Megawatt airborne laser prepares for 'first light'

05 Mar 2004

The ballistic missile-killer is almost complete, and the future of the daring billion-dollar ABL project now rests on the tests it is about to undergo. Oliver Graydon reports.

From Opto & Laser Europe March 2004

Although equipping a Boeing 747 with a giant megawatt-class laser and using it to shoot down ballistic missiles might seem like a far-fetched - or just plain mad - scheme to its critics, that hasn't stopped the US Department of Defense spending $2.1 bn (€1.6 bn) on the idea. And if milestone tests scheduled for this year go well, the sceptics may end up having to eat their words.

Thirty years after a high-energy laser first destroyed an aerial drone the Airborne Laser (ABL) project is preparing for what it calls "first light". Later this year, all six modules of the ABL's giant chemical oxygen iodine laser (COIL) will be switched on simultaneously for the first time. Tests will then show whether the COIL is capable of generating the megawatt 1.3 µm beam that is necessary for shooting down ballistic missiles over a range of hundreds of miles. In theory, if these and subsequent trials are successful the ABL could become an active part of the US missile defence system by 2010.

"This last year has been a busy year delivering the hardware. As we enter into 2004 our focus is to take those pieces that we've demonstrated working individually and put them together, with the goal of shooting down our first missile in 2005," a spokesperson for the ABL project told delegates at IQPC's Directed Energy Weapons seminar in London in January. "We are now focused on getting that first light out of those six COIL modules and propagating it through the beam control system."

The theory Assuming that the necessary beam power is demonstrated, how exactly will the ABL kill a missile in a real-life scenario? The answer lies in a series of sophisticated stages that rely on four different lasers and some of the most demanding optics that have ever been built.

In the first of these stages, while cruising above the clouds at an altitude of more than 38 000 ft, one of the ABL's six infrared search-and-track (IRST) pods will pick up the infrared signature of a ballistic missile launch. As soon as this happens the ABL will train the CO₂ laser of its active ranger system (ARS) onto the missile to track its position and measure its distance from the plane.

Next, the track illuminator laser (TILL), a kilowatt-class pulsed Yb:YAG source, will locate and track the nose of the missile so that the position of its fuel tank can be accurately determined. This is the "sweet spot" at which the high-energy laser will be aimed.

The last step before the COIL is fired is to activate the beacon illuminator laser (BILL), a pulsed Nd:YAG laser. It will bounce an optical beam off the missile so that the atmospheric distortion between the ABL and the missile can be measured and compensated for using an on-board deformable mirror. The COIL will then be fired and the missile, in theory, will be destroyed. All of this must take place in a matter of seconds if the missile is to be shot down during the initial phase of its flight (the boost phase), while it is over enemy territory and open to attack.

The beams from the TILL, BILL and COIL will all leave the plane through the 7 tonne rotatable turret on its nose. The turret contains a large telescope that is used to expand the beam before it exits through a 1.7 m conformal window. The telescope will consist of a 58.8 inch gold-plated mirror and a 12.2 inch secondary mirror.

The front part of the plane contains the beam-guiding optics and electronics, while the back section contains the laser fuel and the six laser modules that make up the COIL. Each laser module is about the size of a car and weighs 2 tonnes. The two sections of the aeroplane are separated by a sealed bulkhead to protect the flight crew in the case of a chemical leak.

So if that's the theory, what's been done in practice? Well, there's no doubt that a great deal of progress has been made since Opto & Laser Europe last reported on the project two years ago. "We have now completed the assembly of the aircraft's beam control system, including TILL and BILL, and are performing end-to-end testing," said ABL's spokesperson. "We are also almost ready to ship the flight ball [the turret interior], which includes the coated 1.7 m conformal window and the 1.5 m primary mirror - two optics that are vital for getting the high-energy laser beam out of the aircraft to the target."

Over the past 12 months, all of the COIL laser modules and 77 tonnes of flight hardware have been installed into the test plane - an old 747-200 fuselage that was rescued from an aircraft junkyard - at the 18 000 ft² System Integration Laboratory (SIL) at Edwards Air Force Base in California. All of the ground-based laser tests will take place at the specially designed $35 m facilities. After tests inside the SIL are complete, the COIL will be dismantled and reassembled inside the ABL.

Next to the SIL is the ground pressure recovery assembly (GPRA) - a giant airtight sphere that simulates the reduced air pressure at 38 000 ft for tests that will mimic the plane's real-life operating conditions.

All of the system's tracking lasers and 127 beam-guiding optics have been manufactured and are now undergoing tests prior to installation. The first batch of laser fuel, a mixture of chemicals including hydrogen peroxide, has also been manufactured in preparation for the first light tests. It is anticipated that the COIL will be able to make about a dozen firings before it needs to refuel, with fuel for each shot costing $10-15 000.

One thing is for sure: regardless of whether or not the ABL ever actually goes into active service, just attempting to build it will have advanced the capabilities of US high-tech firms. More than 30 companies across the US specializing in everything from optical coatings to adaptive optics and laser diodes have been paid to push back the frontiers of technology and solve unique problems.

History of the ABL November 1973: The idea of shooting down missiles with a laser is given a credibility boost when a 100 kW CO₂ laser destroys a 12 ft long airborne drone during military tests. The event is the first time that a high-energy laser has shot down an aerial target.

November 1996: The US Air Force awards a $1.1 bn contract to Boeing, TRW (now Northrop Grumman) and Lockheed Martin to start developing an airborne laser that could destroy ballistic missiles.

January 2000: A Boeing 747-400 freighter purchased by the US Air Force is flown to the Boeing Modification Center in Wichita, Kansas. Two years of modifications commence.

March 2001: Raytheon conducts a "first light" test of the track illuminator laser (TILL) at its High Energy Laser Center in El Segundo, California. The Yb:YAG laser is used to locate and illuminate the nose of the target missile.

July 2001: The ABL's missile seekers, a set of six infrared search-and-track (IRST) sensors, are delivered to Boeing by Lockheed Martin.

March 2002: The first COIL module (LM-1) made by Northrop Grumman is tested, and generates 118% of its anticipated output power. A total of six such modules are required to generate the megawatt beam that the ABL will use to shoot down ballistic missiles.

July 2002: The modified 747 makes its maiden flight carrying bags of metal balls to simulate the weight of the COIL laser. Since then, 14 test flights have taken place.

December 2002: The plane is taken into a hangar at Edwards Air Force Base and grounded for engineers to install its laser modules and optics.

February 2003: Northrop Grumman finishes making the beacon illuminator laser (BILL) and delivers it to the ABL team. The BILL is a pulsed kilowatt-class Nd:YAG laser that is used to measure the atmospheric distortion between the missile and the aircraft.

2005?: First ballistic missile shoot-down is scheduled to take place over the Pacific Ocean.