(ADDENDUM: Apologies, 75833, if necessary. I did search under several terms in the original headline and - based on the time - it appears I was likely reading and obtaining further supporting links while other posts appeared. It's 'all good.' The articles complement each other. Personally, I'm not in a race to post nor am I upset when others duplicate posts I've happened to put up first. The main thing is to 'get the word out' is how I view it. I doubt much of this will soon appear on the nightly news or in such detail. 'Mericuh's more concerned about baseball players taking steroids, ya' know... ~ BMG)
www.fas.org/man/dod-101/sys/smart/gbu-28.htm [link to www.fas.org
Guided Bomb Unit-28 (GBU-28)
The Guided Bomb Unit-28 (GBU-28) is a special weapon developed for penetrating hardened Iraqi command centers located deep underground. The GBU-28 is a 5,000-pound laser-guided conventional munition that uses a 4,400-pound penetrating warhead. The bombs are modified Army artillery tubes, weigh 4,637 pounds, and contain 630 pounds of high explosives. 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.
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. Work on the bomb was conducted in research laboratories including the the Air Force Research Laboratory Munitions Directorate located at Eglin AFB, Florida and the Watervliet Armory in New York. The bomb was fabricated starting on 1 February, using surplus 8-inch artillery tubes as bomb casings because of their strength and weight. The official go-ahead for the project was issued on 14 February, and explosives for the initial units were hand-loaded by laboratory personnel into a bomb body that was partially buried upright in the ground. The first two units were delivered to the USAF on 16 and 17 February, and the first flight to test the guidance software and fin configuration was conducted on 20 February. These tests were successful and the program proceeded with a contract let on 22 February. A sled test on 26 February proved 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. The first two operational bombs were delivered to the theater on 27 February.
The Air Force produced a limited quantity of the GBU-28 during Operation Desert Storm to attack multi-layered, hardened underground targets. Only two of these weapons were dropped in Desert Storm, both by F-111Fs. One weapon hit its precise aimpoint, and the onboard aircraft video recorder displayed an outpouring of smoke from an entrance way approximately 6 seconds after impact. After Operation Desert Storm, the Air Force incorporated some modifications, and further tested the munition. The Fy1997 budget request contained $18.4 million to procure 161 GBU-28 hard target penetrator bombs.
For a visual depiction of how the GBU-28 works view the grapic produced by Bob Sherman and USA Today on-line.
www.usatoday.com/graphics/news/gra/gbuster/frame.htm [link to www.usatoday.com
Mission Offensive counter air, close air support, interdiction
Targets Fixed hard
Class 4,000 lb. Penetrator, Blast/Fragmentation
Service Air Force
Contractor Lockheed (BLU-113/B), National Forge (BLU-113A/B),
Program status Production
First capability 1991
Weight (lbs.) 4,414
Length (in.) 153
Diameter (in.) 14.5
Explosive 6471bs. Tritonal
Fuze FMU-143 Series
Stabilizer Air Foil Group (Fins)
Guidance method Laser (man-in-the-loop)
Range Greater than 5 nautical miles
Development cost Development cost is not applicable to this munition.
Production cost $18.2 million
Total cost $18.2 million
Acquisition unit cost $145,600
Production unit cost $145,600
Quantity 125 plus additional production
Platforms F-15E, F-111F
Sources and Resources
How TTR Helped the Air Force Ready a New Bomb Sandia Lab News, July 26, 1991
www.serve.com/mahood/nellis/ttr/sln4.htm [link to www.serve.com
GBU-28 CHAPTER VI - THE AIR CAMPAIGN Conduct of the Persian Gulf War
es.rice.edu/projects/Poli378/Gulf/gwtxt_ch6.html#GBU-28 [link to es.rice.edu
en.wikipedia.org/wiki/Nuclear_bunker_buster [link to en.wikipedia.org
(GO TO SOURCE URL ABOVE FOR NUMEROUS SUPPORTING LINKS)
Nuclear bunker buster
From Wikipedia, the free encyclopedia
(PHOTO: Subsidence craters left over after underground nuclear (test) explosions)
Bunker-busting nuclear weapons are a type of nuclear weapon which are designed to penetrate into soil, rock or concrete to deliver a nuclear warhead. These weapons would be used to destroy hardened, underground military bunkers buried deep in the ground. These weapons would in theory diminish the amount of radioactive nuclear fallout by reducing the yield of the warhead required to attack a particular target. Warhead yield and weapon design has changed periodically throughout the history of the design of such weapons.
1 Methods of operation
1.1 Penetration by explosive force
1.2 Penetration with a hardened penetrator
1.3 Combination penetrator-explosive munitions
2 Problems with proposed weapons
3 Development of bunker-busting weapons
4 See also
6 External links
Methods of operation
Penetration by explosive force
Concrete design remains little changed since 60 years ago. The majority of protected concrete structures in the US military are derived from standards set forth in Fundamentals of Protective Design, published in 1946 (US Army Corps of Engineers). Various augmentations, such as glass, fibers, and rebar, have made concrete less vulnerable, but far from impenetrable. Raymond T. Moore  was able to create a "human sized hole" in 18 inch (45 cm) thick reinforced concrete in less than 48 seconds with a mere 20 lb (9 kg) of explosive and a bolt cutter.
When explosive force is applied to concrete, three major fracture regions are usually formed: the initial crater, a crushed aggregate surrounding the crater, and "scabbing" on the opposite side of the crater. Scabbing, also known as "spalling," is the violent separation of a mass of material from the opposite face of a plate or slab subjected to an impact or impulsive loading.
The crater volume varies approximately inversely with the square root of the concrete's compressive strength. Therefore, increasing the compressive strength of the concrete by 50% will yield an approximately 25% smaller crater.
As the compressive wave propagates to the opposite side of the concrete and is reflected, the concrete fractures, and scabbing occurs on the interior wall. As such, an asymptotic relationship exists between the strength of the concrete and the damage that will be done between the crater, aggregate, and scabbing.
While soil is a less dense material, it also does not transmit shock waves as well as concrete. So while a penetrator may actually travel further through soil, its effect may be lessened due to its inability to transmit shock to the target.
Penetration with a hardened penetrator
Further thinking on the subject envisions a penetrator, dropped from service height of a bomber aircraft, using kinetic energy to penetrate the shielding, and subsequently deliver a nuclear explosive to the buried target.
The problems with such a penetrator is the tremendous heat applied to the penetrator unit when striking the shielding (surface) at hundreds of meters per second. This has partially been solved by using metals such as tungsten (with a much higher melting point than steel), and altering the shape of the projectile (such as an ogive).
Additionally, altering the shape of the projectile, to incorporate an ogive shape has yielded substantial results. Rocket sled testing at Eglin Air Force Base has demonstrated penetrations of 100 to 150 feet in concrete when traveling at 4,000 ft/s. The reason for this is liquefaction of the concrete in the target, which tends to flow over the projectile. Variation in the speed of the penetrator can either cause it to be vaporized on impact (in the case of traveling too fast), or to not penetrate far enough (in the case of traveling too slow).
Combination penetrator-explosive munitions
Another school of thought on nuclear bunker busters is using a light penetrator to travel 15 to 30 meters through shielding, and detonate a nuclear charge there. Such an explosion would generate powerful shock waves, which would be transmitted very effectively through the solid material comprising the shielding (see "scabbing" above).
Problems with proposed weapons
The largest problem with nuclear munitions is fallout. In theory, fallout may be contained within the shielding of the target. However, underground nuclear testing has revealed a "chimney" or "smokestack" effect, whereby fallout "leaks" through the roof of the cavity created by the explosion.
Another problem is that bunkers can be built farther into the earth to make them more difficult to reach. If a tunnel can be built 300 m into the side of a mountain, then it can be built 1000 m into the mountain using the same equipment and techniques. The target's vulnerability is then limited to openings like the ventilation system, which conventional bombs can handle.
The Union of Concerned Scientists points out that at the Nevada Test Site, the depth required to contain fallout from a nuclear test was between 100 and 500 meters. It is improbable that any type of bomb or missile could be made to penetrate so deeply. With yields between 0.3 and 340 kt of TNT,
it is improbable that the blast would be completely contained.
Current United States weapons penetrate up to 30 meters.
Politically, as well, such nuclear bunker busters are unpopular in some circles. Most targets are near cities, and even minimal fallout will inflict unacceptable levels of collateral damage.
Furthermore, the testing of new nuclear weapons would be prohibited by the proposed Comprehensive Test Ban Treaty, though the United States is not a signatory. Many fear that developing such weapons would lead to a new global nuclear arms race.
Lastly, the requirement to use nuclear weapons in this role is questionable. Very effective conventional ground penetration weapons were designed by the British aerodynamic engineer Barnes Wallis in the 1940s. These weapons were able to destroy very deeply buried or strengthened sites. Additionally, other conventional weapons such as thermobaric weapons and napalm (as in the Vietnam War) have proved effective in defeating buried targets.
Development of bunker-busting weapons
A version of the B61 nuclear bomb was modified by the United States in 1997 for use in bunker busting applications.
As early as 1944, the Wallis Tallboy bomb and subsequent Grand Slam weapons were designed to penetrate deeply fortified structures through sheer explosive power. These were not designed to directly penetrate defences, though they could do this, but rather to slide under a target and dig it up, thus negating any possible hardening. The destruction of targets such as the V2 complex at Wizernes, or the V3 guns at Mimoyecques show that these weapons could destroy any hardened or deeply excavated installation, and modern targeting techniques allied with multiple strikes could unquestionably perform a similar task.
Development continued, with weapons such as the nuclear B61, and conventional thermobaric weapons and GBU-28. One of the more effective housings, the GBU-28 used its large mass (4,700 lb) and casing (constructed from barrels of surplus 203 mm howitzers) to penetrate 20 feet of concrete, and more than 100 feet of earth. The B61 Mod 11, which first entered miltiary surface in January 1997, was specifically developed to allow for bunker penetration, and is speculated to have the ability to destroy hardened targets a few hundred feet beneath the earth.
While penetrations of 20–100 feet were sufficient for some shallow targets, both the Soviet Union and the United States were creating bunkers buried under huge volumes of soil or reinforced concrete in order to withstand the multi-megaton thermonuclear weapons developed in the 1950s and 1960s. Bunker penetration weapons were initially designed out of this Cold War context.
Mountainous terrain in Afghanistan
The weapon was revisited in the post-Cold War during the 2001 U.S. invasion of Afghanistan, and again during the 2003 invasion of Iraq. During the campaign in Tora Bora in particular, the United States believed that "vast underground complexes," deeply buried, were protecting opposing forces.
While a nuclear penetrator (the "Robust Nuclear Earth Penetrator", or "RNEP") was never built, the DOE was alotted budget to develop it, and tests were conducted by the Air Force Research Laboratory. Such complexes were not found.
As well, it has been stated  that Iran may have such deeply buried bunkers to guard its nuclear program.
The Bush administration removed its request for funding of the weapon in October 2005.
Additionally, US Senator Pete Domenici announced funding for the nuclear bunker-buster has been dropped from the Department of Energy's fiscal 2006 budget at the department's request.
While the project for the RNEP seems to be in fact cancelled, Jane's Information Group speculates work may continue under another name.
No first use
Barrier Penetration Tests, Moore, R. T. National Bureau of Standards, ASIN B0006CHZT6
Penetration Resistance of Concrete: A Review, James R. Clifton, The Physical Security and Stockpile Directorate, Defense Nuclear Agency, ASIN B0006E76U2
U.S. Nuclear Weapons: Changes In Policy And Force Structure, Woolf, Amy F., ISBN 1594542341
Nuclear Weapon Initiatives: Low-yield R&D, Advanced Concepts, Earth Penetrators, Test Readiness, Ernest, Jonathan V., et al., ISBN 1594542031
Earth Penetrating Weapons by Lisbeth Gronlund, David Wright and Robert Nelson, Union of Concerned Scientists, May 2005
Bunker-busters set to go nuclear by David Hambling, New Scientist, 07 November 2002
Low-Yield Earth-Penetrating Nuclear Weapons by Robert W. Nelson, Federation of American Scientists, January/February 2001, Volume 54, Number 1
Categories: Nuclear warfare | Anti-fortification weapons | Nuclear bombs | Superbombs