806 - Explosive clearing of jungle
Saturday, January 16, 2010

Project 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.
posted by High Power Rocketry @ Saturday, January 16, 2010  
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