HAWTHORNE, CALIF. -- In a windowless, high-security office here with all the ambience of a bank vault, Northrop Corp. Vice President Thomas R. Rooney is spending countless hours secretly scribbling numbers on a chalkboard, trying to settle on his final bid for the biggest jet fighter program in history.

The same frenzy of activity is occurring in Burbank, Calif., where Lockheed Corp. officials are making preparations to submit their proposal by a Jan. 2 deadline for the full-scale development portion of the Air Force's advanced tactical fighter program.

With more than $75 billion in future business at stake, the roll of the dice doesn't get any more grand in the aerospace industry.

The two companies and their teams have collectively invested $2 billion of stockholders' funds in the aircraft competition since it began in October 1986. Lockheed is teamed with General Dynamics Corp. and Boeing Co., while Northrop is teamed with McDonnell Douglas Corp.

The losing team will end up with nothing. So will Southern California, if Lockheed wins, for it will take the project's thousands of jobs to Marietta, Ga. If Northrop wins, the projects will create thousands of jobs in Southern California into the next century.

The rules of the contest were laid out by the Air Force in a massive, five-volume book, issued Nov. 2, asking the companies to submit their final proposals for building the new weapon.

Now, with a climax only a month away, hundreds of aerospace engineers, mechanics and pilots are working long hours, trying to gain a final edge.

Rooney has chucked his suit and tie, appearing for an interview late one afternoon last week in a flannel shirt, clearly fatigued. But he was nonetheless upbeat. "On the technical side, we are very confident," Rooney said.

Lockheed Vice President Sherman Mullin appeared before a crowded news conference at a fancy hotel the next day in a pin-stripe suit and button-down shirt, extolling the virtues of his company's entry in the fighter competition.

The styles of the two vice presidents mirror the differences in external style between the two companies over the last four years, as well. Lockheed has been forward, formal and public relations-oriented about its program. Northrop has been informal, often reticent and extremely cautious about its statements.

Recently, Calabasas, Calif.-based Lockheed has deluged news organizations with photo packages of its advanced tactical fighter entry, dubbed the YF-22. During a single day last week, the company mailed a half-dozen packages of photographs to the Los Angeles Times.

Northrop has made available photos of its aircraft, the YF-23, but has chosen a more modest approach.

Similarly, Lockheed has made frequent lobbying visits to Capitol Hill this year, while Northrop has been virtually silent on the ATF program in Washington, apparently expending its political energy on protecting its B-2 Stealth bomber from congressional budget-cutters.

"We are concentrating on our program and letting our airplane do the talking," a Northrop spokesman said.

What is known about the two companies' competing airplanes is a carefully concocted, often confusing set of facts released by the Air Force's ATF program manager, Brig. Gen. James Fain Jr., who has gained a solid reputation as a news media adversary.

The Northrop aircraft is believed to be somewhat lighter and faster than the Lockheed aircraft. Northrop has said that it has flown at a maximum speed of Mach 1.8 -- almost twice the speed of sound -- while Lockheed has flown up to Mach 1.7. Both aircraft have flown up to 50,000 feet in altitude.

The ATF will have the capability of cruising supersonically without engine afterburners, devices that devour fuel for extra power. Lockheed has flown its YF-22 equipped with a General Electric jet engine at Mach 1.56 without afterburners, but has not yet "supercruised" another model with a Pratt & Whitney engine.

Northrop has supercruised its Pratt & Whitney aircraft at Mach 1.43, but the Air Force has not authorized the company to say how fast its GE-equipped aircraft flew. Many observers believe that Northrop has the faster supercruise speed with its GE engine.

Virtually nothing is known about the comparative capabilities of the aircraft in such key areas as range, turning rate, roll rate, turning radius, radar function, avionics function or weapons systems. And the aircraft's ability to avoid detection by enemy radar -- its stealth quality -- is an even bigger unknown.

Northrop has completed testing one of its two YF-23s and will complete testing of the other in a week or so, according to Paul Metz, the company's chief test pilot. Many observers also credit Northrop with having a better-looking aircraft -- a seemingly trivial factor, but one that goes to the heart of an old aircraft-design maxim: "Looks good, flies good."

Lockheed, meanwhile, is emphasizing certain capabilities that Northrop does not plan to demonstrate. For example, the company has test-fired a Sidewinder Missile from its YF-22, ejecting the missile from an internal weapons bay. Carrying the bulky missile internally keeps the plane stealthy.

Northrop's fighter also has internal bays, but it will not test-fire a missile. While the Air Force has not required such a demonstration, Lockheed's Mullin said that uncertainties about whether such internal bays will work convinced Lockheed that it should put to rest any worries.

Lockheed chief test pilot David L. Ferguson said that the company will continue to fly both of its aircraft through much of December, right up to the cutoff date for submission of data to the Air Force. Lockheed started flying several weeks after Northrop, so it is not clear which company will have more test flights to show.

The Air Force is scheduled to select a winner of the full-scale development contract for the ATF by the end of April, basing its decision on technical merits, cost and management.

The development of the ATF will cost an estimated $10 billion. The development contract will be a cost-plus type of contract, meaning that the companies will be reimbursed for their costs and do not risk losing money.