The ABL will carry enough reactants for about 20 shots on target, which will be generated in the plane's six rear modules by combining hydrogen peroxide and chlorine gas to produce an excited form of oxygen known as singlet delta oxygen (SDO). Iodine granules are cooked in onboard ovens, forming iodine gas that is injected into a cavity with the SDO, which excites the iodine and "pumps" the laser. As the excited iodine relaxes to its base energy state, photons with the wavelength of 1.3 microns are produced. The photons, bouncing around inside a long cylindrical resonator, are then gathered and sent down a long tube toward the front of the fuselage and the turret. This stacking process-which occurs at fraction-of-millisecond intervals-generates a laser beam.
The Northrop Grumman test firing proved that the laser could generate the heat necessary to destroy a missile from a stationary mounting. The next obstacle was to show that this can be done from a buffeting airplane banking through the sky at 600 miles per hour, 40,000 feet above the ground. This required some serious slimming down.
"One of the key challenges has been to â€lightweight' the system, because we are putting this hardware on an airplane and dealing with the volume and lifting capacity of the airplane," explains Northrop Grumman's ABL Deputy Program Manager Gary Koop at the Systems Integration Lab at Edwards Air Force Base, where the laser components are being inserted into a retired Air India 747 to make sure they fit prior to final installation in the YAL-1A. The weight reduction was achieved with lightweight materials such as titanium, a new range of plastics and new composite materials. But the ABL also needs to survive in a highly dynamic environment. "When we build lasers in a ground facility, we pour thousands of pounds of concrete to make a stiff foundation," Koop says. "Boeing designs airplanes to be flexible to absorb all the aerodynamic loads. So we have one system, the plane, that wants to move, and one, the laser, that wants to be rigid." The solution to that came through the development of multi-axis shock absorbers that keep certain components along the laser's path steady as the aircraft moves around it.
It has taken decades of similar engineering advances to make the ABL feasible. The military has been noodling around with laser weapons since the early days of the cold war. The United States and the former Soviet Union both conducted experiments with nuclear-powered naval and satellite-based lasers, all intended to bring down intercontinental ballistic missiles and bombers. Eventually, the idea became the central component in the now-abandoned Star Wars program floated during the Reagan administration. The technological breakthroughs generated by that failed program were folded into the ABL effort.
Despite the self-assurance surrounding the ABL among Air Force personnel, critics outside the program doubt the effort will pan out. "I'm deeply skeptical about achieving the laser power output required to destroy a missile," says Subrata Ghoshroy, a Harvard research fellow and senior associate at the Federation of American Scientists who has worked on past military laser projects. According to Ghoshroy, Northrop Grumman's ground-test success is only partially conclusive; once the laser is in the air, many factors will interfere with the beam quality, including moisture and air turbulence, which the adaptive optics may not be able to sidestep entirely. "There is very little experience in the whole process of building a high-power laser and intercepting a missile with it," Ghoshroy notes.
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.