By Tom Harris
Harris, Tom. "How Smart Bombs Work." 20 March 2003. HowStuffWorks.com. 26 February 2010.
A conventional bomb consists of some explosive material packed into a sturdy case with a fuze mechanism. The fuze has a triggering device -- typically a time-delay system, an impact sensor or a target-proximity sensor -- which sets the bomb off. When the trigger goes off, the fuze ignites the explosive material, resulting in an explosion.
A "dumb bomb" is a bomb with only these elements, dropped from an airplane such as the B-2 Bomber. The bomb is considered "dumb" because it simply falls to the ground without actively steering itself. A bomber might need to drop dozens, or even hundreds of dumb bombs to take out a target effectively.
"Smart bombs," by contrast, control their fall precisely in order to hit a designated target dead on. A smart bomb is essentially an ordinary dumb bomb that also contains an electronic sensor system, a built-in control system (an onboard computer), a set of adjustable flight fins, and a battery.
When a plane drops a smart bomb, the bomb becomes a particularly heavy glider. It doesn't have any propulsion system of its own, like a missile does, but it does have forward velocity (by virtue of being dropped from a speeding plane). It also has flight fins that generate lift and stabilize its flight path.
The control system and adjustable fins give the bomb a way to steer itself as it glides through the air. While the bomb is "in flight," the sensor system and control system track the designated target on the ground. The sensor system feeds the control system the relative position of the target, and the control system processes this information and figures out how the bomb should turn to steer toward the target.
To actually turn the bomb, the control system sends a message to actuators that adjust the flight fins. These fins work the same basic way as the various flaps on an airplane. By tilting the fins in a particular direction, the control system increases the drag acting on that side of the bomb. As a result, the bomb turns in that direction.
This adjustment process continues until the smart bomb reaches its target, and the fuze mechanism sets off the explosive. Smart bombs generally have proximity fuzes, which set off the explosive just before the bomb reaches the target, or impact fuzes, which set off the explosive when the bomb actually hits something.
The smart-bomb technology of today day is Boeing's JDAM (Joint Direct Attack Munition). The basic idea behind the JDAM program is to outfit existing "dumb" bombs with sophisticated rear guidance sections. The JDAM "tail kit" includes adjustable tail fins, a control computer, an inertial guidance system and a GPS receiver. Both the GPS receiver and the inertial guidance system allow the bomb to locate itself in space. The GPS receiver figures out its position by interpreting GPS satellite, while the inertial guidance system monitors the bomb's movements, tracking its path from its launch position. Costing about $20,000 per tail kit (which can be added to an existing warhead), it's much more economical than $120,000+ laser-guided bombs.
