The answers came in cool, academic formulations until the interview focused on The Bomb. Then emotion escaped from the Secretary of Defense as he revealed some of the visions that drive him.

"I have seen somewhere between 10 and 30 nuclear explosions, including may be 10 of a megaton or more," said Harold Brown.

"When you've seen those," he continued, rolling his open right fist for exphasis, "you don't forget it easily. It has a big effect on your estimate of what a nuclear war would be like."

Whether it is watching a hydrogen bomb erupt from the desert floor or incinerate an atoll in the Pacific, the experience "keeps you from thinking of them as numbers on a piece of paper," Brown told The Washington Post in a recent interview.

He spoke with uncharacteristic emotion while seated in shirtsleeves on a couch in his third-floor office at the Pentagon. Back in the 1960s, as one of the Pentagon's Whiz Kids, he used to come into the same office to discuss everything from The Bomb to how to win the Vietnam war with then. Defense Secretary Robert S. McNamara.

He watched Vietnam defy all the management logic McNamara applied to it. Brown, a nuclear physicist, was similary frustrated as Vietnam refused to be system-analyzed. He still feels the sting of the Vietnam defeat.

The awesome power of The Bomb and the imponderables of Vietnam both drive and brake President Carter's Secretary of Defense as he sets out to reshape the world's mightiest military establishment. The 49-year-old Brown is the 14th secretary to try to do this.

His intimate knowledge of The Bomb, which he helped perfect as a scientist-manager at the Livermore (Calif.) Radiation Laboratory from 1952 to 1960, is driving him to control it.

He was President Carter's most influential adviser on the proposal put before the Soviets at the Strategic Arms Limitation Talks (SALT) in March. Administration sources assert that in White House discussions on SALT Brown displayed far greater virtuosity on the technical issues than Zbigniew Brzezinski, Carter's national security adviser.

Brown fervently believes that both the United States and Soviet Union have far too much nuclear power for their own good. He is pushing for big reductions in the U.S. and Soviet nuclear arsenals, not just a ratification of the current ceilings, which he believes are too high.

He believes just as fervently that sooner or later the Soviet leaders will conclude it is in their interest to make substantial reductions in strategic nuclear weapons. He does not expect the Soviets to accept by October the comprehensive reductions recommended to them in March.

Soviet leaders, in Brown's view, will need more time to digest the idea of deep cuts in the land-based missile force they recently deployed. "In the long run." Brown has said, "the prognosis is favorable. What happens between now and the long run is the issue . . . We're just going to have to go on now and continue the dialogue."

The Defense Secretary has made clear in is first five months of office that he is not averse to pressuring the Soviets to sign an arms agreement by deploying new weapons. This is the same bargaining-chip strategy used by former Secretary of State Henry A. Kissinger in his approach to disarmament negotiation.

The B-1 bomber is in this category. Brown already has given Carter the technical arguments that could be used to justify keeping the B-1 in production, despite campaign pledges.

In addition, the President can contend that since previous administrations had spent about $3 billion on the B-1 already, the nation might as well spend a little more to get some combat models produced to replace the aging B-52 bomber.

Brown was in on the birth of the B-1, and is believed to favor its production, although he has not said so. As McNamara's Pentagon research director in 1961. Brown helped shoot down the Air Force B-70 bomber. The high-altitude bomber, he argued, was too vulnerable to Soviet defenses.

The B-1 is the product of the research that Brown initiated in promising Congress in 1961 to examine a better bomber than the B-70. The B-1 is designed to fly in low under Soviet defenses. Its cost is the highest ever for a combat plane, $102 million and climbing.

Brown's knowledge of weaponry, deeper than that of the previous 13 Secretaries of Denfense, is his primary source of power. He is a highly effective advocate or opponent, whether it be a SALT proposal, the B-1 bomber, conventional weaponry and strategy, the seriousness of the Soviet threat, such as its civil defense program which Brown, knowing the power of The Bomb, deprecates.

Thus, both hawks and doves want Brown on their side as they push a particular position. Members of the Joint Chiefs of Staff in interviews said they feel Brown understands their problems and always give them a fair hearing.

Brown warns that he may look more hawkish as Secretary of Defense than he did before he took the job. Being responsible "for our military capabilities undoubtedly makes me behave in a way that could cause some people to think I was more of a hawk" than he appeared from positions he took as a civilian. "If I am not going to defend the defense budget," he asked rhetorically, "Who is going to do it?

"At the same time," he said in a sentence which may give comfort to the arms-control community, "I don't think that just anything goes. Nor do I think that defense military capability is an end in itself," but must be tailored to specific national purposes.

In that context, Brown is the nation's head tailor. He is deeply involved in a broad review of militray weapons and policies, trying to find ways to make them fit together better. Papers ordered by the National space strategy will start flowing into his in-basket next month.

What he hopes to find in them is some revolutionary thinking. But he does not expect to find it until the government's national security bureaucracy - White House, Pentagon and State Department is persuaded that Carter is looking for new ideas. This will take longer than a year, Brown predicts.

One idea Brown offered as an example of the kind of fresh thinking he is looking comes down to this question: 'why not build lots of little, fast tanks to foil the Soviets all along the NATO front rather than sink so much money in big tanks costing almost $1 million each?

Brown is trying to push himself and others out of old ruts in hopes of making right decisions on the new missiles, planes aircraft carriers the United will build for the 1980's and 1990's

What Brown has going for him, besides his acknowledged expertise and diligence, is the full faith of the President; a Secretary of State who will not outrun him: a public and Congress more conditioned to spending billions for defense in peacetime than they were in the 1960s and early 1970s; military servives accustomed to responding analytically to a strong secretary, and - for the moment anyhow - peace. He has no Vietnam war to derail his efforts to restructure the nation's weapons and strategy.

Brown is determined not to forget the lesson of Vietnam. But he worries about over-reacting to Vietnam by shrinking back from a challenging situation rather than responding to it forthrightly. Said the man who was Pentagon research director from 1961 to 1965 and Air Force Secretary from 1965 to 1969:

"It's a good thing" to be mindful of the Vietnam failure, "but at the same time this has its risks. We may fall into the trap of not recognizing new situations as new."

In a second interview with The Post, this time during the early morning ride from his Crystal City, Va., apartment house to the Pentagon, Brown was asked whether he returned to the Pentagon as an act of anonement for Vietnam.

"That didn't particulary pull me in, but that's in the minds of those of us who were here before."

The main reasons for returning to the Pentagon. Brown said, were to regain "the sense of being at the center of events" and to use "my own abilities in a useful and important way. The fact that the job is hard makes it a challenge."

His Washington lifestyle is work and more work. He starts off with an early-morning swim at the Pentagon and then plunges into meetings, analyses and readings - lots of reading. He leaves the building between 7:30 and 8 at night, returning to the apartment he shares with his wife, Colene. Their two daughters, Deborah, 21, and Ellen, 19, are at college.

"I remember being in here one holiday and Brown telling me it was good it was a holiday because we wouldn't be distracted in our work," said Brown's deputy secretary of defense, Charles W. Duncan Jr.