The U.S. Air Force's SDB (Small Diameter Bomb, a 250 pound JDAM) has passed it's initial flight tests and is on track to finish development and enter service in 2006, or earlier. The SDB looks like an elongated 155mm artillery shell (which weighs 90 pounds). The SDB is 152mm in diameter, but is six feet long. The casing of the bomb weighs 160 pounds (and can penetrate over five feet of concrete), the explosive material is 50 pounds and 40 pounds is used for the guidance system. One time consuming part of developing the SDB was designing and testing the special bomb racks for the different aircraft that will carry SDBs. Current bomb racks can't handle a bomb smaller than the 500 pounder. The B-2 bomber, which uses an internal bomb-bay, and will be able to carry at least 200 SDBs. The F-15E, F-22, B-1, A-10, F-16, B-52, F-117 and Predator UAV will each require special bomb racks for the SDB. The SDB will come in several versions. As a "mini-JDAM" it will use a more accurate guidance GPS system (than the current JDAM) so that most bombs will hit with 25 feet of the target. But about half the SDBs to be built will have a terminal guidance system in the nose (using heat sensing or radar) to pick up and home in on a moving target or simply make a more precise hit of a stationary target (like the mouth of a cave or entrance of a bunker.) The air force is currently planning to buy at least 24,000 SDBs. The bombs are currently expected to cost over $40,000 (for those equipped with a terminal guidance system), with most of the expense being the guidance system. But the U.S. Navy has developed a new terminal guidance technology that could cut the cost of SDBs by at least a third. The basic JDAM version of the SDB will probably be available for combat use in a year or two. But the versions the air force is most interested in are those with terminal guidance, and the longer range one (with a small rocket and pop-out wings) that will glide for up to 120 kilometers. The long range SDB will allow two or three aircraft to devastate an enemy air-defense system.