Navy ships that stand up to missiles? Look for Bath label

America's newest guided-missile frigate and the now-sunk British destroyer Sheffield both have aluminum superstructures. But it's unwise to jump to any hasty conclusions.

So says Capt. Charles L. Mull, who retired last year as the Navy's supervisor of shipbuilding at the Bath Iron Works (BIW).

When the Sheffield was hit by a sea-skimming Argentine missile May 4, its aluminum superstructure (the part above the main deck) melted. The melting point of aluminum -- far lower than steel -- was blamed by some for the severity of the blaze.

Some observers have expressed concern that the new US guided-missile frigates (FFG-7s) also have aluminum superstructures. And other questions have been raised about the ships' defensive capabilities and the relatively exposed position of its sensitive electronic gear.

The US Navy remains reluctant to comment on the Sheffield incident or on the way in which that ship was deployed. But a Navy spokesman does point out that standing orders for more FFG-7s in future years, all with aluminum superstructures (the hulls of the US frigates are steel, as was the hull of the Sheffield) should signal the confidence the Navy holds in its ships.

Captain Mull, who joined in the planning for the FFG-7 in 1973 and has been called the ''father'' of the program, is less reluctant to speak. Captain Mull served as supervisor of shipbuilding for the FFG-7 program at BIW. He notes that the use of aluminum in Navy ships began in the 1950s as complex systems - missile launchers, guns, radars, magazines - continued to be added to topside areas. These required a lighter material than steel for superstructures to keep the ships seaworthy and stable.

''Yes, aluminum melts,'' Mull agrees. But he adds that comparing the FFG-7s with the Sheffield simply because of their common use of that metal is still comparing apples and oranges. ''There are factors in the Sheffield incident that don't apply'' to the FFG-7s, he says. ''The Sheffield was by herself, she had no air cover.'' In many ways, he says, ''she was a sitting duck.''

''The FFG can detect low-flying missiles'' and defend itself against them, Mull points out. And if hit, ''it has fire walls to minimize the spreading of the fire within the superstructure.'' The Navy's chief inspector, a veteran of World World II sea battles, insisted on the change after the first sea trials in 1977, Mull says.

In addition, according to a Navy spokesman, the ships contain both Halon gas and foam fire-control systems. And the aluminum superstructure is strengthed at key points by a polycarbon fabric called Kevlar.

''Ships the size of a frigate are always vulnerable,'' Mull concedes, 'but the probability of a single hit sinking one of these ships is low.''

Most important, he notes, the FFG-7s are designed to work in concert with a task force formed around an aircraft carrier. While the frigates go about their job of protecting the task force from threats from enemy submarines, surface ships, or aircraft, carrier-based aircraft provide added protection for the frigates. The Sheffield had no such air cover.

If a missile -- such as the French-made Exocet that sank the Sheffield -- did elude all other defenses, the FFG-7s are equipped with a ''close in'' defensive system to shoot it down. The Phalanx -- a gattling gun that fires 3,000 rounds per minute -- uses ''closed loop'' radar to guide its shells to their target. It is being installed on new FFG-7s and will be retrofitted onto earlier versions. ''If an Exocet missile came at one of our ships equipped with Phalanx, and all other systems failed to stop it, the Phalanx would destroy it,'' says a Navy spokesman confidently, adding, ''The British don't have this.''

That the ships are being built to the Navy's standards is not a point of contention. Nestled here on the Kennebeck River along Maine's scenic southern coast, Bath Iron Works has an developed an international reputation for producing quality ships on time and under budget. Last fall, the Navy itself termed the FFG-7 program at BIW ''one of the most successful in naval shipbuilding history.'' BIW ''delivered the first seven FFG-7-class vessels at a total of $44 million under budget and a total of 99 weeks (nearly two years) ahead of schedule,'' the Navy report concluded. ''This trend of early-delivery/below-cost construction shows every indication of continuing.''

The FFG-7s are meant to play a key role as part of the Navy's modernization effort, replacing aging destroyers. They carry innovative, computerized defensive and offensive weapons directed from a command room chockful of ''Star Wars''-style video displays. Due to automation, the ship operates with a crew of 185. A crew of about 350 is needed to man the destroyers it replaces.

In May, the yard launched the 10th of the 23 FFG-7s it is scheduled to build. BIW is still ahead of schedule and cost, although the Navy has asked it to no longer release the exact number of weeks or dollars. (The Navy would like to budget for 60 of the ships over the next few years; Todd Shipbuilding Corporation on the West Coast is also building the frigates.)