Discovery won't undergo a fueling test on the launch pad next month, NASA officials have decided. Credit: NASA

Shuttle program manager Wayne Hale ordered engineers to make tentative plans for a tanking test earlier this spring as a way to make sure recently replaced engine cutoff - ECO - sensors would work properly on launch day.

The test would not have included any so-called "drag on" instrumentation in the shuttle's aft compartment and its sole purpose would have been to verify the ECO sensors changed state from dry to wet and back again as expected. In the absence of additional instrumentation, no detailed operational insights would be possible.

During a weekly program meeting today, the management team unanimously decided not to run the test, officials said, because any major problems with the ECO sensors almost certainly would preclude a launch in the July window anyway and because loading the tank with supercold propellant would subject its foam insulation to unwanted thermal stress.

Shuttle tanks are certified for 13 fueling cycles. When a countdown proceeds past the point where the tank is pressurized for launch - part of the plan for the June test - it counts as two cycles.

Discovery's launch on the second post-Columbia mission is targeted for July 1. The ECO sensors will be checked, as usual, when the tank is loaded with liquid hydrogen and oxygen on launch day.

The ECO sensors are part of a backup system intended to make sure the shuttle's three main engines don't shut down early or run too long. All four hydrogen ECO sensors are required to be operational for launch, but NASA managers could waive that requirement in certain narrowly defined cases depending on whether a given sensor failed in the wet or dry state.

But any major ECO sensor problems, whether they occurred June 1 or July 1, almost certainly would rule out a launch before the window closes July 19. The engineering community believes all four sensors currently installed will work properly.

Engineers currently plan to move Discovery from its processing hangar to the Vehicle Assembly Building for attachment to the tank and its twin solid-fuel boosters on May 12. Rollout to the launch pad is targeted for May 19.

Rollover could be briefly delayed if engineers decide to correct a subtle timing problem with a recently installed jet thruster control assembly. But with the elimination of the tanking test, the Kennedy Space Center launch team has 17 days of contingency time available to handle unexpected problems.

The engineering community is still assessing the overall safety of the external tank without foam wind deflectors called protuberance air load - PAL - ramps. The ramps were removed following the loss of foam debris from the hydrogen PAL ramp of the tank used by Discovery during the first post-Columbia mission last July.

A final analysis of the tank's ability to withstand aerodynamic buffeting without the ramps in place, based on wind tunnel testing and complex computer modeling, is expected next month.