Why US is adding 'sting' to Saudi defenses

The military balance of power in the troubled Persian Gulf region increasingly focuses on a kingdom with modern weapons but little experience or appetite for conflict, and a Goliath of sorts that is forced to remain on the sidelines.

The first is Saudi Arabia, and the second is the United States.

As military analysts look at the threats that continue to spread from the lengthy Iran-Iraq war to neighbors in the Middle East and beyond, they see a situation in which theoretical military advantage is not necessarily the key determinant.

If Saudi Arabia were to jump wholeheartedly into the war against Iran, it might easily tip the balance in Iraq's favor. And if the US had only sporadic attacks on Gulf shipping to worry about, it could do the same. But geography and geopolitics are complicating the issue for both countries.

Saudi Arabia is a country of vast territory and obvious wealth, but relatively small population. Its royal family is well regarded, and Islamic law is strictly enforced. But there is concern that Islamic extremism of the kind promoted by Iran's Ayatollah Khomenei could become a problem, especially if the country were drawn into war.

''I would be very uncomfortable if I were a Saudi prince and I saw that prospect developing,'' former White House national security adviser Zbigniew Brzezinski observed last week.

The US, on the other hand, is unequaled in its ability to project conventional military power around the world. The striking power of its amphibious ground force and aircraft carriers cannot be matched. Yet such capabilities alone are not particularly helpful in this case - even though (according to those who have been part of such planning) the US has explored contingencies for military intervention in the Gulf region short of nuclear attack.

The main dampers on US military action are lack of an invitation from friendly Gulf states and the prospect that the Soviet Union could be drawn in if Iran were to become any less stable than it already is. ''We are not in a position to dictate or to conciliate, since we have no relationship with Iran and practically no relationship with Iraq,'' Dr. Brzezinski said. ''The Soviets gain if we do nothing or if we do something on our own.''

Still, attention in Washington is focusing on the military balance in the Gulf and efforts to tip that balance in Saudi Arabia's favor.

The kingdom has an Air Force of about 170 combat aircraft, including about 60 US-built F-15 Eagle interceptors, the US Air Force's most advanced jet. It also has about 90 F-5 jets that are suitable as fighters and ground-attack aircraft. The US has agreed to sell AWACS command-and-control aircraft to Saudi Arabia, and operates four AWACS out of Riyadh.

The Saudi arsenal also includes 18 antiaircraft artillery batteries and 18 surface-to-air missile batteries (16 of which can fire the advanced US-made Hawk missile).

The Saudis also can field 150 M-60 tanks and a variety of mortars, howitzers, and guns of the type used by US forces. Saudi Arabia is expanding its Navy, which now includes attack craft that can fire the Harpoon antiship missile. Its armed forces number 51,500 men, plus a National Guard of 25,000.

Iran (whose population of 41.5 million outnumbers Saudi Arabia by about 4 to 1) has an estimated 2 million men and boys in uniform. But its military hardware is far less formidable. Its Air Force is estimated at ''some 70 serviceable combat aircraft,'' according to the London-based International Institute for Strategic Studies (IISS). It is listed as having 90 F-4 Phantom jets (the workhorse of US forces during the Vietnam war), but the IISS questions whether more than 12 of those are serviceable.

The Iranian Air Force also includes 77 F-14 Tomcat jets (the US Navy's top fighter, which Iran bought before the fall of the Shah). But because the spare-parts pipeline has been cut off for more than four years, no more than a half-dozen of the F-14s are thought to be a threat. The weapons fired against ships in the Gulf by Iranian jets are thought to be relatively small rockets designed to be used against other aircraft rather than more lethal antiship missiles like the French-made Exocet.

Making up for its shortfalls in armament are the fervency and experience with which Iran's forces fight. Surprise attacks by a handful of planes - particularly if flown by those accepting suicide missions - are very hard to thwart.

This is one reason that the Reagan administration has decided to immediately send 400 Stinger antiaircraft missiles - and with congressional approval, as many as 1,200 - to Saudi Arabia. The Stinger is a shoulder-fired, heat-seeking rocket designed to be used against low-flying aircraft.

The missile could be especially effective if deployed aboard oil tankers or Saudi naval vessels accompanying such ships. The US also would like to send two aerial refueling tankers to help the Saudi F-15s protect Gulf shipping. The US already operates three such tankers in conjunction with AWACs aircraft.

But with Israel concerned that mobile antiaircraft rockets could end up in the hands of Palestinian fighters or Syria, Congress is unlikely to approve the larger shipment of Stingers without a fight on Capitol Hill. Israel also opposes making additional aerial tankers available to the Saudis.

US forces near the Gulf center on the aircraft carrier Kitty Hawk and its fighter and attack jets, plus the support ships accompanying it as it operates in the Arabian Sea.

Reaching targets in the Gulf without air refueling or airbases ashore, however, would be difficult. States in the region have rejected US overtures to provide such bases.

The US Central Command (formerly the Rapid Deployment Force), which oversees the 19-nation region, has maintained a ''forward headquarters element'' of several US Navy ships in the Persian Gulf.

But US officials are concerned that these ships could come under fire. They now are being withdrawn to the naval facilities maintained on the island of Diego Garcia, some 2,500 miles away in the Indian Ocean.