Energy & Environment

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15 January 2008
NASA Targets February 7 for Atlantis Launch to Space Station

By Cheryl Pellerin
Staff Writer

Washington -- After two launch delays in December 2007 and weeks of testing and modifying an element of the external tank’s fuel sensor system, NASA has targeted February 7 for the launch of space shuttle Atlantis (STS-122) to the International Space Station.

Atlantis and its crew will deliver to the station the European Space Agency’s (ESA) Columbus laboratory. In mid-March, space shuttle Endeavour (STS-123) will deliver Kibo, the first section of the Japan Aerospace Exploration Agency’s laboratory module, and Dextre, Canada’s new robotics system.

Because the Russian Federal Space Agency is moving the next launch of its Progress space freighter from February 7 to February 5, both shuttle missions will be able to launch before the next Russian Soyuz mission scheduled for April 8.

The Soyuz mission will carry Expedition 17 Commander Sergei Volkov, flight engineer Oleg Kononenko and South Korean spaceflight participant Ko San to the space station.

The new schedule also lets the space station's Expedition 16 crew complete the tasks it trained for, including supporting the launch and docking of Jules Verne, the first ESA automated transfer vehicle.

FALSE READINGS

The shuttle’s December 6, 2007, and December 9, 2007, launches were postponed because of a series of false readings from the shuttle external fuel tank’s engine cutoff sensor system, which indicate whether the tank still has liquid hydrogen fuel during liftoff.

The sensors protect an orbiter's main engines by triggering them to shut down safely if fuel runs low unexpectedly. A false reading could shut the engine down, endangering lives aboard the shuttle.

The sensors also caused problems in March 2006, when the Discovery launch was moved to July, and in July 2005, a week before the shuttle’s first launch since the 2003 Columbia accident. The sensor circuit failed a routine pre-launch check during the countdown July 13, delaying Discovery’s first launch attempt.

At that time, 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 even came out of retirement to help diagnose the sensor circuit failure.

DESIGN FIX

This time, NASA engineers have narrowed down the problem to a feed-through connector that joins the sensors in the liquid hydrogen tank to the shuttle and that tanking and electrical tests targeted as the likely problem.

Inside the connector, 37 metal pins maintain an electrical current by touching metal in the sockets. Engineers believe movement inside the connector causes some of the pins to stop touching the metal, breaking the circuit and causing the flight computers to pick up false signals from the sensors.

The problem is similar to one that occurred in 1994 with the Centaur stage of a Titan-Centaur rocket, said John Shannon, deputy shuttle program manager, during a January 3 briefing at NASA’s Johnson Space Center in Houston.

The “design fix,” Shannon added, was to solder the connecting pins and the socket permanently so the electrical current could not be broken, and the Centaur had no more problems. A similar modification is being made to the feed-through connector.

At Kennedy Space Center in Florida, where Atlantis waits on the launch pad, technicians have installed the replacement feed-through connector in the engine cutoff sensor system and they will examine the connector on the external tank that Endeavour will use for STS-123 in March.

The crew that worked on the modification will not know for sure whether it works until launch day, when the external tank is pumped full of liquid hydrogen, because the connector is designed to work only at those super-cold temperatures.

NASA managers will meet in the coming weeks to address the schedule of shuttle flights beyond STS-123.

More information about space exploration is available at America.gov.

More information about the space shuttle and the International Space Station is available on the NASA Web site.

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