September 1, 2009: The U.S. Air Force has changed the software of the 250 pound Small Diameter Bomb (SDB) so that it can be used like a JDAM. That is, it can now, with the new software, be dropped from an aircraft while directly above the target. The SDB was built as a glide bomb, which was dropped ten or more kilometers from the target, then glided for a bit before diving on the target. This resulted in complaints from troops below, who had to wait longer for a SDB to hit. The SDB was often preferred, especially in urban areas, because it had less bang than a 500 pound JDAM. But not when it took so long to arrive.
It was three years ago that the 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 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 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, smaller, GPS bombs available. This caused the 500 pound JDAM to get developed quickly and put into service.
But what the air force really wanted was to equip the B-1 with SDBs, as this bomber could carry as many as 216 of them. Because the SDB could glide for long distances, one B-2 could come along and take out dozens of targets at once.
The new F-22 and F-35 warplanes are equally 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 non-stealthy 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. But in the meantime, the air force was reminded that the troops on the ground don't care about the fancy tricks your new bombs can do, but more about how fast you can get the bomb on the target.