Bunker buster

A bunker buster is a bomb designed to penetrate hardened targets or targets buried deep underground.

Germany
Röchling shells were bunker-busting artillery shells, developed by German engineer August Cönders, based on the theory of increasing sectional density to improve penetration. They were tested in 1942 and 1943 against the Belgian Fort d'Aubin-Neufchâteau

United Kingdom
In World War II the British designer Barnes Wallis, already famous for inventing the bouncing-bomb, designed two bombs that would become the conceptual predecessors of modern bunker busters: the five tonne Tallboy and the ten tonne Grand Slam "Earthquake" bombs. The designs were very aerodynamic with a tail which caused them to spin. This allowed them to exceed the speed of sound as they fell from 22,000 ft (6,700 m). They had casings of high grade steel, much stronger than the typical WWII bomb so that they would survive hitting a hardened surface, or penetrate deep into the ground.

Though these bombs might be thought of as 'bunker busters' today, in fact the original 'earthquake' theory was more complex and subtle than simply penetrating a hardened surface. The Earthquake bombs were designed not to strike a target directly, but to impact beside it, penetrate under it, and create a 'camouflet' or large buried cavern at the same time as delivering a shock wave through the target's foundations. The target then collapses into the hole, no matter how hardened it may be. The bombs had strong casings because they needed to travel through rock rather than reinforced concrete, though they could perform equally well against hardened surfaces. In an attack on the Valentin U-Boat pens at Farge two Grand Slams went through the 15 ft (4.5 m) reinforced concrete hardening — equalling or exceeding the best current penetration specifications.

The British Disney Rocket-Assisted Bomb was another WW2 device to be used against U-boat pens and other super-hardened targets. Devised by Captain Edward Terrell  RNVR of the Admiralty's Directorate of Miscellaneous Weapons Development it had a streamlined hardened case and weighed some 4500 kg including the rocket assembly. Actual explosive content was about 500 lb.

For accuracy, the bombs had to be dropped precisely from a pre-determined height (usually 20000 ft). They would free-fall for around 30 seconds until, at 5000 ft, the rockets were ignited, causing the tail section to be expelled. The rocket burn lasted for three seconds and added 300 ft/s to the bomb's speed, giving a final impact speed of 1450 ft/s, approximately Mach 1.29. Post-war tests demonstrated the bombs were able to penetrate a 14 ft thick concrete roof, with the predicted (but untested) ability to penetrate 16 ft of concrete.

United States
Post war the US added a form of remote guidance to the Tallboy to create the Tarzon, a 12,000-pound bomb deployed in the Korean War against an underground command center near Kanggye.

Modern
During Operation Desert Storm (1991) there was a need for a deep penetration bomb similar to the British weapons of WWII, but none of the NATO air forces had such a weapon. As a stop-gap, some were developed over a period of 28 days, using old 8 inch (203 mm) artillery barrels as casings. These bombs weighed over two tons but carried only 647 lb of high explosive. They were laser-guided and were designated "Guided Bomb Unit-28 (GBU-28)". It was proved to be effective for the intended role.

An example of a Russian bunker buster is the KAB-1500L-Pr. It is delivered with the Su-24M and the Su-27IB aircraft. It is stated to be able to penetrate 10-20 m of earth or 2 m of reinforced concrete. The bomb weighs 1500 kg, with 1100 kg being the high explosive penetrating warhead. It is laser guided and has a reported strike accuracy of 7 m CEP.

The US has a series of custom made bombs to penetrate hardened or deeply buried structures: More recently, the US has developed the 30,000-pound  GBU-57.

Fuzing
The traditional fuze is the same as a classic armor-piercing bomb: a combination of timer and a sturdy dynamic propeller on the rear of the bomb. The fuze is armed when the bomb is released, and detonates when the propeller stops turning and the timer has expired.

Modern bunker busters may use the traditional fuze, but some also include a microphone and microcontroller. The microphone listens, and the micro controller counts floors until the bomb breaks through the desired numbers of floors.

ATK is working on a Hard Target Void Sensing Fuze (HTVSF) for 2000 and 5000 pound weapons to explode when they reach an open space in a deeply buried bunker.

Missiles
The extra speed provided by a rocket motor enables greater penetration of a missile-mounted bunker buster warhead. To reach maximum penetration (impact depth), the warhead may consist of a high-density projectile only. Such a warhead carries more energy than a warhead with chemical explosives (kinetic energy of a projectile at hypervelocity).

Nuclear
The nuclear bunker buster is the nuclear weapon version of the bunker buster. The non-nuclear component of the weapon is designed to greatly enhance the penetration into soil, rock, or concrete to deliver a nuclear warhead to a target. These weapons would be used to destroy hardened, underground military bunkers buried deep in the ground. In theory, the amount of radioactive nuclear fallout would be reduced from that of a standard, air-burst nuclear detonation because they would have relatively low explosive yield. However, because such weapons necessarily come into contact with large amounts of earth-based debris, they may, under certain circumstances, still generate significant fallout. Warhead yield and weapon design have changed periodically throughout the history of the design of such weapons. An underground explosion releases a larger fraction of its energy into the ground, compared to an explosion at or above the surface which releases most of its energy into the atmosphere.