With the seventh flight of the space shuttle poised for takeoff Saturday, several large aerospace companies are preparing to launch programs that could compete with the reusable space vehicle.

Reacting to both the shuttle's crowded launch schedule and Europe's Ariane rocket-booster program, the companies are gearing up to offer to commercial customers the boosters that have been the workhorses of the government-sponsored space age.

These boosters, known collectively as expendable launch vehicles (ELV), differ significantly from the shuttle: they cannot be reused.

The companies in the best position to benefit from the new industry include Martin Marietta Corp., which makes the Titan booster; General Dynamics Corp., maker of the Atlas-Centaur, and McDonnell Douglas Corp., which produces the Delta. Based on military designs more than a quarter-century old, these boosters have been updated several times and have launched hundreds of satellites. Another possible player is Space Services Inc., a small Texas company that is testing an entirely new booster called the Conestoga.

The companies' plans got a big boost a month ago when the Reagan administration said it would encourage private industry to build and operate rockets for commercial use, and indicated that it would make government facilities such as the Kennedy Space Center and Vandenberg Air Force Base available for launches.

Ironically, the shuttle originally was intended to render ELVs obsolete, by providing a cheap, reusable launch vehicle. But delays in the shuttle program and increased demand for launching capability caused the government to give in to the aerospace industry's request for permission to offer their boosters to commercial customers.

Previously, the government bought and operated boosters, even when they launched private payloads.

"Partnership between the U.S. private sector and the U.S. government will strengthen the U.S space launch capability, develop a major new industry, contribute favorably to the U.S. economy and maintain U.S. leadership in space transportation," the White House said in a statement endorsing the concept.

But analysts are more down to earth in their assessment of the idea. They say the market for commercial rockets may not develop for years, and could be limited at best.

Even if it were successful, they add, it could put business in competition with the government's space shuttle program, an uncomfortable position for the big aerospace companies that would be producing the shuttle's competition at the same time they were selling shuttle equipment to the government.

"I think it still has to be demonstrated how large the market for commercial launches will be," says John Logsdon, a space policy expert on leave from George Washington University.

"It's a market that's developing, and nobody's quite sure how you compete in it," says Paul Nisbet, an aerospace-industry analyst at Bache Halsey Stuart Shields. "It is touchy as to how aggressively they pursue the business if it means taking business away from NASA."

Some analysts see the possibility of stiff competition for payloads between NASA's shuttle program and the commercial rocket companies. The ELV-makers could hold a slight cost advantage over the shuttle, and improve their edge if technical problems delay shuttle launches, forcing satellite operators to look elsewhere for a way to get into space. But analysts say the shuttle seems to be proving itself as a practical, reliable space vehicle.

Still, the companies are putting up "Rocket for Sale" signs. Martin Marietta, for instance, already has put out a slick, full-color brochure extolling the virtues of the latest version of the Titan booster, the model 34B.

Many satellite companies already have indicated an interest in the Ariane program, and the government's commitment to private ELVs is seen by some experts as a way to keep the European program from cutting into America's space leadership. Yesterday's successful launch of the Ariane, experts say, gave that troubled program a big lift.

If the Ariane "is very successful, then the market may not be large enough to create a sufficient demand for American rockets," Nisbet says. "If there's uncertainty there with the Ariane , it may be that there's sufficient room for some commercialization over here."

But the experts also say that the commercialization of the rocket business is a good way to keep together the teams that designed and launched the pioneer workhorse boosters, in anticipation of the day when space flight becomes as common as airplane travel, and demand for rockets of all types booms.