How Do Bunker-Buster Bombs Work?

Between October 1942 and October 1943, the United States Army Air Forces (USAAF) - the precursor to the U.S. Air Force- carried out more than 2,000 heavy bomber sorties targeting the submarine pens established by Nazi Germany along the coast of occupied France. These pens provided German submarines with rapid access to the Atlantic Ocean, where they sank millions of tons of military and civilian supplies en route from America to the besieged British Isles, claiming the lives of thousands of sailors. Aware that targeting the submarine pens would be a top priority for the Allies, the Germans fortified the pens with massive concrete structures featuring walls three meters thick and reinforced concrete roofs exceeding five meters in thickness. Altogether, the five main submarine bases on the French coast were reinforced with more than four million cubic meters of concrete.

The heavy bombings of the submarine bases took a heavy toll, primarily on the Air Force. About 1,200 aircrew members were killed or taken prisoner as their planes were targeted by enemy aircraft or ground-based anti-aircraft fire. However, the damage inflicted on the submarine pens was minimal. Most bombs failed to penetrate the concrete defenses, and in the few instances where breaches occurred, the Germans swiftly repaired the damage. The Allies eventually tackled the submarine threat by employing a combination of other technologies designed to target the submarines directly, including long-range reconnaissance and attack aircraft, improved sonar on ships, effective depth charges, and accurate intelligence.

The issue of fortified bunkers remained and even intensified as the Germans constructed facilities for storing and assembling V1 and V2 missiles toward the end of the war. The Americans embarked on an ambitious project to load heavy aircraft with tons of explosives and remotely control them to crash into the bunkers. However, this effort also ended in failure, primarily due to technical difficulties and malfunctions, claiming, among others, the life of pilot Joseph Kennedy, the older brother of future U.S. President John F. Kennedy.

The concrete that thwarted the U.S. Air Force. The fortified German submarine pens in Saint-Nazaire, France, April 1942 | Photo: Bundesarchiv, Wikipedia

With the Help of Physics

What the Americans failed to achieve, their British allies did, led by the English engineer Barnes Wallis. Wallis's bunker-penetrating bomb was based on a German development of armor-piercing shells and relied on three key principles: structure, mass, and speed. The bombs he developed featured a narrow, elongated cylindrical design with a conical nose made of extremely strong material – the British recycled steel from cannon barrels for this purpose. The bomb's body was equipped with fins, causing it to spin around its axis like a screw. From there, they let physics do its work: these heavy bombs, weighing several tons, were dropped from great heights, accelerating to tremendous speeds – exceeding the speed of sound. The combination of speed, mass, rotation, and the pointed, durable nose allowed the bomb to burrow into the ground, even through concrete layers, before exploding.

Wallis's bombs were even more effective when they struck near the target rather than directly on it: they would embed themselves in the ground, sometimes more than ten meters deep, and when they detonated there, the shock waves would propagate through the dense medium, undermining the structure's foundations and causing it to collapse. For this reason, they were sometimes called "Earthquake bombs" or "Seismic bombs."

Using these bombs, the British succeeded where the Americans had failed. They destroyed a V2 missile launch facility, collapsed bridges, demolished fortified installations, and caused the collapse of a railway tunnel, in the course a bombing that prevented the Germans from moving tank reinforcements to the front during the Allied invasion. They even inflicted heavy damage on the submarine bases in France that the Americans had previously failed to strike. Wallis's bombs became the foundation for all bunker-buster bombs developed thereafter.

Despite their proven effectiveness, the Americans were slow to adopt this technology. They did develop a massive earthquake bomb weighing twenty tons, but by the end of the war, with nuclear weapons already in their possession,they did not prioritize further investment in the development of bunker-penetrating bombs. However, they did adopt one of Wallis's designs, incorporating remote-controlled guidance capabilities, which were used to a limited extent during the Korean War.

The first "bunker busters." A video about Barnes Wallis's "Earthquake bombs" in World War II: 

The Era of Precision

The American need for bunker-buster bombs emerged only in 1991, during Operation Desert Storm, also known as the First Gulf War. U.S. intelligence had identified numerous Iraqi military bunkers that were heavily fortified and constructed deep underground. To effectively target these complexes, the Americans needed a conventional weapon capable of striking such robust structures. Within a few weeks, they developed their own version of Wallis's bombs, utilizing modern technology. This bomb, the GBU-28, weighed about 2,000 kilograms, with less than 300 kilograms dedicated to explosives. Despite its relatively small explosive payload, the GBU-28 was highly effective, capable of penetrating six meters of concrete or roughly thirty meters of earth. The main innovation was a laser-guided targeting system that allowed the bomb to be directed to a very precise location by illuminating the target with a laser from an aircraft or from the ground. An additional enhancement included a delay mechanism that prevented premature detonation, ensuring the bomb exploded only after reaching its maximum penetration depth.

Although other countries, such as Russia and India, are developing similar bombs, the United States has taken the lead in this field, continuously developing larger, more powerful, and more precise bunker-buster bombs. Israel also develops its own bombs, which are usually relatively smaller in size. While these smaller bombs cannot penetrate meters of concrete like their larger counterparts, they are better suited for use in densely populated urban areas, where highly  precise strikes are necessary to minimize risks to nearby civilians.

Regardless of their size, the fundamental physical principles behind bunker-buster bombs remain the same as those used during World War II. However, advancements over the years have further enhanced their capabilities.

 A U.S. Air Force F-15 drops a GBU-28 bomb during training | Photo: Tech. Sgt. Michael Ammons, USAF, Public Domain

Stronger and Smarter

To enhance a bomb's penetration through concrete, armor, soil, or other hard mediums, its energy must be increased. This can be achieved by adding mass—the heavier the bomb, the more energy it accumulates, and the greater its penetrating power. Another approach is to increase the bomb's speed; instead of relying solely on free fall, the bomb can be equipped with a rocket engine that accelerates it to high speed, thereby improving its penetration capability. Additionally, designing the bomb as a narrow, long cylinder helps concentrate its energy on a smaller area, further enhancing its penetration ability.

In the early bombs, engineers used steel for the "nose" to provide the mechanical strength needed to break through concrete or other hard materials. Today, even harder materials, such as depleted uranium, are used. Natural uranium primarily consists of two isotopes: 99.3% uranium-238 and a small fraction (only seven-thousandths) of uranium-235, the fissile material used in nuclear bombs. During uranium enrichment, the concentration of the fissile isotope, uranium-235, is increased. In the opposite process, known as depletion, the concentration of uranium-238 is increased, resulting in a particularly dense and hard metal. It can be further hardened by alloying it with metals such as titanium or tungsten. Another advantage of using uranium in bombs is its flammability: it ignites at temperatures above 600 degrees Celsius, which are easily reached when the explosive is triggered, further increasing the bomb's destructive capacity.

Moreover, using strong yet relatively light alloys allows more explosive material to be packed into the bomb without significantly increasing its mass. While a large mass does enhance penetration power, it also reduces the number of bombs a plane can carry and decreases its flight range. Naturally, the more explosive material a bomb contains, the greater its destructive power.

An American B-52 drops a GBU-57 bomb weighing 14,000 kilograms and measuring six meters in length during training | Photo: US DoD, Public Domain

Further refinements to the bunker buster bombs have come from advancements in fuzes, the mechanisms that trigger the explosive. Modern bombs incorporate smart, computerized fuzes capable of measuring the bomb's progress through the ground or whatever medium it penetrates and timing the explosion for maximum destructiveness. For example, if the bomb penetrates through a building's roof, the fuze can count the floors it passes through and activate the bomb at the desired level.

Another significant advancement is in bomb targeting precision. Today, bombs are guided using satellite navigation systems, such as GPS or equivalent technologies. In addition, they are equipped with inertial navigation systems, which enable them to calculate their exact position based on the launching aircraft's coordinates without relying on external sources like satellites. This feature helps overcome disruptions or blockages of satellite signals.

Technological improvements in bombs relieve the pilot of the need to guide the bomb manually. Once the pilot reaches a certain range from the target, they only need to press a button, release the bombs, and let technology and physics handle the rest. This allows multiple bombs to be dropped simultaneously, sometimes on the same target, ensuring they explode at the same time. This greatly increases the destructive power and significantly reduces the target's chances of withstanding the attack.