Saturday, April 08, 2006

GBU 28 "Bunker Buster" bombs may attack Iran's nuclear facilities

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GBU-28 Bunker Buster

"Bunker Buster" bombs are 19' long, and a heavy enough to penetrate 100' of earth or 20 ' of rock or concrete, with time-delayed fuses to detonate the explosives seconds later, after entering the actual target. These bombs may very likely be used to target Iran's nuclear installations by Israel and/or, the US.

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Guided Bomb Unit-28 (GBU-28)

The GBU-28 laser guided bomb was developed, built, tested, and used in combat in a 17 day period. The deepest Iraqi bunkers were secure from the best penetrating bomb, the GBU-24A/B, with the I-2000 warhead. Coalition leaders required the capability to destroy these vital command and control facilities. Texas Instruments and Lockheed combined their efforts to build the 18 ft long bomb. TI adapted the seeker from the GBU-24 and Lockheed built the bomb body from discarded eight inch howitzer barrels. The Air Force initially contracted for 30 bombs but the cease fire started after only two were employed. Two more of the bombs were used in testing before the bombs were dropped in combat and the Air Force expended two or three more in additional tests after the war. The Air Force ordered an additional 100 GBU-28s with the BLU-113 (8 inch gun barrel) bomb body and stocks remained low due to the limited number of targets and the only fighter capable of employing it initially was the F-111F.

The Guided Bomb Unit-28 (GBU-28) bomb is designed to penetrate hardened targets before exploding, capable of penetrating 100 feet of earth or 20 feet of concrete. The GBU-28 was initially developed in 1991 for penetrating hardened Iraqi command centers located deep underground. This "bunker buster" was required for special targets during the Desert Storm conflict and was designed, fabricated and loaded in record time. The GBU-28 is a laser-guided conventional munition that uses a modified Army artillery tube as the bomb body. They are fitted with GBU-27 LGB kits, 14.5 inches in diameter and almost 19 feet long. The operator illuminates a target with a laser designator and then the munition guides to a spot of laser energy reflected from the target.

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Some considerable confusion exists in the literature concerning the weight of this bomb. Although nominally a 5,000 pound bomb, it appears that the actual weight is somewhat less than this, and that the 5,000 figure is arrived at by rounding up [the 250, 500, 1000 and 2000 pound figures for the Mk80 family are also such approximations. Statements that it is a 4,000 pound bomb reflect a similar rounding, but rounding down and rounding rather further from the exact number. Reports that the bombs weigh 4,637 pounds, and contain 630 pounds of high explosives, are too precise to be in error. Reports that the bomb weighs 4,700 pounds are in close agreement with this more precise number. A report that the bomb incorporates a 4,400-pound penetrating warhead may reflect the weight of the filled bomb body, minus guidance head and tail kit.

The GBU 28 "Bunker Buster" was put together in record time to support targeting of the Iraqi hardened command bunker by adapting existing materiel. The GBU-28 was not even in the early stages of research when Kuwait was invaded. The USAF asked industry for ideas in the week after combat operations started. The bomb was fabricated starting on 01 February 1991, using surplus 8-inch artillery tubes. The official go-ahead for the project was issued on 14 February 1991, and explosives for the initial units were hand-loaded by laboratory personnel into a bomb body that was partially buried upright in the ground outside the laboratory in New York.

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This new system was needed to deal with deeply buried command and control bunkers that were beyond the reach of existing systems. The need was great, the time was short, and the only solution was to innovate a solution in an unprecedented short period of time. A team of government and industry people came together sharing the common objective of solving a difficult technical challenge in a breakneck race against time.

Personal interests were set aside as were traditional approaches, with long hours being the norm. The team worked to trade time against everything (cost, risk, performance). Reuse of existing subsystems offered the only answer. However, the pieces would have to be integrated in a very innovative way to achieve the desired results. The GBU-28 Bunker Buster that was conceived, developed, tested, and deployed in approximately 28 days. This was less time than had ever been dreamed possible.

The first two units were delivered to the USAF on 16 and 17 February 1991, and the first flight to test the guidance software and fin configuration was conducted on 20 February 1991. These tests were successful and the program proceeded, with a contract let on 22 February. A sled test on 26 February 1991proved that the bomb could penetrate over 20 feet of concrete, while an earlier flight test had demonstrated the bomb's ability to penetrate more than 100 feet of earth.

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Watch the flash animation to see it in action.

USA Today also has an animation that's worth watching.



While laser-guided bombs have dramatically increased the accuracy and efficiency of aerial bombing, they are not the miracle weapon that many believe them to be. A laser beam quickly loses its integrity when its is shown through fog, rain, clouds or dust. If the beam is scattered by these obstructions, then it is not powerful enough to be seen by the bomb's guidance unit, and the bomb cannot be guided. The cloudy weather over Kosovo in 1999 proved a significant hindrance to the use of laser-guided bombs. Despite impressive Gulf War video of bombs falling down ventilation shafts, there were very few targets hit accurately enough the first time; an average of four laser-guided bombs were required to destroy most targets.

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An F-117 Nighthawk engages its target and drops a bunker buster during a testing mission at Hill Air Force Base, Utah.

The U.S. military uses a calculation known as circular error probability (CEP) to determine bomb accuracy. A circular error probability number is the radius of the circle in which a bomb will land at least half of the time. The circular error probability of a typical Vietnam-era dumb bomb was 150 feet; half the time it was dropped, the bomb hit within a 300 foot-wide circle. The circular error probability of the GBU-28/B, one of the most accurate laser-guided bombs, is about 25 feet (other American smart bombs have a circular error probability of between 10 and 40 feet). This means that half of the time, the GBU-28/B falls somewhere within a 50-foot wide circle around its target.

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An F-15E Strike Eagle pilot and a weapons system officer inspect a GBU-28 laser-guided bomb.

While the GBU-28/B's circular error probability is technically impressive, the bomb may still hit unintended targets, especially when the U.S. is bombing an urban area like Mogadishu or Kabul. In addition, even the most advanced technology can be defeated by incorrect targeting and pilot error--the bombing of the Chinese embassy in Belgrade and Red Cross warehouses in Kabul are two recent examples.

Due to its unusual length, the GBU-28/B cannot be carried by most of the U.S. aircraft that are otherwise suited to fly bunker-busting missions. Of the three active American heavy bombers, only the B-2A "Spirit" (or stealth bomber) is able to drop a variant of the bunker buster, called the GBU-37/B. America's other stealth aircraft--the F-117A "Night Hawk" light bomber and F-22A "Raptor" fighter--are unable to carry the GBU-28/B because of the planes' relatively short bomb bays.

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Air-to-air view of GBU-28 hard target bomb on an F-15E Eagle

The only U.S. plane currently cleared to drop the GBU-28/B is the F-15E "Strike Eagle," a long-range fighter bomber. While the F-15E's sophisticated radar aids it during the low-level attacks common to bunker-busting missions, the heavily-defended environments in which bunkers are often found pose a severe threat to this non-stealthy jet.

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