For the first time in almost six years, the roar of rocket engines carrying American astronauts into space will rock the beaches of Florida's Cape Canaveral.

Three years late and almost $2 billion over cost, the space shuttle Columbia is finally ready to fly. Shuttle astronauts John W. Young and Robert L. Crippen have been ready for months. A veteran of four space flights and two trips to the moon, Young is so impatient to fly that one gets the feeling he'd take off in a hurricane next week to get spaceborne.

"I figure we'll be 130 percent trained by the time we fly," the 50-year-old Young said in a recent interview. "I've never been this trained before. I mean, all the missions I've been on we launched before we finished our training."

The spacecraft Young and Crippen will fly next week is like none ever built. The size of a DC9 jetliner, Columbia is the first spacecraft to have wings and a tail. Like Mercury, Gemini and Apollo before it, Columbia will streak away from the Earth straight up like a rocket. Unlike any spacecraft before it, Columbia will return to Earth and land on a runway like an airplane.

Developed at a cost of almost $10 billion, Columbia puts the United States squarely into a new epoch of the Space Age. No longer will manned space flights cost more than $100 million apiece; each trip Columbia takes into orbit will cost $32 million, no matter how much or how little it carries.

The reason: Its engines will be recovered after launch to be reused and the spacecraft will return to Earth the same way it left, undamaged by the heat of reentry and the salt water of an ocean landing that left Mercury, Gemini and Apollo unusable at the end of a flight.

The shuttle does more than lower the cost of space flight. It provides the United States with the first real space cargo carrier. The shuttle can carry satellites of all kinds and sizes into space or pick them up in space and bring them home. The shuttle's cargo bay can carry 65,000 pounds into orbit and bring 32,000 pounds back to earth.

The shuttle more than doubles spacecraft crew size. Mercury carried one astronaut, Gemini two and Apollo three. While next week's maiden test flight will be flown by just two crewmen, Columbia has room for as many as seven. Future crews won't even have to be astronauts.Scientists, scholars, statesmen and even journalists will make shuttle flights.

"The Apollo adventure to the moon was like the first covered wagons making it to California," said Robert Thompson, space shuttle manager at the Johnston Space Center in Houston. "The shuttle is more like putting the first transcontinental railroad across the country or flying the DC3 for the first time, which is, after all, what opened things up to airline travel."

While it may be a giant leap for mankind, Columbia's flight around the Earth next week will be measured in baby steps. Young and Crippen will say in orbit for only 54 hours, carry no satellites into space and bring none back with them. The flight is an engineering test flight; the astronauts will go up, test their maneuvering engines, their guidance computers and cargo bay doors in orbit and come back to Earth.

"As far as John and I are concerned, if we get up and down that's a successful mission," the 43-year-old Crippen said last month. "The main thing is to find out if any problems exist with the shuttle so we can put it into an operational state as soon as possible."

Three more flights are planned for Columbia, the next one coming late in September and the last two in the first half of next year. Each flight will last longer than the one before and make more demands of the shuttle crew. The shuttle goes operational in late 1982, when Columbia will be joined by a sister ship named Challenger.

Columbia and Challenger will fly seven missions in 1983, when a third shuttle named Discovery will join the fleet. As many as 17 flights will be made in 1984 and 24 in 1985 when a fourth shuttle named Atlantis comes into service. No fewer than 30 trips into orbit will be flown in 1986. Though the shuttle can stay in orbit for as long as a month, almost all its flights will last no longer than a week.

Two grant spaceports are being built at either end of the United States to handle all this traffic. The one at Kennedy Space Center on the Florida coast, consisting of a launching site and a three-mile-long landing strip, is just about completed. The second is under construction at California's Vandenberg Air Force Base and is due to be ready for the 24th shuttle flight in June 1984, when the shuttle will be flown in polar orbit for the first time.

Space missions cannot be flown around the Earth in a north-south direction from Cape Canaveral because they would fly over too many large cities before reaching orbit, a somewhat dangerous undertaking. North-south flights can be made from Vandenber, because the flight pattern reaches so far out into the Pacific Ocean that spacecraft would not fly over populated regions on their way into polar orbit.

The advantage of polar orbit is military. Any satellite in a polar orbit flies over every spot on the Earth as the Earth turns underneath it. A shuttle launched from Vandenberg into polar orbit quickly reaches the Soviet Union and flies over almost its entire land mass in a few days.

The space shuttle marks the first time that the civilian space agency will be working hand-in-glove with the Pentagon. All the Air Force's satellite traffic will be carried into space by the shuttle, its communications and navigation satellites, its weather satellites and its surveillance satellites. No fewer than 14 Pentagon satellites will be brought into space by the shuttle in the next four years, some of them as large as 23 tons.

The National Aeronautics and Space Administration and the Pentagon haven't always been on such good terms over the shuttle, which had little Air Force support until the Pentagon realized it needed the shuttle to carry the surveillance satellites to verify the SALT treaties. The Air Force has bickered constantly with NASA over the shuttle's size and range, where it would take off and land, how secret its operations should be and even who should be in charge of shuttle operations.

"Since the beginning of the space program, there's always been a bunch of guys in the Air Force saying it was theirs," former administrator Robert A. Frosch said just before he left NASA last January. "They're still saying it."

The Air Force aside, the shuttle will be the mainstay of civilian space traffic for years to come. In the next four years, the shuttle will carry communications satellites into orbit for Canada, Indonesia, Intelsat, RCA, Saudi Arabia, Bell Telephone and the People's Republic of China. The shuttle will also drop off in earth orbit a spacecraft that will fly around the north and south poles of the sun, another that will fly to Jupiter and a 54-foot-long telescope that will see 10 times farther into the heavens than the largest telescope on Earth.

So busy are scheduled shuttle operations in the years ahead that the first 52 shuttle flights are booked solid with satellite traffic, most of it civilian.At least half the cargo space for the next 16 flights are booked, which brings NASA out to 1986.

Sen. Harrison H. Schmitt (R-N.M.), the onetime Apollo astronaut who is now chairman of the Senate subcommittee on space, says he sees the day when the shuttle is so busy the nation will need a fleet of eight, four painted blue for the Air Force and four painted white for NASA. Says Schmitt: "This machine can serve us well for the rest of this century. All we have to do is decide we want to use it."