NASA managers today decided to swap out engine cutoff - ECO - sensors inside the liquid hydrogen section of an external fuel tank slated for use with the shuttle Atlantis in late August. Because of lingering questions about subtle failure modes, engineers at the Kennedy Space Center earlier installed a fresh set of hydrogen ECO sensors in the tank that will be used on the next flight in July.

Technicians work around the manhole cover on the bottom of Discovery's external tank after replacing suspect ECO sensors in April. Credit: NASA-KSC

Engineers were never able to determine the exact cause of those earlier problems, and many favored changing a launch rule to permit a flight if three of the four sensors were working properly. While that might seem like an undue risk, the redundancy built into the system assures safe operation in certain narrowly defined failure scenarios.

But in a bit of a surprise, shuttle program manager Wayne Hale today decided to stick with the four-of-four ECO sensor launch rule, meaning Discovery will be delayed if one or more sensors misbehaves when the tank is loaded with fuel on launch day.

The launch team left open the possibility of revisiting the three-of-four rationale if a failure is actually observed. A similar rationale was in put in place before Discovery's launch last July on the first post-Columbia mission, but the sensors worked normally and the rule was never put to the test.

In any case, Hale told the assembled engineers and managers today that he is confident the carefully screened ECO sensors recently installed in external tank 119 will work properly.

"We have a great deal of confidence we're not going to see any issues," an official said (a detailed description of ECO sensor location and operation is available here).

Now, based on today's discussion, ET-118, the tank that will be used by Atlantis in late August, will get a new set of ECO sensors as well. The tank is scheduled to be shipped from Lockheed Martin's Michoud Assembly Facility near New Orleans on May 30, arriving at the Kennedy Space Center June 5. The new sensors will be installed at Kennedy, although engineers have not yet decided whether to do the invasive work with the tank in a horizontal or vertical orientation.

NASA managers are protecting the option of launching rescue flights for the next two missions. Assuming Discovery takes off July 1 as currently planned, a "launch-on-need" rescue by Atlantis, using ET-118, could be staged as early as Aug. 11. If Atlantis takes off Aug. 29, a rescue flight by Discovery could be launched around Nov. 11 using the next tank in the sequence, ET-123.

But because resources have been focused on getting the next two tanks in the sequence ready for flight, preparations have lagged for ET-123. Current processing schedules show shipment to Florida could be delayed from around Aug. 12 to as late as Sept. 17.

It typically takes about three months to process a tank for launch at Kennedy but the external tank project recently concluded a 62-day flow is achievable. If ET-123 really is delayed to mid September, processing for a Nov. 11 rescue flight would end up, on paper at least, being about a week down.

Hale today told external tank managers to "scrub" the schedule as much as possible to make up for lost time and offered help from other centers if needed.

The concern here is that if Atlantis' crew ran into problems that forced the astronauts to seek "safe haven" aboard the international space station, ET-123 might not be ready to support a rescue flight before station supplies started running short.

This all assumes that Discovery could be "turned around" after its July flight in time to support a rescue mission.

The launch window for mission STS-115 - the fight after Discovery's - opens Aug. 29 and closes Sept. 13. The next window opens Oct. 26 and closes Oct. 29. Those launch windows are based on a requirement to launch the next two flights in daylight and to have good orbital lighting to photograph the external tank after separation half a world away.

Another constraint is the angle between the sun and the plane of the station's orbit, which determines how hot or cold the shuttle-station combination will get during docked operations. Because of payload temperature constraints, NASA may only have two launch days in October due to beta angle issues if Atlantis misses the August-September window.

Foam insulation issues aside, the next two external tanks in the sequence - ET-119 and ET-118 - should be ready to support launchings in July and late August or early September, although the ECO sensor swap out planned for ET-118 will be difficult. More important, both launch dates assume NASA ultimately concludes the decision late last year to remove long foam air deflectors from the tank was sound and that external fittings are tough enough to stand up to increased aerodynamic buffeting.

But any major delays getting ET-123 ready to support a rescue mission could prevent NASA from launching Atlantis in the August-September window. While the schedule is currently out of synch, NASA managers are hopeful they can pull it back in when all is said and done.

Additional coverage for subscribers:
VIDEO: EXTERNAL TANK OPENED FOR SENSOR REPLACEMENT PLAY
VIDEO: EXTERNAL TANK HOISTED OUT OF CHECKOUT BAY PLAY
VIDEO: FOAM REAPPLIED TO EXTERNAL TANK ACCESS PORT PLAY
VIDEO: EXTERNAL TANK MATED TO SOLID ROCKET BOOSTERS PLAY
VIDEO: PAYLOAD BAY DOORS CLOSED FOR HANGAR DEPARTURE PLAY
MORE: STS-121 VIDEO COVERAGE
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