The basic concept of a bomb could hardly be simpler. A conventional bomb consists of some explosive material packed into a sturdy case with a fuze mechanism (yes, that's fuze, not fuse). The fuze mechanism 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. The extreme pressure and flying debris of the explosion destroys surrounding structures (see How Grenades Work for information on explosives and fuzes).
Smart Bomb Image Gallery
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. Needless to say, it's some feat hitting a target precisely with this type of weapon. A bomber might have 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. In this article, we'll find out how the major types of smart bomb accomplish this.
Smart Bomb Basics
A smart bomb is essentially an ordinary dumb bomb with a few major modifications. In addition to the usual fuze and explosive material, it has:
- an electronic sensor system
- a built-in control system (an onboard computer)
- a set of adjustable flight fins
- 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. (See How Airplanes Work for more information).
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 main difference between the different types of smart bomb is how the sensor system actually "sees" the target in the first place. We'll look at how smart bombs have done this in the past in the next section.
Yesterday's Smart Bombs
Up until relatively recently, most smart bombs were either TV/IR-guided or laser-guided. Both types of bomb use visual sensors to locate ground targets.
A TV/IR-guided bomb has either a conventional television video camera or an infrared camera (for night vision) mounted to its nose. In remote-operation mode, the controller relays information through radio signals to a human operator, who is usually onboard the bomber plane. The remote operator relays commands to the control system to steer the bomb through the air -- the bomb acts something like a remote-control plane. In this mode, the operator may launch the bomb without a specific target and sight, and then pick up the target from the video as the bomb gets closer to the ground.
In automatic mode, the pilot locates a target through the bomb's video camera prior to launch and sends a signal to the bomb telling it to lock on to the target. The bomb's control system steers the bomb so that the indicated target image always stays near the center of the video display. In this way, the bomb zeros in on the locked target automatically.
Laser-guided smart bombs work a little differently. Instead of a video camera sensor, the bomb has a laser seeker -- an array of photo diodes. As you might expect, the photo diodes are sensitive to a particular frequency of laser light. For the bomb to see its target, a separate human operator, either on the ground or in the air, has to "paint" the designated target with a high-intensity laser beam. The laser beam reflects off the target, and the laser seeker picks it up.
The laser designator has its own unique pulse pattern. Before dropping the bomb, the bomber aircraft computer tells the missile's control system the specific pulse pattern (via an electronic "umbilical" connection to the bomb). Once the bomb is in the air, the control system is only interested in laser energy with this pulse pattern. The control system's basic goal is to steer the bomb so that the reflected laser beam is hitting near the center of the photo diode array. This keeps the bomb heading straight toward the target.
Both of these systems can be highly effective, but they have one major drawback: The bomb sensor has to maintain visual contact with the target. If cloud cover or obstacles get in the way, the bomb will most likely veer off course.
We'll explore today's smart bombs next.
The preeminent smart-bomb technology of the day is Boeing's JDAM, which stands for Joint Direct Attack Munition. The basic idea behind the JDAM program is to outfit existing "dumb" bombs with sophisticated rear guidance sections. The U.S. Air Force is currently using JDAM with the 2,000-pound (907-kg) BLU-109 or MK-84 warhead or the 1,000-pound (454-kg) BLU-110 or MK-83 warhead.
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 signals (see How GPS Receivers Work), while the inertial guidance system monitors the bomb's movements, tracking its path from its launch position.
Before dropping the bomb, the aircraft uses its own GPS receiver to pinpoint particular targets on the ground. Just before releasing the bomb, the aircraft's computer feeds the bomb's computer its current position and the GPS coordinates of the target.
In the air, the JDAM's GPS receiver processes signals from GPS satellites to keep track of its own position. As with other smart bombs, the control system adjusts the flight fins to "steer" the bomb in the right direction. According to the U.S. Air Force, the system is accurate to within 40 feet (13 meters). When everything goes exactly right, the bombs generally hit within a few feet of their targets.
This system works fine even in bad weather, because the JDAM gets all its information from satellite signals, which aren't blocked by cloud cover or obstacles. The bomb doesn't have to see anything at all to find its way to the target. And at around $20,000 per tail kit (which can be added to an existing warhead), it's much more economical than $120,000+ laser-guided bombs.
The JDAM played a major role in the U.S. invasion of Afghanistan in 2001, and it will certainly play a significant role in any U.S. bombing campaigns in the near future. While the newest smart bombs aren't 100 percent accurate, they are such an improvement over their predecessors that they're quickly taking over the U.S. arsenal.
For more information about smart bombs and other types of ordnance, check out the links in the next section.