March launch of Discovery possible, but not certain
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: February 24, 2009;
Updated with decision to swap out valves
NASA managers today ordered engineers to replace suspect hydrogen flow control valves aboard the shuttle Discovery with valves that have less flight time in a bid to reduce the chances of in-flight cracks that could lead to debris in a pressurization line. If ongoing tests and higher fidelity computer models continue to show positive results, Discovery could be cleared for a delayed launch attempt by around March 12, sources said today. That would give NASA just two or three launch opportunities before standing down until April 7 to avoid conflict with a Russian Soyuz mission to the international space station.
A decision to set a new target launch date could come as early as Wednesday, but sources said it was not a done deal because engineers are still debating the root cause of the valve problem that has grounded Discovery. The results of ongoing tests, however, along with the predictions of more realistic computer modeling, may convince skeptics the shuttle's internal plumbing can withstand impacts from valve debris should cracks develop in flight.
While that remains to be seen, a brief update on NASA's web site late today indicated a plan for moving forward could be in place by Wednesday.
"Though the plan has not yet been completed, technicians will install flow control valves that have flown fewer times than the ones currently in Discovery's main propulsion system," NASA said in a statement.
"The plan should be finalized by Wednesday and once senior managers are in agreement, a Flight Readiness Review will be rescheduled to assess the readiness for launch and to set a formal launch target date."
The Discovery astronauts were allowed to break quarantine Friday after mission managers put the flight on hold and ruled out a Feb. 27 launch date. At the time, Launch Director Mike Leinbach said Discovery could be launched five days or so after a decision was made to proceed, assuming the shuttle was cleared for flight as is.
But the valves are being replaced and the crew still must put in a full week of quarantine time. Several sources said today it was unlikely Discovery could be ready for launch before March 12, and that assumes engineers and managers can get comfortable enough with the test data to press ahead without any design changes for the valves in question.
To avoid conflict with the upcoming launch of a Russian Soyuz spacecraft carrying two fresh space station crew members and a tourist, Discovery cannot take off after around March 13 unless the Russians agree to delay their own launch. As of this writing, a shuttle launch on March 14 is possible, but it would require the crew to eliminate one of their four planned spacewalks and shorten the flight by a day.
The next available launch date after that would be April 7, the day the station's current commander, flight engineer and the tourist depart in an older Soyuz.
If Discovery launches in March, before the Soyuz "cutout," officials say the shuttle Atlantis will stay on schedule for launch May 12 on a long-awaited mission to service and upgrade the Hubble Space Telescope. If Discovery's flight slips to April, the Hubble mission likely would slip to around June 2.
Complicating the picture in the near term, United Launch Alliance plans to launch NASA's Kepler science mission aboard a Delta 2 rocket on March 5 and an Atlas 5 rocket carrying a military communications satellite on March 9. It does not appear the Kepler mission will be a factor in shuttle planning, but the Atlas flight is in direct conflict.
The Air Force Eastern Range, which provides radar tracking and other services for all rockets launched from Florida, supports one flight at a time and each mission gets at least two launch opportunities on successive days. As of this writing, there are indications the Atlas flight may slip a few days, but no such delay has yet been announced.
Even if the Eastern Range is clear for Discovery, a launch in March assumes no major repairs or redesign work are required to resolve concerns about the suspect hydrogen flow control valves. Originally scheduled for launch Feb. 12, the flight has been repeatedly delayed because of concern a valve could break in flight, sending metallic debris into a pressurization line.
The shuttle is equipped with three hydrogen flow control valves that work like pop-up sprinklers to route hydrogen gas to the external tank to keep the hydrogen section properly pressurized at 32 to 34 pounds per square inch during the climb to space.
During the most recent flight last November, a small piece of one valve poppet broke off in flight. It was the first such incident in the 124 shuttle flights to date, but testing and computer analysis indicates cracks in the valves have been a long-standing, but unknown, threat with at least one valve design.
Engineers are carrying out tests to determine what caused the valve to break; how cracks propagate and how large resulting debris might be; and to make sure the pressurization line is tough enough to withstand debris impacts from any future failures.
It's not yet clear whether cracks in the valves are the result of accumulated stress over multiple flights or the result of single-flight events related to unexpected harmonic and structural interactions between the pressurization lines in question and the orbiter.
Managers decided today to replace the current valves, which have flown 11 to 12 times each, with similar 1301-design units that have logged fewer flights. That will protect against multi-flight stress-related cracks and poppet failures. At the same time, older 1301 valves likely will be inspected to make sure there are no signs of cracks that could be age related. None are expected.
Engineers are hopeful more realistic computer models, based on actual impact tests, will show the odds of a catastrophic failure are remote, even in a worst-case scenario.
Testing to date seems to indicate a 90-degree bend in the external tank pressurization lines just five inches from the valves can withstand the sort of impacts one could expect in an actual failure. Earlier less positive assessments were based on knife-edge impacts from debris with worst-case velocities and unrealistically stable paths down the pipe. Using those assumptions, the pressurization line elbow bend could sustain impact velocities of around 560 feet per second.
It now appears the elbow bend may be able to withstand impact velocities higher than 900 feet per second - more than 600 mph - if the debris is relatively small and following a more realistic trajectory. But engineers do not yet know enough about how the valves can break to predict the maximum allowable size of any released debris.
Given the complexities involved, it is perhaps not surprising the issue is not yet clear cut and that additional time may be needed to resolve the matter one way or the other.