Australia has ordered 2,950 of the American SDB (Small Diameter Bomb also known as GBU-39). This is the earlier model (SDB I) that can only hit stationary targets. The U.S. Air Force has already ordered over 10,000 SDBs.
A major reason for this Australian purchase is the development of a special internal bomb rack that enables the new F-35 to carry eight SDBs internally. Australia is buying a hundred F-35s in part because when flying with all weapons carried internally F-35 is nearly invisible to radar.
By 2014 the U.S. Air Force had completed development and testing the JMMBRU bomb rack for the F-35 so that this stealth aircraft could carry eight SDBs internally, plus (in a less stealthy configuration) another 16 externally. This makes the F-35 a much more effective bomber, especially since the SDB has been upgraded. In 2014 the air force completed tests of SBD II (130 kg/285 pound, also known as the GBU-39/B). This model is equipped with hardware and software to enable it to hit moving targets. The SBD II is now ready for mass production and Australia may be an export customer.
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 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.
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 and missiles. But all these smaller 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 U.S. 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.
Meanwhile a lot of controversy surrounds the F-35. The U.S. Air Force still expects to get production models of its 31 ton F-35A in late 2016. This is the cheapest version, costing about $159 million each. The U.S. Navy version (the F-35C) will arrive in late 2019 and cost about $264 million each. This version has a stronger landing gear to handle carrier landings and components that are more resistant to corrosion from constant exposure to salt water. The vertical take-off version for the marines, the F-35B, will cost $214 million each. All of these prices are expected to be much higher (20 percent or more) in reality. This is happening despite more and more delays as well as questions about reliability and cost. At the moment the F-35 costs 60 percent more (than the F-16, per flight hour) to operate.
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.
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.