In mid-2018 the U.S. Air Force ordered production of the BLU-137/B as a replacement for the older BLU-109/B penetrator (“Bunker Buster”) bomb. BLU-109 has been in use since 1985 and is usually mated with a Paveway laser-guidance kit like the GBU-27. Currently the most frequently use penetrator bomb is this laser guided, 909 kg (2,000 pound) BLU-109/B. This “bunker buster” can penetrate five meters (16 feet) of concrete and even more earth. This is accomplished using a 25mm thick steel cashing filled with 240 kg (530 pounds) of Tritonal. In the rear of the bomb is a time delay fuze.
Details of how the BLU-137/B is different were not revealed but it is known that the air force has been concerned for years with the number of BLU-109/Bs that failed to detonate after penetrating. This is not a new problem. Back in 2011 the U.S. Air Force spent $36 million to develop a new Hard Target Sensing Fuze for its large (BLU-109 and larger) penetrator bombs. The new fuze would also be reprogrammable by the pilot, while in the air. The pilot can specify how deep the bomb should go before detonating, as well as how many voids (levels of an underground bunker) to go through before detonating. A time delay can also be specified. The new fuze must be able to survive a penetration force of 15,000 pounds per square inch (one ton per square cm). The new fuze was to be used with the BLU-109, 113 or 122 bombs. Improvements to the BLU-109 fuze and detonation system were not sufficient. As more BLU-109s were used against underground targets in Iraq and Syria (and Gaza, by Israeli aircraft) BDA (Bomb Damage Assessment) teams eventually got to target areas and confirmed what prisoner interrogation and other intel reports were describing as an “unacceptable” level of failed detonations. So the air force ordered a major redesign of its 2000 pound penetrator bomb and apparently tested it sufficiently to be satisfied the reliability problems were fixed and now ordered it into mass production. The BLU-137/B will be interchangeable with the BLU-109/B so that equipment on aircraft or guidance kits won’t have to be modified.
The failure rate of the BLU-109/B was not so high that could not be used. Instead, to ensure destruction of an underground target multiple BLU-109/Bs would be used on certain targets. Many BLU-109/B targets were based on imperfect intel and were hit just to take the chance that the enemy bunker would be there and it would be destroyed. Many ISIL (Islamic State in Iraq and the Levant) members believed themselves protected by Allah when the penetrating bomb did indeed penetrate but did not detonate. Not exactly the sort of morale impact such weapons are supposed to have. If the BLU-137/B proves as reliable in combat as it did in testing Islamic terrorists will find their divine intervention has diminished when it comes to penetrating bombs. ISII made extensive use of deep tunnels and bunkers. In Gaza Hamas has been getting expert advice from Iranian tunnel builders on how to build more effective tunnels and bunkers. Israel had watched (from the air, and via spies on the ground) as Hezbollah used lots of its Iranian money to build underground bunkers in the areas of southern Lebanon that Israel withdrew from in 2000.
In 2012 Israel developed its own penetrator bomb design. This MPR-500.was for a 500 pound (227 kg) penetrator bomb. The MPR-500 can smash through more than a meter (39 inches) of concrete or four 200mm (8 inch) concrete barriers (floors or bunker walls) and then detonate. When the MPR-500 explodes it releases 26,000 fragments, which will wound or kill out to 100 meters. Satisfied with the effectiveness of the MPR-500 Israel went on to design 1,000 pound and 2,000 pound versions. It is unclear if the BLU-137/B design owes anything to the MPR line but Israel and the United States have freely exchanged design info and user experiences for decades and many American designs are heavily influenced and improved by Israeli technology.