With as many as a million people watching from the beaches, astronauts Jack R. Lousma and C. Gordon Fullerton rode the space shuttle Columbia into earth orbit today on its third flight in the last 11 months.

Trailing 600 feet of flame into bright blue skies, Columbia began its most trouble-free flight since its maiden orbital voyage last April. The only snag was its hour-late takeoff at 11 a.m., hardly a sign of serious trouble in a 100-ton space plane still being test flown.

"If we'd been on our toes, we could have gone a little sooner than we did," Shuttle Operations Director George F. Page said in a deadpan voice at Kennedy Space Center after Columbia left the earth. "But I'm real pleased with the performance of the launch team."

Lousma, 46, and Fullerton, 45, were just as pleased. Fullerton is making his first space flight and Lousma his second, although his first as commander. Although not talkative, Lousma and Fullerton were no less enthusiastic than the 46 other American astronauts who have preceded them in space.

"The first part of this ride was a barn burner," Lousma said as the delta-winged spacecraft sped upward and his heartbeat rose to 132 beats a minute. Three hours later, Lousma looked down on his home state of Michigan and marveled at the view he beamed back with a television camera aimed through the shuttle's open cargo bay doors.

"Looks like we've got some snow up there," Lousma said, looking through the shuttle's towering tail at snow-covered land east of Chicago. "But while things look a little chilly in Michigan today, I want to wish all my home-state folks who didn't get to the launch a warm welcome."

Speeding across the United States at five miles a second, Lousma and Fullerton flew across the eastern half of the country and televised a cloudless view of the Delmarva Peninsula, the Chesapeake Bay and the coastline from the Outer Banks of North Carolina to the shores north of Cape May, N.J.

"That's a spectacular sight," astronaut Terry Hart said from the Mission Control Center in Houston. "You can see forever."

If things keep moving along as free of trouble as they did today, Lousma and Fullerton will spend seven days in space--twice as long as the first two flights combined. The first lasted a little more than two days last April, and the second was cut to three days after a fuel cell failed.

Making the third of four planned test flights, Lousma and Fullerton will push the space plane through a demanding week of tests that will expose the spacecraft's tail, nose and cargo bay to the sun for hours at a time. The payload bay will be open to the sun for 26 hours, the tail for 30 hours and the nose for 80 hours.

On their eighth orbit tonight, the astronauts turned Columbia's tail toward the sun, then began their first eight-hour sleep.

The point of the turn is to see how Columbia behaves as its temperature rises and falls from 250 degrees Fahrenheit to 215 degrees below zero. A key test is to restart the engines, which are located at the rear of the craft, both after they've been heated and chilled to determine their reaction to the temperature extremes.

Another test is to expose the 65-foot-long cargo bay, which is carrying 21,000 pounds of instruments and experiments, to the sun to find out how much heat the instruments can take and how the experiments respond to it.

The shuttle will land at the White Sands Missile Range in southern New Mexico instead of California's Edwards Air Force Base. The Mojave Desert lake beds that are the runways at Edwards are still quagmires of mud after being flooded by heavy spring rains almost 10 days ago.

As they flew across California for the first time near the start of their second orbit, Lousma and Fullerton could see the lake-bed runway where they would have landed.

Said Lousma: "I think we did the right thing. It looks like you could see a lot of water on those lakes."

Today's launch was delayed an hour because of what appeared to be a human error. A tank that feeds liquid nitrogen through a heater that warms it into a gas to purge some of the lines feeding fuel to Columbia failed early in the morning, apparently because someone failed to throw the switch that turns on the heaters.

The countdown was stopped and the launch moved back from 10 a.m. to 11 a.m. while technicians went to the launch pad to throw the right switch.

Seven to eight minutes after Columbia left its pad, one of the power units that swivels the rocket engines, moves its elevons and guides its tail rudder heated up unexpectedly and had to be turned off. Since the shuttle had two other working hydraulic power units, the crew was told to press on into space.

Later, the unit cooled down so well that flight directors dismissed the trouble as a temporary aberration. All three power units were turned off in orbit since none will be needed again until Columbia returns to earth Monday.