Shipping natural gas under polar ice; General Dynamics has a dream - a fleet of gigantic submarine tankers

You've heard the question, ''But will it fly, Orville?''

There is an underwater version making the rounds in the drafting rooms of naval architects - ''But will it sink, GD?'' The new twist, plus the initials, stems from General Dynamics Corporation's proposal to build a submarine version of the Empire State Building.

General Dynamics is the nation's leading builder of nuclear submarines for the US Navy. It hopes to transform that military submarine concept into a commercial supertanker and build a fleet of underwater behemoths that will ferry liquefied natural gas (LNG) on a 3,200 mile voyage beneath the Arctic icecap to ports on the east coast of Canada and in Europe.

It is considered the most fascinating concept yet advanced as a solution to the major problem of the Alaskan North Slope: Now that the producing wells are working, how do you move the product to world markets in large enough quantities to make the whole effort worthwhile?

The key question that must be answered, says Capt. William McMullen, head of the Department of Nautical Science at the US Merchant Marine Academy, is, ''Do heavy tonnages move under water more cheaply than on the surface, in a polar environment. It is not so much a cargo handling problem, it is a problem of economic justification.''

General Dynamics says it can provide the justification. ''The primary advantage offered by a submarine system over a surface-ship system is the ability to deliver a constant cargo volume at uniform, predictable schedule intervals the year-round, regardless of surface ice and weather conditions,'' says L.E. Holt of General Dyanmics.

General Dynamics says it believes the concept would be competitive economically with a surface icebreaking tanker system. It is considerably lower in cost than any of the proposed natural gas pipelines at present languishing in economic limbo for lack of some $45 billion-plus in financing.

The shipbuilders are proposing two versions of the tanker, one nuclear powered, the other nonnuclear. The nonnuclear version would be 1,470 feet long. The nuclear version would be 220 feet shorter. Each would have a beam width of 228 feet and a depth from deck to keel of 92 feet. They would remain underwater all year and have a crew of 32.

The nonnuclear propulsion plant would burn methane in supercharged boilers for turbines capable of delivering 50,000 horsepower through two shafts. Speed would be 12 knots or about 14 m.p.h. Plans are to steam at an operating depth of 600 feet below the ice. Each ship would be equipped with various kinds of sonar to read not only bottom configurations but top ice as well where spikes sometime drop down more than 1,000 feet.

The LNG, kept at -260 degree F., would be carried in six 341-foot cylindrical cargo tanks with a total volume of 140,000 cubic meters per ship (or enough natural gas to meet the present needs of the city of Philadelphia for a month.) A main, variable ballast tank running down the center of the submarine would provide the necessary buoyancy and handling stability for submerged running with or without cargo.

The General Dynamics concept proposes a fleet of 17 nonnuclear subs rather than a single vessel, which the company says would be uneconomical. At $700 million per ship - or 14 nuclear at $725 million per ship - the fleet would have a combined capacity of 2 billion cubic feet of natural gas per day, every day. In effect, this would result in a floating pipeline, allowing subs to load and unload LNG at underwater terminals in an estimated and economical 24-hour turnaround time.

''I can't imagine the amounts of steel that will go into it,'' says Prof. Norman Hamlin of the Webb Institute of Naval Architecture on Glen Cove, N.Y. The US steel industry, along with ailing domestic commercial shipyards, certainly like trying to. But the boon for US shipyards would be mixed as the final assemblege would occur in Europe. No US shipyard is physically big enough.

''You can't overcompartmentalize a tanker, and the complexities of carrying LNG are difficult enough. By adding it to submarines makes it even greater,'' cautions Professor Hamlin.

William L. Fink, program manager for the Maritime Administration in the Department of Transportation, says he thinks General Dynamics has the expertise to meet the challenge of carrying LNG cargo beneath the ice. The company builds its case on the marriage of its Electric Boat Division, the leading designer and builder of submarines for the US Navy, with its sister division, Quincy Shipbuilding, a leader in surface LNG tanker design and technology. Negotiations with West German shipbuilders are in progress to form a consortium to handle the project.

The LNG plan also has generated interest in some quarters of the US State Department since it encourages alternate gas supplies to European markets other than the proposed Soviet gas pipeline into West Germany.