After many delays, the U.S. Air Force has finally begun production of the 250 pound Small Diameter Bomb (SDB). The SDB was supposed to enter service in 2005, in the wake of the 2004 introduction of the 500 pound 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. It's shape is more like that of a missile than a bomb (70 inches long, 190 millimeters in diameter), with the guidance system built in. The smaller "bang" from the SDB will result in fewer civilian casualties when used in an urban area. 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.
But what the air force really wants is to equip the B-1 with SDBs, as this bomber could carry as many as 216 of them. The new F-22 and F-35 warplanes are stealthy and normally carry their bombs internally. This limits how many they can carry, but with the SDB, an F-22 can carry eight of them. The Navy F-18 could easily carry 24 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.
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 each.) JDAM (a guidance kit attached to a dumb bomb) only cost about $26,000. 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 several feet of 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.
There was never any no point in building a 250 pound dumb bomb, as they would be too inaccurate to be useful. So it made sense to merge the guidance kit and the bomb itself. But the superiority of guided bombs is such that the next generation of heavier (500-2000 pound) smart bombs will probably be like the SDB.
The problem is that the SDB has required more testing that expected, and it's availability for combat has been delayed several times. When the SDB does appear this Fall, carried by F-15Es, it will have to compete with the army's new Excalibur 155mm GPS guided shell. This is a hundred pound weapon. The army prefers its GPS guided munitions as small as possible, so that there is less risk of civilian casualties, and so U.S. troops can be closer to the target. In ground combat, it's often important to get your troops into the bombed out target as soon as possible, to take prisoners or recover documents and other valuable intelligence material. Often, the bombed out target just provides another position for advancing friendly troops. Thus, in urban fighting, the smallest smart bomb currently available, the 500 pounder, was often too much. The army also has a 227mm GPS guided MLRS rocket, but this weapon has a 200 pound warhead that packs a pretty large bang. Thus the hundred pound Excalibur is more eagerly anticipated than the 250 pound SDB. But since both will arrive in the combat zone at the same time, it will be interesting to see how the troops evaluate both of them. The infantry don't care who a smart bomb, or shell, belongs to, as long as the damn thing gets the job done.