October 30, 2024: The American GMLRS (guided Multiple Launch Rocket System) missile used so frequently in Ukraine since 2022 has been joined by GLSDB (Ground-Launched Small Diameter Bomb). This weapon combines the 227mm rocket motor that propels the GMLRS and uses it to launch the latest version of the U.S. Air Force SDB (Small Diameter Bomb), the GBU-53 Stormbreaker.

Initially, the SDB was seen as the next generation of smart bombs and is a more compact design of the classic 227 kg/500-pound unguided bomb. The streamlined shape of the SDB, which carried the GPS guidance system and other guidance components internally allowed more to be carried by F-15/16/18 type aircraft. These aircraft can carry 24 or more SDBs each. 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 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.

SDB has a hard front end that can punch through nearly three meters of rock or concrete and a warhead that does less damage than the usual dumb bomb using explosives in a metal casing. That’s because the SDB warhead carries only 17 kg (37.4 pounds) of explosives, compared to 127 kg (280.9 pounds) in the conventional 500-pound bomb.

The U.S. delivered GLSBD to Ukraine in early 2023. Ukraine was eager to use this weapon because GLSBD can be fired from HIMARS and has a range of 150 kilometers. Moreover, it is more precise than GMLRS because Stormbreaker has multiple guidance systems that enable it to hit moving targets, including ships at sea and do so at night and in any weather.

Stormbreaker entered service in 2018 as the latest version of the SDB II/GBU-53. This model can identify, track and hit a moving target on the ground in any weather and at night. Originally this was just called SDB II. Initially there were problems getting the three targeting sensors to work reliably and the manufacturers worked developing different technologies to make it work. That took three years of tweaking and testing, a cycle repeated several times until 2018 when the GBU-53 designers got their novel targeting system to work reliably. In recognition of this the GBU-53 was given an official nickname; Stormbreaker.

SDB II has 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 combat.

When first announced in 2005 SDB II was described as possessing multiple sensors and a data link that enabled it to hit vehicles going at high speed and in bad weather. Sounded great but the repeated failures to get past testing efforts SDB II meant a long wait for impatient users. SDB II used an encrypted data link that enabled F-35 pilots to guide the SDB with great precision to hit moving targets. This communications capability enabled the SDB movement to be controlled via Link 16, an airborne Internet.

This capability meant that delivery of a bomb could be controlled by anyone with access to the network. That included air controllers on the ground with the troops.

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.

Meanwhile, sales of the SDB were picking up, especially export orders. In late 2017, Australia ordered another 3,900 American GBU-39 for $209,000 each. This order was for GBU-39B, which is used on F-35s. In early 2016 Australia ordered 2,950 of the older model SDB I that could only hit stationary targets. These cost $131,000 each. So far nearly 30,000 SDB I and SDB II have been delivered or are on order.

Another reason for the success of SDDB is the development of JMMBRU. This is an internal bomb rack that enables the F-35 to carry eight SDBs internally. In that configuration F-35s are nearly invisible to radar. In a less stealthy configuration, another 16 SDBs can 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 of the JMMBRU bomb rack in 2014. This made the F-35 a much more effective bomber, especially since the SDB was 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 within 5-8 meters of the aiming point the SDB II had guidance system options that enabled it to land within a meter. The SBD II began mass production in 2016 and several export customers placed orders.

SDB is basically an unpowered missile which can glide long distances. This makes the SDB even more compact, capable, and expensive. While the original SDB I cost about $70,000 each, that went up to $300,000 for Stormbreaker. The small wings allow the SDB to glide up to a hundred kilometers when launched from high altitudes.

Launching Stormbreaker from the ground or a ship using any GMLRS launcher provides long range precision firepower at relatively low cost. In 2017 the American marines demonstrated that GMLRS rockets could be launched from the flight deck of amphibious assault ships against land targets. GLSDB can also be used this way against enemy ships or land targets.

Ukrainian warplanes have been heavy users of SDB while ground forces find GLSDB very effective against distant Russian targets. Russia also has GPS guided glide bombs, but nothing as effective as SDB and GLSDB.