NASA's Shuttle Strategy

This article was written by CBS News Space Consultant Bill Harwood

The shuttle Discovery was cleared for launch Tuesday after senior managers agreed on a strategy that would permit liftoff even if - and only if - the shuttle experiences a fuel sensor problem like the one that grounded the ship July 13.

That strategy, built around the agency's willingness to consider relaxing a long-standing launch rule to get around the fuel sensor problem, has raised eyebrows in some quarters and questions about whether safety is taking a backseat to schedule pressure and "go" fever.

NASA managers insist their approach is based on sound engineering and even though the sensor problem remains an "unexplained anomaly," exhaustive testing has convinced the community the problem is not a generic safety-of-flight issue.

Wayne Hale, chairman of NASA's mission management team, put it like this: "I wake up every day and I ask myself are we pushing too hard, are we doing this thoroughly, have we done the right technical things, have we asked the right people, have we built the tests properly?

"I think we're all still struggling a little bit with the ghost of Columbia and therefore we want to make sure we do it right," Hale said Monday. "Based on the last 10 days worth of effort, the huge number of people and the tremendous number of hours that have been spent in testing and analysis, I think we're coming to the right place."

NASA Administrator Michael Griffin agreed, saying "I've delved into this situation as thoroughly as my intellect permits. And I'm quite comfortable with where we are. ... Even if (a sensor failure) does recur, we're still two-failure tolerant, so it's not a safety of flight issue."

Asked if he was concerned about any public perception that NASA has "launch fever" and was not giving the sensor issue the attention it might deserve, Griffin said, "I think what you want coming out of the Columbia accident and the loss of Columbia and the soul-searching examination that NASA has undertaken since then, what you want of NASA is that we make the right technical decisions, that we do the right thing to the extent that we can figure that out. Which is hard."

"We can't restrict the range of our options to those things which are going to present well. We have to figure out the right thing and try to do that and then work hard to try to explain to you why it is the right thing. And these are rather arcane matters, I would admit. They're rather difficult. And sometimes they don't always present well. (But) you want us doing what's right, not what's necessarily obvious or popular," Griffin said.

NASA hopes to launch Discovery at 10:39 a.m. Tuesday, weather permitting, on a flight to service and resupply the international space station. If the shuttle is not off the ground by July 31 or Aug. 1, the flight will be delayed to September.

The launch window is defined by the space station's orbit, which is tilted 51.6 degrees to the equator. The shuttle has enough power to launch five minutes to either side of the moment Earth's rotation carries the launch pad into the plane of the station's orbit.

The issue is more complicated than usual in Discovery's case because of a NASA-imposed requirement to launch in daylight - and to make sure the external tank separates in daylight half a world away - to ensure good photography of the shuttle.

Given the inherent pressure of a tight launch window, the fuel sensor problem has been particularly vexing.

One of four liquid hydrogen main engine cutoff - ECO - sensors failed to operate properly during a routine pre-launch test July 13, forcing NASA managers to delay the first post-Columbia shuttle flight. The ECO sensors are part of a backup system that ensures the shuttle's main engines shut down normally before the tank runs out of fuel. NASA launch rules require all four ECO sensors to be operating before a countdown can proceed.

Despite exhaustive, around-the-clock tests, engineers were never able to find the problem that caused ECO sensor No. 2 to misbehave. Even so, they believe the testing completed to date proves the problem, whatever the cause, is not a generic defect and as such, they are confident it will not affect the other sensors.

"We have literally run every check that we can think of that people could suggest to us to try to find this problem and so far, no repeat," said Hale. "So we have developed a plan that says we have to go to cryogenic temperature to find out what's going on next.

"We also need to turn on all the equipment in the orbiter and the launch pad area to see if there's any electromagnetic interference that we could not check piece wise earlier. So both of those things together say we are ready to go the launch countdown configuration to see our next level of checking."

NASA's original launch rule required three operational ECO sensors for a countdown to proceed. But in the wake of the 1986 Challenger disaster, the LCC was amended to four-of-four because of concerns two sensors could be knocked out by a single failure in an upstream electronic black box known as a multiplexer-demultiplexer. The single-point failure was corrected during Discovery's last overhaul, but the four-of-four launch rule remained on the books.

And that rule will be in place Tuesday. But the mission management team is prepared to sign an exception to the rule - permitting Discovery to launch with three of four ECO sensors - depending on what happens after the tank is loaded with fuel.

To help isolate the problem, engineers swapped the wiring between ECO sensors 2 and 4. If a problem shows up during fueling with sensor No. 4, engineers will have high confidence the problem is in the sensor itself or somewhere in the wiring between the sensor and an electrical component called a point sensor box. If sensor No. 2 misbehaves, they will have high confidence the problem is in the point sensor box.

Here's the ECO sensor launch strategy at a glance: