Another scrub for space shuttle Discovery's launch
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: August 25, 2009
Engineers plan to test a suspect hydrogen fill-and-drain valve aboard the shuttle Discovery late Wednesday to determine if the valve or a sensor system that measures its position is to blame for readings that forced NASA to call off the shuttle's countdown Tuesday, delaying launch from early Wednesday until Friday at the earliest.
If it turns out the position sensor was to blame - and if NASA managers can get comfortable launching Discovery without full instrumentation in a critical system - then a launch attempt Friday at 12:22 a.m. EDT might be feasible. But if engineers are forced to open the shuttle's engine compartment and replace any suspect components, launch could be delayed to around Oct. 17.
That's because Discovery's current launch window closes after Aug. 30 because of upcoming Japanese and Russian space station launches and because the Air Force Eastern Range, which provides tracking and telemetry support for all rockets launched from Florida, is booked for another operation in early September.
"Right now, I'm pretty confident in the schedule of attack that says we'll be really
good to go on Friday if we come out with a good technical story that says we can fly
without instrumentation," said MIke Moses, chairman of NASA's pre-launch Mission
Management Team. "So we both have to first prove it is instrumentation and then
prove we're OK to fly without instrumentation."
"Basically around this time tomorrow, we'll be ready to have a technical discussion amongst the experts to say did that look like what we thought it did, do we think we're in good shape?" Moses said. "So heading into the midnight timeframe tomorrow, we'll probably know whether we are going to keep pressing forward (for a Friday launch) or we've learned we're not going to make it."
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 Mission Management Team plans to meet at noon Thursday to assess the progress of troubleshooting and to make a formal decision on whether to proceed with launch.
The problem developed around 5:52 p.m. Tuesday when the 8-inch-wide liquid hydrogen inboard fill-and-drain valve in Discovery's aft engine compartment was commanded to close during fueling, a routine step to slow the rate at which hydrogen flows into the shuttle's external tank. One position indicator showed the valve was no longer open, but the closed indicator never provided a reading and engineers were unsure of the valve's actual position.
The valve must be fully closed for launch. In the event of a delay, the valve also must be able to open back up to drain the giant external tank. IN addition, the valve is opened in orbit to blow residual propellant out of the engine plumbing and into space.
NASA flight rules forbid cycling the valve for troubleshooting once fueling has started out of concern about galling, debris creation and the possibility a failing valve could get stuck or break in the closed position. Following the rules, NASA managers called off the countdown and ordered engineers to drain the external tank.
"As we got into tanking we ran across a failure signature today that really was one of those that's in our LCC (launch commit criteria) that doesn't give us a lot of options," said Moses. "Basically, it was an LH2 inboard fill-and-drain valve. these are the big valves in the 8-inch lines that feed the liquid hydrogen into the system. ... If we needed to drain the external tank, that's how we'd drain back out.
"In that 8-inch duct, there's a valve flap, and the valve is really kind of complicated. If you think about it, it's at cryogenic temperatures, it's liquid hydrogen in an 8-inch diameter pipe. It's a very big, complicated system. There's a gear drive mechanism that makes this valve lift off the little cover and rotate out of the way, it's kind of a visor type of valve. We have to have good indications that that valve is either open or closed, you want to make sure you know exactly where it is."
Based on two past failures, engineers found galling can create debris in the line that can cause a valve to jam.
"That's the thing we want to avoid," Moses said. "So the LCC's really set up to say you want to make sure that valve goes closed because it needs to be in a good closed position for launch, that's where you want it to be, but don't go cycling around because if it is broken or if it's about to break, you really need to make sure in a drain situation you can open that valve back up again. Otherwise, it's a very challenging job to drain the external tank. ... That's really the hazard we're trying to control."
He said engineers "really do think this is just telemetry. ... We have the pressure traces, we know it looks like the valve is going its full range of motion. But we have a much bigger database at ambient temperatures when we do valve checkouts on the ground pre-launch than we do here at cryo conditions. So we want to get back to that pre-launch state where we know it's an inert system, go run some valve cycle tests, compare those two sets of data and show we really do have just a position indicator problem.
"That's postulating," he said. "The teams are going to spend the next two days gathering the technical analysis that says if that's truly the situation, we're comfortable launching in that scenario. And then go gather the data to make sure it really matches with what we think.
"If we find that's not the case ... or the team just doesn't get comfortable that we really can tell truly that this valve is actually open or actually closed without this position indicator, then we would probably not be in position to launch 48 hours from now."
As if engineers didn't already have their hands full, telemetry during Tuesday's fueling indicated elevated levels of hydrogen gas in the tail service mast on the left side of the shuttle where the liquid hydrogen feedline attaches to the orbiter.
Launch Director Pete Nickolenko said engineers planned to take advantage of the launch delay to troubleshoot that issue as well.