The Functionality of Bunker Busters

Essential Insights

Globally, countless military installations are built to withstand standard attacks. In Afghanistan, caves are carved into mountains, while Iraq houses massive concrete bunkers buried beneath the desert. These fortified sites often contain command hubs, weapon storage, and research facilities critical to military operations. Their subterranean nature makes them challenging to locate and nearly impossible to destroy with conventional methods.

The U.S. military has engineered a variety of weapons to target underground fortifications. Referred to as bunker busters, these explosives are capable of drilling deep into the ground or piercing through thick layers of reinforced concrete before detonating. Such bombs have enabled the destruction of facilities that were previously considered invulnerable.

This article explores various types of bunker busters, providing insights into their mechanisms and the future trajectory of this technology.

Traditional Bunker Busters

During the 1991 Gulf War, allied forces identified several heavily fortified and deeply buried military bunkers in Iraq that were beyond the reach of existing weapons. The U.S. Air Force launched an urgent research and development initiative to design a new bunker-busting bomb capable of neutralizing these targets. Within weeks, a prototype was developed, featuring the following characteristics:

The completed bomb, referred to as the GBU-28 or BLU-113, measures 19 feet (5.8 meters) in length, has a diameter of 14.5 inches (36.8 cm), and weighs 4,400 pounds (1,996 kg).

Busting a Bunker

As outlined in the previous section, the principle behind bunker-busting bombs such as the GBU-28 relies on fundamental physics. The design involves a robust, narrow tube that is exceptionally dense and heavy relative to its size.

When released from an aircraft, the bomb gains significant velocity, accumulating kinetic energy as it descends toward its target.

Upon impact, the bomb functions like a giant projectile fired from a nail gun. During testing, the GBU-28 has demonstrated the ability to penetrate up to 100 feet (30.5 meters) of soil or 20 feet (6 meters) of solid concrete.

In standard operations, intelligence data or aerial/satellite imagery identifies the bunker's location. A GBU-28 is then loaded onto a B2 Stealth bomber, F-111, or comparable aircraft.

The aircraft approaches the target, the target is marked with a laser, and the bomb is released.

The GBU-28 has historically been equipped with a delay fuze (FMU-143), allowing it to detonate after penetration rather than upon impact. Significant research has also been conducted into smart fuzes, which utilize a microprocessor and accelerometer to monitor penetration dynamics and trigger the explosion at the optimal moment. These advanced fuzes are referred to as hard target smart fuzes (HTSF). For more information, visit GlobalSecurity.org: HTSF.

The GBU-27/GBU-24 (also known as BLU-109) is almost identical to the GBU-28, with the key difference being its lighter weight of 2,000 pounds (900 kg). This makes it more cost-effective to produce and allows bombers to carry a greater number of them per mission.

Making a Better Bunker Buster

To enhance the penetration capabilities of bunker busters, designers have three primary options:

To increase the weight of a bunker buster without expanding its cross-sectional area, a denser metal than steel can be used. While lead is denser, its softness makes it unsuitable for penetrators, as it would deform or break apart upon impact.

Depleted uranium is an ideal material for penetrating weapons due to its exceptional strength and density. For instance, the M829, an armor-piercing projectile fired from an M1 tank's cannon, weighs 10 pounds (4.5 kg), measures 2 feet (61 cm) in length, and has a diameter of about 1 inch (2.5 cm). Traveling at over 1 mile (1.6 km) per second, these darts possess enough kinetic energy and strength to penetrate the toughest armor.

Depleted uranium is a byproduct of the nuclear power industry. Natural uranium consists of two isotopes: U-235 and U-238. U-235 is essential for nuclear power production (see How Nuclear Power Plants Work for details), so uranium is refined to extract U-235, resulting in "enriched uranium." The remaining U-238 is termed "depleted uranium."

U-238 is a radioactive metal emitting alpha and beta particles. In solid form, it poses minimal risk due to its 4.5-billion-year half-life, indicating very slow atomic decay. Depleted uranium is commonly used as ballast in airplanes and boats. Its suitability for penetrating weapons stems from three key properties:

These three characteristics make depleted uranium an ideal material for developing advanced bunker-busting bombs. Its properties enable the creation of heavy, durable, and slender bombs with exceptional penetrating power.

However, there are significant drawbacks to using depleted uranium.

Tactical Nuclear Weapons

The primary issue with depleted uranium is its radioactivity. The U.S. military deploys large quantities of depleted uranium in combat zones, leaving behind significant amounts of radioactive material in the environment post-conflict. For instance, as reported by Time magazine in "Balkan Dust Storm":

During the first Gulf War, an estimated 300 tons of DU weapons were deployed. When DU burns, it produces uranium-oxide smoke, which can be inhaled and spreads over vast areas, settling far from the original site. Inhaling or ingesting this radioactive smoke poses significant health risks due to its radioactivity. For more information, see How Nuclear Radiation Works.

The Pentagon has created tactical nuclear weapons designed to target heavily fortified and deeply buried bunkers. These weapons combine a small nuclear bomb with a penetrating casing, enabling deep ground penetration followed by a nuclear explosion. The B61-11, introduced in 1997, represents the latest advancement in nuclear bunker-busting technology.

The primary advantage of a small nuclear bomb lies in its ability to deliver immense explosive power in a compact form. (Refer to How Nuclear Bombs Work for details.) The B61-11 can be equipped with a nuclear payload ranging from 1 kiloton (equivalent to 1,000 tons of TNT) to 300 kilotons. For context, the Hiroshima bomb had a yield of about 15 kilotons. The resulting shockwave from such an underground explosion would cause extensive subterranean damage, likely obliterating even the most fortified bunkers.

However, the B61-11 raises significant environmental and diplomatic concerns. No existing penetrating bomb can bury itself deeply enough to fully contain a nuclear explosion, leading to massive craters and widespread radioactive fallout. Diplomatically, the B61-11 conflicts with global efforts to reduce nuclear weapon usage. For further details, see FAS.org: Low-Yield Earth-Penetrating Nuclear Weapons.