The space agency's goal of total reliance on the shuttle for all space operations by 1988 is unsafe and must be abandoned if the nation is to have a reliable space transportation program, several senior space agency safety advisers now say.

In interviews over the past several days, seven of the outside advisers, who sit on the 13-member Aerospace Safety Advisory Panel, said the National Aeronautics and Space Administration should turn to unmanned, disposable rockets to put many of its payloads in orbit and send spacecraft into deep space.

The panel members -- aviation and aerospace veterans -- said NASA's launch schedule problems, which had already become severe well before the Jan. 28 Challenger tragedy, cannot be solved by relying on the shuttle alone.

"I really think they must look hard at expendable launch vehicles," said John G. Stewart, a panel member. "Everybody needs to take a deep breath, swallow their pride and look again."

"It would have been correct and sensible to retain a certain number of unmanned rockets," said panel member Seymour C. Himmel. "It would have been better than using a still-developmental orbiter."

Added Stewart, "Don't put people at risk just to put a routine communications satellite into orbit. In fact, I think they would have a much better manned schedule if they did not have this terrible shuttle schedule."

NASA scheduled 15 shuttle flights for 1986, 19 in 1987, and 24 a year into the 1990s. But not one member of the safety panel, which was set up after the 1967 Apollo capsule fire killed three astronauts, believes such a schedule is possible.

The panel has repeatedly cautioned that it regards the complex shuttle system as an experimental vehicle unsuited to routine operations.

NASA has set five unmanned rocket launches for this year, and four in 1987. "After that, everything goes on the shuttle," said agency spokesman Hugh T. Harris.

The views of the safety panel are becoming the focus of increasing interest as the investigation in the causes of the Challenger disaster gets underway.

Panel member John F. McDonald, an aviation consultant, said that "cannibalization" of spare parts from one orbiter to prepare another for flight is "a very difficult problem. It is a small fleet, and impossible for NASA officials to hold the kind of unlimited inventory of spare parts they'd like."

NASA decided in the mid-1970s to turn away from unmanned rockets, in part because shuttle cost overruns forced the agency to concentrate on the reusable orbiter system. But the debate has simmered for years, and seems likely to boil again as the agency's operations come under scrutiny from Congress and the Reagan administration.

One strong proponent of the shuttle program is panelist Richard H. Battin, of the Charles Stark Draper Laboratory in Cambridge, Mass. In Battin's opinion, "one should not put all the eggs in one basket. Even Aesop knew that." Battin, who was a principal mathematician for the Apollo voyages to the moon, said he strongly favors building a fifth orbiter.

Gerard W. Elverum, an executive with TRW, a major aerospace and defense contractor, said he believes NASA should "begin to make judicious use of expendable launch vehicles. The shuttle probably should be dedicated more to those critical payloads that can only go on the shuttle."

"My feeling is that NASA was tending to be too optimistic about the schedule," Elverum addid. "There are too many things to do to get ready for each launch, the payload options are too varied . . . . NASA probably took too many expendable launch vehicles out of the system. That's still to be debated."

Said Stewart, an executive of the Tennessee Valley Authority, "I think the idea of building a fifth orbiter deserves very careful study. You could reopen a lot of expendable launch vehicle production lines for that $2 billion orbiter pricetag."

Himmel, an aviation consultant, said, "I still think they shouldn't have done that. But people were playing numbers games, and basing things on numbers they had no experience with. This is a very dangerous practice.

"The more complicated the system, the more difficult to get things to function routinely. It would have been prudent to provide an alternative means of getting into orbit."

A Rand Corp. study on shuttle fleet operations prepared for the Air Force in 1984 strongly challenged NASA's assumptions about how reliable the shuttle could be.

Using computer simulation techniques, RAND researchers stated that "it would take a long history of successes to establish firmly a high shuttle reliability."

More importantly, the report said, "improvements in reliability of between one and two orders of magnitude" over regular rocket launches "are required for the four [shuttle] fleet to complete all its missions in a timely manner."

David Leinweber, who authored the report, has stated that the shuttle's reliability problems could be traced to the complexities involved in its "turnaround" preparation between flights.