In 2003 the Air Force began testing a weapon with a range measured in hundreds of miles, accuracy measured in inches, a flight time measured in fractions of a second, and uses enough hydrogen peroxide with each shot to give every man and woman serving in the military blonde highlights. No, it's not an over-the-horizon blow dryer, but rather a potentially major component in America's Ballistic Missile defense: the Airborne Laser.

Housed inside a highly modified Boeing 747-400F cargo plane (which, incidentally, set a record in 1988 for the heaviest mass (405,659 kg) ever lifted by an aircraft) the mission of the ABL (designated by the Air Force as the YAL-1A) is simple: pick off enemy missiles in flight before they do any damage. The ABL cruises at 40,000 feet, looking for launch signatures from theater range (non-ICBM) ballistic missiles. Once a missile launch is detected, three lasers, directed by a dual-axis mirror housed in the aircraft's nose, paint the target. Once this critical information is collected, it is relayed to the mirrors that tune the primary laser. At that point, look out, as the megawatt-class COIL (Chemical Oxygen Iodine Laser) lets loose with a 1.315 micron (invisible to the naked eye) beam that heats and ruptures the pressurized fuel tank of the outbound missile. From start to finish the entire process is measured in seconds. Star Wars, indeed.

A member of Team ABL -- - The U.S. Air Force, Boeing, Lockheed Martin and TRW -- - works on a scaled Laser Beam Control System built by Lockheed Martin Missiles & Space, Sunnyvale, Calf., that demonstrates the functional performance needed for the Air Force's Airborne Laser (ABL) (Photo provided courtesy of Boeing).

Finding the Needle

Of course, if it were actually as easy as all that, the Air Force wouldn't need 2.1 billion dollars to develop the system. Suffice to say, the tasks to be accomplished are enormous.

The first step in shooting down a ballistic missile is to detect the launch of said missile. To accomplish this, the ABL will have an array of six long range Infrared Scan and Track (IRST) sensors (originally developed for use on the Grumman F-14 Tomcat interceptor in the 1970s) arranged around the aircraft to provide 360 degrees of coverage. Once a launch is detected and one or more of the IRST sensors has acquired the missile, data is fed to a modified Low Altitude Navigation and Targeting Infrared for Night (LANTIRN) pod, which will in turn, use laser ranging to determine the three dimensional coordinates of the launched missile. Then the fun begins, as this information is fed to 2 low-power Track Illuminating Lasers (TILLs).

One of the TILL lasers tracks the nose of the target, establishing a baseline for calculating how far away the target fuel tank is, while the second TILL laser tracks the target area calculated by the first TILL. The TILL lasers are CO2-based Yb:YAG lasers, and were selected because their mission requirements required the least amount of power to accomplish.

Once the TILL lasers track the target, a more powerful Beacon Illuminating Laser (BILL) is used to sample the atmospheric conditions between the ABL and the target. The Nb:YAG laser was selected to fill this role because of its high power output and the need to differentiate it from the returning pulses of the TILL lasers. The information gathered by the BILL is then sent to the optically adaptable deformable mirror. This computer-driven reflector is equipped with 341 individual actuators that operate at 1,000 Hz to shape the outgoing COIL pulse to reduce the effects of optical turbulence, beam diffraction and wavefront distortions. In short, virtually all movement and atmospheric variables are accounted for by the time the primary laser is ready to fire. Once the target "sweet spot" has been designated and the deformable mirror attenuated, the COIL fires, sweeping the target area for several seconds until the enemy missile's heated casing ruptures, and the missile explodes.

Laze and Blaze

At the heart of the ABL is the megawatt-class Chemical Oxygen Iodine Laser. When reviewing potential laser systems to be used on the ABL, the COIL was selected specifically because of the numerous advantages offered by the beam's wavelength. At 1.315mm, the beam is relatively unaffected by atmospheric conditions, and is relatively small when compared with other high-powered lasers, which makes it easer to focus on distant targets. In addition, very little of the beam's energy is absorbed by fused silica (the medium through which the beam is projected through the modules), while metals readily absorb it. This means that not only will little of the beam's energy be lost as it is projected through the long axis of the ABL aircraft (which will also have the benefit or reducing thermo-optical beam degradation), but much of it will be readily absorbed by the metal skinned rockets the laser is being shot at.

An artist's conception of the ABL in action (Image provided courtesy of Boeing).

Since the ABL, as the name implies, is an airborne system, a non-conventional (and non-nuclear) power supply was needed. Rather than rely on batteries, capacitors, electric generators or really long extension cords, the COIL is powered by liquid chemicals and ionized gas. First tested at the Kirtland AFB Phillips Lab in 1977, the COIL, now being built by Northrop Grumman Space Technologies, uses atomized liquid hydrogen peroxide (H2O2) and potassium hydroxide (KOH) -- - essentially beauty salon highlighter and Liquid Plumber -- - and chlorine gas (CL2) to form an energized (ionized) form of oxygen known as singlet delta oxygen (O2(1D).) SDO, in turn is mixed with molecular iodine gas (I2) to form ionized iodine gas. As the ionized iodine gas returns to its resting state it releases a photon pulsing at 1.315mm. As the photons are released, they are collected and amplified by a pair of parallel laser cavity mirrors and finally discharged as a pulse of coherent light. When finally installed, the COIL will consist of six individual lasing modules (each weighing 4,500 pounds, and as big as a panel van) linked in series (so that the beam from one module can be amplified further as it passes through subsequent modules.)

To properly mix the chemicals to produce enough photons to be effective, the SDO and iodine mixture is injected into the lasing cavity at a near supersonic speed (the turbine-like pump that performs this task is small enough to fit on an office desk and can fill a typical backyard pool in less than 10 minutes.) To reduce weight and chemical waste, unused H2O2 is recycled until exhausted. The byproducts from this beam generation process are harmless heat, potassium salt, water, and oxygen.

A schematic of the ABL's mounted mirror turret (Image provided courtesy of Boeing).

The Big Eye

There's no argument that the COIL can do its job -- - in December of 2003, a ground test utilizing a single COIL module generated 118% of anticipated power -- - but generating a megawatt rated pulse of light is meaningless if it isn't pointed in the right direction. That's where the ABL's 7-ton nose mounted mirror turret comes into play. In effect a large (1.5m) two-axis telescope, equipped with a 58.8 inch gold plated primary mirror and a 12.2 inch secondary mirror, the turret has a 120-degree field of view and is protected from the elements by a 1.8m, 330 pound Nose Cone Conformal Window. The NCCW is the largest optical quality domed conformal window ever manufactured and is made from unique materials intended to be "transparent" to the outgoing laser (to reduce the effects of optical distortion).

Smooth Sailing

Designing such a powerful system is difficult enough -- now add the "Aerial" to ABL, and you have a real challenge. Up to now, the entire system has been limited to ground testing, but now it must be mounted in the host aircraft and successfully tested under realistic "combat" conditions. To function properly, the six COIL modules in the ABL must be absolutely rock stable, which means that the resonating chamber that links the modules, the deformable mirror that shapes the beam, and the beam control station which releases the beam must all be isolated from its 747 host, which, by design, flexes and twists as it flies through the air.

An artist's conception of the 747-400 that serves as the "aerial" in ABL (Photo provided courtesy of Boeing).

Enter the highly modified 747-400 purchased directly from Boeing. It is powered by four CF6-80C2BSF 61,000 pound thrust engines (some of the largest that General Electric makes for aircraft use) and made its first flight on July 18th 2002. Mounting the ABL components in the aircraft presents two challenges. First, and most important, the systems must be stabilized against in-flight vibrations, which can be achieved through the use of spring loaded vibration isolation benches. Stabilizing the lasers as the aircraft flies through the air at 600 knots will be the responsibility of the nose mounted tracking turret, which must be able to make rapid multi-axis corrections to the beam's attitude to compensate for aircraft motion.

The second issue concerning the ABL installation is weight. While the 747-400 has a maximum payload weight of 320 tons, in its current configuration the complete ABL system weighs in at about 300 tons. Reducing the system's overall weight will not only provide a larger margin for safety, but will allow the aircraft to carry more laser reactant (for more shots), fuel (for longer loiter times) or improved crew accommodations (which will increase mission loiter times and reduce crew fatigue).

Obviously, all of the above innovations and creations have come with a major price tag, and while plans are moving forward to bring the ABL into full military service, several questions remain (see the "Reality Check" below). But even if the ABL's utility as an effective anti-missile system may be open to debate, there is no denying the benefits to be reaped from the research invested in the individual ABL subsystems. The advances made in flowing gas laser systems (which is what the COIL is) have been astronomical. TRW (Now Grumman Space Technologies) has established a world record for chemical efficiency with the COIL laser, technology that can be applied to commercial industrial applications. The IRST acquisition and tracking system, developed to lock on to and track missiles, can be applied to other ABM systems, such as space based lasers or space/surface based electromagnetic mass drivers (rail guns), or even passive anti-aircraft ship and facility defenses. Does America need the ABL? Maybe, maybe not. What the world does need however, is the technology behind it.