By Jason Szep

SINGAPORE, Feb 24 (Reuters) - Commercial jets could carry military-grade anti-missile systems within months rather than years, aerospace industry officials said on Tuesday, as governments strengthen defenses against portable rockets.

Washington picked three firms last month to design systems for protecting commercial airliners against shoulder-fired missiles, adapting defenses now used on President George W. Bush's Air Force One and on military planes.

"I think you'll start seeing this measured in months, not years. I wouldn't be surprised if in 90 days or 120 days you might see them," said Art Heckler, a director of international business development at Britain's BAE Systems Plc .

The U.S. unit of BAE, Europe's largest defense firm, was one of the three companies selected by the U.S. Homeland Security Department to test infrared jammers that redirect heat-seeking rockets away from aircraft engines.

The others are American defense contractor Northrop Grumman Corp and United Airlines , the world's second-largest airline. Washington negotiated contracts worth $2 million with each firm.

"We're now in the study phase," said Everett Pratt of Northrop Grumman.

"Over the next six months or so these studies will be done and the results will go to the Department of Homeland Security, who will decide whether to proceed," said Pratt, a vice president of Northrop's RF combat and information systems division.

Singapore Airlines Ltd and affiliate SilkAir could be fitted with anti-missile devices within two years, the city state's Deputy Prime Minister Tony Tan said this week, adding that a Singapore government contractor was developing the device.

Northrop was also in talks with Singapore Airlines, said Mark Kilduff, the firm's director of business development, speaking on the sidelines of an aerospace trade show.

GROWING THREAT

Shoulder-fired rockets are considered among the most serious threats facing commercial aircraft. One struck a DHL cargo plane over Baghdad in November, forcing it to make an emergency landing with its wing aflame. All three crew were safe.

Plentiful on the black market, the missiles can be bought for a few thousand dollars each and fired from a launcher small enough to fit on a shoulder.

A near miss involving an Israeli airliner taking off from Mombasa, Kenya, in November 2002, has added to the alarm.

But installing and maintaining the defense devices could cost billions of dollars in an industry desperate to slash operating expenses. This has kept many airlines and governments from committing to the technology despite concerns about safety.

The defense system, which Pratt says looks a bit like a "canoe" under the rear of the plane, could cost $1 million per aircraft, raising questions of whether Washington would help subsidize the bill for America's jetliner fleet of 6,800 planes.

Though designed to work autonomously, the devices may also require training for pilots and maintenance crews, raising the possibility of union opposition and extra expense.

Kilduff said Northrop's system would cost about $1 million each, though the price could vary with different plane sizes. BAE's system was below $1 million with some configurations priced under $500,000, Heckler told Reuters.

Both systems work in much the same way, directing lasers or flashes of light to fool a heat-seeking missile into going off course and exploding at a safe distance.

"You can spoof it," said Heckler. "You can confuse the missile so it can't really tell where the exhaust or the engine is." Pilots would need to bank the plane away after the device is used, he added.

Northwest Airlines Corp and Federal Express are part of Northrop's study group. Honeywell International Inc and Delta Airlines are part of BAE's group.