The presidential commission on the Challenger accident said yesterday it may recommend major changes in the shuttle program, including an ejection system that could help astronauts escape a doomed spacecraft and an independent safety board to ensure that hazards are identified and corrected.

Two senior astronauts told the panel at a public hearing that the shuttle orbiter would not survive an emergency landing in water. As a result, the National Aeronautics and Space Administration already has begun studying modifications of the orbiters, ranging from simple parachute ejections through hatches to elaborate rocket-propelled escape systems, one of the astronauts said.

"If you put the right people on it, with the right money and the right effort, you ought to be able to do it pretty darn quickly," said chief astronaut John Young. "But I'm not sure we have the kind of capability it takes."

Young stressed that no conceivable escape system could have saved the crew of the Challenger. But the new look at the issue is one indication that the Challenger investigation has sensitized NASA to a wider range of safety concerns that extend far beyond the specific causes of the Jan. 28 accident.

Modifications to eliminate the chief concerns are likely to prove costly, and could deal a major blow to NASA's budget, made tighter by the Gramm-Rudman-Hollings budget-balancing law. Modifications could also set back NASA's hope of resuming shuttle flights in 12 months to 18 months, further eroding the likelihood the shuttle will become commercially viable and also support the Pentagon's need for space launches.

Astronauts and top NASA officials also told the commission of a variety of other longstanding problems affecting flight safety. Among them were:

*The inability of the orbiters' brakes to stop the vehicle reliably. Arnold Aldrich, manager of shuttle systems, said the brakes have been a problem on almost every landing. He said new brake designs have been ordered but will not be ready for two years.

*A rough, narrow runway at the Kennedy Space Center that, when combined with unpredictable Florida weather, has led many astronauts to conclude it is too dangerous to land there. Young said future landings should be at Edwards Air Force Base in California's Mojave Desert. NASA built the Cape Canaveral runway chiefly to save the five days it takes to mount orbiters atop a special Boeing 747 and return them from Edwards to the Cape.

*A lack of spare parts that has often forced technicians to "cannibalize" one orbiter to supply another.

*Deficiencies in spacecraft simulators that astronaut Henry Hartsfield said has sometimes led to "negative training" because the simulators did not truly reflect shuttle conditions.

Commission Chairman William P. Rogers, repeating his concern that neither astronauts nor high NASA officials knew before the explosion the seriousness of recurring problems with the O-ring seals on shuttle booster rockets, said one of the panel's major recommendations will be an independent safety board.

Although the precise nature of the body has yet to be determined, Rogers suggested it would have access to all data affecting flight safety and be charged with seeing that safety problems are solved before they endanger crews.

Endorsing the idea, commission member Richard P. Feynman compared NASA's attitude about the O-ring problem to playing Russian roulette. He noted that when O-rings were found damaged in earlier shuttle flights, there was some concern, but that it gradually abated as the problem recurred without triggering a disaster.

Feynman suggested NASA's stance was like that of someone spinning a revolver's cylinder, pulling the trigger, surviving and concluding that the experience has determined it is safe to continue.

"An argument is always given that last time it worked," Feynman said. "It's a kind of Russian roulette . . . . There was a risk, but you got away with it . . . . When I look at the [flight readiness] reviews, I find perpetual movement heading for trouble." He said a safety board would "work as hard as possible to keep everybody awake."

"I think all of us believe there should be an independent safety review panel of some type," Rogers said.

The comments came after the panel of astronauts and officials said they were unaware of the severity of the O-ring problem. Investigators believe the Challenger explosion resulted from a failure of O-rings to seal a joint in the right-hand booster rocket. Astronaut Robert Crippen, veteran shuttle commander, said that although the problem had come up before other flights, "it wasn't presented like it was that much of a problem. It really wasn't that big of a deal."

On a related matter, White House spokesman Larry Speakes announced that an internal White House review had found no substantiation for rumors that presidential aides had telephoned the space agency and urged the launch of the Challenger to coincide with the president's planned State of the Union address.

Speakes said the White House has responded to a request from Ernest F. Hollings (D-S.C.) for logs of calls between the White House and the space agency before the shuttle explosion.

Speakes said the White House will not provide the phone records but is giving Hollings a report of its own internal review. He said some calls were made between the White House and NASA in this period, but the calls did not seek to interfere in the "launch approval process."Staff writer David Hoffman contributed to this report.