Friday, July 23, 2010

Earthquake bomb series: Tallboy and Grand Slam

T56 M121 10,000-lb
T39 M123 12,000-lb Tallboy
T14 M110 22,000-lb Grand Slam

During the Second World War the British designer Barnes Wallis developed the largest conventional bombs used in combat. Wallis first designed the "Upkeep" bouncing bombs that were used during the Dambusters Raid on 16th May 1943. The rotating bouncing bomb exploded at the base of the retaining wall of the dam, producing heavy floods and damaging German production in the Ruhr.

Wallis next produced the 12,000-lb Tallboy, also known as the earthquake bomb. Tallboy was 21" long, with an overall diameter of 3'8", while the bomb body itself was 10'4" long and 3'2" in diameter. It weighed a total of 11,855 pounds, of which 5,200 pounds was Torpex D1 explosive. The weight of the case was thus a high proportion of the weight of the bomb. Dropped from 20,000 feet, a Tallboy made a 80ft deep crater, 100ft across. The bomb had a high terminal velocity, variously estimated at 3,600 and 3,700 feet a second [much faster than sound], and at these speeds it could go through 16ft of concrete. It was used for attacks on tunnels, V-1 Flying Bomb launch sites, and other high-priority targets. Its most important use was in the sinking of the German battleship Tirpitz at anchor in Norway, on 12 November 1944. Over 700 Tallboys were dropped during the War.

In 1945 Barnes Wallis developed the 22,000lb Grand Slam, which remains the largest conventional bomb ever used in action. The 10 ton (22,000 pound) "Grand Slam" was 26-feet, 6-inches long. Its hardened casing was cast in a single piece in a sand mold, using a concrete core. The "Grand Slam" could reportedly penetrate though 20+ ft of concrete. The first one was dropped on Germany on 14th March 1945. It hit the ground about 80 feet from the target, but it created a crater over 100 feet deep. This bomb was used with great effect against viaducts or railways leading to the Ruhr and also against several U-boat shelters. In one raid on 27 March 1945 against the U-Bootbunkerwerft "Valentin" submarine pens near Bremen, two Grand Slams penetrated 7 meters (23 feet) of reinforced concrete, bringing down the roof. In total, 41 Grand Slams were dropped during the war.

The M-121, sometimes called the "Earthquake" bomb, was more often referred to as the "Grand Slam" bomb, a totally misleading nickname. Actually "Grand Slam" was the code name of a highly classified modification project strictly concerned with atomic matters. The "Grand Slam" modifications would allow the Convair B-36 to carry atomic bombs, which the Air Force believed might weigh more than 40,000 pounds. Since the 10,000-pound M-121, when properly dropped, could inflict the damage of a 40,000-pound bomb, curiosity and rumors most likely explained the ensuing confusion. As a matter of fact, the "Grand Slam" designation was also loosely applied to other conventional bombs of the M-121 category.

Combat Trap

Because of the extensive use of helicopters in the Republic of Vietnam, landing zones had to be rapidly constructed in heavily forested areas, like those surrounding the Kim Son and Soui Ca valleys. The engineers in Vietnam were thus challenged to reduce the landing zone construction time, in order to meet the needs of the quickly shifting tactical situation. Landing zone requirements ranged from the hasty construction of a helicopter pad, from which to provide emergency resupply or medical evacuation, to the development of large landing zones, able to handle sufficient aircraft to support battalion or brigade operations.

Experience gained by engineer units in Vietnam led to the development of landing zone construction kits that contained the necessary tools and demolitions to prepare a landing zone for one aircraft. If the engineer team could be landed near the new construction site, they would rappel from the helicopter or climb down rope ladders. When sufficient area had been cleared, air-portable construction equipment or additional tools and demolitions were lifted in to expand the new landing zone.

Large HE detonations would dear trees and brush from an area, leaving a zone suitable for helicopter landings. As applied to the HLZ problem, it was clear that the HE had to be detonated at some height above the ground to avoid cratering, for two reasons: First, the ground should not be disturbed so much as to make it difficult or impossible for a helicopter to land safely. Second, even shallow bomb penetration would result in the blast being directed at an upward angle, greatly reducing the total surface area affected by the blast.

In the search for a suitable high-explosive package to perform the mission, the M-121 10,000-pound bomb was found in an ordnance depot in sufficient supply to carry out the tests. This bomb had been developed in 1954 to be dropped by the B-36 but had never been employed. It was decided to test out the HLZ concept in the United States before proceeding to Southeast Asia. The M-121 was taken to Fort Benning, Georgia, where a stand of mixed hardwood and conifers had been designated as a test area. The bomb was emplaced by an Army CH-54 helicopter at a height corresponding to that planned for an airdrop burst and was statically detonated on 10 June 1968. When the smoke had cleared, the area was surveyed, and an Army Huey helicopter was flown in to land in the cleared area. The zone created had usable space approximately 100 feet in diameter.

The Combat Trap project began preparations for aerial delivery of the M-121 for operational tests in South Vietnam. The M-121 was fitted with a drogue parachute for stability, and a special tail fuze was developed to serve as backup to the nose fuze. To provide for a burst height of about three feet, a standard nose fuze and M-1 fuze extender (a tube packed with explosives, which was attached to a detonator inside the bomb) were used. The contact nose fuze was protected with a brush deflector, a locally designed iron basket to enable the bomb to penetrate the tops of the trees without detonation. The sequence of events is as follows: As the bomb separates from its carrier, pins are pulled from both nose and tail fuzes, and the drogue chute is deployed. The fuzes are armed at a preset time to provide safe separation, and the chute quickly stabilizes the trajectory of the bomb. Penetrating the top of the canopy, the brush deflector pushes aside the smaller branches and is crushed by the impact with the earth. The fuze detonates the explosive in the extender tube, which in turn ignites the booster in the bomb, which sets off the main charge. All this occurs rapidly enough to ensure that the detonation will occur with the nose of the bomb only slightly less than three feet above ground level.

The 10,000-pound bomb seemed to work much better in the Southeast Asia jungle than in the Georgia pine woods. The typical Combat Trap HLZ consisted of an area about 120 feet in diameter completely devoid of vegetation, including stumps. Beyond that, the height of the remaining stumps gradually increased, so that at some 70 feet from ground zero their height was approximately six feet, the limiting height for helicopter operations. Damaged and defoliated trees extended to approximately 180 feet from ground zero. The MSQ radar demonstrated a high degree of accuracy in working with the C-130. Drops were made with miss distances from 30 to 150 yards. After the combat trap had finished its job, a construction party and equipment were taken by helicopter to the new landing zone to expand it to the desired size.

The rate at which Combat Trap was using the 10,000-pound M-121s as a clearing device for helicopter landing zones was rapidly depleting the limited supply of bombs. In the search for a suitable substitute, methods for developing a cheap, big bomb were explored. Slurry explosives, chiefly ammonium nitrate and water, have been used for many years by the oil and mining industries, and tests were conducted on various mixtures. A 1000-gallon propane tank was used for the container, and appropriate flanges and openings arranged for. When filled with the slurry mixture, which solidified into a rubbery mass after pouring, the device weighed 15,000 pounds.

The BLU-82/B first saw use in Vietnam on March 23, 1970. Throughout the rest of the war, the USAF used them for tactical airlift operations called “Commando Vault.” After the war, the BLU-82/B was used during the Mayaguez rescue in May 1975, but the remaining BLU-82/Bs went into storage until the mid-1980s, when the Air Force Special Operations Command began using them again in support of special operations. During Operation DESERT STORM, MC-130E “Combat Talon” aircraft from the 8th Special Operations Squadron dropped 11 BLU-82/Bs, primarily for psychological effects. The USAF also used these weapons against terrorist strongholds in Afghanistan during Operation ENDURING FREEDOM.



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