The Reagan administration is discarding the plan it announced in October to "super-harden" the underground silos in which it will put the first 40 new MX intercontinental ballistic missiles, according to civilian and military officials at the Pentagon.

The administration said at the time it could super-harden the silos to protect the missiles from enemy nuclear attack over the next few years, while it decides on a permanent basing scheme to keep them invulnerable to attack over the long run.

The president had criticized former president Carter during the 1980 campaign for not having a feasible plan to protect the new MX, thereby creating a "window of vulnerability" in U.S. defenses.

Abandonment of the hardening idea is the second important departure in four months from the administration's original $19 billion-plus plan for the interim deployment of the MX, a huge new ocean-spanning missile that will carry 10 nuclear bombs in its nose and is meant as the backbone of the U.S. nuclear retaliatory force for the 1990s and beyond.

The administration indicated in October that it intended to put the first 40 MXs in former Titan II silos, which meant there would be no diminution of U.S. defenses, since the older Titans were scheduled to be withdrawn anyway.

On Dec. 31, however, the Pentagon announced that it had decided instead to put the first MXs in some of the silos now occupied by the 1,000 U.S. land-based Minuteman missiles. These are the silos officials have now decided will not be super-hardened.

One senior specialist said that, with the MXs inside, these Minuteman silos may be even less resistant to enemy attack than they are now. This is because the MX is a lot bigger than the Minuteman and there will be less "rattle space," or room to absorb shock, in these silos with the MXs in them.

From a political standpoint, the hardening and Titan site aspects of the original plan were key elements in that they made the president's controversial decision on the MX seem somewhat more logical to Congress and the public.

There is considerable irony surrounding the changes in the interim MX basing plan and the overall status of the MX project now.

The hardening plan was dropped in part because Congress proved skeptical and put restrictions on the use of money for it. But civilian and military officials acknowledge that there also has been a loss of interest in the idea within the administration.

Specialists point out that there were many sound military reasons to shift from Titan to Minuteman silos, that it is technically questionable whether silos could be super-hardened effectively, and that this would be quite expensive.

At the time the Reagan plan was announced, Carter's defense secretary, Harold Brown, a technical expert, described the hardening idea as "whistling in the dark."

Yet the specialists who thus approve of the changes in the original plan also acknowledge privately that these shifts are contributing to a general problem of credibility and confidence that the missile project continues to have in Congress.

The other irony is that, by all Pentagon and Air Force accounts, the development of the missile is coming along "smashing well," as one officer put it. The rocket engines reportedly have been fired successfully several times, and the highly advanced engine control and guidance system seem amenable to mass production.

"It's a highly successful development program," one official said, "surprisingly trouble-free, on schedule, on cost and with no surprises" so far.

But the MX specialists still shrug their shoulders when asked if it is any more clear now than it was a year ago how the full 100-missile force will be based.

On Oct. 2, in conjunction with a five-year, $180 billion plan to modernize the nation's land-based missiles, missile-firing submarines, bombers and military communications network, the president and Defense Secretary Caspar W. Weinberger announced that 100 MX missiles would be produced. The first 36 to 40 would be installed in existing silos on an interim basis while studies continued on finding a permanent home that would be invulnerable to attack.

The official documents mentioned using Titan or Minuteman silos, but Weinberger, in his public comments, clearly indicated that it would be Titan silos, an indication that the specialists say they wish he had not made.

Some officials say that an overriding quest for secrecy among the handful of top White House and Pentagon leaders who knew what the final MX decision would be prevented a full airing of final technical details before they were announced publicly, and thus contributed to the subsequent reversals.

The decision to field the first 36 to 40 missiles in existing silos was calculated on two key points. It was felt that it was better to get the missiles into the field rather than put them in a warehouse while the Pentagon continued to seek a long-term basing solution.

The Reagan administration had rejected the Carter administration's "shell-game" program for MX, moving the missiles constantly among a number of silos to confuse the enemy, and a mini-shell game proposed by the Air Force. But after nine months of its own studies, it still hadn't found a different answer, and then embarked on further studies of three possiblities.

The second reason, officials say, lies in the pure threat of nuclear terror with which Washington and Moscow confront each other in the name of deterrence. Putting between 30 and 40 of these 10-warhead missiles in the field quickly meant there would be enough individual warheads to target every one of the 308 big Soviet multiple-warhead SS18 missiles, which are the main threat to the U.S. force.

While this does not mean Washington was planning a first-strike strategy, it was meant to balance the threat Moscow now poses.

The MX missiles are slated to begin coming off the production line in mid-1986, and officials say all 40 of the first batch should be installed within a year.

Officially, the Pentagon has until July, 1983, to figure out what to do as a permanent solution and tell Congress about that plan.

Top Pentagon civilian specialists say they are optimistic that by deadline time they will have decided to put MX in a new aircraft that can stay aloft for days, or bury it in extremely deep and new silos, or put it in other super-hardened and deceptively based silos and defend it against attack with a new anti-ballistic missile (ABM) system.

Officials also say more than one of these choices might be selected in a mixture that could complicate Soviet plans to attack these missiles and make it more expensive for Moscow to develop counterforces.

On the other hand, these same officials acknowledge that there are major unanswered technical and political questions about each of the proposed ultimate solutions to the MX problem.

Some say that the MX, despite $4.4 billion in the new budget, probably will fade from public and congressional scrutiny this year as the studies continue, but that the battle over what to do with these weapons will be fought all over again in 1983, because any of the available choices is certain to be extremely controversial.

Within the Pentagon, interviews produce no great sense of enthusiasm for the so-called "continuous patrol aircraft," once a favorite of Weinberger. Even getting money to study it is uncertain because of opposition in Congress.

The so-called "Citadel" idea of basing the MX in silos 3,000 feet below the surface also does not seem very popular, mostly because it would take a long time to get the missiles out and thus reduces the ability to return enemy fire promptly.

Major field tests are necessary, and there are potential political problems similar to those that hampered Carter's plans to field the MX in Nevada and Utah. Some officials say they believe the Citadel plan could be included if it is decided to choose a mixture of solutions.

Without any doubt, the greatest effort and most enthusiasm is linked to the idea of trying to defend the MX with an anti-missile defense system of some kind.

The budget for this development work doubles next year, and it is in conjunction with such a defense, if it could be made to work, that new super-hard silos may be built to protect both the missiles and the defensive radars and interceptors.

But here, too, there are huge problems, including figuring out how to shoot down one speeding missile with another.

There also would be the likely need to amend or junk the 1972 U.S.-Soviet treaty limiting such systems, which could produce big political controversies here and among Western European allies. It also could add greatly to the final cost of the MX.