With the maiden flight of the space shuttle Columbia, the first returnable and reusable spacecraft, the United States will arrive at a crossroads in its space program Friday morning.

For the first time in almost six years, American astronauts are to be spaceborne. Not since three American astronauts flew with two Soviet cosmonauts in the summer of 1975 have Americans been in space. It has been a Soviet province, with no fewer than 43 Russians flying in space since then. a

That was all about to change at 6:50 a.m. Friday. If all went as planned, astronauts John W. Young and Robert L. Crippen were to spend the next 54 1/2 hours 170 miles away from the Earth testing the first spacecraft built to return to Earth like an airplane, a first giant leap toward revolutionizing space travel. Gone forever are the days of the expendable rocket and the single-use spacecraft.

Young and Crippen were scheduled to make 36 revolutions of the Earth doing nothing more taxing than opening and closing the shuttle's cargo bay doors and exercising its 46 maneuvering engines before returning to Earth.

Failure or catastrophe would set the U.S. space program back for years, possibly as severely as it was at the start of the Apollo program 13 years ago, when three astronauts burned to death less than a mile from where Young and Crippen were to board Columbia Friday morning.

"It would probably cost us two years," said John F. Yardley, associate administrator of the National Aeronautics and Space Administration. "But the shuttle will come back from it, and the space program will come back from it."

Nobody was forecasting failure here at the Kennedy Space Center, where the mood was one of tension and expectation. The countdown toward liftoff moved forward without a hitch, the weather was better than it had been all week and crowds estimated in the hundreds of thousands had begun to gather on the beaches to watch a spacecraft that had never flown before rocket into the skies.

"I have a good feeling," Yardley said at a pre-launch news conference. "I have a feeling we're going to go tomorrow."

Making it means the United States will move into a new epoch of the Space Age, where the cost of flying into orbit comes down as the size of the cargo that can be flown into orbit goes up.

The shuttle means the military can make more use of space, flying bigger navigation, communication and surveillance satellites into Earth orbit than they ever did before.

Science also benefits. Among the satellites the shuttle will take into space in the next five years is a 54-foot-long telescope that will peer 10 times deeper into the heavens than the largest telescope on Earth.

Before any of that happens, Young and Crippen must prove that Columbia can do all that NASA trusts it can do. No spacecraft has gone though the tests and trials Columbia has. Its engines are the most tested engines in history. The foamed glass black-and-white tiles that protect the airframe from the heat of reentry have undergone three years of tests and changes that have cost over $300 million.

Columbia is also the most computerized spacecraft ever flown. Young and Crippen weren't scheduled to command it to do anything except put its wheels down at landing Sunday. If the flight goes as planned in ascent, orbital flight and reentry, four computers developed by IBM for the F15 fighter plane will fly Columbia.

They are programmed so that every bit of flight information -- speed, spacecraft attitude, position, altitude -- is fed into all four computers, which will interrogate one another 440 times a second to make sure they're all getting the same numbers. Only one of the four computers will do the flying at any one time. The other three will serve as standbys in case the on-duty computer fails.

The idea behind the four computers is to give Columbia what shuttle engineers call a "fail-operational, fail-safe" condition. That is, if one computer fails, the shuttle can still complete its mission.

Should a second computer fail, the flight is still safe. Young and Crippen can reenter the Earth's atmosphere and glide to a safe landing even with only two computers at work checking each other.

If a third computer fails, the shuttle must come home at once, but it can return safely to Earth on one computer. The reason is that it has a fifth computer on board, never used except in emergencies but programmed to take the craft through the most critical times of ascent and reentry if it is ever called on to do so.

To hear Young tell it, he didn't mind at all that IBM was to do all the flying. Said Young not long ago: "You use a man in the shuttle to manage the computers. Man thinks and anticipates, and no matter how hard you try you can't make a computer anticipate."

Young said Columbia is so complex a flying machine, that a man couldn't fly it by himself. The thruster jets that give it roll, pitch and yaw in flight all point in so many different directions that a man could never remember how many different ways to fire the jets to keep the shuttle on the precise course needed in space.

"There's 38 thrusters on that rascal, and they don't all point orthogonally down," Young said. "Like when you fire a down thruster you don't just get down, you get yaw out of it, too. Only a computer could operate such a device."

By late today, crowds were gathering along the stretch of beach from Cape Canaveral to Melbourne Beach, 40 miles to the south, to witness history.

As preparations went on at the space center, President Reagan sent a message to the two astronauts saying, "You go forward this morning in a daring enterprise, and you take the hopes and prayers of all Americans with you."

The president added, "Through you, today, we all feel as giants once again. Once again we feel the surge of pride that comes from knowing we are the first and we are the best, and we are so because we are free."