The Air Forca this past weekend formally requested defense conctractors to sumit designs within 45 days for the MX intercontinental missile, the nuclear blockbuster that could be deployed by 1985 if arms negotiations rail to ban such weapons.

Although the request represents another step toward building the MX, President Carter has not made the decision to advance the missile from its current paper design phase to a flying prototype. He could postone that decision until next year because where is still much paperwork to do, including the evaluation of completing designs the Air Force has just requested.

Defense Secretary Harold Brown embraced the concept of building a mobile MX by earmarking $245 million for it in draffting the Pentagon budget for fiscal 1979, which begins Oct. 1, 1978. But in House hearings recently made public, Brown indicated that both the United State and the Soviet Union would be better off if the United State deployed the cruise missile instead of the silo-busting MX.

If, however, the United State decided to modernize its ground-launched intercontinental missile force, "I think that the MX is the bestoption I have seen for retaining a land-based intercontinental ballistic missile as part of the triad," Brown told the House Defense Appropriations Subcommittee.

The MX would be the United States' first mobile intercontinental ballistic missile. It would roll back and forth inside an underground tunnel 10 to 20 miles long so that Soviet rocket forces could not pinpoint its location. The nation's current force of 1,054 land-based lmissiles stands still in underground silos.

The United State and Soviet Union both are developing intercontinental missile accurate and powerful enough to knock out the other's fixed missiles, despite tons of protective concrete. And some arms specialists argue that the United State must deploy 300 mobile MX missiles to make sure the "triad" of land-based missiles, submarine-based missiles and bombers remains a convincing threat.

The triad is designed to deter nuclear war by making a first strike against the United States look like a losing proposition to the would-be attacker.

Opponents of the MX counter that building such nuclear blockbusters would put a hair trigger on nuclear warfare because each side would worry about losing its land-based missiles to such accurate and powerful weapons. Rather than risk losing them, continues the argument, each superpower would be more tempted than ever to fire off its land missiles at the first warning of attack - perhaps in response to an accisental launch.

Brown, in discussing with the subcommittee how to protect the triad in this age of nuclear sharpshooting, said he felt uneasy about putting a larger proportion of the nation's missiles in submarines where they are currently invulnerable to surprise attack. He said there is always the chance that the Soviets might find a way to detect submarines or develop an effective defense against their missiles.

Since "we could not introduce the MX as a mobile missile before about 1985," Brown continued in his congressional testimony, the United States needs some additional weaponry between now and then to maintain this strategic balance.Arming bombers with cruise missiles, which be ready by 1980, he said, is the best way to improve the balance for the early 1980s.

Brown added that the MX would complicate arms control efforts by the United States and the Soviet Union . The Soviet reconnaissance satellites could not tell how many MX missiles were inside one long tunnel.

From the Soviet point of view, said Brown, the bomber-cruise missile combination would provide hours of warning time and thus should be less worisome to Soviet leaders fearing a strike at their missiles by the MX blockbuster.

"If both sides have major forces that cannot be destroyed in a first strike," Brown said, "it argues somewhat against adding first to our intercontinental ballistic missile force or for the matter our" submarine missile force "if we can do what we need to now" by improving the bomber force by adding cruise missiles.

Aithough "there may well be a need to deploy" some advanced land-based missile like the MX in the mid-1980s, he said, "none of the land-based ballistic missile options" looks better than the bomber-cruise missile combination "as a way of retaining our retaliaotory capability at parity with the Soviets through the 1980s."