Busting a Bunker
From the description in the previous section, you can see that the concept behind bunker-busting bombs like the GBU-28 is nothing but basic physics. You have an extremely strong tube that is very narrow for its weight and extremely heavy.
The bomb is dropped from an airplane so that this tube develops a great deal of speed, and therefore kinetic energy, as it falls.
When the bomb hits the earth, it is like a massive nail shot from a nail gun. In tests, the GBU-28 has penetrated 100 feet (30.5 meters) of earth or 20 feet (6 meters) of concrete.
In a typical mission, intelligence sources or aerial/satellite images reveal the location of the bunker. A GBU-28 is loaded into a B2 Stealth bomber, an F-111 or similar aircraft.
The bomber flies near the target, the target is illuminated and the bomb is dropped.
The GBU-28 has in the past been fitted with a delay fuze (FMU-143) so that it explodes after penetration rather than on impact. There has also been a good bit of research into smart fuzes that, using a microprocessor and an accelerometer, can actually detect what is happening during penetration and explode at precisely the right time. These fuses are known as hard target smart fuzes (HTSF). See GlobalSecurity.org: HTSF for details.
The GBU-27/GBU-24 (aka BLU-109) is nearly identical to the GBU-28, except that it weighs only 2,000 pounds (900 kg). It is less expensive to manufacture, and a bomber can carry more of them on each mission.