With a roar like muffled thunder, a hugh section of prefabricated subway tunnel for the Washington Metro was launched into the Susquehanna River at a shipyard here today.

After being partly filled with concrete that will form part of the eventual trackbed, the hollow 2-million-pound stl hulk that is longer than a football field will be towed down the Chesapeake Bay and up the Potomac River and installed underwater in the Metro system.

Along with two nearly identical sections that are still being fabricated here, the tube will be sunk later this spring into a 40-foot-deep trench that is now being scooped from the muck on the floor of the Washington Channel, off the Maine Avenue waterfront.

Thousands of motorists passing on the nearby Southwest Freeway view the scooping operation each day.

After the three sections are joined together, forming a continuous link similar to an underwater pipe between the mainland of Southwest Washington and the Hains Point Peninsula, the metal tube will be covered again with the muck.

Then tracks will be installed in the tube as part of line permitting trains to run directly from 7th Street in Washington to the Pentagon in Arlington County. A bridge has been built to carry the Metrorail across the Potomac. The line will not be opened before 1981.

Metro's first crossing of the Potomac, scheduled to be used by passenger-carrying trains starting July 1, is a tunnel dug by orthdox mining methods through bedrock between the Foggy Bottom station in Washington and the Rosslyn station in Arlington.

Today's launching took barely five minutes at the Wiley Manufacturing Co. shipyard that occupies a slender shelf of land at the base of sheer granite bluffs on the Susquehanna's north bank some 400 miles northeast of Baltimore.

The yard has been here since 1919 producing tugboats, barges and, in recent years, subway and highway tunnel station for Norfolk, Boston and New York City. The Metro tunnel sections are being produced by Wiley at a cost of $39 million under a contract with Perini-Horn-Morrison-Knudson, the prime contractor for Metro on the Washington Channel crossing.

Although the section launched today is the first of its type for Washington, the same method of sunken trench installation was used successfully on the much longer underwater tube that carries Bay Area Rapid Transit (BART) trains between San Francisco and Oakland.

When the workmen and spectators arrived today for the allaunching, scheduled for shortly after 8 a.m. because of a favourable high tide, the tunnel segment was poised on a shipway looking like a huge oval-shaped cigar 340 feet long, 21 feet high and 37 feet across.

Temporary steel coverings are fastened to both ends, making it watertight until it can be submerged in Washington.

At 8:15, on a signal from a superintendent, welders took torches and, in a carefully synchronized manner, began cutting through steel beams that held the hulk on the shipway. Then other workmen began pumping on jackhandles that exerted pressure on the side of the tunnel.

St 8:18, the hulk began sliding sideways, almost imperceptibly at first, then gaining speed as it moved down the sloping ramp covered with a thick layer of orange grease.

As it hit the water sideways, waves splashed up on both sides.There was a brief sound, like muffled thunder.

Before the hulk stopped bouncing in the water, tugboats picked up heavy lines and began towing it to a Concrete will pumped into it, prodock where it will remain for a week, viding enough weight to keep it from capsizing on the four-day voyage to Washington.

Arriving in Washington in about 10 days, it will be tied up at Hains Point and more concrete will be pumped in, preparing for the eventual sinking in the trench.