By Ed Stoddard and Irene Klotz

HOUSTON, March 20 (Reuters) - A pair of astronauts ended a spacewalk late on Thursday in which they tested a repair procedure for the heat shields on the space shuttle fleet -- a technique NASA hopes it never needs to use.

"I think it went really, really well," astronaut Rick Linnehan, who was overseeing the spacewalk from inside the shuttle Endeavour, radioed to spacewalkers Mike Foreman and Robert Behnken after they finished the testing.

During the over six-hour outing the pair also replaced a faulty circuit breaker and removed a thermal sock from the station's new Canadian-built handyman robot, which was no longer needed since it now has power from the station.

The heat shield test -- part of a strategy to improve shuttle safety -- was the highlight of the fourth of five spacewalks scheduled during Endeavour's busy 12-day repair and construction visit to the International Space Station.

Since the shuttle Columbia's demise five years ago due to heat shield failure, NASA has devised damage prevention and repair procedures to give future shuttle crews a better chance of surviving a similar accident.

Columbia's seven astronauts were killed as it broke apart during its return to Earth for landing.

Using a tool that is like a high-tech caulk-gun, the spacewalkers filled gouged tiles with a gooey gel. Some tiles were intentionally damaged while others were unintentionally broken during previous space flights.

"Mr. Goo, you're in control today," Linnehan said as the spacewalkers set up equipment for the tile repair tests.

Engineers are concerned to see if the goo will bubble up inside, creating a lip around a repair that could trigger excessive heating as the shuttle plunges through the atmosphere on its way back to Earth.

The patched tiles will be packed up on the shuttle and returned to Earth for analysis.

NASA wants to have the results before dispatching a shuttle crew to work on the Hubble Space Telescope later this year, as the space station will be too far away to shelter the crew if their ship is too damaged to return home.

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