The management of Morton Thiokol Inc., apparently responding to pressure from the National Aeronautics and Space Administration, recommended launching the space shuttle Challenger after rejecting the impassioned and unanimous objections of its own engineers, the presidential commission investigating the Jan. 28 disaster heard yesterday.

Several of the engineers who argued against the launch, fearing that cold temperatures would cause solid rocket booster O-rings to fail, said space agency officials tried to pressure them to change their recommendation.

NASA officials told Thiokol that they were "appalled" by the engineers' recommendation against launching, according to the testimony. When the engineers stood firm, their company's management, then trying to win a new booster contract from NASA, overruled them and gave NASA the go-ahead it sought, company officials told commission members.

At that point, one of the engineers, Allan McDonald, told NASA that "If anything happens to this launch, I sure wouldn't want to be the person who had to stand in front of a board of inquiry to explain" the decision to proceed.

Thiokol officials also revealed yesterday an internal July 1985 memorandum, in which an engineer warned that a "catastrophe of the highest order" could occur if the booster's seal problems were not corrected.

The shuttle exploded shortly after liftoff Jan. 28, killing its crew of seven. A failure of the O-rings, which would allow hot gases to burst through the side of the booster, is a prime suspect in the Challenger investigation.

The engineers said NASA's actions reversed a longstanding practice in which manufacturers of shuttle components have had to prove their systems were safe to launch. This time, said McDonald, director of Thiokol's booster project, "the tone of the meetings was just opposite of that."

In the past, McDonald told the commission, "the contractor always had to get up and prove that his hardware was ready to fly. In this case he had to prove it wasn't. That's a big difference. I felt I was pressured."

The commission also heard from Thiokol engineers who had worried about the vulnerability of the O-rings for many months and repeatedly tried to warn Thiokol management.

Roger Boisjoly, a Thiokol engineer in charge of a special task force to study and solve the seal problem, warned his boss in a memo last July of a "catastrophe of the highest order -- the loss of human life" if the O-ring problem was not fixed.

Boisjoly, among those whose advice not to launch Challenger eventually was overruled, sent the memo to his superior, Robert K. Lund, who also testified yesterday. In the memo, which Boisjoly read yesterday, he declared, "It is my honest and very real fear that if we do not take immediate action to dedicate a team to solve the problem . . . then we stand in jeopardy of losing the flight along with all the launch pad facilities."

Another veteran Thiokol rocket structures engineer, Arnold Thompson, warned his superiors in a memo last August that the shuttle must "stop flying" until the joints problem was solved. The commission did not ask for details but Thompson said he never got an answer.

McDonald said that when Thiokol engineers at the plant in Utah learned the night before the Challenger launch that temperatures would fall into the low 20s, they arranged a telephone conference call with Lawrence Mulloy, chief of the booster program at NASA's Marshall Space Flight Center in Alabama, and NASA officials at Cape Canaveral.

The engineers told NASA that the cold would stiffen the rubbery O-rings, making them slower to seal the joint between booster segments. They reminded NASA that O-rings were damaged in several previous shuttle flights and that the worst case was a year earlier, when the shuttle took off in 53-degree air, the coldest temperature experienced at launch.

McDonald said the engineers believed it was too dangerous to launch in colder weather. "The bottom line," McDonald said, "was the engineers could not recommend a launch below 53 degrees."

The engineers' position was met with "a lot of strong comments from the NASA officials," McDonald told the commission. "I believe Mr. Mulloy made some comments about when we'll ever fly if we have to live with that."

McDonald quoted another NASA official, George Hardy, deputy director of science and engineering at the Marshall center, as saying he was "appalled" by the engineers' stance.

After much further discussion, McDonald said, Thiokol's vice president Joe Kilminster, in Utah, told the engineers that if they could not say for certain that the O-rings would fail, the decision would be made by management. Kilminster then polled Thiokol management officials and got a unanimous decision to approve the launch.

Unanimity was achieved, according to McDonald, when Lund, Thiokol vice president for engineering, who had previously sided with the engineers, was told to "take off your engineer hat and put on your management hat." Lund then approved the launch decision.

Commission Chairman William P. Rogers asked Lund, "How do you explain the fact that you seemed to change your mind when you changed your hat?"

Lund responded by blaming NASA's reversal of its launch-readiness review policy. "We got ourselves in the thought process that we were trying to find some way to prove to them it wouldn't work," Lund told Rogers. "We were unable to do that. We couldn't prove absolutely that it wouldn't work . . . . That's the kind of boat we got ourselves into that evening."

Pressed to explain why he didn't stick to his no-launch decision, Lund replied after a long silence, "As a quarterback on Monday morning, that's probably what I should have done. You know, you work with people and you develop some confidence and I have great confidence in the people at NASA."

Boisjoly said, "There was never one comment in favor [of launch] by any engineer before or after." When it became a management decision, "I was not even asked to participate. I did not agree . . . . I was never asked or polled, it was clearly a management position. There was no point in me doing anything further. I really did all I could to stop the launch . . . . "

During the hearings, a young Thiokol engineer, Brian Russell, sketched the conflicts he felt as the discussions progressed to the fateful decision.

"I remember distinctly at the time [wondering] whether I would have the courage if asked, what I would do . . . whether I would be alone . . . I didn't think I'd be alone but I was wondering if I would have the courage, I remember that distinctly, to stand up and say 'No.'

"However, we were not asked . . . It was a management decision at the vice presidents' level, and they had heard all they could hear, and I felt there was nothing more to say to change anything . . . .

"I knew we were in an increased risk, and I didn't feel comfortable doing that . . . . " That night, he said, "I thought I slept okay" but "my wife differed with that . . . . There was a nervousness there that we were increasing the risk, and I believe all of us knew that if it were increased to the level of an O-ring burnthrough, what the consequences would be . . . . And I don't think there's any question in anyone's mind about that."

Although Thiokol officials repeatedly claimed the booster joints were secure because of a backup O-ring, commission members reminded them that NASA found the secondary O-ring ineffective three years ago. At that time the seals were declared a "Criticality 1" item, meaning that a single failure could destroy the shuttle.

The primary ring had partly burned through during several previous shuttle flights, but Jerald Mason, senior vice president of Thiokol's Utah plant, testified that this was not a serious problem. Mason said the rings would work after three times as much damage as before.

"It sounds as if you're willing to accept damage" to an essential part of the shuttle that has no backup, said Sally Ride, the only commission member who has flown aboard a shuttle.

"The answer has to be yes," Mason replied. "We recognized that we might get some erosion but we recognized that it was a low-frequency event."

"Once something is classified as a Criticality 1," Ride said, "that sets a red flag in everyone's mind. I'm not sure you're allowed to go back and rationalize."

The decision to launch came when various pressures were working on both the company and the agency. Morton Thiokol was discussing a new contract with NASA for additional boosters. The agency had been studying the possibility of getting another company as a second source. Challenger faced a fast turnaround for its next mission, the time-critical launching of the Ulysses solar probe.

Richard H. Truly, new head of the space shuttle program, said yesterday that he has ordered a complete review of NASA's launch procedures as a result of recent disclosures from the Challenger investigation.