing to better define consequences if a valve piece were to break off and strike pressurization lines between the shuttle and external fuel tank. Hardware modifications may be made to the pressurization lines to add extra protection in the unlikely event debris is released."
The proposed modification is a clamp-like fitting that would strengthen a 90-degree bend in the line five inches from the valves.
Engineers still do not fully understand what causes cracks to develop, how fast they might propagate and how big liberated pieces might be in worst-case scenarios. As such, the March 12 launch date is a tentative target as of this writing.