April 25, 2012: While the American 130 kg (285 pound) Small Diameter Bomb (SDB, also known as the GBU-39/B) was not as successful in combat as expected, it did accomplish the rare feat of costing less than it was supposed to. The latest version, the SDB II (GBU 40) came in 19 percent under budget, with the 17,000 SDBs ordered costing $277,000 each. The SDB also performed as expected. Unfortunately, cheaper weapons were competing with SDB and were often more in demand for other reasons as well.

The SDB has frequently been ignored because of the competition. It was not supposed to work out this way. The SDB was designed from the bottom up as a smart bomb. It was only six years ago that the U.S. Air Force finally got the SDB into service, in Iraq. The SDB was supposed to enter service in 2005, in the wake of the 2004, introduction of the smaller (227 kg/500 pound) JDAM (GPS guided bomb kit). But there were many technical problems with the SDB. That's because this was not just another "dumb bomb" with a GPS guidance kit attached. The SDB had a more effective warhead design and guidance system. Its shape is more like that of a missile than a bomb (nearly two meters, as in 70 inches, long and 190mm in diameter), with the guidance system built in. The smaller blast from the SDB resulted in fewer civilian casualties. Friendly troops can be closer to the target when an SDB explodes. While the 500, 1,000, and 2,000 pound bombs have a spectacular effect when they go off, they are often overkill. The troops on the ground would rather have more, and smaller, GPS bombs available. This caused the 500 pound JDAM to get developed quickly and put into service. But it wasn't small enough for many urban combat situations. The SDB carries only 17 kg (38 pounds) of explosives, compared to 127 kg (280 pounds) in the 500 pound bomb. But it was found that the JDAM kit could be attached to 500 pound bombs with most of their explosives removed (and replaced with concrete) to provide a bomb with the same small explosion as the SDB but costing over 80 percent less.

The SDB is basically an unpowered missile which can glide long distances. This makes the SDB even more compact, capable, and expensive (about $70,000 for SDB I and four times that for SDB II). JDAM (a guidance kit attached to a dumb bomb) only costs about $30,000. The small wings allow the SDB to glide up to 70-80 kilometers (from high altitude). SDB also has a hard front end that can punch through nearly three meters (eight feet) of rock or concrete and a warhead that does less damage than the usual dumb bomb (explosives in a metal casing). The SDB is thus the next generation of smart bombs. The more compact design of the SDB allows more to be carried. Thus F-15/16/18 type aircraft can carry 24 or more SDBs. The SDBs are carried on a special carriage which holds four of them. The carriage is mounted on a bomber just like a single larger (500, 1,000, or 2,000) pound bomb would be. However, this feature was rarely needed in combat situations.

The most recent model, the SDB II, has an encrypted data link that enables the SDB to hit moving targets. This communications capability enables the SDB movement to be controlled via the air force's airborne Internet (Link 16).