Any idea what makes the 747 the ideal craft for such a system, as opposed to something lighter and/or faster? Would it likely be range, reliability, or carrying capacity?
I guess the laser system takes a lot of room. Maybe the rest of the space will be used for extra fuel storage for endurance.
The six-unit COIL laser generator takes up most of the space inside the YAL-4. It's the only aircraft in the U.S. inventory big enough (and otherwise suited) for the job.
It is a specially modified version of the 747-400F which is the largest freight aircraft built in the US. The airborne laser requires massive quantities of very toxic chemical reactants to provide power for the laser. After all we're talking about laser powerful enough to heat up and destroy ICBMs in flight. Most of the space on the plane is reserved for the reactant tanks. There is a vapor proof wall to separate the passenger compartment from the reactants, because if they were to leak into the passenger compartment, everyone would die.
Because it's friggin' huge ... They've released "cartoons" of the ABL system inside the 747, and the fuselage is pretty much filled with the laser and its supporting hardware ... the turret in the nose is just the aiming/focusing device.