The Soviet Union is testing a model of a reusable space vehicle -- similar in concept to the U.S. space shuttle -- the chief of a Soviet space research institute confirmed today.

Roald Sagdeyev confirmed that Cosmos 1614, which orbited the Earth on Dec. 19 and splashed down in the Black Sea, was an "experiment" in reusable space crafts. However, he said that his agency is not convinced of the shuttle's economic viability.

" . . . I can confirm we are considering technical and economical aspects of" reusable space crafts, he said. "We will certainly do much more research, because we are not completely convinced that the shuttle is economically . . . justified."

In answer to further questions, he added, "I don't think we will eventually have a copy of the American type."

In Washington, however, U.S. experts have said they believe the Soviets are preparing a testing program of a craft similar in design and purpose to the American shuttle.

Sagdeyev answered reporters' questions after a press conference on the Soviet "Vega" project, which is studying Halley's Comet with scientists from other countries.

Pictures of the Soviet model were taken by Australians in 1983 when a Soviet ship was seen retrieving a 10-foot-long craft, which resembled the U.S. space shuttle in outline, from the Indian Ocean.

The craft tested by the Soviets is considered to be a one-third-scale model of a space plane.

While the Soviet Union is also believed to be developing a larger reusable vehicle, in public statements, it has criticized the U.S. program as risky, expensive and part of a design to militarize space.