It's a bird, it's a plane, it's the secret Soviet space shuttle

The Soviet Union has confirmed what Western experts have long suspected: that it is testing a new generation of reusable space vehicles. The vehicles are thought to include a space shuttle - essentially a copy of the American shuttle - a space plane, and a space ''tug.''

Such vehicles would undoubtedly move this country closer to its goal of permanently manned space stations. These vehicles and space platforms could be used to attack satellites, or perhaps even targets on earth.

Because of their military potential, Western experts are carefully monitoring Soviet progress in reusable space vehicles. It is yet another sign of the emergence of space as a new arena of military competition between the superpowers.

Roald Sagdeyev, an official of the Soviet Academy of Sciences, recently admitted to reporters that a scale model of one of the craft - presumably the space plane - underwent a test flight on Dec. 20. His comments were the first official confirmation that testing of reusable space vehicles is under way here.

While Western analysts say the military capability of such vehicles is undeniable, the Soviets say they are contemplating only peaceful activities in space.

But doubts persist in the West. Satellite photos of the Soviet space shuttle show it to be ''quite a bit like the American shuttle,'' says a Western diplomat. ''Curious, isn't it?'' he adds, smiling.

The Soviets have essentially copied the American space shuttle, Western analysts say. Apparently, the only major difference is that the main engines would not be reusable.

These analysts expect a Soviet shuttle launch before 1990. There is some speculation, however, that the Soviets may speed up the date so as not so as not to fall too far behind the US.

''They're certainly keeping busy down there,'' says one Western analyst, referring to the shuttle launch facility at Tyuratam, in the Central Asian republic of Kazakhstan.

The Soviets deny they've copied the US shuttle, and say they are not even sure that reusable space craft make economic sense. That, said Mr. Sagdayev, is why the current tests are being conducted. When they are over, he said, it's doubtful whether the Soviet Union will end up with ''a direct copy of the American model.''

Konstantin Feoktistov, one of this country's top spacecraft designers, wrote last month that the American shuttle was unreliable, difficult to land, and too costly. A better approach, he said, would be wingless craft that are launched on single-stage rockets and that rely on parachutes to land.

To be sure, the Soviets are building new, more powerful booster rockets to carry bigger payloads into space. One is expected to be able to lift loads from five to seven times heavier than the US space shuttle, perhaps hinting at the scope of Soviet ambitions in the cosmos.

But the Soviets are giving few clues as to the nature and extent of their experimentation.

The test flight of the scale-model space plane is a case in point. Tass, the official Soviet news agency, originally reported only that a satellite, which it called Cosmos-1614, orbited the earth and made a controlled descent into the Black Sea.

Western experts quickly guessed it was the scale model space plane making a test flight. It was more than a week later, and only under prodding by reporters , that Sagdayev confirmed the speculation (after first claiming he had no information on the subject).

It is believed this month's test was the craft's fourth test flight. During two earlier tests, Australian Navy observers were able to photograph the recovery of the model after it splashed down in the Indian Ocean.

The pictures show a stubby spacecraft, only about 10 feet long, that looks like the American space shuttle in miniature. Presumably, results of the test flight will be used to modify the full-sized space plane.

Ironically, these developments are taking place even as Moscow continues to call for a halt to what it terms the ''militarization of space'' by the US. There is, according to Tass, evidence of an ''ever growing militaristic direction of the (American) space shuttle program.''

Indeed, a Jan. 23 launch of the American shuttle Discovery will reportedly carry a new military-intelligence satellite into orbit over the Soviet Union. And the American military has made strenuous efforts to prevent the news media from reporting on specifics of the coming mission.

But those measures pale beside the Soviet Union's own efforts to keep its space program under wraps. Western journalists are never allowed to cover Soviet space launches from the launch sites. Indeed, the latest edition of the Soviet road atlas does not even show Tyuratam, the main launch facility. Similarly, the Soviets do not even admit to the existence of a facility at Plesetsk, north of Moscow, from which many military satellites are launched. There are unconfirmed reports that, even now, the Soviet shuttle is being tested there.