Engineers are working around the clock and through the weekend, putting on a "full-court press" to recreate, isolate and eliminate the fuel sensor problem that grounded the shuttle Discovery Wednesday, NASA officials said late today.

Reporters and photographers clamor around NASA officials following today's news conference. Credit: Stephen Clark/Spaceflight Now

If troubleshooters get lucky and find an obvious problem with a quick fix, Discovery could be ready for another launch attempt by late next week, according to deputy shuttle program manager Wayne Hale. If not, mission managers are studying the possibility of extending Discovery's launch window from July 31 to Aug. 4. Either way, the next launch window opens Sept. 9.

"We are not in any sense of the word becoming pessimistic about making the July launch window," Hale said. "We are here for the duration, we're committed to giving this the good old college try until we get the problem resolved."

Discovery was grounded Wednesday when one of four hydrogen fuel sensors at the bottom of the shuttle's external tank failed a pre-flight test. The sensors serve as a critical safeguard in the event of other problems that could cause a main engine to shut down early or burn too long. All four must be operational for a countdown to proceed.

NASA ran into problems with the engine cutoff - ECO - sensors and associated electronics earlier this year, trigging extensive tests and troubleshooting (see below for details about sensor operation and logic). Engineers never succeeded in duplicating the problem or identifying what caused it. Instead, they replaced virtually all of the cables, connectors and the so-called point sensor box in the shuttle's engine compartment that relays sensor data to the ship's computers.

Still, NASA went into Discovery's countdown with the sensor problem characterized as an "unexplained anomaly." As the countdown cutoff demonstrated, the problem was aptly named (see below for a chronology of the ECO sensor issue with Discovery).

John Muratore, a senior manager and troubleshooter based at the Johnson Space Center in Houston, said 12 teams of experts are studying the problem "very diligently to try to understand what are the set of conditions that are causing this problem so we can recreate it, isolate it and eliminate it from the system."

"To do that, we have 12 teams of people working across the country, they're going to work all weekend," he said. "We have engineering tag-ups scheduled Saturday, Sunday, Monday, and we're planning to go basically every day until we have got the problem identified and isolated and a solution in work."

Discovery's next launch opportunity is not known. Credit: NASA-KSC

Asked if he was optimistic about a quick fix in light of the team's failure to solve the original sensor problem, Hale said he remains "very hopeful because we're taking this troubleshooting to a significantly higher level than we took it the first time."

"The first time through, we didn't involve nearly as many folks, we didn't have the data as well coordinated as we have it now," he said. "I feel very confident that we will get a solution to our problems. Now it's not out of the woods yet, I don't want to mislead you. But I'm very hopeful."

Over the weekend, engineers plan to re-enter the shuttle's aft engine compartment to gain access to avionics bay No. 5 where Discovery's point sensor box is located. Wiring will be inspected and various connectors will be checked to make sure the system's electrical continuity is intact and there are no obvious shorts. They also plan to use a test device to simulate sensor data to gain additional insights into the behavior of the point sensor box electronics.

"We are putting a full-court press on this to resolve this anomaly," Hale said.

The ECO sensor system is ingenious, complex and until this point, very reliable. Here's a brief overview:

ECO sensor background

Twenty four propellant sensors are used in the shuttle's external tank, 12 each in the oxygen and hydrogen sections. Eight are used in each tank to measure the amount of propellant present before launch. Four in each tank, known as engine cutoff - ECO - sensors, are part of a backup system intended to make sure the ship's engines don't shut down too early, resulting in an abort, or run too long, draining the tank dry with potentially catastrophic results. All four ECO sensors in each tank are required to be operational for a countdown to proceed.

The hydrogen ECO sensors are located at the very bottom of the tank near the entrance to the pipe that carries hydrogen into the shuttle's engine compartment.

The cutoff sensors are armed late in the ascent when a relatively small amount of rocket fuel remains in the tank. Once armed, the shuttle's computer system checks the status of each sensor, which is still immersed in cryogenic propellant, to make sure it is "wet." To protect against a faulty sensor, the first "dry" indication from any one of them is discarded.

During normal operations, the shuttle's flight computers continuously calculate the orbiter's position and velocity, using that data to figure out when the engines should be shut down to achieve the desired target. As a backup, the computers also monitor the ECO sensors as the tank empties to protect against unexpected problems that might affect the performance of the propulsion system.

The shuttle is launched with more fuel than it needs and in normal operation, the ECO sensors would never be "dry" before the normal guidance-based engine shutdown sequence begins. But if a problem does occur, and the computers detect two "dry" sensors, they will shut the engines down to keep from running the tank dry. As long as at least three sensors indicate "wet," however, fuel is assumed to be in the tank and the engines will keep running.

Once the system is armed, two sensors must fail "dry" to trigger an inadvertent engine shutdown. Before arming, three sensors must fail "dry." If three sensors fail "wet," the engines could run the tank empty.

The odds of such multiple failures are "extremely remote," according to internal NASA documents describing earli er problems. In fact, no cutoff sensors have failed in flight since the sixth shuttle mission in 1983 when the design was changed.

But the consequences of an early or late engine shutdown are extreme. A premature shutdown could prevent a crew from reaching orbit while a late shutdown could result in an engine fire or explosion. Even though the cutoff sensor system is considered a backup to the shuttle's flight computers, NASA's launch commit criteria require four operational cutoff sensors in each tank to provide multiple layers of redundancy.

The engine cutoff sensor system has been put to the test only two times in the history of the shuttle program.

During the shuttle Challenger's launching July 29, 1985, on mission STS-51F, a main engine shut down five minutes and 43 seconds after blastoff because of an internal temperature sensor failure. The fuel consumption of the two engines that kept running was affected and the end result was an ECO sensor engine cutoff.

The only other such shutdown in shuttle history occurred during Discovery commander Eileen Collins' last flight, mission STS-93, when a hydrogen leak in the coolant tubes making up main engine No. 3's nozzle caused more oxygen to be consumed than expected. In that case, oxygen ECO sensors went "dry," triggering engine shutdown.

In both cases, the shutdowns happened late in the ascents and both shuttle crews were able to complete their missions (Challenger's crew ended up in a lower-than-planned orbit due to the.

At today's briefing, Hale was asked why NASA required four operational sensors at launch given the seemingly small chance that multiple failures could occur on any given flight.

"Going down the logic path, one of our safety requirements on this vehicle is that we are two-fault tolerant in our electronics," he said. "We can take two failures and can continue to keep on flying safely. And anytime you step away from that standard, you incur risk and you'd better make sure you have an air-tight story to step away from that posture. If we get to the end of all this troubleshooting and everything's working fine, we may come around to the discussion of what if. But we're not ready to go there yet."

Here is a brief chronology of the problems encountered during tests of Discovery's ECO sensor system:

Discovery ECO sensor chronology

NASA has encountered a string of problems in recent weeks with the ECO sensor system in Discovery, glitches that have proven to be surprisingly difficult to resolve. The problems began during a tanking test in April when ECO sensors 3 and 4 operated intermittently.

Engineers removed an electronic controller, called a point sensor box, from Discovery and replaced wiring to the two sensors in question (wiring to sensors 1 and 2 wasn't touched). But the controller checked out OK and troubleshooters were unable to trace the cause of the problem A point sensor box from the shuttle Atlantis was installed and a second fueling test was conducted.

This time around, the sensors worked normally. But during additional post-test troubleshooting, the replacement sensor controller box malfunctioned. It was replaced by one taken from the shuttle Endeavour. NASA already had decided to replace Discovery's tank to address ice debris issues. With a fresh controller, replacement cabling, a new tank and solid test results, NASA managers decided to treat the sensor issue as an "unexplained anomaly" that presumably had been fixed.

But during Wednesday's countdown, the No. 2 low-level hydrogen sensor failed to switch from "wet" to "dry" during a test in which computers send signals to simulate a dry tank. When the tank was drained, the other three sensors changed from wet to dry as expected. The No. 2 sensor remained "wet" for another three hours before switching back to "dry."

A major concern for NASA is whether the problem affecting Discovery is generic or an isolated issue. The shuttle Atlantis is being prepared for launch in September and it also will serve as a rescue craft in case of problems with Discovery once it reaches orbit. The status of its cutoff sensor system is equally critical.

"Until you understand what the problem is, the potential exists for a generic or fleet-wide problem," Hale said. "That's one of the reasons why we want to resolve this, not just fix the particular problem that happened with the tank and orbiter combination we've got."

Said Muratore: "The bottom line is, we don't know if we're having a problem in the tank, we're having a problem in the wiring, we're having a problem in the electronics box."

"We don't know if the equipment is fine and it's just the environment that we're operating in is somehow subtly different, or we don't know if there's a problem in the equipment," he said. "And until we know that, everything is suspect. When we clear items by test and analysis, then we'll move on and decide whether we've got to deal with problems on other vehicles or if it's just limited to this one vehicle."

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