June 26, 2011: The U.S. has resumed production of the 250 pound (113 kg) unguided bomb. Actually, this bomb will rarely be used as a "dumb bomb", but with a new Paveway laser guidance kit, which turns the Mark 81 250 pound bomb into the GBU-58 smart bomb. France was the first to buy and use the GBU-58, mainly because this 113 kg bomb only has 44 kg (96 pounds) of explosives. That's less than half what is contained in the 500 pound (127 kg) bomb. The GBU-58 is also half the price of the other U.S. 250 pound (actually 285 pound/129 kg) bomb. This is the Small Diameter Bomb (SDB, or GBU-39), which costs about $77,000 each. The SDB has only 17 kg (38 pounds) of explosives, but also is designed to penetrate 2.4 meters (8 feet) of concrete.
Three years ago, the U.S. Air Force finally got the SDB into service. The SDB was supposed to enter service in 2005, in the wake of the 2004 introduction of the 500 pound (227 kg) GBU-38 JDAM. 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 (1.8 meter/71 inches long, 190 millimeters in diameter), with the guidance system built in. The smaller blast from the SDB results 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, smaller, GPS bombs available. This caused the 500 pound JDAM to get developed quickly and put into service, followed by the SDB and now the GBU-58.
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. This allows each fighter-bomber to take out up to four times as many targets per sortie. The latest version (SDB 2/GBU 40) has a glide range of up to 110 kilometers and lands within 8 meters (25 feet) of its aim point. The original SDB has a glide range of 40 kilometers.
The SDB is basically an unpowered missile, which can glide long distances. This makes the SDB even more compact, capable and expensive. JDAM (a guidance kit attached to a dumb bomb) only cost about $30,000. The small wings allow the SDB to glide long distances, especially from high altitude. SDB also has a hard front end that can punch through rock or concrete, and a warhead that does more damage than the usual dumb bomb (explosives in a metal casing.) The SDB is thus the next generation of smart bombs.
But France, and many other air forces, believe that the old 250 pound dumb bombs, with a laser guidance kit, make more economic sense. You don't always need the special abilities of the SDB, just accuracy and a smaller bang.