The Army said yesterday that an experimental flight vehicle with the potential for providing a last-ditch defense against incoming enemy warheads scored its first bull's-eye against a stationary target 12,000 feet above the Earth.

The vehicle, known as FLAGE, for Flexible Lightweight Agile Guided Experiment, was shot Sunday at the target from a launcher at White Sands Missile Range in New Mexico. It found the target, an aluminum sphere 44 inches in diameter, by radar and sailed through it, the Army said.

The target was suspended from a balloon and kept immobile by a line anchored to an Army tank on the ground.

Army officials said that FLAGE represents breakthroughs in miniaturizing a radar "seeker" and in packing solid rockets the size of shotgun shells around the vehicle in such a way as to provide accurate steering at hypersonic speed.

While stressing that FLAGE is not a weapon and may never become one, Army officials said the vehicle holds promise for stopping shorter-range missiles targeted by the Soviet Union on Europe and for destroying longer-range warheads seconds before they would explode on the United States.

In these roles, FLAGE would constitute what the Pentagon calls a "terminal" defense because it would be used near the end of the flight of incoming warheads.

Defense Secretary Caspar W. Weinberger yesterday embraced FLAGE for its potential as part of President Reagan's Strategic Defense Initiative, or "Star Wars," research program.

Hailing success of the FLAGE shot, Weinberger in a speech at Virginia Polytechnic Institute & State University said, "This program is part of SDI research toward an effective counter to the tactical missile threat which we and our allies face. This fifth test in the Army's FLAGE series takes us closer to the guidance accuracy needed for non-nuclear intercepts of missiles within the atmosphere."

In contrast to Weinberger's speech at Blacksburg, Va., many Army officials have complained that their programs have been shunned by SDI project directors searching for more-exotic missile defenses than terminal ones such as FLAGE. Yesterday, for example, a high Army official, who declined to be quoted by name, said FLAGE has "no place in SDI's architecture."

Some critics have suggested that a terminal defense could be useful in defending small strategic targets, such as missile fields, but would not provide the broad umbrella of protection that Reagan has envisioned for cities.

Some SDI enthusiasts warn that unless some achievable missile defense such as FLAGE is shown to be effective between now and 1989 when Reagan leaves office, the next president many scale down or abandon the "Star Wars" program.

The FLAGE program was formerly called Sure Hit, for Small Radar Homing Intercept Technology. The $100 million program began in January 1983 and is scheduled to end Oct. 1. Army officials plan to move up to more-realistic experiments, such as attempting to shoot down a simulated warhead from an F4 aircraft.

If this test, now scheduled in the next couple months, proves successful, the Army plans to move the target to a higher altitude. Army officials said they hope FLAGE will show the capability to stop warheads 10 miles above the Earth.

Weinberger said SDI has made more technological breakthroughs than detractors thought possible. He cited development and testing of a megawatt continuous-wave laser; test of a charged-ion beam; "a major step," which he did not further identify, toward developing a mirror that could reflect destructive beams of light precisely, and a "remarkable breakthrough" at Auburn University for storing energy within lightweight capacitors.