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A Light Dawns The Airborne Laser - U.S. Air Force weapons system program
Aerospace Power Journal, Spring, 2001 by Gilles Van Nederveen
GILLES VAN NEDERVEEN [*]
Editor's Note: PIREP is aviation shorthand for pilot report. It's a means for one pilot to pass on current, potentially useful information to other pilots. In the same fashion, we intend to use this department to let readers know about aerospace-power items of interest.
THE AIRBORNE LASER (ABL), or YAL-1A is the second-largest aircraft program in terms of funding (the F-22 being the largest). [1] The modified 747-400F ABL (fig. 1) is designed to serve as a theater-ballistic-missile-defense platform by engaging missiles in their boost phase. After Operation Desert Storm, the Air Force stood up the ABL Program Office in 1992 at Phillips Laboratory, located at Kirtland AFB, New Mexico.
In order to carry out a successful intercept and shootdown, ABL will operate above the clouds at 40,000 feet, where its boost-phase attack profile offers several advantages. First, the target missile moves slowly in this phase of flight, and the missile frame is highly stressed, making it vulnerable to attack. Second, the missile's infrared plume is easy to detect so that targeteers do not have to worry about distinguishing between decoys and warheads. Finally, destruction of the missile over enemy territory minimizes the threat to US and allied positions from falling debris.
The technologies used in the ABL were first developed in the Airborne Laser Laboratory (ALL), an NKC-135 that successfully used an ABL to shoot down air-to-air missiles and drones in the 1980s. The ALL's limited laser range, however, made the system militarily insignificant. [2] Yet, the ALL program prompted several new technology initiatives for the ABL.
For example, chemical mixtures were reformulated to produce a more powerful version of the chemical oxygen iodine laser (COIL), invented at Phillips Laboratory in 1977. The laser fuel consists of hydrogen peroxide, potassium hydroxide, chlorine, iodine, and ammonia--all of which are combined with water to produce the beam. The laser operates at 1.315 microns, an infrared wavelength invisible to the naked eye. By using plastics and titanium and by recycling chemicals, laser contractor Thompson Ramo Wooldridge (TRW) was able to make the module lighter but at the same time increase the laser's power output by 400 percent. The one-megawatt laser will have a range of four hundred kilometers, and an ABL will be able to fire the laser 30 times per sortie.
Another significant technological development is adaptive optics, developed to combat fluctuations in air temperature and consequent atmospheric turbulence that weakens and scatters the laser's beam. Adaptive optics relies on a deformable mirror, sometimes called a rubber mirror, to compensate for tilt and phase distortions in the atmosphere. The mirror has 341 actuators that change one thousand times per second, enabling the mirror to modify the laser beam so that it can travel further through turbulent air. Finally, the development of non-water-cooled optics resulted in enormous weight savings.
In 1995 the ABL transitioned out of Phillips Laboratory, becoming a major defense-acquisition program. In order to mitigate risk, the chief of staff of the Air Force changed the prototype aircraft from a used 747-200 to a new-production 747-400 freighter. The added weight capacity of the new aircraft allowed for more flexibility, and several risk-reduction experiments conducted by Phillips Laboratory in 1996 showed promise. The TRW COIL laser, which demonstrated chemical efficiencies well beyond requirements, used adaptive optics to propagate a low-power beam between two aircraft to establish the feasibility of the ABL. In 1997 the Air Force awarded Boeing a $1.4 billion six-year contract to design, build, and test the ABL. The test aircraft will have six laser modules, and the production version will have 14. The schedule calls for the first ABL to shoot down a target representative of a theater ballistic missile in 2004. The Boeing team includes TRW, which builds the laser, and Lockheed Martin, which devel ops the optics.
The year 1997 also saw the formation of a team to gather atmospheric data in theaters of interest--specifically, Korea and the Middle East. The data, collected seasonally, confirmed the models used by the ABL. Due to the difficulty in measuring atmospheric turbulence directly, however, the Office of the Secretary of Defense requested further data collection through fiscal year 2000. The Air Force continues to build atmospheric databases for ABL, using a star scintillometer to gather light from certain stars that simulate ABL targets. The process uses a modified C-135E--code-named Argus--as the test platform, from which the scintillometer locks onto a star and then measures the amount of optical turbulence between the sensor and the star. By knowing the amount of distortion present, the ABL can predistort the laser-beam weapon so that it will be most intense when it hits the target.
In order to help with the tracking of the laser beam and target acquisition, the ABL is fitted with an active ranging system (ARS), composed of an F-15 LANTIRN pod with a [CO.sub.2] laser. The ARS, cued by the infrared search-and-track sensor, points the [CO.sub.2] laser for a highly accurate ranging and three-dimensional track of targets. Six infrared search-and-track sensors, located along the fuselage of the 747-400, provide 360-degree surveillance, initial detection, and tracking of missiles in boost phase.
