Shuttle Discovery on track for Sunday launch, unless...

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On March 12, NASA managersscrubbed the launch of Discovery and its seven-member crew to the International Space Station. Now managers say they are shooting for a launch Sunday evening.

The reason for the scrub: a leaky valve at the fuel-tank end of a line that draws excess hydrogen gas out of the tank to be burned off at the end of a tall pipe some distance from the launch pad. You can read more about the particulars behind the delay here.

Have they found out why the valve gave them trouble and fixed it? No. They'll certainly try to. During a briefing this morning, launch director Mike Leinbach expressed confidence that engineers will identify the problem before launch.

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But just to be sure Discovery stands a fighting chance of launching on time to achieve most of its objectives, mission managers have decided to replace the balky valve. The mission's major task is to install the station's final set of solar panels.

"If we can't find the smoking gun, it will be declared an unexplained anomoly, " Mr. Leinbach said.

Because it must rendezvous with the space station, Discovery is on a tight schedule. Because of the orbits involved, that means it has only a small window each month in which to launch.

This month's window is especially short. Delays involving another valve pushed the launch date from February 12 to March 11. Meanwhile, Russia is launching a Soyuz capsule with two new crew members  to the station March 26. If Discovery had launched on the 11th as planned, it would have left the space station in time for the arrival of the Soyuz craft.

But a shuttle launch between now and March 17 for a full 15-day mission would boost the chance that the two would overlap. The formula mission managers want to avoid is: V + S = Tmgo .

Ah yes, let us define our terms. V is a very tightly packed demanding set of tasks while the shuttle is at the station. S is a space-station crew replacement, with its own tightly timed hand-off procedures. Tmgo is too much going on at once.

NASA managers say they are will push Discovery's launch off to next month if necessary. But they'd rather get the solar array up and installed before then if they can launch safely. A Sunday launch would trim the mission to 13 days and three spacewalks instead of four. A  Monday or Tuesday launch would mean an 11-day mission with only one spacewalk.

What might push the launch off to April? If the replacement valve exhibits the same tendency to leak as its predecessor when the hydrogen fuel tank is topped off, that would be a show-stopper.

Weather also could be an issue, if a launch can't take place until Tuesday. Currently, forecasts suggest a 60 percent chance that the weather will be unacceptable for launch that day.

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