A bunker buster is a bomb designed to penetrate hardened targets or targets buried deep underground.
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In World War II the British designer Barnes Wallis, of bouncing-bomb fame, designed two bombs that would today be called 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 break the sound barrier as they fell from a height of 22,000 ft (6,700 m). They had casings made of high grade steel, much stronger than the typical World War II 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 U-boat pens at Farge two Grand Slams went through the 15 ft (4.5 m) reinforced concrete hardening[1] — equalling or exceeding the best current penetration specifications.
Post war the U.S. added a form of remote control guidance to the Tallboy to create the Tarzon, a 12,000-pound bomb which was deployed in the Korean War against an underground command center near Kanggye.
The Disney Rocket-Assisted Bomb was another World War II device to be used against U-boat pens and other super-hardened targets. Thought up by Royal Navy Captain Edward Terrell,[2] it had a streamlined hardened case bomb weighing some 4,500 lb (2 tonnes). The bomb was dropped from 20,000 ft (~6,000 m). At 5,000 ft (~1,500 m) a barometric fuse fired the rocket in the tail to give it a velocity at impact of up to 2,400 ft/second (730 m/s). It was first used by the 92nd Bomb Group on 10 February 1945 on U-boat pens at IJmuiden in The Netherlands.- one bomb under each wing of 9 B-17 Flying Fortress. On that occasion a single direct hit was scored. A total of 158 "Disney Bombs" were used operationally by the end of hostilities in Europe.
During Operation Desert Storm (1991) there was a need for deep penetration bomb similar to the British weapons of World War II, but none of the NATO air forces had such a weapon, though the RAF possesses several of Barnes Wallis' bombs as museum pieces. As a stop-gap, some were developed rapidly 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)", and worked very effectively.
More recently the US has developed a series of custom made bombs to penetrate hardened or deeply buried structures:
| Depth of Penetration | Weapon Systems | |
|---|---|---|
| Penetration of reinforced concrete: 1.8 m (6 ft) | BLU-109 Penetrator | GBU-10, GBU-15, GBU-24, GBU-27, AGM-130 |
| Penetration of reinforced concrete: 3.4 m (11 ft) | BLU-116 Advanced Unitary Penetrator (AUP) | GBU-15, GBU-24, GBU-27, AGM-130 |
| Penetration of reinforced concrete: 3.4 m (11 ft) | BLU-118/B Thermobaric Warhead | GBU-15, GBU-24, AGM-130 |
| Penetration of reinforced concrete: more than 6 m (20 ft) | BLU-113 Super Penetrator | GBU-28, GBU-37 |
The traditional fuse 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 fuse 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 fuse, but some also include a microphone and micro controller. The microphone listens, and the micro controller counts floors until the bomb breaks through the desired numbers of floors.
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).
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