It was always something of a mystery why the U.S. Air Force developed the 285 pound (129 kg) Small Diameter Bomb (SDB). The official story was that this GPS guided smart bomb was needed for urban warfare. The smaller blast (17kg/38 pounds of explosives, compared to 127 kg/280 pounds for the 500 pound bomb) 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 the smaller SDB was always a mystery, with many produced, but few actually used.
But the SDB also has a hard steel, ground penetrating, front end, that can penetrate nearly two meters (six feet) of concrete. Not much use for that in urban warfare. But such a capability is very useful for taking out underground installations, particularly the entrances and air intakes. North Korea, for example, has twenty airfields with underground hangars for the aircraft. Usually tunneled into a nearby hill or mountain, the underground hangar allows fighters and bombers to quickly taxi out onto the runways and take off. Since North Korea doesn't have that many operational warplanes, it's believed that some of these "airfields" actually have long range rockets and ballistic missiles, mounted on trailers equipped to erect the missile into launch position and fire it off, in the underground hangars. The trailers are hauled out of the tunnels, onto the air field, the missile fired, and then the trailer is taken back inside to be reloaded. The North Koreans also have hundreds of other, smaller, underground facilities, close to the South Korea border, containing artillery and rocket launchers. These weapons are meant to be quickly hauled out and fired south.
That's where the SDB comes in, but the U.S. Air Force isn't saying much about it. The SDB would be the ideal weapon for launching a surprise attack on North Korean underground facilities, both the airfield hangers and the artillery bunkers. American B-2 and F-22 aircraft can dodge North Korea radar and drop a lot of SDBs all at once. A B-2 can carry over 200 SDBs. An F-22 can carry eight, and still protect the B-2s against any North Korean fighters that might have been in the air at the time of the attack. A half dozen B-2s carry over 1,200 SDBs, which is sufficient to cripple North Korean air defenses and twenty key air bases. A few dozen F-22s carry another 300 SDBs to hit smaller, spread out targets. The SDBs not only shut down the entrances to the hangars, but also blow deep holes in the airfields. While North Korea has thousands of troops trained and equipped to quickly come in and clear the hangar entrances and repair the airfields, they are not quick enough to do so before unstealthy B-1s and B-52s come in with more smart bombs (and cluster bombs, carrying thousands of small booby traps, that explode when stepped on or rolled over by vehicles or engineering equipment).
The North Koreans (and Iranians, and so on) know about the SDB, and what it can do. If they have come up with a solution, they are going to keep it quiet. As will the U.S. Air Force about the details of their attack plans. But it's pretty obvious that the versatile, ground penetrating SDB, is a key part of those plans.
It was only four 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 more complex 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 (1.78 meters/70 inches long, 190 millimeters in diameter), with the guidance system built in. In the last few years, development work has continued. Last year, for example, there was a change in the software of the 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 over a hundred kilometers from the target, then glided most of that distance 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.
The B-2, F-22 and F-35, in order to maintain their stealth, normally carry their bombs internally. This limits how many SDBs they can carry. The non-stealthy fighters can carry 24 or more SDBs. The SDBs are carried on a special carriage, which holds four of them, and takes up about as much space as one large (2,000 pound) bomb. 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 110 kilometers (from high altitude.) The SDB is thus the next generation of smart bombs, and just what was needed to wreck the plans of any nation that depends a lot on underground facilities.