Along with its crew and 4.5-million-pound payload, the space shuttle Columbia will be carrying a large part of America's bruised technological reputation aloft on its scheduled takeoff today.

Much of the sting of a publicized string of failures and setbacks, from the Three Mile Island nuclear accident to the breakdown of the rescue helicopters in the Iranian desert to the eclipse of the Americans auto industry by Japanese competition, could be eased by a sucessful flight by the Columbia, the first U.S. manned space flight in five years.

But the development of the most ambitious American space project ever has been haunted by engine fires, cracked turbines, fragile insulating titles, faulty welds, mechanical breakdowns and other engineering mishaps.

Trouble has become the space shuttle's middle name.

The National Aeronautics and Space Administration and Rockwell International, the shuttle's prime contractor, say the bugs have been worked out and the shuttle is safe to fly.

"All in all, it looks like it's turned out darn well," said Noel Hinners, a former top NASA official. "But it will still be a sweaty-plam launch. There's a lot of new stuff on the shuttle."

Unlike the Apollo moon program, which was given a blank check by President Kennedy, the shuttle has had to live hand-to-mouth, sometimes on funds "barrowed" from the next year's budget.

"The program was underfunded at the start and has been underfunded ever since," says Robert Frosch, NASA administrator.

Because the shuttle is required to pay its way by carrying civilian and military payloads into space on repeated launches, the design requirements were far beyond anything that NASA and its contractors had attempted in the moon program.

The student's rocket engines had to produce an unprecedented burst of power, and yet be lightweight and compact to leave more room for the craft's cargo, noted a special National Research Council committee that reviewed the shuttle's problems at the request of Congress three years ago.

"These pumps we are talking about are roughly the size of a trash can, and the amount of energy being released during combustion is equivalent to 5 million horsepower," said Eugene E. Covert of the Massachusetts Institue of Technology, who headed the council committee.

The need for compactness meant that lines carrying the oxygen and hydrogen propellants had to be crowded close together, aggravating the risk of a disastrous fire in case a line or valve failed and the two gases came into contact."

An unprecedented computer system was designed to assure precise operation of the pumps and valves in the main engine, with sensors placed throughout the engine to "read" critical temperatures and pressures. Even so, the engine's development has been marred by breakdowns and fires.

The pressures of tight money and time schedules also forced NASA to abandon the successful approach followed in the moon program of parallel development of alternative designs and technologies.

In building the shuttle, engineers had to commit themselves to a "success-oriented" development strategy which, put bluntly, meant that things had to work the first time.

When they didn't, the result was delays and more expense. In his critique of the program, Covert observed that "managerial pressures build up, particularly when things seem to be going less rapidly than desired. There is a lot of pressure on people to cut corners. . . . All schedules of this kind are built on the expectation of success."

The most famous of the shuttle's snafus involved the 31,000 delicate foamed silicate insulating tiles that form its heat shield. A technological marvel, they disperse heat so rapidly that they can be heated with a blowtorch and handled immediately afterward. But the tiles weren't tested adequately before being manufactured and bonded to the craft. When some proved strain of flight, all had to be checked painstakingly. About one in seven eventually were replaced at considerable expense in time and money.

At another point, NASA discovered that a supplier inadvertently had shipped the wrong kind of solder, raising the possibility of faulty welds throughout the shuttle. All of these welds had to be tested.

"We have so many vendors and suppliers . . . Reaching back and assuring that we have good-quality materials and parts is tough," says Walter Dnakhoff, head of engine programs for NASA.

Robert Anderson, chairman of Rockwell, said recently that, with the successful flight of the shuttle, "Our technological capability will be displayed to the world."