After an unsuccessful July 5 test, the head of America’s missile defense program promised Congress to “aggressively attack” any quality control problems and figure out what went wrong.

But lawmakers expressed alarm about a track record of unsuccessful tests with global threats growing and the administration seeking more than a trillion dollars in additional interceptors.

Vice Adm. James Syring, director of the Missile Defense Agency, appeared today before the Senate Appropriations Subcommittee on Defense to answer questions about his department’s 2014 budget request.

The interceptor missile was launched from Vandenberg AFB, Calif., but failed to take down a long-range ballistic missile target launched from the U.S. Army’s Reagan Test Site on Kwajalein Atoll, Republic of the Marshall Islands.

“We have started an extensive review to determine the cause of the failure,” Syring told the panel today. “We did demonstrate all possible secondary objectives… however, the overall test was a failure because the primary objective of intercepting the target was not met.”

In later questioning, he said the failure came at the booster separation.

The director said the future plan would include more regular testing and accelerated upgrades after testing. “Regardless of the path we embark on, we will aggressively attack any substantiated quality control problems coming out of the failure review board that need to be corrected through the program,” he said.

The agency plans to increase the operational fleet of ground-based interceptors from 30 to 44 by 2017, and is evaluating possible locations in the continental U.S. “to determine a site suitable for possible future deployment of homeland defense interceptors,” Syring said.

Chairman Dick Durbin (D-Ill.) noted that the Pentagon has a 50 percent success rate in intercepting test targets “and the system’s track record has not improved over time.”

He also highlighted the fact that the ground missile defense system has not been tested against an intercontinental ballistic missile and no such test is planned until 2015.

“Of the 30 deployed GMD interceptors, it has been reported that half included obsolete parts while an additional 10 have been taken off operational status because of a known design flaw,” Durbin added.

“I won’t stipulate the number due to classification, but there are a number of GBIs that are available to the war-fighter, but in a lesser readiness condition, but still usable by the war-fighter,” Syring said.

“Since President Reagan announced this concept 30 years ago and we started making rather substantial investments, there are still serious questions as to whether or not we have a missile defense system that can protect America against threats that [we] believe could be coming our way from Iran, North Korea or other enemies of our country,” Durbin said. “…How can you say that you’re confident that America could be defended if we’ve never tested our system against an intercontinental ballistic missile?”

“We have extensive model and simulation capability that projects the results of our conducted intercept testing into the longer range environment,” Syring responded. “Speed and distance is important and, as we have a target that’s available for intercept testing starting in 2015, we will actually demonstrate that. But our models and simulations and ground testing that we have done indicate that we would be successful.”

The FY 2014 budget request includes two intercept tests.

“The threat is continuing to proliferate. The threat is becoming more sophisticated in both numbers and capability. It’s important that we stay ahead of the threat, as we are today. And the way we do that — and let me break it down into two separate areas: regional defense and homeland defense,” Syring said.

“We need to start to change the costs calculus and the costs curve. We can’t just keep building bigger interceptors and more interceptors. And that’s where we’ll get into some of the advanced technology work that we’re pursuing.”

Syring confirmed that the agency is currently testing “in a way that’s representative of a trajectory or a threat missile that would come from a country such as North Korea.”

“The last test that we did was very similar to that in terms of speed and altitude. And it was actually the longest range intercept test that we had tried,” he said.

Sen. Susan Collins (R-Maine) lobbied for her home state, a possible site mentioned for new ground-based interceptors.

“A 2012 report by the National Research Council concluded that there are gaps in our nation’s ballistic missile defense system, particularly when it comes to protecting the East Coast. Alaska’s going to be fine, but Maine is — there’s a real gap,” she said.

Durbin said the conversations about more silos in Alaska and missile defense for the East Coast still didn’t answer why we haven’t had a successful intercept test in five years.

“I’m trying to reconcile the appetite of Congress to keep spending more money with the actual results of testing. I want this country to be safe, and I believe if we had a ground missile defense system that worked, we would be a safer nation, period,” he said.

“The notion that we’re committing ourselves to 14 more interceptors in a year or two when we haven’t really flown, we’re buying before we fly, we haven’t really proven that these interceptors can work. How do you reconcile that?” The cost of the 14 additional interceptors is $75 billion each.

Syring argued that the intercept tests “all the way up to the point of failure did everything they needed to do.”

He acknowledged that North Korea’s successful launch of a space-bound version of the Taepodong-2 last December showed that the DPRK’s technology “took a step.”

“We must continue to monitor that, sir, and not count that it won’t be successful,” Syring said. “We must plan that it will be successful and we must be able to maintain our defense of the country.”