Space


Airborne Laser Laboratory

The Airborne Laser Lab is a modified KC-135 used for flight testing. Similar to the commercial Boeing 707, the slightly smaller KC-135 was designed to military specifications and operated at hight gross weights. The initial flight of a KC-135A took place on August 31, 1956, and the USAF accepted its first one on January 31, 1957. By 1966, 732 KC-135Aa had been built and the aircraft had become the USAF's standard tanker. It was also used for transporting cargo or personnel and by 1970 was serving in other roles, too, including reconnaissance, electronic intelligence and project testing.

The NKC-135A (S/N 55-3123) is one of 14 KC-135As permanently converted for special testing. It was extensively modified by the Air Force weapons Labratory at Kirtland AFB, New Mexico, and used in an 11-year experiment to prove a high-energy laser could be operated in an aircraft and employed against airborne targets. During the experiment, the Airborne Laser Lab destroyed five AIM-9 Sidewinder air-to-air missiles and a Navy BQM-34A target drone.

In the early 1950s, Air Force Office of Scientific Research [AFOSR ] funded research led to the development of the MASER (Microwave Amplification by Stimulated Emission of Radiation) by Dr. Charles Townes. Further research by Dr. Townes and a colleague, Dr. Arthur Schawlow, led to work on the principles of the LASER (Light Amplification by Stimulated Emission of Radiation). For his work on the maser and laser, Dr. Townes shared the Nobel Prize in physics in 1964.

In 1962 the Department of Defense's Advanced Research Projects Agency (ARPA) funded research at the Air Force Special Weapons Center (AFSWC) on the potential military use of lasers. After early tests with ruby lasers proved disappointing the hopes of laser advocates surged in the mid 1960s when advances were made in the development of a gas carbon dioxide laser. In 1967, scientists made another large step forward with the invention of the gas dynamic laser, an improvement on the carbon dioxide laser that utilized nitrogen and water vapor. This spurred the Air Force belief that a laser could be used successfully as an antimissile system. However, testing this hypothesis proved to be a drawn out procedure.

In 1968, the Air Force Weapons Laboratory (AFWL), formerly the AFSWC and now the Directed Energy Directorate of AFRL, was authorized to begin a new program on building and testing a CO2 gas dynamic laser. Shortly thereafter, research into the idea of mounting a laser on an airplane as an anti-missile system began. Before any proposed weapon could be used aboard an aircraft it first had to be tested on the ground, a process that made its first noticeable advancements between October and December, 1972 when technicians fired a ground-based 100 kilowatt CO2 laser that propagated at 10.6 microns against a variety of stationary targets. The tests went so well the project elevated to firing the laser at a moving airborne target.

On November 13, 1973, the laser was used against a 12 foot long Northrop MQM-33B radio controlled aerial target, a drone, in an attempt to knock it out of the air. Indeed, the drone did drop, but not precisely as planned. The laser beam burned through the drone's aluminum skin, frying the control system. The Air Force had hoped the beam would ignite the drone's fuel tank. The next day, the laser performed according to expectations. The beam found and locked onto the area of the drone where the fuel tank was located for 1.2 seconds, long enough to raise the temperature on the exterior of the fuel tank to ignite the interior vapors. This time the drone went down in a fireworks-like burst of smoke and flame. The two tests marked the first time that aerial targets had ever been destroyed by a highenergy laser.

Flushed with success, the Air Force decided that the next step would be to mount the laser on an aircraft, then shoot down targets while circling above the clouds. Earlier, in March 1972, the organization that was managing the project, the Air Force Weapons Laboratory, had secured a 15-yearold KC-135A, the military version of the Boeing 707. Once it was turned over to the Weapons Lab, it was designated as "nonreturnable" by the donating unit, thus getting a new designation as NKC-135A. However, it would win its place in history known as the Airborne Laser Laboratory (ALL).

ALL flight tests began in January 1975 but the early ventures were merely shakedown operations to see how the aircraft would fly with the laser aboard and test support equipment such as tracking and alignment hardware. It was eight years before the ALL scored its first "kill." The next stages of the ALL revolved around three key cycles: demonstrating that an airborne pointing and tracking (APT) system could accurately track an aerial target; aligning a low-power beam with the APT and then directing the beam out of the turret on top of the aircraft to an aerial target; and combining a high-powered beam with the APT to shoot down the targets. The three cycles were finally completed in 1983.

On May 26, 1983, the ALL shot down an AIM-9B Sidewinder air-to-air missile over California's China Lake, a feat it quickly repeated. On May 31, it destroyed another Sidewinder, then, on June 1, two more [Other accounts report that in these tests in May and June 1983, the ALL destroyed four other AIM-9B missiles]. The final test for the ALL took place on 26 September 1983. In a joint experiment with the Navy, the ALL shot down and destroyed three 23-foot-long, ground-launched, BMQ-34A Navy drones, representing a Russian cruise missile, a weapon of deep concern to the Navy. The interception and destruction of the three drones signaled that the ALL program was a resounding success, proving that the goal of airborne anti-missile defense was indeed realistic.

A high energy laser system focuses large amounts of radiant thermal energy in a small area to dustroy targets. When the beam is subjected to jitter or optical distortions, a longer time on target is required to achieve the desired destructive results. To insura that rhe high energy laser is an effective weapons systems, aerodynamic refinements must be made to enhance its effectiveness.The density fluctuations due to unsteady flow degrade the optical quality of the laser beam due to propagation through the turbulent medium. The major challenge is to solve the unsteady flow in the aft hemisphere of the laser turret This degradation in the aft hemisphere is due primarily to unsteady flow resulting from boundary and shear layers as well as vortex shedding. Adaptive optics cannot fully correct the problem so an aerodynamic modification must be obtained. The unsteady pressure loading on the turret also causes jitter which spreads the laser beam and requires longer time on target to achieve the same destructive results as under more stable conditions.

The high subsonic flowfield around a laser turret was the subject of considerable research. The Air Force Weapons Lab at Kirtland Air Force Base, New Mexico, instituted much of the research in the study of turbulent flow about a laser turret and how it related to the optical guality of a high energy laser system currently installed on the NKC-135 aircraft. Several wind tunnel tests were conducted at NASA Ames 14-foot wind tunnel and were the subject of four thesis projects at the Naval Postgraduate School 5 x5 foot wind tunnel.

The use of directed energy systems requires precision in both tracking the target and pointing the weapon at the target. This precision must be maintained over sustained periods of time as the effect of the weapon is thermal - the energy beam must be focused on a fixed region of the target long enough to deliver the amount of energy necessary to destabilize the target. Historically, the greatest challenge to maintaining the required precision is controlling the inertial motion of the optics that direct the beam. Mechanical motion of the steering and shaping optics induce jitter - the dynamic deviation of the beam from an inertially stable trajectory through space. An airborne platform is by design compliant - it is not the type of stiff, stable platform required for a precision optical system.

ALL was designed and built to demonstrate that microradian-precision pointing and tracking could be obtained in an airborne operational laboratory. The results of ALL testing revealed several important lessons learned concerning jitter and its mitigation. Foremost among these were the degrading effects of acoustically induced jitter. Acoustic disturbances, such as those caused by the pressure recovery system of the high power laser, are a significant jitter source. Several technologies may be appropriate for reducing the acoustically induced jitter. The first choice for mitigation would be passive approaches, such as acoustic blankets. There is, however, some uncertainty whether these approaches will provide sufficient attenuation and there is concern about the weight of these approaches.

Static and dynamic strain measurements which were taken during test stand operations of the gas dynamic laser (GDL) for the AF Airborne Laser Laboratory indicated that higher than expected vibrational stress levels may possibly limit the fatigue life of the laser structure. Particularly the diffuser sidewall structure exhibited large amplitude random vibrations which were excited by the internal gas flow.

Since the ALL was only an experimental aircraft, the Air Force decided that its mission had been completed after shooting down the BMQ-34A. The ALL was retired in 1984 and four years later was flown to Wright-Patterson Air Force Base in Dayton, Ohio, where it is now on display at the Air Force Museum.

Despite its success, the ALL was ignored by weapons planners, mainly because its missions had been classified as "proof of concept" exhibitions rather than demonstrations of a viable warfighting tool. Although it had shown that a laser mounted on an aircraft could be a formidable defensive weapon, it was generally viewed as impractical. Its carbon dioxide laser was too bulky and it did not generate enough power to be effective at extended ranges. Its long, 10.6-micron-wavelength gas dynamic laser, combined with limited optical component dimensions, led to poor laser beam propagation over distances greater than 10 km. Just as importantly, the system was not designed to be operated or maintained by a war fighter. However, it did give us a glimpse of the kind of devastating damage HELs could produce when operated from an airplane and coupled with the inherent flexibility and mobility of air power.

However, almost a decade later, after Saddam Hussein began firing Scud theater ballistic missiles at US troops and their allies in the Persian Gulf War, the concept of an antimissile laser was revitalized. By then, technological advances had dictated the replacement of ALL's gas dynamic laser with a vastly superior chemically operated device that had been invented at the Air Force Weapons Laboratory at Kirtland Air Force Base, N.M., in 1977. Called a Chemical Oxygen Iodine Laser (COIL), it resolved many of the doubts planners had about the ALL system. A number of times more powerful than the ALL's CO2 laser (a megawatt-class laser as opposed to the ALL's kilowatt-class), it is much more compact, and it was capable of producing a lethal beam over long distances.

As a result, rather than reviving the ALL, the Air Force decided to build an entirely new system, changing not only the laser but the type of aircraft that would carry it. Plus, it got a brand new concept of operations. Dubbed the Airborne Laser (ABL), the new system includes multiple COIL modules installed in pairs in the rear of a Boeing 747-400 freighter. Also, there is one important addition: ABL, unlike the ALL, has a sophisticated optical system capable of projecting a beam over hundreds of kilometers and compensating for any atmospheric disturbances that might exist between the aircraft and its target.





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