Lots of Mirrors; Lots of Lenses - The Airborne Laser Beam Control/Fire Control System
If Battle Management is the Airborne Laser's (ABL) brain, and the lasers the brawn, then Beam Control/Fire Control (BC/FC) is the ocular system, the super-sensitive complex of lenses and mirrors that assure the lethality of the revolutionary directed energy missile-killing machine.
Stretching from the turret on the front of the Boeing 747-400 Freighter to an area behind an airtight bulkhead that protects the crew from possible chemical leaks, the BC/FC system is the most intricate of all the ABL systems. It is composed of 127 optics (mirrors, lenses and windows) ranging in size from one inch to five and one-half feet in diameter, many of which are unique to ABL and represent the state of the art in glass manufacturing and optical coating.
The BC/FC system performs two general functions: It safely controls the high- and low-energy laser beams propagated through the aircraft, and it is crucial to ABL's kill mechanism. Once a missile is locked in as a target, the system tracks it, compensates for atmospheric distortation between the target and aircraft, and, finally, points the high-energy laser onto the fuel tank, causing the missile skin to rupture. In essence, it makes the missile kills itself.
The complex BC/FC system is composed of three major assemblies and extends for half the length of the aircraft.
One assembly -- the Multi-Beacon Illuminator (MBIL) bench -- is located over the wings. It houses ABL's Track Illuminator Laser (TILL), which stabilizes the target in the onboard cameras and the Beacon Illuminator Laser (BILL), which corrects for the atmospheric disturbance between the aircraft and the target.
The Beam Transfer Assembly (BTA) -- the second of the three -- is forward of the MBIL, approximately under the cockpit. It holds ABL's unique, highly sensitive cameras, sensors, deformable and steering mirrors, and other specialized optics and electronics that enable the system to track and kill its target.
The third major assembly is the seven-ton rotatable turret. It contains, among other hardware, three of the system's vital elements: the primary mirror, the secondary mirror, and the conformal window.
The 58.8-inch, gold-plated primary mirror (left) and the12.2-inch secondary mirror (right) form a large telescope, which is the heart of the ABL kill chain. Beams from all three of ABL's lasers -- the TILL, the BILL and the high energy Chemical Oxygen Iodine Laser (COIL) -- all of which propagate at different wavelengths -- must touch these two mirrors on their journey to engage the target missile. By design, the laser beams enter the turret through an elbow-like passageway called a coude path. If the system had been set up where the beams fired directly out of the aircraft, the flight crew would have had to turn the plane to aim the lasers. By using the rotating turret the aircraft
remains on a steady course. When exiting the aircraft, the beams first strike the secondary mirror and bounce to the primary mirror, where they are expanded and reflected through the conformal window (below), a five and one-half foot "contact lens," which took almost four years to shape and grind. It is durable enough to withstand the tremendous aerodynamic forces exerted on it in its position at the front of the turret and still remain transparent to the megawatt-class beam from ABL's high-energy laser as it passes through it.
One of the greatest challenges that ABL has successfully overcome is in the manufacturing and coating of specialized optics, which include almost the entire BC/FC system. Each lens and mirror in the system has to be coated -- a highly complex process that is as much an art form as a science -- by applying fine layers of gold and other materials in an extremely delicate integrated fashion. Simply painting a lens as if it were a wall is ineffective. Since many of the optics must be able to perform dual duty by reflecting and transmitting high intensity laser beams at different wavelengths, the coatings must be multi-purpose to keep the optics from being damaged or destroyed. One coating typically makes an untreated glass reflective (turns it into a mirror), while another coating on the same optic might make it transmissive (turns it into a window).
Other crucial cogs in the BC/FC system are the atmospheric compensation deformable mirrors. They have a thin membrane surface backed by hundreds of tiny, finger-like actuators that move thousands of times per second in reaction to readings from the BILL, which serves as a sort of radar to paint a picture of the atmosphere between the aircraft and the target.
The Lockheed Martin Corp.'s facility at Sunnyvale, Calif., is creating and assembling ABL's BC/FC system. Once it has been completed and tested it will be disassembled and shipped to Edwards Air Force Base, Calif., where it will be re-assembled on the aircraft just prior to installation of the COIL. After all systems are in place they will be tested as a whole, on the ground and in the air. The tests are scheduled to culminate in the shootdown of a ballistic missile over the Pacific Ocean in 2005.
Airborne Laser System Program Office, Office of Public Affairs,
3300 Target Road, Building 760, Kirtland AFB, NM