No jet or tank carries its streamlined logo. No Wall Street analysts tout its stock. No weapons are built by its 5,700 workers.

Yet in little more than a decade, Science Applications International Corp. of Tysons Corner and La Jolla, Calif., has become one of the nation's fastest-growing and most successful defense firms. As industry and the Pentagon look to the stars for profit and protection through the Strategic Defense Initiative, SAIC has come of age as what may become the archetypal military contractor of the 21st century.

"Our shipping platform," said executive vice president David R. Heebner, "is a Xerox machine."

SAIC's unheralded entry into the top rank of defense contractors reflects its reputation for producing excellent analyses. It also reflects, critics suggest, the Defense Department's growing reliance on paper warriors who both advise on military matters and then stand to benefit if their advice is heeded.

Never has the Defense Department leaned more heavily on its contractors than in their joint pursuit of the Strategic Defense Initiative (SDI), President Reagan's proposed shield against nuclear attack. From the moment Reagan announced his "Star Wars" program in 1983, the Pentagon turned to industry -- and particularly to "systems integrators" such as SAIC -- to ask how to convert the president's multibillion-dollar dream into some sort of reality.

"It's not peculiar to Star Wars, but it's more scandalous than normal," said Kosta Tsipis, director of MIT's Science and Technology for International Security program. "Saying it's feasible will clearly help line their pockets; saying it can't be done would deprive them of a source of income."

For SAIC, a close relationship with the Pentagon is nothing new. From the colonel SAIC hires from the Defense Intelligence Agency to the former defense secretary on its board of directors to Heebner himself, who chairs the Navy's research advisory committee, the company and its chief customer have long been snugly intertwined.

Jasper Welch, a retired Air Force two-star general who now coordinates SAIC's Star Wars efforts, said that any self-serving advice would quickly be exposed by competing firms. Heebner added that his company, which makes almost no hardware, has even less reason for bias than major aerospace firms as it evaluates SDI's prospects.

"Our economic welfare is not dependent on SDI being rushed into full-scale deployment," Heebner said. "One of the reasons we're credible is that our advice isn't colored by the motivation to fill a factory."

But SAIC's studies and software would be ubiquitous midwives to the space lasers and battle stations that larger factories would produce. Already in fiscal 1985, Pentagon records show, SAIC won more Star Wars-related contracts than the General Dynamics, Northrop, Grumman, Raytheon and Rand corporations combined. Assuming Military Tasks

For SAIC, Star Wars began long before President Reagan launched his effort for a strategic defense on March 23, 1983.

Eager to escape the meddling and bureaucracy common to larger firms, J. Robert Beyster and a handful of other scientists established SAIC in San Diego in 1969. It was to be, as it remains, employe-owned and nurturing of innovation.

Within a few years, SAIC was one of the more highly regarded companies that thrived in part by assuming tasks the military used to do for itself: thinking about how different parts of a weapon system fit together, writing computer software, managing complex purchases for the services.

The firm wrote computer programs to steer cruise missiles and studied the vulnerability of Trident missile submarines. It specialized in "nuclear warfare planning," according to its annual reports -- predicting "nuclear weapon effects," establishing "target damage criteria" and modeling "national recovery capabilities."

In its own version of one-stop shopping, the Pentagon might ask SAIC's Center of Soviet Studies to define a threat to U.S. security, then seek SAIC's advice on how to parry that threat and, finally, pay the firm to coordinate the prescribed countermeasures.

From $250,000 in 1969, SAIC's annual revenues climbed passed $46 million in 1974 and hit $420 million last year, with nearly nine of 10 dollars coming from the federal government. The company's privately traded stock increased in price from $1.11 per share in 1974 to $22.12 last spring.

Along the way, the company grew especially wise in the field of ballistic missile defense. After the United States and Soviet Union banned most defense systems in the 1972 Antiballistic Missile (ABM) Treaty, many concluded that strategic defense wasn't the most promising avenue for business growth.

But a dedicated band of scientists and true believers in the military and outside never gave up. With a branch near the Army's missile defense office in Huntsville, Ala., and another outside Air Force Space Command in Colorado Springs, SAIC snared much of the obscure technical work.

The company had always stressed "establishing and maintaining close liaison between key members of the company's professional staff and its customers," as it said in one prospectus.

To that end, SAIC recruited a stellar board of directors: former defense secretary Melvin R. Laird; retired admiral Bobby R. Inman, former chief of the supersecret National Security Agency; Lucy W. Benson, former undersecretary of state; former undersecretary of energy John M. Deutch, now provost of MIT; retired general Welch, who had served on the National Security Council staff; and Northrop Corp.'s Donald A. Hicks, who resigned recently to become the Pentagon's research czar.

Similarly, the company hired scores of retiring officers, from majors to major generals. Among them: a brigadier general who left as deputy director of the National Strategic Target List to become vice president for strategic matters; a colonel from Eglin Air Force Base who now "keeps companies informed on Eglin programs" as consultant to SAIC and a few other firms; a Navy commander who headed the undersea warfare technology section of the Naval Intelligence Support Center, now a senior scientist at SAIC.

And the company maintained "close liaison" in the field of missile defense, too, even in its years of adversity. When the Pentagon formed a survey team in 1978 to study the potential of particle beam weapons -- now a key SDI technology -- SAIC was the best-represented contractor.

When the Army Ballistic Missile Defense Agency (BMDA) orchestrated a series of seminars to build support for strategic defense between 1978 and 1981, SAIC was among the contributors; one SAIC expert spoke on "BMD's role in national survival: a first assessment."

And when the new administration began reassessing the ABM Treaty in March 1981, SAIC won a $224,000 contract to help the Army prepare for a treaty review.

Then, in the summer of 1981, came what Welch now recalls as "the first strategic defense initiative." Defense Secretary Caspar W. Weinberger, discounting the skepticism of some aides, asked the Defense Science Board, his top technical advisory group, to study the potential of missile defense.

The board, studded with executives from General Dynamics, TRW, Lockheed and General Electric, met for several weeks in San Diego. Welch, who chaired the policy panel while still on active duty, said its conclusions were "encouraging and supportive of a broad range of strategic defense."

When the Pentagon convened yet another panel on strategic defense the following year, Julian Davidson -- former deputy for the Army's missile defense program who by then worked for SAIC -- headed the "blue team," studying U.S. capabilities.

That same fall of 1982, White House science adviser George Keyworth asked his advisory council to study the potential of advanced technologies that now form part of SDI. Among his experts was Edward A. Frieman, an executive vice president of SAIC.

So when Reagan astonished most of the nation in 1983 with his March 23 speech, SAIC was prepared. And when the Pentagon appointed a committee to study the feasibility of Reagan's sweeping goals, chaired by former NASA director James C. Fletcher and peopled almost exclusively by Pentagon employes and contractors, SAIC again was the best-represented firm.

Davidson, Inman and three SAIC vice presidents all contributed to the panel's generally optimistic report. And Frieman, SAIC executive vice president, chaired its scientific review group.

Throughout industry, the Fletcher report was a welcome green light. Aerojet General, a California contractor, also had a vice president on the committee.

"When he returned, he gave a briefing to the company and said, 'This is likely to be a big thing for some time to come, and how can we posture our company to take advantage and make a contribution?' " recalled Aerojet's Philip A. Buckley. "We started as soon as he came back."

SAIC swung into action, too, both publicly and in the Pentagon's classified world. Davidson wrote a magazine piece calling the defense initiative "long overdue" and stating that missile defense is "feasible" with components that could be demonstrated "within this decade."

SAIC's Sovietologist Daniel Goure explained to Military Space newsletter how the Soviets would attempt to derail the U.S. SDI program. "I also wouldn't be surprised," he said, "to see SDI opponents coaching the Soviets on how to approach the U.S. government and how to 'sound right' on SDI."

Meanwhile, ten contractors won contracts to begin roughing out the design of a space defense system. SAIC was one of those ten and then, last month, one of five to survive the Pentagon's first cut. To perform the work, SAIC has allied itself with Boeing Co., Westinghouse Electric Corp. and Booz-Allen & Hamilton of Rockville -- where Davidson now heads an SDI team.

Heebner said his company's success with SDI -- it has won about $38 million in contracts so far, according to the Federation of American Scientists -- owes little to contacts built up on past advisory committees.

"We have to compete for our business, and when we compete I don't know that I get a point for being chairman of the Naval Research Advisory Committee," he said. "I do think it provides us with a perspective of what's going on in the defense community and the service community . . . and that does allow us to allocate technical resources accordingly. But that's information that's open and not given exclusively to us."

Aerojet executives, whose team didn't make the second round of five contractors working on an integrated Star Wars plan, said ruefully that winners like SAIC nonetheless will have an advantage in future bidding. "Not that they'll be able to skew the program," Buckley said, "but they'll have a better sense of what's coming." Critics Worry About Advice

Even critics acknowledge that SAIC and similar firms attract some of the sharpest minds in the business. As weaponry becomes more complex, they agree, the Pentagon will become ever more dependent on analyses rolling off SAIC's copying machines.

The critics worry, though, that industry advisers may tell the military mostly what it wants to hear. David L. Parnas, a computer scientist who joined an advisory panel on SDI's computing needs, said most of his colleagues were defense contractors or, like him, recipients of Pentagon grants.

When Parnas suggested that Star Wars' goals might in fact be impossible to achieve, he said, the panel and the Pentagon were not very interested in his doubts.

"I said, 'Could we just tell them it can't be done?' " Parnas said. "They said, 'Oh, no, they won't accept that. We have to find ways to spend the money.' "

But Welch said SAIC is as quick to disabuse the Pentagon of ideas that won't work as it is to recommend inventions that might. And Julian Davidson, who has served on four advisory panels on missile defense during the past six years, said the Pentagon has little choice but to rely on the expertise of its growing army of systems integrators.

"It's the same old problem," he said. "If you want a completely objective opinion, you have to get someone who knows nothing about it."