The U.S. Air Force has nearly completed tests of its SBD II (130 kg/285 pound Small Diameter Bomb, also known as the GBU-39/B). This model is equipped with hardware and software to enable it to hit moving targets. When all tests are completed by the end of 2014 the SBD II will be ready for mass production. The problem is that there is not a lot of demand for the SDB II and not a lot will be ordered. This despite the fact that SDB II can hit vehicles going at high speed and in bad weather. SDB II has an encrypted data link that enables the aircrew to guide the SDB to hit moving targets with great precision. This communications capability enables the SDB movement to be controlled via the air force's airborne Internet (Link 16), which means the “bomb driver” can be anywhere. The SDB II has three different guidance systems: radar, heat seeker, and homing on laser light bounced off the target. That means no matter what the weather or time of day there is a guidance system that will find the target. Even without human intervention the three sensors enable SDB II to find targets in a cluttered and obscured (by weather or darkness) environment.
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 for SDB I and four times that for SDB II). 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 nearly three meters (eight feet) of rock or concrete and a warhead that does less damage than the usual dumb bomb (explosives in a metal casing). That’s because SDB carries only 17 kg (38 pounds) of explosives, compared to 127 kg (280 pounds) in the 500 pound bomb. The SDB is, thus, the next generation of smart bombs and the more compact design allows more to be carried. Thus, F-15/16/18 type aircraft can carry 24 or more SDBs. The SDBs are carried on a special carriage which holds 4 of them instead of one bomb of more traditional shape. The carriage is mounted on a bomber just like a single larger (500, 1,000, or 2,000) pound bomb would be. However, this feature was rarely needed in combat situations because one smart bomb does the work of hundreds of unguided ones. There are also a lot of other guided weapons out there.
As effective as the SDB is it must operate in a very competitive environment. The U.S. has several long range guided bombs as well as cruise missiles for this sort of thing. The long range bombs include the JASSM and JSOW, which are both basically GPS guided smart bombs. The original JDAM bomb kit (added to 500, 1,000, and 2,000 pound bombs) cost $26,000 each. The longer range JSOW (JDAM with wings and more powerful guidance system) cost $460,000 each. The even longer range JASSM cost $500,000 (the 400 kilometer version) to $930,000 (the 900 kilometer JASSM ER) each. The AGM-158 JASSM missiles are 1,045 kg (2,300 pound) weapons that are basically 455 kg (1,000 pound) JDAMS (GPS guided bombs) with a motor added. JASSM was designed to go after enemy air defense systems or targets deep in heavily defended (against air attack) enemy territory. The reason for buying these is to have something to deal with air defenses of a nation like China.
Finally, there’s the new Tomahawk. The RGM-109E Block IV Surface Ship Vertical Launched Tomahawk Land Attack Missile weighs 1.2 tons, is 6 meters (18 feet) long, has a range of 1,600 kilometers, getting to the target at a speed of 600-900 kilometers an hour, flying at an altitude of 17-32 meters (50-100 feet), and propelled by a jet engine generating only 600 pounds of thrust. Accuracy is on a par with JDAM (10 meters/ 31 feet). The Block IV Tomahawk can be reprogrammed in flight to hit another target and carries a vidcam to allow a missile to check on prospective targets. There’s also the new JMEW (Joint Multi-Effects Warhead System) warhead for the Tomahawk. This is a 450 kg (1,000 pound) warhead designed mainly for penetrating underground bunkers, but it will also provide excellent blast effect for less robust targets. Exact penetration was not revealed. JMEW uses laser terminal guidance, enabling it to hit within a few meters (ten feet) of its aiming point. JMEW can also hit moving targets.
The 130 kg SDB more often finds itself competing with smaller weapons like the 49 kg Hellfire missile rather than with the larger smart bombs. But all these weapons have precision in common and the SDB is the only one of the “smart bombs” that can go after moving targets on land or sea. One problem with this, and the SDB in general, is that smaller guided missiles, especially Hellfire, are still the preferred weapon for this sort of thing. The problem for the air force is they don’t have a small weapon like Hellfire for their jets. Britain does (Brimstone, a version of Hellfire tweaked to work on fast aircraft) and now the air force has the smaller 70mm guided missile that will work on jets. So far SDB has been an excellent weapon in search of a mission that isn’t already being taken care of by something that gets it done and has been around longer.