In late 2017 Australia ordered another 3,900 American SDB (Small Diameter Bomb also known as GBU-39) for $209,000 each (including training and maintenance accessories and support). This order is for the SDB II (GBU-39B) for its F-35 fighters. In early 2016 Australia ordered 2,950 of the earlier model (SDB I) that can only hit stationary targets. These cost $131,000 each. The 130 kg (285 pound) SDB entered service in 2006 and so far over 20,000 have been ordered or delivered, most by the U.S. Air Force.
A major reason for Australia buying so many SBDs is the success of JMMBRU, a special internal bomb rack that enables the F-35 to carry eight SDBs internally. Australia is buying a hundred F-35s in part because when flying with all weapons carried internally the aircraft is nearly invisible to radar. In a less stealthy configuration another 16 SDBs cab be carried externally giving the F-35 a maximum capacity for 24 of these smart bombs.
The U.S. Air Force completed development and testing the JMMBRU bomb rack in 2014. This made the F-35 a much more effective bomber, especially since the SDB has been upgraded that same year with hardware and software to enable it to hit moving targets as well as being more accurate. While the original SBD would land 5-8 meters (16-25 feet) of the aiming point the SDB II had guidance system options that enabled it to land within a meter (three feet). The SBD II began mass production in 2016 and Australia was already interested.
The SDB was supposed to be a revolutionary weapon and in many ways it was. But there was not as much demand as expected because there so many other small, precision weapons available. Even the availability of the SDB II, which can hit vehicles going at high speed and in bad weather, did not boost sales as much as anticipated. SDB II has an encrypted data link that enables the F-35 pilot to guide the SDB, with great precision, to hit moving targets. 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, even another aircraft or on the ground. 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 the SDB warhead carries only 17 kg (38 pounds) of explosives, compared to 127 kg (280 pounds) in the 500 pound bomb.
The SDB is considered 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.
The F-35 is armed with an internal 25mm cannon and, before the SDB, four internal air-to-air missiles (or two missiles and two smart bombs) plus four external smart bombs and two missiles. All sensors are carried internally and max weapon load is 6.8 tons. The aircraft is very stealthy when just carrying internal weapons. The user reports so far have been very positive and the F-35s are performing as advertised and then some. Details on that are often classified, which is sort of a good sign.
Like the F-22 fighter, the F-35 is stealthy and stuffed with a lot of new technology. Most (about 60 percent) of the F-35s built will be used by foreign nations. The rising cost of the F-35 brings with it reluctance to buy as many aircraft as currently planned. The success of smart bombs in Iraq and Afghanistan has also made it clear that fewer aircraft will be needed in the future. In any event, it's likely that F-35s will end up costing more than $100 million each. But with SBDs F-35s become a very potent bomber that can get at well protected targets.