After two successful tests of an Army weapon designed to blast enemy missiles out of the sky, the Pentagon announced yesterday that it will skip further prototype testing and move quickly toward final development of the $15.4 billion project.

The acceleration of the Theater High Altitude Area Defense (THAAD) program reversed a Defense Department decision only last year to require three intercepts before moving ahead. The stiffer requirement had been imposed after THAAD failed its first intercept attempts, raising questions about the weapon's design and about Lockheed Martin Corp.'s management of the project.

Defense officials said the two recent successes--in June and earlier this month--reinforced confidence in Lockheed and the program. But making the prototypes work, they added, had diverted too much attention from the ultimate goal of designing a production model.

"Rather than spending months and millions of dollars on another THAAD prototype launch only to prove a point, we have decided to get on with the business of engineering development of the real thing," Jacques Gansler, the Pentagon's chief of acquisition, said in a letter to Congress. "This will also accelerate the ultimate fielding of THAAD."

Still, critics of the project, which is several billion dollars over initial projections and years behind schedule, have cautioned that two intercepts under tightly controlled conditions and after six failures in four years is hardly proof that all hurdles have been overcome. They have urged testing THAAD against more challenging targets, under more variable conditions.

In fact, the next phase will involve as many as 40 more flight tests, according to Army Maj. Gen. Peter Franklin, deputy director of the Ballistic Missile Defense Organization.

Current plans call for fielding THAAD in 2007, although defense officials are looking at the possibility of deploying some interceptors earlier.

Just how soon THAAD can be put into the hands of U.S. troops to protect them from missile attack depends, in part, on whether the Army or Navy--which also is developing a high-altitude, antimissile interceptor--is chosen by top Defense Department officials to take the lead.

Worried about the cost of funding both efforts, Defense Secretary William S. Cohen put the Army and Navy on notice in January that one of them might have to take a back seat.

THAAD relies on a powerful new tracking radar, land-based launchers and missiles equipped with special sensors to home in on enemy warheads. Its purpose is to guard U.S. and allied troops from medium-range ballistic missiles such as those being developed by North Korea, Iran and Iraq.

But THAAD's development also has implications beyond battlefield defense. The operational concept on which it based--hitting a speeding missile with another speeding missile--is the same "hit-to-kill" concept that is at the core of an even more ambitious system under development to protect the entire United States, a successor to the Reagan-era Strategic Defense Initiative, also known as "Star Wars."

While the timing and cost of deploying a national system remain a source of tension between the Clinton administration and Republican lawmakers, there is strong consensus on the need to equip troops as soon as possible with antimissile defenses more effective than the Patriot system used against Iraqi Scud missiles during the 1991 Persian Gulf War.

Even one leading skeptic of America's ability ever to produce a fail-safe missile defense system offered little more than a shrug at the news that THAAD is skipping ahead.

"The tests done so far haven't said much about how the system would perform in combat, so doing one more wouldn't have made much difference," said John Pike, a defense analyst with the Federation of American Scientists. "I may be prepared to cut the Pentagon more slack on this one because I have such low expectations about what they'll be able to eventually deliver."

Skipping a Stage

The Pentagon gave a tentative go-ahead for the Theater High Altitude Area Defense system to enter final development phase. Here is how THAAD is designed to work.

1. THAAD radar detects incoming missile, transmits target position and trajectory to tactical operations.

2. Tactical operations relays signals to remote launchers.

3. Mobile launchers can launch a dormant missile within seconds of command.

4. Radar communicates target and interceptor positions. Missile adjusts in flight using own sensors, zeroing in on target. Radar determines if kill is made.