Aviation Week & Space Technology, 01/16/2006, page 435

Michael A. Dornheim, Los Angeles

After development on a ground testbed, Northrop Grumman is disassembling its chemical oxygen-iodine laser for transfer into the Missile Defense Agency's YAL-1A airborne laser aircraft, with the intent of shooting down a missile about three years from now.

The megawatt-class laser produced the required power and duration for a missile kill on Dec. 6, says Col. John Daniels, director of the MDA's Airborne Laser System Program Office. "We exceeded our goal for lasing duration and we consistently achieved power levels that will allow for approximately 95% of predicted lethal range against all classes of ballistic missiles." That includes solid-rocket missiles, which have a thicker skin than liquid-fueled ones.

The tests came after extensive tuning to get the chemical oxygen-iodine laser (COIL) and associated systems to work properly, and a long period to install the six beam-generating modules on the converted Boeing 747-400 freighter. The original concept was to have 14 modules in the aircraft, but getting just six installed was a tight fit. There is less power with six modules, and officials now believe 14 are impractical--the high power would probably destroy the optical train, for example.

SEPARATELY, Northrop Grumman delivered a solid-state kilowatt-class beacon illuminator laser (BILL) to the Boeing plant in Wichita, Kan., on Dec. 2, where the 747 is being converted into the YAL-1A prototype airborne laser (ABL) aircraft. The BILL shines a spotlight on the missile target to measure atmospheric distortion along the laser path, which is used to command a deformable mirror in the beam control section to correct the distortion.

The BILL is part of the beam control/fire control system, which is being installed by Lockheed Martin onto the YAL-1A at Wichita. This system also includes a tracking illuminator laser (TILL), built by Raytheon Electronic Systems, that puts a spot on the target for precise tracking. The beam control/fire control is to be flight tested later this year with the BILL and TILL operating.

The Dec. 6 COIL lethality demonstration ran more than 10 sec. During one test, a beam splitter was damaged. A spare was installed and used for several long-duration lases without damage, Daniels says. The laser is installed in the ABL Systems Integration Laboratory (SIL), which is a scrapped 747-200 on the ground at Edwards AFB, Calif.

The COIL is in the aft section of the SIL and is shown in the photo, which is looking forward in the aircraft. The 1.3-micron wavelength infrared laser beam is generated in a U-shaped beam tube, of which two legs are labeled "5" in the image. Along the beam tube are the six lasing modules that transversely inject excited iodine that is the lasing medium.

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Each lasing module is about the size of a sport-utility vehicle. The beam tube is folded to fit the six modules within the length of the aft fuselage, and the corners of the "U" are to the rear of the viewer of the photo. Three modules are on the left side of the image, and three are on the right. The two modules in the foreground obscure the others behind them.

The process starts with heated helium gas, supplied in insulated pipe "1," which vaporizes chlorine supplied in pipe "2." The chlorine is reacted in singlet oxygen generators "3" with basic hydrogen peroxide, which is hydrogen peroxide mixed with base compounds. This produces single energetic atoms of oxygen, along with by-products.

The gas flows sideways out of the singlet oxygen generators into gain isolation valves, one of which is the white cylinder labeled "4." The valves turn the gas downward into the gain cavities, which are sections of the beam tube "5." At laser startup, the valves are shut until downstream pressures and flows in the gain cavity and exhaust system are correct.

AT THE EXITS of the gain isolation valves are nozzles that inject iodine that has also been vaporized by the hot helium. The singlet oxygen atoms transfer their energy to the iodine, and the excited iodine is the lasing medium in the gain cavity. The iodine and by-products exit the cavity downward, assisted by steam ejector pumps under the floor in the cargo bay. This gas stream is then dumped overboard.

The ends of the beam tube go into the white vacuum chamber "6" at the end, which contains the laser resonator and pickoff mirrors. From there, the high-power light goes into the beam control/fire control system in the forward part of the aircraft.

The 747 is a very flexible structure compared with the granite optical benches that optical engineers are used to. The concern is both slow gross bending of the airframe and high-frequency jitter created by vibrations of the laser itself. The red pads on the beam tube cover bellow joints that allow the tube to flex. Such motion would normally ruin laser operation, if not cause destruction from a high-power beam gone awry, but an alignment laser runs in the parallel tubes marked "7." Its measurements control mirrors to keep the laser working correctly. Jitter control was demonstrated in flight tests last year when a guidance laser was able to align the full optical path of the COIL, a Lockheed Martin official says.

Item "8" in the photo is a crane rail and not part of the laser system. The black rectangles along the center are work platforms for technicians. The tight fit illustrates the difficulty they had installing the COIL.

TESTING IS LIMITED to about 10 sec. at the SIL because the facility's vacuum system that attempts to simulate a 40,000-ft. outside atmosphere gets overwhelmed by the laser exhaust. The high-power beam goes into a calorimeter for measurement.

Tuning of the COIL involved adjusting the chemical flows for good laser operation, as well as extensive refinement of the optical alignment controls that compensate for the flexible fuselage. More than 70 firings have been made, with initial durations of a fraction of second. Another challenge was getting the "hardwire abort" safety shutdown system to work correctly. For short tests, it was not needed; but for longer duration firings, it was required in case something went out of alignment or otherwise wrong early in the run. The system tended to abort firings unnecessarily, but "now is very reliable," an official says. "It was a complex task."

Since the Dec. 6 COIL demonstration, work has begun to remove the laser from the SIL and clean and prepare it for installation in the YAL-1A. Some of the less delicate pieces--such as propellant tanks and refrigeration systems--will be installed in the YAL-1A at Wichita. But the more delicate pieces, such as the lasing modules and optics, will await the return of the aircraft to Edwards in the fall for transfer into the flying testbed, probably starting in early 2007. The Missile Defense Agency hopes to demonstrate an inflight missile kill in late 2008.

"I'M SURPRISED they are putting it in the aircraft now," says Philip E. Coyle, 3rd, a senior adviser at the Center for Defense Information and the director of Pentagon test and evaluation from 1994-2001. He suspects more development is needed of the COIL itself. "It's easier to do research and development on the ground than on an aircraft."

The ABL budget was $474 million in Fiscal 2005 and is $465 million in Fiscal 2006. The total program cost is expected to be $3.6 billion with one aircraft.