YAL-1A Airborne Laser Testbed (ALTB)
Airborne Laser

On 14 February 2012, the YAL-1A Airborne Laser Test Bed aicraft made its final flight to Davis-Monthan Air Force Base, Arizona marking the end of the ALTB test program. The ALTB aircraft was to be processed into storage at the Air Force's Aerospace Maintenance and Regeneration Group and its associated systems would be screened in accordance with DOD procedures for reutilization.

The Airborne Laser Testbed (ALTB) is a pathfinder for the nation's directed energy program and its potential application for missile defense technology. The use of directed energy is very attractive for missile defense, with the potential to attack multiple targets at the speed of light, at a range of hundreds of kilometers, and at a low cost per intercept attempt compared to current technologies.

On 11 February 2010 the Airborne Laser Testbed (ALTB) successfully destroyed a boosting ballistic missile. At 8:44 p.m. (PST) a short-range threat-representative ballistic missile was launched from an at-sea mobile launch platform. Within seconds, the ALTB used onboard sensors to detect the boosting missile and used a low-energy laser to track the target. The ALTB then fired a second low-energy laser to measure and compensate for atmospheric disturbance. Finally, the ALTB fired its megawatt-class High Energy Laser, heating the boosting ballistic missile to critical structural failure.

The ALTB research and development platform successfully fired the onboard High Energy Laser (HEL) to engage an instrumented target missile, called a Missile Alternative Range Target Instrument (MARTI). This test engagement was not intended to lethally destroy the missile. The entire engagement occurred within two minutes of the target missile launch, while its rocket motors were still thrusting.

The experiment, conducted at Point Mugu Naval Air Warfare Center-Weapons Division Sea Range off the central California coast, served as a proof-of-concept demonstration for directed energy technology. This test demonstrated the full functionality of the ABTBL system to successfully acquire, track, and engage a boosting target. Test instrumentation aboard the MARTI collected data to evaluate ALTB laser system performance. The MARTI was launched from San Nicolas Island, located in the Naval Air Warfare Center-Weapons Division Sea Range, off the central California coast. This test provides data to support the ALTB platform's attempt of the first lethal shootdown of a boosting ballistic missile using directed energy technology, scheduled for 2010.

This was the first directed energy lethal intercept demonstration against a liquid-fuel boosting ballistic missile target from an airborne platform. Less than one hour later, a second solid fuel short-range missile was launched from a ground location on San Nicolas Island, Calif. and the ALTB successfully engaged the boosting target with its High Energy Laser, met all its test criteria, and terminated lasing prior to destroying the second target. The ALTB destroyed a solid fuel missile, identical to the second target, in flight on February 3, 2010.

On April 6, 2009 Defense Secretary Robert Gates announced that: "We will cancel the second airborne laser prototype aircraft. We'll keep the existing aircraft and shift the program to an R&D effort. The ABL program has significant affordability and technology problems, and the program's proposed operational role is highly questionable." At that time the ABL program was eight years behind schedule and $4 billion over cost. On April 24, 2009 the Boeing Company, industry teammates and the U.S. Missile Defense Agency began Airborne Laser (ABL) flight tests with the entire weapon system integrated aboard the ABL aircraft.

The Airborne Laser (ABL) weapon system was designed to detect, track, and destroy all classes of ballistic missiles in the boost phase of their flight. To this end, the weapon system comprised of a megawatt class high-energy, chemical oxygen iodine laser (COIL) mounted on a modified Boeing 747-400F (freighter) aircraft. As of 2007 Boeing was designated as the primary contractor, overseeing systems integration and testing. Northrop Grumman and Lockheed Martin were also been involved supplying the high energy laser and beam contro/fire control equipment respectively.

The ABL's COIL, which was invented at the Phillips Laboratory at Kirtland Air Force Base, New Mexico in 1977. The Phillips Laboratory was absorded into the Air Force Research Laboratory consortium in 1997, and while it retains its location, AFRL's main headquarters is at Wright-Patterson Air Force Base, Ohio. The COIL relies on the excited state of molecular oxygen, O2(1D), is generated by the chemical reaction between chlorine gas and an aqueous mixture of hydrogen peroxide and potassium hydroxide (basic hydrogen peroxide). The byproducts of this reaction include salt (potassium chloride) and heat. Water vapor in the gas flow is removed because it interferes with the laser gas kinetics. Molecular iodine is then injected and mixed with the gas flow, and some of the energy in the oxygen is used to dissociate the iodine. Resonant energy transfer from the excited oxygen to the atomic iodine excites the iodine, and the gas flow is accelerated to a supersonic velocity in an expansion nozzle to create the laser gain region. Light is extracted with a laser cavity positioned transverse to the gas flow, and the exhaust gases are scrubbed to remove the residual chlorine and iodine.

Kirtland AFB is now home to AFRL's Directed Energy Directorate, overseeing development of systems such as the COIL and ABL. The laboratory successfully demonstrated the world's highest power subsonic gas flow COIL in 1982 followed by the first supersonic gas flow COIL in 1984. Additionally, under contracts from the US Air Force the TRW Corporation and the Rockwell Aerospace Corporation developed rotating disk oxygen generator and COIL supersonic mixing nozzle equipment respectively in 1984.

The COIL operates at an infrared wavelength of 1.315 microns, which is invisible to the eye. By recycling chemicals, building with plastics and using a unique cooling process, the COIL team was able to make the laser lighter and more efficient while at the same time increasing its power by 400 percent in five years. The flight-weighted ABL module was be similar in performance and power levels to the multi-hundred kilowatt class COIL Baseline Demonstration Laser (BDL-2) module demonstrated by TRW in August 1996. As its name implies, though, it was to be lighter and more compact than the earlier version due to the integration of advanced aerospace materials into the design of critical hardware components. The maximum individual COIL that had been achieved by 1990 was forty kilowatts, meaning that the ABL would require multiple COILs in order to produce the necessary power to destroy a ballasitic missile target.

As of 2007 the primary features of the ABL were its nose mounted turret, Beam Control System, Active Ranging System, Battle Management equipment, Illuminator Optical Bench, Advanced Resonator Alignment System, the high energy laser (HEL), and the laser's fuel system. The 12-15,000 pound nose mounted turret is 1.5 meters in diameter and was designed to focus the beam and collect return images and signals from other equipment. The turret can be rotated into a stowed position to protect the lense and sensors from foriegn object damage and inclement weather. The Beam Control System controls target aquisition, tracking, fire control, aim point selection, and the shape and intensity of the HEL beam. The Active Ranging System is contained in a dorsal pod and contains the Track Illuminator. The entire system was based on the LANTIRN laser designator equipment already in service with the United States Air Force. The Battle Management equipment is the human control interface for the system, and was designed to make heavy use of commercial software in targeting, engaging, and kill assessment. The Illuminator Optical Bench was developed as a single piece of equipment for beam shaping and alignment, containing the aligning optics, and capable of being removed as a single modular unit for easy maintenance. The Advanced Resonator Alignment System was required to isolate focusing optics from disturbances that could poentially scatter the laster beam.

The 2007 version of the HEL had set the record for chemical efficiency and used plastics and titanium to reduce overall component weight. Component weight is a major factor as the COIL's being used in tests since 2003 have been listed as "size of a SUV turned on its end...[weighing] about 6,500 pounds exclusive of the plumbing and support equipment." A total of six COILs were being used on the test aircraft by 2007 in order to achieve the requisite power. Like the Illuminator Optical Bench the design was also modular to allow for easier maintenance of select components. Unlike the original COIL, the design as of 2007 used a combination of hydrogen peroxide and ammonia as the fuel source, along with helium as a pressurant. The ABL's exact range under various conditions has been classified, but the officially the USAF has said "hundreds of kilometers" and "more than 200 miles." Adaptive optics have also been developed for the HEL. This is because of atmospheric turbulence produced by fluctuations in air temperature and the same phenomenon that causes stars to twinkle can weakens and scatters the laser's beam. Adaptive optics rely on a deformable mirror, sometimes called a rubber mirror, to compensate for tilt and phase distortions in the atmosphere. The HEL's mirror was designed with 341 actuators that change at a rate of about a 1,000 per second.

By 2006 the Airborne Laser program had hit hard times, beset by delays and major technical problems. For the following two years, the laser program was relegated to a technology demonstration status while a planned five-aircraft purchase by the Air Force was put on hold. As of early 2009 the beam had been fired in the air and was performing well to ranges beyond 100km. According to an American Physical Society report in 2004, the Airborne Laser could shoot down a typical liquid-fuel intercontinental ballistic missile (ICBM) from up to 600km away. However, against solid-fuel ICBMs, which are more resistant to heating, the useful range would be about 300km. The weapon system's ability to compensate for atmospheric conditions between it and its target was a make-or-break matter, and at that time it was doing fine. ABL may be able to carry out as few as 6-10 "shots" before refuelling, though a reload of the toxic chemical fuels for an ABL would fill two C-17 transport planes. The ABL was expected to achieve effective range of at most 400km. While impressive, the system would be hard to use against Russia and China, since the ABL would be unlikely fly into these countries' airspace during crisis.

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