June 24, 2019: The United States has an acceptable problem with its new versions of the SDB (Small Diameter Bomb). Efforts to develop advanced features resulted in two new distinct versions of the SDB; GBU-40 and GBU-53. Both are in demand. American forces are standardizing on the more capable, and more expensive, GBU-53. For the moment the manufacturers of these two models are willing to continue production of both as long as there is demand. This situation came about because developing the “SDB II” ran into more problems than anticipated and some drastic solutions had to be found. That included letting another firm try to get the wonderful features planned for SDB II working. That turned out to be a good idea, because in 2018 the U.S. Air Force finally had the preferred version of SDB II, also known as the GBU-53. This model is equipped with hardware and software to enable it to hit moving targets. The GBU-53 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. It took three years of tweaking and testing, a cycle repeated several times until the GBU-53 team won because they finally got a promising concept to actually work. The GBU-53 design was finally doing everything SDB II promised but never quite delivered and the air force was so pleased that the GBU-53 was given an official nickname; Stormbreaker. Much catchier than SDB, or even (or especially) SDB II. Publicizing the new, improved Stormbreaker does cause a bit of confusion for those who do not follow the SDB saga regularly.

Most of those who looked into Stormbreaker eventually realized it is not just similar to the SDB, it also has a better press agent. At the same time Stormbreaker is a visibly, and operationally, different small bomb than SDB and SDB II. Yet there will continue to be demand for the original GBU-39 SDB and GBU-40 SDB II. Both of these are cheaper and less capable than GBU-53. Yet more warplane fire control systems can handle GBU-39/40 and do what their users require and cost a lot less than GBU-53. The GBU-40 version has laser guidance added to GPS but lacks all the additional features of the GBU-53. While the U.S. Air Force and Navy are eager to switch from to GBU-53, many foreign customers are not sure they need the much more capable, and expensive, GBU-53. One problem with the Stormbreaker is that the aircraft using it has to have its fire control system upgraded considerably to handle it and that upgrade is more complex than needed for SDB II GBU-40. At the moment only the F-15E and F-22 have Stormbreaker upgrades. The F-35 won’t be able to use Stormbreaker until 2022 when its software is updated to handle the many additional capabilities of the GBU-53. Similar upgrades for the F-18E won’t be ready until 2023.

SDB II had been in development since 2005 and announced as ready several times, at least until the last round of Operational Tests were conducted and, as often happens during these realistic (“operational”) tests something goes wrong. In other words, SBB II was, for a long time, ready to go, but not quite ready for prime time (actual combat). At least not for missions requiring certain features that were having Operational Test problems.

When first announced in 2005 SDB II was described as possessing multiple sensors and a datalink that enabled it to hit vehicles going at high speed and in bad weather. Sounded great but the repeated failures to get past the Operational Testing meant SDB II did not boost sales as much as anticipated. From the beginning, SDB II had an encrypted data link that enabled the fighter (especially F-35) pilot to guide the SDB, with great precision, to hit moving targets. This communications capability enabled 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 original SDB II spec called for three different guidance systems: radar, heat seeker, and homing on laser light bounced off the target. That meant 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. Now it actually works but it was felt that a new name might overcome the shaky reputation the SDB II had earned.

Meanwhile, sales of the SDB has been picking up, especially export orders. Australia wants 3,900 GBU-30 SDB II for their F-35s. Australia and expects to pay $209,000 each (including training and maintenance accessories and support) for each one. In early 2016 Australia ordered 2,950 of the earlier model (GBU-39) that can only hit stationary targets. These cost $131,000 each. The original SDB entered service in 2006 and so far over 20,000 GBU-39, 40 and 53 have been ordered or delivered, most for the U.S. Air Force.

A major reason for some countries (like Australia) are 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 GBU-40/53 had guidance system options that enabled it to land within a meter (three feet).

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 are so many other small, precision weapons available. With the appearance of Stormbreaker, the F-35 will be an even more impressive ground attack aircraft.

The original SDB is basically an unpowered 129 kg (285 pound) missile which can glide long distances. This makes the SDB even more compact, capable, and expensive (about $100,000 for GBU-39 and more than twice that for GBU-53). The small wings allow the SDB to glide up to a hundred 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. Actually, several different warheads were developed for the SDB, one of which lacked the metal penetrator and instead had a composite casing that produced fewer lethal fragments and reduced collateral damage to nearby civilians or friendly troops.

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 the 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.