The Pentagon’s Airborne Laser platform on Oct. 21 failed in its second consecutive attempt to shoot down a boosting ballistic missile with its high-energy laser, the U.S. Missile Defense Agency (MDA) announced Oct. 21.

The modified 747 aircraft, built by Boeing Defense, Space & Security Systems of St. Louis, was supposed to destroy a solid-fueled, short-range Terrier Black Brant missile in the most recent test. The target was launched, and the Airborne Laser acquired and tracked the missile’s exhaust plume, but the transition to track the missile itself was never made, and the shoot-down attempt was aborted, according to an MDA press release. Program officials are investigating a valve within the laser’s tracking system that may have not have performed properly, it said.

This test failure follows a Sept. 1 test in which the Airborne Laser attempted but could not destroy a short-range liquid-fueled missile at a distance three times greater than that of a previous, successful test. A communications software error was blamed for steering the system’s high-energy laser slightly off target in that instance.

Once envisioned as a steppingstone to an operational missile defense system, the Airborne Laser has been relegated to a stand-alone technology development and test platform.