A NEW MISSILE GAP is opening up, this one of the United States' own making. Pentagon manager, not the Kremlin nor a presidential candidate looking for head-lines, are to blame. Simply put, the Navy's present fleet of Polaris and Poseidon missile submarines will wear out faster than they can be replaced by the giant Trident missile subs now beset with cost and construction difficulties.

This confronts U.S. policy makers with two alternatives, neither of them attractive: keep the aging Poseidon-Polaris fleet of 41 subs at sea longer than planned to reduce the gap or gamble that such a risk need not be taken because the United State can safely settle for a smaller sea-based nuclear deterrent.

Defense Secretary Harold Brown said at his March 10 press conference that the way to stave off the forth-coming missile gap "is to make plans, if necessary, to extend the life of the Poseidons.That is not simple. It is likely to cost money, and it is likely to mean that they have to be out of commission a somewhat longer time, but it is an alternative approach."

Although B-52 bombers have been kept flying long after their scheduled retirement dates, the depths of the ocean are mercilessly unforgiving of any kind of mechanical failure on a submarine. Sending men down into the sea on an over-age submarine would involve risks no leader would like to take. And even if such risks were taken, Navy projections still show there would be a big gap.

The second option, of tolerating a reduction in the sea-based deterrent of submarine missiles, would bring down a number of problems on any administration - unless the Soviets made offsetting cuts, a dim propect at the moment.

Historically, both hawks and doves in Congress have enthusiastically support the missile submarine force. The idea of hiding missiles under tons of water where they can neither be found nor destroyed has made eminent sence through the years to politicians of the left and right. These submarine missiles would be fired at Russia to retaliate for a surprise nuclear attack on the United States - or would deter the Kremlin from pushing the button in the first place.

The missile submarine is clearly a retaliatory weapon, which has made it especially attractive to arms control specialists. The missile now carried by submarine would not be used to launch a surprise, first-strike attack, because they are neither accurate nor powerful enough to crack Soviet missile silos. But they could devastate Soviet cities in a retaliatory second strike. (That old distinction between first-strike and second-strike weapons is now being blurred however, by improvements in the submarine missiles and in the satellite systems that would guide them to their targets.)

Any president who announced that the United States was going to reduce unilaterally its sea-based missile force would draw heavy political fire from all sides. Therefore, the president would be tempted to add to another part of the U.S. nuclear arsenal.

This is where a land-based mobile called MX comes in, a nuclear blockbuster that would be carted back and forth inside tunnels 10 to 15 miles long. The MX to the distress of hawks, has been kept at a moderate rather than all-out development pace by President Carter. But the Navy's problems on Trident are likely to give the Air Force MX program a boost.

Once Congress perceives the extent of the projected gap between the Polaris-Poseidon and Trident submarine fleets, the demands for going ahead with MX are sure to intensify. If MX does go into full-scale development by virtue of that argument, the gap in the nation's second-strike submarine force will have helped tilt the American nuclear offense toward first-strike. This is because the MX would be so accurate and powerful that it would fall into the first strike category of "silo busters," especially in the eyes of the Soviets.

How did Pentagon leaders, past and present, fail to anticipate the gap between Polaris-Poseidon and Trident? And is there anything more that can be done to minimize it besides extending the life of the Poseidon?

The short answer to the first question is that, despite record high Pentagon budgets of recent years, there was not enough money to buy all the Tridents needed to avoid a gap. Adm. Elmo R. Zumwalt Jr., former chief of naval operations, wrote about Trident's first delay in his book, "On Watch": "In order to soften the financial impact" of Trident on the Pentagon's Fiscal's 1973 budget to go to Congress in January, 1972, former Defense Secretary Melvin R. Laird decided that the target date for deploying the first boat should be 1980 rather than 1978" as the Navy had planned.

Today, despite earlier efforts to return to the 1978 deployment date, the Navy estimates the first Trident will not go to sea until 1981. So this replacement missile submarine is off to a late start and still slipping

Compounding the slippage problem, the cost of this first Trident is running 50 percent above Navy estimates - $1.2 billion rather than $800 million. More increases are anticipated. President Carter and Defense Secretary Brown have concluded that the Trident building rate can be no higher than 1 1/2 submarines a year. Otherwise the Navy would have money for little else in the way of ships besides Trident.

To further cloud Trident's future, the only shipbuilder contracted to build this largest-ever submarine is so angry at the Navy that it has threatened to stop work on its other big submarine project, the 688 class of attack submarine. General Dynamics' Electric Boat division of Groton, Conn., has threatened to abandon the 688 project on April 12 on the ground the Navy has refused to make an equitable settlement of $544 million in disputed back bills.

Until last week, the official line was that problems with the 688 class of attack sub would not jeopardize Trident. But in Electri Boat's letter to the Navy dated March 13, the company said: "The Navy's material breaches of the 688 contracts have materially increased the cost of performance on the Trident contract and seriously delayed of the Trident vessels . . ."

Even if the Navy and Electric Boat manage to resolve their differences and get Trident back on schedule, the planned building rate still will leave a gap. Vice Adm. C.H. Griffiths, deputy chief of naval operations for submarine warfare, detailed this gap in testimony to a House Armed Services subcommittee last month:

"Retirement of the Poseiden submarine at their normal service life of 20 years or at the presently planned maximum extended life of 25 years will result in a significant reduction of (submarine missile tubes) in the 1980s and early 1990s since the Polaris-Poseidon force was built at a much faster rate than that planned for Trident."

The admiral added that if the Navy does not spend the money to renovate Poseidon subs to make them last 25 instead of 20 years, the big submarine missile gap will occur in 1986, when the United States would have only 312 launchers available instead of today's 656. (Each of the 41 Polaris and Poseidon subs carries 16 missiles for a total of 656. Each Trident will carry 24 missiles.)

If the Poseidons are made to last 25 years, the biggest submarine missile gap will be pushed back to 1992, when the United State would have 480 submarine missile tubes to deploy in the depths, according to Griffiths. "A Continued Soviet Advantage"

EVEN THOUGH Brown has said Trident subs and their missile will be more lethal than the Polaris-Poseidon fleet, the Navy estimates of the gap are ready-made for political thundering. One likely formulation: "The Navy itself admits that by 1986 the submarine missile gap will be so big that the United State will have only half as many missiles at sea as are deployed today. We must go ahead with new land missiles or build a new bomber."

A possible counter-argument is that the more powerful Trident missile plus the Trident II missile under consideration for future deployment, could hurl more individual nuclear war-heads, called MIRVs, longer distances, and that the gap in submarine missile launches therefore is not as alarming as it seems.

Griffiths buttressed the Hawkish side of the argument by stating: "The combination of a steadily increasing Soviet SLBM [submarine-launched ballistic missile] force level, block obsolescence of the U.S. Polaris-Poseidon force and a slow Trident shipbuilding rate clearly predict acontinued Soviet advantage in SLBM launchers, the most invulnerable element of each nation's strategic forces, for the foreseeable future."

Currently, the Pentagon is planning to build a total of 14 Trident sunmarine for $25 billion, including new bases for the giant sub, making it the most expensive weapon in the history of warfare. (The MX would cost about $40 billion.) It would take 27 Tridents, each carrying 24 missiles for a total of 648, to come close to duplicating the current force of 656 submarine missiles.

Adm. James L. Holloway III, chief of naval operations, said in an interview with The Post that he sees "no alternative" to the Trident for keeping up the nation's sea-based nuclear deterrent. But undersea warfare specialists, who declined to be quoted for fear of retaliation by Navy leaders, argued that there are ways to plug the coming submarine missile gap if the defense establishment will only stand up to Adm. H. G. Rickover and change course. Rickover, by insisting on building bigger and bigger nuclear power plants, they claimed, has saddled the Navy with giant submarines it can neither afford nor build in a hurry.

Alternatives recommended by these specialists in a series of interviews included: Converting the troublessome 688 class attack subs to missile boats. This, it was argued, would help plug the missile gap while giving the Navy breathing time to design better subs than the 688 and Trident. One drawback here: The 688, despite all the praise President Carter heaped on it when he was given a demonstration ride, cannot go as deep as the old Polaris missile boat. Depth is protection for subs.

Building a mix of do-everything and single-purpose attack and missile subs. The Navy, specialists argued, cannot afford to keep building all its subs as big and as expensive as the Trident and 688. Non-nuclear propulsion deserves a new look in this context, they said.

Changing course away from fast missile subs like Trident to slower and cheaper ones that would rely primarily on depth and quietness for protection. Even the huge Trident power plant will not provide the sub with enough speed to outrace modern Soviet killer subs.

Compressing today's nuclear propulsion plants into smaller units without sacrificing horsepower, thus increasing performance while reducing the size of new subs.

Some of these specialists predicted that as the extent of the coming submarine missile gap begins to be appresiated by the public and Congress, the Navy will be forced into an overdue look at alternatives to the giant attack and missile submarines now threatening to leave the service short of money and the nation short of vital weapons.