posted 12-06-2007 06:19 AM               

Commander Steve Frick and his crew are now awake, eating breakfast and receiving a final medical check from the flight surgeons.

posted 12-06-2007 08:08 AM               

Testing will continue as fueling of the external tank is completed. A decision as to whether to scrub today's launch will be made at that time.

posted 12-06-2007 09:14 AM               

quote:

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Originally posted by cspg:
Wasn't this an issue for a previous mission?

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posted 12-06-2007 09:21 AM               

Tests are performed during fueling to insure the sensors are functioning properly. As controllers know that the tank has fuel in it (a.k.a. it's "wet"), they test the sensors by ordering the sensors to read "dry".

Today, two of the sensors, numbers 3 and 4, refused to read "dry" despite being commanded to do so as part of the test. They continued to read "wet".

The two sensors failed simultaneously, so there is some hope that it may be the circuitry connecting them rather than the sensors themselves that have failed.

NASA previously experienced issues with ECO sensors on STS-114, STS-121 and STS-115. After STS-114, manufacturing defects were discovered and a new batch of ECO sensors were tested and installed.

posted 12-06-2007 09:29 AM               

quote:

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Originally posted by NavySpaceFan:
I take it that the sensors can be replaced on the pad, correct?

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posted 12-06-2007 09:59 AM               

"The failure was an LH2 low level liquid sensor," explained Lyons, "we also refer to them as our ECO sensors.

"We had a failure of sensor number 3 and number 4. In the system we have a total of four sensors.

"The failure occurred during tanking around 16 minutes into fast fill.

"We picked it up while implementing our standard checkout of the system," Lyons continued. "As soon as we get these sensors wet, we go through a battery of checks to make sure that they are operating nominally and properly. We were at a point in the test where we sent commands to take all four sensors 'dry' and when we did that, sensors 1 and 2 went dry as expected, and sensors 3 and 4 went 'wet'. Right then we knew we had an issue."

Lyons said that his team is now troubleshooting the issue, with early signs pointing away from the sensors themselves.

posted 12-06-2007 06:40 PM               

The decision to stand down for 48 hours after a scrub on Thursday was driven by the need for more time for technicians to evaluate the problem with two low fuel sensors inside the shuttle's external tank.

"We want to sleep on it," said Leroy Cain, NASA's launch integration manager. "We want to encourage the engineers and the rest of the team to sleep on it and think about what we might not be thinking about yet."

NASA will use the time to troubleshoot the issue, as well as develop flight rationale by which Atlantis could fly with one or more failed or malfunctioning sensors.

"We do have some additional data on this system over what we had in previous years and some 120 flights we have flown," said Cain. "On the last two missions, we flew for the first time some additional instrumentation... that gives us some insight into the system that we didn't previously have.

"We have the mission operations team, the flight crew and the safety folks off working together to put together what the operational plan would be to be able to fly with any number of failures in this LH2 ECO system and be able to mitigate the risk with operational workarounds essentially by having this additional insight on the fly," he said.

posted 12-07-2007 07:14 PM               

After standing down for 48 hours on Thursday, NASA mission managers spent Friday weighing their options with regards to a pair of suspect low level fuel sensors.

Their meeting having run into the early evening, managers ran out of time to attempt a launch on Saturday, as had been earlier targeted.

"We have encountered a problem that we had thought we had solved," said Wayne Hale, NASA's space shuttle program manager, referencing earlier problems with the low level ECO sensor system. "So, we have spent the last day or so thinking about what we might do about it."

"As of today, no one has come forward with a good plan to improve our situation in the real hardware sense," continued Hale. "Part of this is because the failure is an intermittent failure. Right now, out at the launch pad, all the sensors and their wiring are working perfectly. When we fill the tank up with cryogenic hydrogen again, the next time we fill the tank up, past history says they are all likely to work. This makes it very difficult to troubleshoot."

"I think we've all had the experience of having a problem on our automobile and taking it to the mechanic and by the time you get it to the shop, it's working just fine an they can't find anything. That's kind of the situation we're in right now," said Hale.

According to Hale, it was the astronaut office, including the STS-122 crew, that has proposed a potential way forward.

"The crew office, the flight crew actually came with a very positive proposal in my view today, that said let us tighten up our launch commit criteria and require all four of these sensors to be working, plus the new instrumentation that we've added that measures voltage in the middle of this system, which was new on the last couple of flights, require all four of those measurements to be working, and if all that stuff is working and we restrict our launch to a very short window around the most optimum time to give ourselves the maximum performance benefit, then, with some new flight rules that the mission operations people back in Houston are implementing, the flight crew believes that it would be acceptably safe for us to go fly, with some additional risk," Hale explained.

"I personally believe that this is a great proposal, however we wanted to run down a number of the technical aspects. We are still facing a situation where we do not know root cause, and if we tank the vehicle up and we have these sensors act up again, we would certainly want to stand down because that is outside of our experience base.

"So, the proposal is on the table," Hale said, "We asked the team to go home and think about it. We'll come back tomorrow at one o'clock... and if we find that this proposal that the flight crew office has offered allows us to fly with an acceptable risk, we'll try to tank up the vehicle and launch on Sunday."

If the flight crew's proposal is accepted, then on launch day, all four of the ECO sensors will need to pass all three tests during tanking.

"We're going to go back to four of four for the ECO sensors," said Mike Leinbach, NASA's launch integration manager. "We had gone down to 3 of 4 on that system about a year or so ago, I believe it was."

"If all four of the sensors are working at liftoff," Hale added, "we think there is a good probability that the system will be working late in the ascent when you need it to work. That's kind of the logic behind the whole subject."

Hale believes that all four sensors will be working based on past history.

"Our previous experience has been that when we have one of these low level cutoff sensors not perform properly and we scrub, de-tank, come back the next day or a couple of days later, and tank again, our experience has been that they magically work," Hale said.

The other change to the countdown would be a shorter launch window.

"If you recall, we shoot for a 10 minute period and we usually launch in the middle of the window to allow us a five minute launch window," said Leinbach. "We going to shorten that to about a minute, we haven't concluded exactly how long the new window is going to be, but it's going to be on the order of a minute or so as opposed to five minutes, and that saves us some fuel for going uphill and is actually a better trajectory for the shuttle to get to the International Space Station."

"It's a more direct trajectory to the space station and therefore the shuttle system uses less fuel to get to the station," explained Leinbach. "Therefore, you have more reserve in the tank. One of the scenarios that would actually drive us to use the ECO system is fuel depletion or a low level anyway, so the more fuel you have in the tank, the better off you are because it is less likely you use the system."

posted 12-07-2007 09:28 PM            

quote:

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Originally posted by Robert Pearlman:
Though the day started on a disappointing note, the scrub did allow for an unexpected photo opp out with Atlantis...

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posted 12-07-2007 09:43 PM               

quote:

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Originally posted by robsouth:
Beautiful photo, who took it?

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posted 12-08-2007 01:49 PM               

Meanwhile, the weather forecast has been upgraded for Sunday with now only a 20% chance of conditions preventing a launch.

As an aside but relevant to the overall theme of collectSPACE, Wayne Hale last night put the history of ECO sensors into perspective.

posted 12-08-2007 02:15 PM               

quote:

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Originally posted by art540:
What is an alternative design to monitor for low level propellants?

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posted 12-08-2007 03:03 PM               

------------------
Scott Schneeweis

posted 12-08-2007 03:21 PM               

The proposal put forth by the flight crew division yesterday, including requiring all four ECO sensors to pass pre-flight tests and shortening the 10-minute launch window to just a minute, was approved by managers today.

"We had basically a unanimous decision to go forward, so we will proceed to tanking tomorrow," said space shuttle program manager Wayne Hale.

Fueling of Atlantis' external tank is scheduled to begin as the countdown exits a planned T-6 hour hold at 5:55 a.m. EST. At 6:41 a.m., the four ECO sensors should become immersed in liquid hydrogen, providing the first opportunity to test the system and assess whether the countdown will proceed.

posted 12-08-2007 07:48 PM               

Further, if it was only the temperature that was causing the problem, then you would expect the sensors to fail every time they were subjected to the cryos, which they do not. STS-116, STS-117, STS-118 and STS-120 all were fueled without any problems with the ECO sensors, as has many more missions before them.

posted 12-08-2007 08:09 PM               

quote:

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Originally posted by Jay Chladek:
Is that the apparent rationale I am seeing?

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quote:

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Which member originally proposed it?

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posted 12-08-2007 11:13 PM               

quote:

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Originally posted by LCDR Scott Schneeweis:
Of course if there is unanticipated loss of LH2 from the tank leak following launch then all bets are off...

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