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NASA Announces July Launch Window for Next Space Shuttle Mission

15 March 2006

Engineers will remove and replace external tank fuel sensors, agency says

By Cheryl Pellerin
Washington File Staff Writer

Washington – NASA announced March 14 that July 1-19 is the new launch planning window for space shuttle Discovery's mission (STS-121).

The decision to target July followed a two-day meeting on the external fuel tank's engine cutoff (ECO) sensors, said space shuttle program manager Wayne Hale during a press briefing from NASA's Johnson Space Center in Houston.

The sensors indicate whether the tank still has fuel during liftoff.  During testing, one of the four ECO sensors had a slightly different reading than what was expected.  That discrepancy prompted a decision by shuttle officials to remove and replace all four liquid hydrogen sensors.

"We've been saying for months that our engineering work would determine when we fly our next mission,” said Bill Gerstenmaier, NASA associate administrator for space operations. “Targeting July is the right choice in order to make smart decisions."

The sensors also caused problems in July 2005, a week before the space shuttle’s first launch since the 2003 Columbia accident.

The sensors protect an orbiter's main engines by triggering them to shut down safely if fuel runs low unexpectedly. The sensor circuit failed a routine prelaunch check during the countdown July 13, delaying Discovery’s first launch attempt.  (See related article.)

A dozen teams, with hundreds of engineers across the country, worked through a troubleshooting plan to find the source of the intermittent sensor problem.

The original designer of the sensor box in the 1970s came out of retirement to help diagnose the sensor circuit failure.

OTHER FACTORS AFFECTED DECISION

For the current launch, other issues factored into the decision to adjust the STS-121 launch window, including required testing and analysis on the shuttle's modified external tank.

The testing will help verify that the tank is safe to fly without the protuberance air load (PAL) foam ramp, which is designed to prevent unsteady air flow underneath the tank’s cable trays and pressurization lines during launch.

The PAL ramp was removed after a large piece of foam fell from that area during Discovery's July 2005 launch.

More analysis is needed to decide whether changes are needed on the tank's ice frost foam ramps – areas of foam around brackets supporting the pressurization lines that are intended to prevent ice formation before launch.

Another issue affecting timing of the next launch involves repair work that must be completed on the shuttle's robotic arm.  Technicians on a work platform accidentally bumped the arm last week, causing a tiny crack. The arm will be removed for repair.

The STS-121 mission will take shuttle commander Steve Lindsey and six crewmembers to the International Space Station.

This is the second mission in the return-to-flight sequence to evaluate new heat shield inspection and repair techniques and to deliver supplies and equipment to the space station.

Additional information about the space shuttle program is available on the NASA Web site.

(The Washington File is a product of the Bureau of International Information Programs, U.S. Department of State. Web site: http://usinfo.state.gov)