If space shuttle astronauts John W. Young and Robert L. Crippen have to abort their flight Friday, they will be banking on a pair of ejection seats that have never been used before.

The seats are the same ones the Air Force uses in its supersecret SR71 spy plane. They are built to work at altitudes up to 100,000 feet and velocities up to three times the speed of sound.

As far as anyone knows, no SR71 pilot has ever had to use his ejection seat. More troublesome, the seats are not built to eject Young and Crippen on the launch pad, when they could need them the most.

If Young and Crippen have to eject from the launch pad, they will be propelled straight back about 200 feet at a height of no more than 300 feet. The question: would the ejection seat's parachute inflate fully enough at that height?

"They're far more likely to abort off the launch pad -- crawling out the escape hatch and using the slide wire to the ground," an Air Force spokesman said.

Should Young and Crippen have to trigger their ejection seats in the first few seconds of flight, they'd stand a better chance of surviving.

The seats are good for the first 90 seconds, when Columbia will have reached an altitude of almost 100,000 feet and a speed of more than 2,000 mph. Beyond that, Columbia will be too high and moving too fast for Crippen and Young to use the ejection seats.

If they must abort in the next few minutes, the astronauts must separate Columbia from the huge external fuel tank, turn it around and land on the three-mile-long runway at Kennedy Space Center.

Later on, they can drive the shuttle into a low Earth orbit and begin a maneuver to bring them down at one of six emergency runways scattered around the Earth.