WASHINGTON _ If North Korea ever unleashed nuclear-armed missiles against America, the defense of U.S. cities and towns would depend to no small degree on something called a divert thruster.
These small rocket motors would be counted on to keep U.S. anti-missile interceptors on target as they hurtled through space toward the incoming warheads.
If the thrusters malfunctioned _ and they have a record of performance problems _ an interceptor could veer off-course, allowing a warhead to slip through. The consequences could be catastrophic.
So a lot was at stake when the Pentagon's Missile Defense Agency conducted the first flight test of a new and supposedly improved version of the thruster on Jan. 28.
An interceptor launched from Vandenberg Air Force Base in California was to make a close fly-by of a mock enemy warhead high above the Pacific. The interceptor's four attached thrusters would provide precision steering.
The missile agency issued a news release that day touting a "successful flight test." The agency's lead contractors were no less effusive. Aerojet Rocketdyne Inc., maker of the thrusters, said the new model "successfully performed its mission-critical role."
Raytheon Co., which assembles the interceptors, said the "successful mission proved the effectiveness of a recent redesign of the ... thrusters, which provides the control necessary for lethal impact with incoming threats."
In fact, the test was not a success, the Los Angeles Times/Tribune Washington Bureau has learned. One of the thrusters malfunctioned, causing the interceptor to fly far off course, according to Pentagon scientists.
The mishap raises fresh questions about the Ground-based Midcourse Defense system, the network of ground-launched interceptors that is supposed to protect the United States against a nuclear attack.
The system, known as GMD, has cost taxpayers more than $40 billion since it was declared operational in 2004. It is being expanded at a rapid pace despite stubborn technical problems that have undermined its reliability.
In carefully scripted tests, the system has failed to intercept and destroy mock warheads about half the time.
Project engineers for the Jan. 28 test had planned for the interceptor to fly within a narrow "miss distance" of its target to test the new thrusters' effectiveness.
That is not what happened. The closest the interceptor came to the target was a distance 20 times greater than what was expected, said the Pentagon scientists, who spoke on condition they not be identified.
"The mission wasn't successful," one of the scientists said. "Did the thruster perform as expected? No, it did not provide the control necessary for a lethal impact of an incoming threat."
A second scientist said the claims of success by the Missile Defense Agency and the contractors were "hyperbole, unsupported by any test data."
Asked for comment by The Times, the agency acknowledged, for the first time publicly, that a problem surfaced during the Jan. 28 exercise.
"There was an observation unrelated to the new thruster hardware that has been investigated and successfully root-caused," the agency said in a written response to questions. "Any necessary corrective actions will be taken for the next flight test."
Among aerospace scientists, "root cause" refers to the source of a system failure or malfunction. It is typically identified through an exhaustive review by an expert panel called a "failure review board."
The agency statement did not offer any detail on the "observation," or explain what went wrong during the test that required an investigation. It did say that "the new thrusters performed as designed" and within "critical performance parameters."
Asked whether any of the thrusters stopped working at any time during the exercise, the agency declined to comment.
It also declined to say how close to its target the interceptor was supposed to fly, or how close it had actually flown. That information is "classified and not releasable," the statement said.
A spokeswoman for Raytheon referred questions to the missile agency. Aerojet Rocketdyne representatives did not respond to emailed questions.
In appearances before Congress, the agency's director, Vice Adm. James D. Syring, has expressed no concerns about any aspect of the test, including the thrusters' performance.
On April 13, he told a Senate defense appropriations subcommittee that the test had "enhanced" the agency's confidence in the GMD system.
Syring was unavailable to comment for this article, according to an agency spokesman.
When Syring was delivering his positive message on Capitol Hill, engineers and technicians at the Army's Redstone Arsenal in Huntsville, Ala., were analyzing data from the test to figure out what went wrong, according to the Pentagon scientists interviewed by The Times.
The scientists said a possible culprit was a faulty solenoid valve _ a circuit that supplies power to the thrusters from the interceptor's battery.
If that circuit turns out to have caused the malfunction, it would represent a serious technical problem in the complex web of components that the thrusters depend on. A breakdown at any point can spell failure.
Philip R. Coyle III, a former director of operational testing for the Defense Department, said any malfunction involving the thrusters should prompt serious concern.
"It tells you that there is a problem that you really need to get your arms around, or else it's going to bite you," Coyle said.
