By the turn of the century, even the hottest of today's U.S. tactical combat aircraft - the F-14s, F-15s, F-16s and FA-18s - will be aged, if not technological relics, by military standards.

The average length of good service for a fighter airplane, we say, is 20, 25 years, said Gen. Robert Russ, commander of the U.S. Air Force Tactical Air Command.With the country's air combat arsenal nearing retirement age by the year 2000, the Pentagon has set its sights on a new generation of fighters and ground-attack aircraft. The Air Force and Navy want their new planes to incorporate stealth design and construction to let them evade enemy radar. And they want to cram them with advanced electronics to outfly enemy planes and spot and attack their targets before they can be seen.

But as the cost of such gadgetry mounts and dollars for defense shrinks, the Air Force and Navy have embarked on a buying strategy that is forcing some U.S. aircraft companies into unusual cooperative ventures and forcing others out of the business of building combat aircraft.

Rather than replace each of today's highly specialized tactical aircraft, giving companies a chance to bid on a wide variety of contracts, the Air Force and Navy have bowed to Congressional demands and are developing a single tactical plane each. And each service is pledged to buy a version of the plane the other service develops.

So rather than compete one-against-all for the contracts, as they might have done in the past, the manufacturers - with Pentagon prodding - have formed competing teams to vie for contracts to build the Air Force's Advanced Tactical Fighter and the Navy's Advanced Tactical Aircraft.

David J. Smith, a former Air Force test pilot who is now an aerospace industry analyst with Robert Ellsworth & Co., an international consulting firm, said the teamwork is dictated by the need to survive in a shrinking market.

As a result, though there are only two major new tactical aircraft programs for the seven major airplane manufacturers to divvy up, five are getting a shot at one or both of the multibillion-dollar ATF and ATA jackpots by teaming up.

General Dynamics Corp.'s Fort Worth (Texas) Division and St. Louis-based McDonnell Douglas Corp. have a chance to be double winners. So far, Grumman Corp. of Bethpage, N.Y., and Pittsburgh-based Rockwell International Corp. are the big losers.

In October 1986, the Air Force awarded contracts for $691 million to each of two teams to develop two ATF prototypes each. One team is led by Lockheed California Co. of Burbank and includes GD's Fort Worth Division and the Boeing Military Aircraft Co. of Wichita, Kan. The other is led by Northrop Corp. of Los Angeles, teamed with McDonnell Douglas.

For General Dynamics and McDonnell Douglas, however, the aircraft market has rarely been better.

Aerospace industry analyst Mr. Smith said GD's Fort Worth Division was in enviable shape with its F-16 still in production, a piece of the Navy's ATA contract and a shot at the Air Force ATF contract as well.

Northrop and Boeing also are healthy, having a chance at the ATF contract while being teamed together to build the new stealth bomber. Northrop also is McDonnell Douglas' partner in building the FA-18.

The Air Force wants its ATF to cruise and fight at supersonic speeds - an unprecedented and unproven capability. It seeks to use stealth technologies to hide the aircraft from enemy radars. The ATF also is to employ sophisticated, computerized flight controls and warfare electronics to enhance survival chances.

The Navy's ATA is to be a stealth attack aircraft, designed to swoop down and strike at land and sea targets rather than engage in cloudtop dogfights. Beyond that, the Navy is keeping nearly everything about the ATA secret.

The result of the ATF and ATA competitions - as the fate of Grumman and Rockwell already reflects - will be a dramatic winnowing of U.S. aircraft manufacturers in coming years, the experts say.

You're going to see a significant consolidation in the industry over the next 10 years, the Senate aide said. Is that a bad thing? No. We've just got too much capacity.

Given the checkered history of interservice cooperation, some experts question whether the services will stick by their pledge to jointly develop the ATF and ATA and buy each other's planes. Despite hurdles, the Senate aide said the declining defense budget was likely to force the services to cooperate on the ATF and ATA. Neither of them has the money to choose a different course right now, the aide said.

In addition to forcing the services to trim their shopping lists to only two new types of aircraft, the budget crunch and the awesome costs of the ATF and ATA themselves are forcing the Air Force and Navy to buy fewer planes than they have purchased in the past.

But the main reason the ATA and ATF are going to cost so much is their complex avionics, or aviation electronics. It is the driving factor in the airplane, Gen. Russ acknowledged.