Fuel sensor glitch a mystery as countdown begins
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 23, 2005
"We believe our flight systems and ground support hardware are ready, we know our flight crew and support teams are ready and we're all eagerly anticipating and looking forward to a successful launch and mission," said NASA test director Pete Nickolenko.
With forecasters predicting a 60 percent chance of good weather, NASA's shuttle team started a fresh countdown at noon, setting up a launch attempt at 10:39 a.m. EDT (1439 GMT) Tuesday, the same time shuttle Columbia blasted off on its final mission two-and-a-half years ago.
A detailed countdown timeline is posted here.
Shuttle forecaster Kathy Winters said the primary concern will be the development of a sea breeze Tuesday and showers that might move into the launch area from offshore. The outlook for Wednesday is 60 percent "go," improving to 70 percent should the flight slip to Thursday.
"In general, we've been keeping a close eye on tropical storm Franklin, but we're starting to get more confident in this northeasterly turn," Winters told reporters today . "For launch, basically what we're concerned about is the placement of that sea breeze and the timing of the development of those showers."
Discovery 's launch window, based on the international space station's orbit and a requirement to launch in daylight to verify post-Columbia fuel tank modifications, extends through July 31. If the weather or some other problem delays launch Tuesday, NASA can make attempts July 27, 29 and 31 or July 27, 30 and 31, depending on the weather and work to service the shuttle's electrical generators.
Engineers were hopeful earlier this week that electrical interference caused by subtle grounding problems might explain why one of four hydrogen fuel sensors in Discovery's external tank failed to respond properly to a pre-launch test July 13.
At it turned out, the three grounding problems were very minor - 0.2 milli-ohms of resistance or less - and engineers were never able to duplicate the unexpected behavior of ECO sensor No. 2. As such, the problem remains an "unexplained anomaly."
But NASA managers believe they have developed a strong rationale for proceeding with the 114th shuttle mission after an exhaustive battery of tests and work to swap the electrical cables routing commands and data from sensors 2 and 4 to an electronic component called a point sensor box.
If a problem shows up during fueling with sensor No. 4, engineers will have high confidence the problem is somewhere in the wiring between the point sensor box and the sensor itself, and not a generic problem that could affect the other three sensors. In that case, NASA's mission management team could consider making an exception to a launch commit criterion that calls for four operational sensors before a countdown can proceed.
The ECO sensor system is a backup that protects against other failures that might result in running the shuttle's main engines long enough to drain the external tank. Complete details on sensor operation and logic are available on the CBS News/Spaceflight Now ECO sensor page.
NASA's original flight rule, in place for the first 25 shuttle missions, only required three of four sensors because the system is redundant - three of the four sensors would have to "fail wet" to drain the tank after some other problem required their use in the first place - and to protect against the possibility of a faulty sensor, which cannot be easily replaced.
But after Challenger, engineers discovered a single-point failure mode in an electronics black box upstream of the point sensor box that could take out two fuel sensors at once. That failure mode was corrected several years ago, but the four-of-four LCC was never changed back to three of four.
If sensor No. 4 - the one now connected to the wiring that originally went to sensor No. 2 - acts up Tuesday, NASA may be forced to sign an exception to the LCC to permit Discovery to take off with three of four operational ECO sensors. If any other sensors misbehave, the launch will be called off and troubleshooters will go back to the drawing board.
"Certainly, if we get anything else new, that would certainly be a cause for a scrub condition," said NASA test director Pete Nickolenko. "If we were to see (problems with sensor No. 4), we can reasonably conclude it was related to that circuitry that was downstream of the point sensor box and then we could entertain that 3-of-4 flight rationale, which we have been working on developing. But anything other than that might implicate the point sensor box ... or something else in the wiring."
NASA's mission management team will meet Sunday afternoon to assess Discovery's readiness to launch, the status of the ECO sensor troubleshooting and whether or not to adopt a three-of-four strategy if sensor No. 4 does, in fact, act up. It's not yet clear whether the MMT has consensus to make an exception to the LCC or whether new NASA Administrator Mike Griffin will go along.
In any case, the sensors will be tested around 7:15 a.m. Tuesday as the astronauts are heading to the launch pad to strap in. The sensors will be tested again during a final hold at the T-minus nine-minute mark.
Engineers believe they have done everything possible to ensure all four sensors will work properly.
"The battery of testing and analysis that we've done so far leads us to believe we are confident that we've got good sensors," Nickolenko said. "The true proof will be when we perform the tanking operation for the launch attempt Tuesday morning. But so far, based on what I understand, we've got good sensor paths, we've got a good point sensor box, we've tested it as exhaustively as we possibly can."
The grounding problems discovered earlier this week were minor, but they were fixed anyway. The resistance across one ground was .2 milli-ohms, Nickolenko said, when the specification called for .1 milli-ohms. Two other grounds measured around .11 and .14 milli-ohms. All three were disconnected, the mating surfaces were sanded and the wires reconnected and bonded in place. Measurements showed all three were back within specifications.
Engineers said earlier this week that even subtle grounding problems could result in electromagnetic interference that might affect the signals to and from the ECO sensors. But again, engineers were not able to duplicate the failure signature and the problem remains an unexplained anomaly.
"We performed the EMI checks and we saw no anomalous indications and after reviewing that data, the troubleshooting team gave us the concurrence to proceed with the electrical ground repairs," Nickolenko said. "We're optimistic we're going to see all good sensors."
See the Status Center for full play-by-play coverage.