The United States and the United Kingdom are discussing the joint development and production of a combat airplane that military branches of both nations would purchase, Sir Kenneth Keith, chairman of Rolls-Royce Ltd., disclosed yesterday.

Under the project, British Aerospace and McDonnell Douglas Corp. would jointly build the AV-8B, a "super" version of the Harrier, a British Aerospace fighter that can take off and land vertically, Sir Kenneth said.

The Super Harrier, which would have double the payload of the current model and would incorporate the latest technology, would integrate the needs of both the United States Marine Corps and the Royal Air Force for a V/STOL aircraft for the 1980s Sir Kenneth said.

"This would bring together into a single airframe the latest technologyy and operational capability of British Aerospace and McDonnell Douglas," he said. "The initial assessments of this objective are encouraging."

Sir Kenneth's remarks were made in a speech to the National Aviation Club here with numerous high-level USMC officials in the audience. The Marines currently operate some Harriers.

A Rolls-Royce official said after the speech that the discussions are being held at ministerial levels. He said if the plan goes ahead, the new plane would be built with the assurance that the Marines would purchase up to 330 of them and the RAF would buy about 60 over the life of the program.

Rolls-royce produces the engines that power the Harrier.

Sir Kenneth also disclosed yesterday that Rolls-Royce plans to establish its first manufacturing and engineering facility in the United States possibly as soon as January.

Although he declined to elaborate, another source said the plant -- possibly located in Miami -- would produce components for the Rb-211 engine that is used on Lockheed's £1011 Tri-Star jets.The components would be flown to Britain to be fed into Rolls-Royce's assembly plant, the source said.

In explaining the reasons for setting up a facility in the United States, Sir Kenneth noted that the U.S. airlines buy almost 50 percent of the world's total output of planes and engines and U.S. aircraft companies build more than 80 percent of the free world's commercial aircraft.

Further, Rolls-Royce major two competitors in the sale of aircraft engines are the U.S.-based General Electric Co. and Pratt & Whitney Aircraft Group.

"It appears that the business is big enough to require three major engine manufacturers for as far ahead as one can see," Sir Kenneth said.