NASA managers today ruled out a third tanking test for the shuttle Discovery, keeping launch of the first post-Columbia mission on target for July 13. The launch window extends to July 31 and as of this writing, engineers have five days of contingency time in the launch processing schedule to handle unexpected problems between now and the opening of the window.

Space shuttle Discovery is lowered to the ground after being removed from its original external tank and solid rocket boosters. Credit: NASA-KSC

Still to come: final disposition of questions about the risk posed by ice debris during launch; a final report by the Stafford-Covey Return to Flight Task Group regarding NASA's implementation of post-Columbia safety upgrades; and a formal flight readiness review to clear Discovery for flight.

The debris verification review, which will quantify the threat posed by ice debris based on a flurry of recent tests and extensive analysis, is targeted for June 24, followed by the Stafford-Covey group's final meeting and the two-day flight readiness review, which will be held at the Kennedy Space Center.

Among the debris threats still on the table is the question of possible ice buildups around brackets near the top of the external tank.

Assuming no show stoppers emerge during the meetings later this month, commander Eileen Collins and her crew will fly to Kennedy July 9 for the start of the countdown to blastoff of the 114th shuttle mission. Liftoff July 13 is targeted for 3:51 p.m. EDT (1951 GMT).

NASA had hoped to launch Discovery in May, but problems cropped up during a fueling test April 14 that contributed to a decision to delay the flight to mid July. During that test, two of four liquid hydrogen fuel depletion sensors (also known as engine cutoff sensors) failed to operate properly and a pressure relief valve that helps maintain hydrogen tank pressurization in the final two minutes of the countdown cycled more often than usual.

At the same time, the ongoing ice debris verification review raised questions about potentially dangerous buildups of ice around a liquid oxygen feedline bellows assembly that could shake off during launch and damage the shuttle's fragile heat shield tiles or wing leading edge panels.

NASA managers ultimately decided to delay launch and to move Discovery to a different set of boosters and a modified external tank equipped with a heater on the feedline bellows to prevent ice formation. Engineers are still debating the threat posed by ice around brackets at the top of the tank.

The new tank also features an older-style single-screen diffuser, which injects a jet of helium gas into the hydrogen tank to help keep the supercold fuel circulating at the proper temperature. It also provides the pressurization needed after the tank is isolated from ground systems one minute 52 seconds prior to launch.

Engineers believe the unusual valve cycling during the April 14 tanking test was due to a newer style dual-screen diffuser. During the April 14 test and a second tanking test May 20, the valve cycled 13 times. Based on past experience, it was expected to cycle eight or nine times.

By switching back to the older single-screen diffuser, engineers are confident the problem will not recur during the launch countdown. But testing continues to make sure.

As for the engine cutoff sensors, NASA managers now believe the intermittent operation in April most likely was do to a wiring issue that was resolved during troubleshooting. The sensors worked properly during the May 20 tanking test and engineers believe the sensors in Discovery's new tank will behave normally as well.