KENNEDY SPACE CENTER, Fla.--NASA managers Thursday ordered another delay for launch of the shuttle Discovery, from early Friday morning to late Friday night, to give engineers additional time to make sure an 8-inch hydrogen valve in the orbiter's engine compartment will operate normally during fueling.
NASA's Mission Management Team plans to meet at noon EDT Friday to review the work, consider a waiver to flight rules governing how the valve system operates, and make a final decision on whether to clear Discovery for fueling and launch at 11:59:39 p.m. Forecasters are calling for a 60 percent chance of acceptable weather at launch time.
Discovery is bound for the International Space Station, carrying a crew of seven and more than 7.5 tons of supplies and equipment. Because the space station launch window moves about 23 minutes earlier per day, Discovery has two launch opportunities Friday, the first at 12:22 a.m. and the second at 11:59 p.m. The MMT on Thursday gave up the first opportunity in favor of the second.
"First off, just to put the right spin on it, we're here to not talk about a scrub, in fact we're still going to launch on Friday," MMT Chairman Mike Moses joked during a news briefing.
The problem cropped up during fueling Tuesday for a day-late launching Wednesday morning. As Discovery's external tank was filled with liquid oxygen and hydrogen rocket fuel, the inboard hydrogen fill-and-drain valve was commanded to close. Data from a position sensor never indicated the valve was, in fact, closed and NASA managers called off the countdown.
They were responding to a flight rule that prohibited real-time troubleshooting under supercold cryogenic conditions out of concern a valve already near failure might break in the closed position. If that happened, it would be difficult to drain the huge fuel tank after a launch delay.
Engineers tested the valve under ambient conditions late Wednesday and in five complete cycles, the valve -- and the position indicator -- worked normally.
Engineers restarted Discovery's countdown early Thursday in hopes of getting clearance to proceed to an early Friday launch. But the Mission Management Team ordered another delay to give the team time to complete the complex analysis without being under the pressure of a countdown.
"The teams have been working really hard over the last several days," Moses said. "They got back together this morning and it became pretty apparent pretty quick we had really good rationale, we had a really good story, but the teams that needed to polish it, do the double checks, were the same teams that within another hour had to go on console and do the tanking. And then I needed them to be on console for launch.
"When we looked at that and we started to judge our readiness, we basically said you know what? The better part of valor here is to take a day, let us go polish that off, make sure we really understand what's going on."
Engineers believe the valve is operating normally and that for some reason, the position indicator, which measures the movement of a pneumatically-driven gear that pushes the valve open and closed with pressurized helium, failed to operate normally during tanking Tuesday.
It may work normally during fueling Friday afternoon, or it may malfunction again. If engineers do not get a closed indication, Moses said procedures will be in place to evaluate secondary cues to confirm the valve is, in fact, closed. In addition, they will be cleared to cycle the valve, if necessary, to make sure it is seated.
"We want to make sure we're ready," Moses said. "If we're wrong, we want to make sure we really do understand that there's not some other mechanism that we're not quite smart enough to think of yet that might fake us out on some of these other signatures."