Shuttle set for crucial test, then launch

Alan Boyle Science editor • Profile • E-mail

CAPE CANAVERAL, Fla. - For the second time in two weeks, the space shuttle Discovery is ready for launch to the international space station, on the first shuttle mission since the Columbia disaster two and a half years ago.

It may seem like deja vu all over again, but there's one big difference for Tuesday's launch attempt: On a flight designed to signal NASA's heightened commitment to safety, mission managers are geared up to grant a launch-pad exception to its rules, for the first time in recent memory.

The exception would clear Discovery for launch if a fuel-gauge glitch that arose during the previous countdown turns up again. The last time, on July 13, bad readings from one of the fuel sensors forced a postponement of liftoff, setting off a full-bore effort by hundreds of engineers to track down the glitch's cause.

They narrowed down the cause to grounding problems in the circuitry and electromagnetic interference — but in the end, they could only assume that they had resolved the problem.

"We've done an extensive degree of troubleshooting and analysis ... to really as best we can understand what we've got," test director Pete Nickolenko told reporters Monday at NASA's Kennedy Space Center. "I believe that we're ready, and I feel very confident going into the terminal countdown that the entire team's ready to execute this mission. And we fully expect that it should work as designed."

Nickolenko said there were "no significant issues or problems" being worked, and preparations are on track for launch at 10:39 a.m. ET Tuesday.

But first, Discovery has to pass a crucial test beginning in the wee hours of Tuesday morning, soon after the launch team starts loading the external fuel tank with liquid hydrogen and oxygen. Mission managers admit there's a chance that the fuel-gauge glitch might arise only when the tank contains the superchilled propellants — and so they'll start duplicating the conditions that set off the glitch almost as soon as the liquid hydrogen comes up to the sensors' level.

Last time, one of the four low-level sensors appeared to give bad readings during a simulated "empty" condition. Under NASA's current rules, all four sensors have to work in order for launch to proceed — even though the sensor system is designed to work with just two good sensors.

FREE VIDEO Launch worries July 25: Some engineers are concerned about the circumstances of the rescheduled shuttle launch, NBC's Jay Barbree reports. MSNBC

This time, the connection to the questionable sensor, known as No. 2, has been swapped to a good sensor, No. 4. And this time, the launch team will simulate the "empty" or "dry" condition for most of the time that the tank is filled.

If either the No. 2 or the No. 4 sensor registers bad readings in the simulation — that is, "wet" when it should read "dry" — mission managers will be able to narrow down the problem further. In fact, deputy shuttle program manager Wayne Hale said "some of us are kind of hoping" that the glitch reappears, so that engineers can figure out definitively whether it lies in the shuttle's circuitry or the tank's wiring.

With a single glitch, the managers are prepared to sign a waiver and allow the countdown to go forward with three good sensors, Nickolenko said. But they'd hold up the launch if other, unexpected problems crop up — for example, if both No. 2 and No. 4 show bad readings, or if the other sensors exhibit problems.

Nickolenko said he couldn't recall a time when a waiver had been granted under such conditions on launch day, and NASA representatives said they were reviewing their records to find out whether there was any precedent.

MORE FROM HUMAN SPACEFLIGHT

Human Spaceflight Section Front

Space station trio lands safely in Kazakhstan Crew shrinks to two on space station Shuttle fliers land with families on their minds Astronauts surprised by holiday turkey dinners New dad-in-space: Focusing on shuttle job easy Plan for human mission to asteroid gains speed Space shuttle lifts off amid tons of tweets Countdown begins for NASA's uncertain future Space hotel says it's on schedule to open in 2012 Space clown comes back down to Earth Human Spaceflight Section Front   Add Human Spaceflight headlines to your news reader: