WASHINGTON -- The Pentagon is pushing ahead with an expansion of the nation's homeland missile-defense system instead of replacing rocket interceptors affected by a newly recognized deficiency.
The problem threatens the performance of small thrusters attached to the interceptors. In the event of a nuclear attack, the thrusters would be relied on to steer interceptors into the paths of enemy warheads, destroying them.
If a thruster malfunctioned, an interceptor could fly off course and miss its target, with potentially disastrous consequences. The interceptors are the spine of the Ground-based Midcourse Defense system, the nation's primary protection against a missile strike by North Korea or Iran.
The problem affecting the thrusters came to light as a result of the system's most recent flight test, on Jan. 28, 2016, when an interceptor was launched from Vandenberg Air Force Base in California.
It did not go as planned. One of the interceptor's four thrusters shut down during the test, causing the interceptor to veer far from its intended course.
The U.S. Missile Defense Agency and its lead system contractors nevertheless touted the exercise as a success, making no mention of the malfunction.
Now, the first public explanation for the thruster malfunction has emerged in Pentagon documents and from interviews with missile-defense specialists.
A review board formed by the missile agency linked the failure to a circuit board that powers the thrusters. The most likely explanation, the panel said, was that a "foreign object" in the interceptor's internal guidance module came loose, fell onto the board and caused a short circuit.
The review board did not say what the foreign object was, but government and independent scientists say it could have been a wire fragment, a piece of soldering material or other debris.
Of the Ground-based Midcourse Defense system's 37 operational interceptors, 34 are equipped with older circuit boards vulnerable to the same kind of problem, according to missile defense specialists, including former and current government officials.
The missile agency is in the midst of expanding the system to a planned total of 44 interceptors by the end of this year. The 10 newest interceptors will have circuit boards made with "improved manufacturing processes," an agency spokesman said.
But agency officials do not plan to retrofit or repair the older circuit boards, which were manufactured differently. Spokesman Christopher Johnson, in written responses to questions from The Times, said that based on the review board's analysis, "no corrective actions are needed."
Independent missile-defense experts, however, said the review board's findings signal a weakness in the 34 older interceptors, which now make up more than 90 percent of the fleet. When the expansion is finished, they will still account for three-fourths.
Former Assistant Defense Secretary Philip Coyle III, who led the Pentagon's office of operational testing and evaluation for six years, said the January 2016 test failure was cause for serious concern about the Ground-based Midcourse Defense system's reliability.
"If the circuitry caused a failure, that's a bad thing," Coyle said. "One out of four [thrusters] failed, and that's important. I don't think anybody should whitewash that."
The nation's defense against a nuclear attack by China or Russia relies on deterrence: the Cold War doctrine that none of the major nuclear powers would strike first out of fear of retaliation.
The Ground-based Midcourse Defense system, in contrast, was designed to thwart a "limited" strike by a nonsuperpower such as North Korea by intercepting and destroying incoming warheads.
After a January 2010 test in which a thruster shut down, officials blamed a missing fastener in the thruster assembly. In December of that year, an interceptor again missed its target, and the failure was attributed to severe vibrations caused by the thrusters' "rough combustion" of fuel.
In 2011, Lt. Gen. Patrick O'Reilly, then the missile agency's director, shut down production of interceptors in an effort to correct technical shortcomings exposed by the test failures.
O'Reilly retired in late 2012, and production resumed under his successor, Vice Adm. James Syring, as President Barack Obama's administration prepared to expand the Ground-based Midcourse Defense system fleet -- a decision supported by bipartisan majorities in Congress.
In the January 2016 test, the interceptor launched from Vandenberg was supposed to perform maneuvers before making a close flyby of a mock warhead in space.
As the flight unfolded, one of the thrusters stopped firing. Project engineers had planned for the interceptor to fly within a narrow "miss distance" of its target. In fact, the closest it came was a distance 20 times greater than expected, according to Pentagon scientists who spoke on condition of anonymity.
The findings of the January 2016 exercise are summarized in the most recent annual report of the Pentagon's Office of Operational Test and Evaluation.
Regarding the exercise, the report said all four of the interceptor's thrusters "turned on and off as commanded" at the outset. But one thruster could not be restarted after the circuit board that powered it "experienced a short," the report said.
The report cited the review board's finding that "foreign object damage" was the most likely cause.
Tensions arose over how the office would characterize the January 2016 flight test.
The missile agency insisted that the report say the redesigned thrusters -- technically known as alternate divert thrusters -- worked flawlessly, and that the failure stemmed from an unrelated problem with the interceptor's electronics.
"They made a big deal about, 'The ADTs worked as designed.' And in fact, when you look at it, no, they didn't," said a government official familiar with the matter. "Because the one thruster didn't fire. ... If this was actually an intercept, it probably would have missed."
A Section on 02/27/2017