The $40 billion Ground-based Midcourse Defense system has hit only eight of 17 targets, with the last successful test in December 2008. Roll Call reported Tuesday.
The last test firing, on July 5, failed after the kill vehicle failed to separate from its booster engines, Vice Adm. James D. Syring, director of the Missile Defense Agency, has told the Senate Appropriations Defense Subcommittee.
While the test accomplished its secondary objectives, Syring said the Pentagon was investigating the cause of the failure.
He encouraged the subcommittee to continue funding the tests, noting "we have obtained three now out of four intercepts with the version that we just flew in July."
The Defense Department has indicated it plans to add 14 of the $75 million missiles to the national defense system by 2017.
Sen. Richard Durbin, D-Ill., chairman of the subcommittee, questioned Syring whether half of the 30 interceptors already deployed have obsolete parts and if another 10 weren't working because of a design flaw.
The system has undergone extensive upgrades since the test earlier this month, Syring said.