The navy has concluded for the first time that it could save billions by building smaller nuclear submarines which could perform just about as well as such giants as the Trident missile sub now under construction.

David E. Mann, assistant secretary of the Navy for research, said in an interview yesterday that this was the key finding in a secret study he directed, which took seven months and cost about $1 million.

He said technical advances made since blueprints were drawn for the Trident and another underwater giant, the Los Angeles class attack submarine, suggest that the Navy now could switch safely to something smaller.

Although these and other conclusions in the Navy study are expected to provoke demands within the Pentagon and Congress for a change of course in submarine construction, this will not happen right away.

Mann said further analysis still must be done, adding that it would take 10 years to design and build the smaller submarines if they are chosen over Trident and Los Angeles class subs.

"My personal opinion," he said, is that the Trident submarine building program could be stopped after 12 boats are finished.

The Trident, being built to replace the aging fleet of 41 Polaris and Poseidon missile submarines, is costing so much that some Navy leaders claim it is eating up money critically needed for surface ships.

The first Trident, according to Pentagon cost estimates, will run $1.5 billion. Mann said the newly completed study indicates a smaller submarine which could do the same job would cost 30 percent less, a potential saving of $450 million a copy.

A Los Angeles submarine costs about $500 million. Mann said a smaller submarine could do the same job and cost about 20 percent less to build, a possible saving of $100 million on each boat.

The study panel, he said, found ways to make the nuclear reactor and its propulsion machinery more compact, saving both weight and space.

Critics long have maintained that Adm. H.G. Rickover, director of nuclear propulsion for the Navy, has resisted making the submarine power plants smaller in his emphasis on reliability.

The only penalty for building smaller missile and attack submarines, Mann said in discussing the panel's findings, is a loss of "less than five knots in speed. This is not a critical difference."

He said the smaller submarines would be just as quiet as today's Trident and Los Angeles subs. Quietness is considered the key to survival in the depths.

Mann acknowledged that submariners argue that speed is critical to put their subs on station in a hurry or to evade pursuers. But he said Trident's main job will be to run slowly on station, awaiting the word to fire its missiles if war came.

An attack sub is used primarily to hunt down and sink enemy subs. Mann said the suggested alternative to the Los Angeles class would carry the same punch.

The Trident, 560 feet long - or five feet longer than the Washington Monument is high - displaces 18,750 tons. Mann said the panel concluded a submarine of 15,000 tons could carry the same load of 24 Trident I or Trident II missiles.

Similarly, said Mann, the submarine panel chartered by Navy Secretary W. Graham Claytor Jr. last May, reported that an attack sub 1,000 tons lighter than the 7,000 ton Los Angeles looked feasible. Another Navy study group has roughed out the design of an even smaller attack sub of 5,000 tons.

In focusing on Trident, the most expensive sub ever built, Mann said: "The study has shown for the first time that a significantly less costly ballistic missile sub can be designed and built without having to give up safety, vulnerability or the missile" Trident was designed to carry. CAPTION: Picture, First Trident submarine Ohio at christening. UPI