The winged spaceship Columbia, a craft unlike any other ever built, in undergoing its final hours of countdown poised on its launch pad, ready to inaugurate the age of living and working in space.

When it leaves the Earth Friday morning, it will roar away faster than any manned spacecraft had done.

Trailing streaks of flame and clouds of gray smoke, Columbia will clear its 347-foot tower in five seconds, be traveling at the speed of sound in less than 30 seconds, reach an altitude of almost eight miles in one minute and be more than 20 miles high and out of sight in two minutes.

The DC9-sized shuttle craft will burn out its two solid rocket motors two minutes and 11 seconds after liftoff, escape the Earth's gravity and be in orbit in a little more than 10 minutes.

The launch is scheduled for 6:50 a.m. Friday, assuming the weather remains good, as is expected. It will be televised live and watched by millions of Americans.

Columbia is unique because it has wings and a tail for gliding to Earth and wheels for landing on a runway. It will rise from Earth propelled by the two largest solid rocket motors ever built and three liquid hyrdrogen engines producing 420,000 horsepower. It is the first manned spacecraft ever to burn hydrogen, the most powerful liquid fuel, at liftoff.

Reuseability is what the space shuttle is all about. When Columbia's solid rocket motors burn out, they'll parachute into the Atlantic Ocean, where they will be recovered at sea. refueled and reused 20 more times.

The hydrogen engines have been built to be flown no fewer than 55 times, the shuttle craft as often as 60 times. The only thing Columbia will discard when it flies Friday is its huge external fuel tank, which holds the liquid oxygen and hydrogen burned by its three main engines.

By tonight the countdown to a Friday liftoff was on schedule.

Astronauts John W. Young and Robert L. Crippen arrived at Cape Canaveral from Houston's Johnson Space Center just after 2 p.m., flying their T38 jet trainers in formation low over the Kennedy Space Center and landing at nearby Patrick Air Force Base.

Young and Crippen began rehearsing liftoff procedures, then went into semi-seclusion to avoid any contagious disease that could ground them at flight time.

The weather today and the outlook for Friday was better than it had been all week. While today began winding and overcast, the skies quickly cleared as the wind died down. The forecast for Friday was scattered clouds, no rain, southeasterly winds of 10 knots and visibility of at least seven miles.

Columbia's maiden flight will last 54 hours and 36 revolutions of the Earth. In that time, the pilots will do nothing fancy. The purpose of the flight is to see how well Columbia flies into orbit, flies in orbit and flies back from orbit.

In orbit, the astronauts' main task will be to open the doors to Columbia's cargo bay, which measures 60 feet long and 15 feet wide.

Inside the doors are a pair of radiators, whose glycol-filled pipes carry away the heat generated by the shuttle's three big fuel cells and radiate it into space.

If Young and Crippen can't open the doors, they must return to Earth almost immediately. The shuttle has only enough backup ammonia-and-water cooling tanks to make five orbits.

Once they've gotten the doors open, Young and Crippen will keep them open almost their entire time in orbit. They will close them when they're ready to come home, then move to the water and ammonia that will keep them and the shuttle cool during the flight back to Earth.

If for some reason the astronauts can't close the doors, one will don a spacesuit, grab some tools and move into the cargo bay to force the doors closed.

One thing is clear: Young and Crippen could not fly back to Earth with their cargo bay doors wide open. Flying home at 26 times the speed of sound would rip the doors off their hinges, almost surely tearing the shuttle's tail and fuselage heat tiles with it.

Most of the time in orbit will be spent testing Columbia's 44 maneuvering engines, which roll, pitch and yaw the craft in flight. This tests the shuttle's four computers, which steer the craft the entire time it's in orbit.

At re-entry time, Columbia pitches down and begins to fly backward.Two big engines are fired, the craft slows down and begins to fall out of orbit toward Earth.

Over the mid-Pacific, northwest of Hawaii, Columbia re-enters the atmosphere and begins a 4,000-mile glide towards Edwards Air Force Base in California.

Computers navigate the shuttle all the way to Edwards, firing tiny engines and moving the tail and elevons on its wings to keep the course. The only time Young will handle the controls will be to put the wheels down, 90 feet from the runway. The Age of the Shuttle will be be at hand.