NASA officials poised for shuttle launch date debate
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: February 19, 2009
Senior NASA managers and engineers headed to the Kennedy Space Center today for a second flight readiness review Friday to assess testing of suspect hydrogen flow control valves and to make a decision on whether or not to press ahead with launch of the shuttle Discovery Feb. 27 on a space station assembly mission.
Some managers are opposed to launch at present, sources say, arguing the valves should be redesigned to eliminate the possibility of cracks that could lead to potentially catastrophic in-flight failures. Others believe exhaustive testing shows Discovery can be safely launched as is while a redesign is implemented for downstream flights. Still others believe a redesign is not necessary, arguing if it's safe to launch Discovery as is, it's safe to launch any mission using carefully inspected valves of the current design.
The stakes are high. With the shuttle fleet facing retirement in 2010 after nine more flights, any significant delays now, for a redesign or any other reason, could result in one or more lost missions.
But the Obama administration has not yet named a new NASA administrator and Friday's flight readiness review will be run by acting administrator Chris Scolese, NASA's former chief engineer, and Bill Gerstenmaier, associate administrator of space operations. As of this writing, a unanimous decision to proceed with launch Feb. 27 appears unlikely, but it's not yet clear how the discussion might play out.
Scolese and other senior managers could decide to press ahead with launch Feb. 27 despite objections; they could opt to put the mission on hold pending a redesign; or they could opt to simply delay the decision another few days, and push launch back accordingly, pending additional analysis.
Discovery originally was scheduled for launch Feb. 12 on a mission to deliver a final set of solar arrays to the international space station. 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 concerns about the integrity of the shuttle's three hydrogen flow control valves.
There are three such valves on every shuttle and they work like pop-up sprinklers, extending as required during ascent to route hydrogen gas to the shuttle's external tank to keep the tank properly pressurized at 32 to 34 pounds per square inch.
During the most recent shuttle launch last November, a valve cracked and a piece of debris was liberated that presumably made its way into the tank. While more hydrogen passed through the damaged valve than normal, the other two valves compensated to maintain a normal pressure and Endeavour's climb to space was uneventful.
Inspection of the damaged valve indicated high-cycle fatigue was the culprit. Engineers were surprised, because they believed the environment inside the pressurization line was relatively benign. Using computational fluid dynamics and extensive testing and inspections, they now understand harmonics can set up inside the line that can subject the valves to the sort of stress that could lead to fatigue. If a crack is present in a valve, under some circumstances it can propagate to the point small sections can break away.
While this scenario has played out only once in 124 missions, tank pressurization is critical and any debris-related punctures to a pressurization line could be catastrophic, depending on when an incident occurred.
Over the past two weeks, engineers using full-scale mockups of pressurization lines flowing air and hydrogen gas have run a variety of tests to find out whether valve debris can puncture the pressurization line or cause any other serious problems should a failure similar to Endeavour's happen again.
Engineers also are trying to find out whether valves can be confirmed to be crack free, how cracks develop in the first place and how fast any such cracks might propagate in flight. In the wake of Endeavour's flight, Discovery's valves were replaced with flight-tested valves that were inspected and believed to be crack free. But other valves presumed to be pristine were deliberately tested to failure and engineers discovered pre-existing cracks that had eluded detection.
Testing to date seems to indicate the valves do not pose a catastrophic threat even if an unseen crack propagated and led to a small piece breaking off. The testing shows such debris, ricocheting down the pressurization line with the same speeds and trajectories one might expect during flight, does not cause punctures or other critical problems.
But the issue is not clear cut and several assumptions are built into the analysis, including how fast cracks can propagate and what size debris can be expected to break off in worst-case scenarios.
The situation is reminiscent of an analysis that took place in late 2002 when NASA managers decided to keep launching shuttle missions while engineers worked on a fix to prevent foam insulation from breaking away from so-called bi-pod ramps on the external tank. The shuttle Columbia was destroyed during re-entry Feb. 1, 2003, because of a bi-pod foam impact during launch.
With the memory of Columbia still fresh even after six years, sources say at least some managers want to ground the fleet until the hydrogen flow control valves can be redesigned.
But others argue the two cases are not so similar. In the case of foam, engineers were misled by a long history of foam shedding and a resulting false sense of security. Foam shedding had evolved into an accepted risk that engineers thought they understood.
In this case, there is no such history. The first indication of any significant trouble with flow control valves in 124 flights occurred during the November shuttle launch.
In any case, the thrust of the recent testing has been to assess the safety of launching Discovery as is. NASA managers have not yet addressed whether a redesign might be needed or what impact such a decision might have.
If Discovery ultimately is cleared for launch on or near Feb. 27, the crew would fly to the Kennedy Space Center this Sunday to begin final preparations. The countdown would begin at midnight Monday, setting up a launch attempt at 1:32:06 a.m. Friday, Feb. 27.
Mission planners are revising the crew's flight plan to help the space station astronauts avoid major sleep cycle changes after the shuttle departs.
When launch was scheduled for Feb. 12, Discovery's mission was designed to end with what is known as an ascending node approach for landing, i.e., along a trajectory from the southwest to the northeast. That's become a standard approach in post-Columbia planning because it minimizes the time a shuttle spends flying over populated areas.
But to avoid having the station astronauts radically change their sleep cycles between the shuttle's departure and arrival of a Russian Soyuz spacecraft in late March, flight planners are now designing a descending node approach that would carry Discovery across the heartland of America from northwest to southeast.
Orbits that permit Florida landings occur roughly 12 hours apart in alternating ascending and descending trajectories. A switch to a descending node approach will move major mission events six to eight hours later each day than originally planned.
Updated launch windows have been posted here. But NASA has not yet developed a revised flight plan reflecting landing change.