The MX - again

ONCE more the MX missile is tangled in a battle between a President who wants it and a Congress which increasingly suspects that it is an expensive waste of money.

It is expensive. Since the launching of the project in 1974 a total of $10.5 billion has been spent. The Air Force estimates that it will cost $26 billion more to complete the existing program for building and deploying 100 MX missiles. There has been a preliminary flight test of one MX. It was called successful. But none is yet ready for deployment.

What is the controversy all about?

The most helpful exercise to me in my own efforts to understand it is to start with one particular feature of the big intercontinental strategic missiles. The experts rate missiles in many ways; throw-weight, range, numbers of warheads - and ''circular error probable,'' or CEP.

CEP means accuracy. If a warhead can be delivered within a certain radius of the launch pad of an opponent's missile then, it can, in theory, knock out that opponent's missile.

The newest and latest American long-range missile (ICBM) is the Minuteman III. It comes in two versions. There are 250 missiles (launchers) with 3 Mark 12 warheads each and with a CEP rating of 280 meters (308 yards). There are 300 more with 3 Mark 12A warheads each, and a CEP rating of 220 meters (242 yards). That gives the US 2,250 land-based, long-range warheads with an accuracy rating of better than 300 meters.

The Soviets' best-rated missiles are at 300 meters. They have 308 SS-18s, some of which have a CEP of 450 meters and others at 300 meters. They also have 330 of the SS-19s with a CEP rating of 300. But the SS-18 carries 10 warheads. The 33-19 carries 6.

So the United States leads in accuracy in this super-accurate area, but the Soviets have a lead of nearly 2 to 1 in numbers of warheads.

The MX is designed to have the same super-accuracy as the Minuteman III, or better. And each one will carry 10 warheads. So the 100 MX missiles the Reagan administration wants would give the US another 1,000 warheads able, in theory, to take out the big Soviet SS-18 and SS-19 missiles. That would make a total of 3,250 warheads aimed at 638 Soviet missile silos. In theory, this ought to be more than enough to give the US adequate ''counterforce.''

But the MX is too big to travel. It is to be planted in fixed silos. Hence it is vulnerable to any Soviet missile with a high accuracy rating. Also, any missile with 10 warheads is an attractive target. That is why the Scowcroft Commission, which studied the problem of the MX, recommended that research begin at once on a smaller, mobile missile with a single warhead. It is nicknamed the ''midgetman.''

The Navy's new Trident missiles have an accuracy rating of 450 meters, which is very good for a sea-launched ballistic missile. There are 264 Trident missiles deployed now and each carries 8 warheads. That means 2,112 warheads on submarines, which move so quietly that the Soviets are unable to intercept and track them. These are the most secure and least vulnerable nuclear weapons in the world today.

Add that the US Navy expects its next generation of Trident missiles to equal in accuracy the present Minuteman IIIs. If so, the US would have a wide advantage over the Soviets in strategic nuclear ''counterforce.''

So why put another $26 billion into the MX, which makes a tempting target for the Soviets, when the money could get us ''midgetmen'' or better Tridents?

Then there is the idea favored by the NATO command of improving NATO's conventional capability to the point where the West could be defended without recourse to nuclear weapons. It is estimated that the $26 billion needed to finish the 100 MXs would be enough to defend NATO with conventional weapons.

The House of Representatives agreed last week to authorize funding for another 15 MXs (21 were previously authorized). The argument was that it would give the President bargaining power to use in negotiating with the Soviets. But why would the Soviets trade something of value against 15 MXs? The Soviets have plenty of extra warheads in the 300-meter CEP range with which to take them out.

The case for putting the money instead in Tridents, ''midgetmen,'' or conventional weapons is impressive.