WASHINGTON (Reuters) - An intercept test of a
missile-destroying laser aboard a converted Boeing Co 747
aircraft has been postponed for a fourth time because of
technical problems, the Pentagon's Missile Defense Agency
said.

"Troubleshooting indicates that a hot bypass valve on the
aircraft is in an abnormal condition, thus not allowing for
proper component cooling," the agency said in a statement on
its website. "The team is evaluating the potential causes."

The test involving the Airborne Laser Test Bed was to have
taken place Sunday morning off the California coast. A new date
for the experiment will be set for the coming week, the
announcement said.

The goal is to destroy a simulated enemy ballistic missile
in flight more than 100 miles away, or about twice the range
demonstrated in a maiden test on Feb. 11 using a chemical
oxygen iodine laser aboard the jumbo jet. The laser heats the
boosting ballistic missile's skin, weakening it and causing
failure from high-speed flight stress.

The previous postponements took place over roughly the past
three weeks. They were attributed by the agency, in turn, to a
problem with a stand designed to hold the target before launch,
a tracking system software glitch and, on Tuesday, a problem
with a tracking camera's cooling system that prompted a system
reboot.

Army Lieutenant General Patrick O'Reilly, the agency chief,
said the first two delays reflected safety concerns and a
limited pool of target ballistic missiles that represent the
perceived threat from countries such as Iran and North Korea.

Boeing provides the aircraft, battle management and
overall systems integration for the flying ray run. Northrop
Grumman Corp supplies the megawatt-class laser and
Lockheed Martin Corp supplies the beam control.

The successful test in February prompted calls on the
Pentagon to restore funding for further development of the
system. Defense Secretary Robert Gates cut it to a research
effort last year from a development program headed for possible
deployment.

Boeing, the prime contractor, said in February that the
system's initial success, in a test against a short-range
missile in its boost phase, had "blazed a path for a new
generation of high-energy, ultra-precision weaponry." Some
experts have said it could have potential use against enemy
fighter aircraft, cruise missiles or even low-earth orbit
satellites.

(Reporting by Jim Wolf; editing by Bill Trott)