The Airborne Laser (ABL) is a boost-phase system consisting of a laser mounted in the nose of a 747-400F aircraft, and is currently being developed for theater missile defense. Rather than attempting to burn through the missile skin, the ABL heats the skin to a critical temperature at which external forces cause it to break. If the skin is heated at a single point, pressure from the internal fuel tanks will likely cause a rupture at that point. If the skin is heated along an arc, axial stresses resulting from the missileís high acceleration will cause the booster to collapse. Either way, the warhead will be diverted from its intended target.
The ABL will first see the missile plume several hundred kilometers away using its infrared seekers. Its two low power illuminating lasers will then determine the target range and get initial information about the atmosphere between the ABL and the missile. They will then track the missile and provide aiming data to the ABL. The beacon laser then takes over, getting precise data about the atmosphere along the laser path, allowing the kill laser to compensate for atmospheric disturbances. Finally, the primary COIL laser locks onto the target and remains on target until the missile is sufficiently damaged.
Each ABL will require escort by F-22s. In it may require escort by an electronic countermeasures aircraft as well. All of these aircraft will require tankers for in-flight refueling.
The ABL program has been highly successful in meeting its current objectives, and has attracted interest as a possible national missile defense system. In migrating the system from a theater to a national system there are several critical technical considerations.
The range of the ABL against ICBMs will be significantly different from that against the short-range missiles it was designed to intercept. Several factors influence this.
When engaging an ICBM, atmospheric effects will have a different influence from in a TBM engagement. Generally, the more air the laser must travel through, the less the power of the beam incident upon the missile. Distance also spreads the beam, attenuating the intensity of the beam on the target. The ICBM case differs here from the TBM case in two respects.
As with other boost-phase defenses, the fate of the warhead after the booster is disabled needs to be carefully considered. Unlike kinetic-kill defenses, a laser will not destroy the booster. If the ABL punctures a hole in the skin surrounding a fuel tank (which describes most of the booster area) the fuel will simply leak. While it is possible that thrust would terminate immediately, this is not certain. In particular, judging from past space accidents, it appears improbable that the fuel would ignite and destroy the missile. With current understanding, one cannot be certain of where the warhead would land. A lower bound on the distance traveled by the warhead may be found by assuming that thrust simply terminates when the tank ruptures; for example, an 8000 km range ICBM with thrust terminated ten seconds early would travel 3000 km. This is in stark contrast with the conventional wisdom which pictures the missile falling back on enemy territory. Political sensitivities of those under the flight path (Russia and most of Western Europe) would be an issue.
The other kill mechanism occurs if the ABL weakens an arc of the booster skin rather than a simple point. Axial stresses resulting from the rocketís high acceleration will then result in the collapse of the booster. The threshold arc-length and acceleration for such a collapse are unclear, so at this point it is difficult to determine the shortfall of the warhead.
There are two possible deployment approaches for using ABL in an NMD role. The first would require activating ABL aircraft only in a crisis situation, while the second would call for full time presence of ABL aircraft in the vicinity of states of concern. While only the second would offer full time coverage, there are several reasons that the first might be preferred. It would be much cheaper. Additionally, with the large number of flight-hours associated with the first option, the chances of a crash might increase. This is an important consideration as there is a massive quantity of chlorine onboard the aircraft; any crash would be equivalent to the dropping of a very large chemical bomb.
There are several countermeasures that the attacker might use to make the ABLís task more difficult: