M121 (T56) Demolition Bomb
Project Combat Trap
Project Commando Vault
The M121 10,000 pound demolition bomb, sometimes called the "Earthquake" bomb, was more often referred to as the "Grand Slam" bomb, a totally misleading nickname. Grand Slam had been a British codename during World War II for a similar 22,000 pound weapon. During the early years of the Cold War, the US Air Force used "Grand Slam" as a 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. Modified B-36 bomb bays could also fit the 42,000 pound T12 demolition bomb. Since the 10,000-pound M121, 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 M121 category. The US Air Force acquired a number of M121s, initially given the experimental designation T56, and similar weapons as a conventional alternative to nuclear weapons for heavy bombers such as the B-36.
The M121 bombs found new life after Military Assistance Command, Vietnam began investigating methods for explosively clearing helicopter landing zones in heavy jungle terrain. The extensive use of helicopters in the Republic of Vietnam meant that landing zones had to be rapidly constructed in heavily forested areas, like those surrounding the Kim Son and Soui Ca valleys. 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. W hen sufficient area had been cleared, air-portable construction equipment or additional tools and demolitions were lifted in to expand the new landing zone.
Large high explosive detonations would dear trees and brush from an area, leaving a zone suitable for helicopter landings. An added benefit was that the overpressure would often detonate mines and boobytraps. As applied to the helicopter landing zone problem, it was clear that the high explosives had to be detonated at some height above the ground to avoid cratering, for 2 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. The first attempts to use aircraft bombs to achieve this began in late 1967 using a modified M118 bomb. However, the setup, developed as part of Project Combat Trap, still had to be emplaced on the ground by engineers. MACV subsequently directed Seventh Air Force to develop the capability of delivering specialized ordnance for clearing such heavily-jungled areas. Seventh Air Force conducted tests with M118 and Mk 84 bombs using fuze extenders. The Armament Development and Test Center at Eglin Air Force Base, Florida conducted tests using BLU-1B/B bomb casings filled with explosives.
The US Army began tests of the M121 bomb at Fort Benning, Georgia. M121s had been found in storage, having been developed in 1954 for the B-36 and then never used operationally. In the tests, an M121 was statically detonated in a wooded artillery range. The bomb was emplaced by an Army CH-54 "Flying Crane" 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 UH-1 helicopter was flown in to land in the cleared area. The zone created had usable space approximately 100 feet in diameter.
While the results were satisfactory, practical considerations demanded aerial delivery for any employment in Southeast Asia. As a result, the M121 was modified and redesigned to contain 2 independent fuzing systems, one forward and one aft, as well as a stabilization parachute. Continued testing with inert bombs on western test ranges, using both the Army's CH-54 helicopter and the Air Force's C-130, demonstrated the feasibility of using the latter as a "bomber," but eventually resulted in a decision to abandon further employment of the big helicopter in this role.
Subsequently, preparations were made to deliver M121s to South Vietnam for operational tests. In December 1968, the 834th Air Division, headquartered at Tan Son Nhut Air Base in South Vietnam conducted a 10-weapon operational test of the M121 in South Vietnam as part of Combat Trap. For air drop, the bombs were fitted with a standard nose fuze and M1 fuze extender (a tube packed with explosives, which was attached to a detonator inside the bomb), to provide for a burst height of about 3 feet. 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 for employing the weapon involved a number of actions. As the bomb separates from its carrier, pins were pulled from both nose and tail fuzes, and the drogue chute was deployed. The fuzes were armed at a that time to provide safe separation, and the chute quickly stabilized the trajectory of the bomb. Penetrating the top of the canopy, the brush deflector pushed aside the smaller branches and was crushed by the impact with the earth. The fuze then detonated the explosive in the extender tube, which in turn ignited the booster in the bomb, which set off the main charge. All this occurred rapidly enough to ensure that the detonation would occur with the nose of the bomb only slightly less than 3 feet above ground level.
The 10,000-pound M121 bomb seemed to work much better in the Southeast Asia jungle than in the Georgia pine woods. The typical Combat Trap helicopter landing zones 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 6 feet, the limiting height for helicopter operations. Damaged and defoliated trees extended to approximately 180 feet from ground zero. The MSQ-77 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 weapon 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 M121 was given its first tactical employment in support of Operation Taylor Common in I Corps Tactical Zone in December 1968.
This test evolved into Project Commando Vault. Commander, US Military Assistance Command, Vietnam's (COMUSMACV) original request, SEAOR 168, had stated that the desired capability was for a weapon that could rapidly create landing zones of 5-helicopter size and that development efforts should be concentrated on the largest weapon compatible with C-130 delivery. Though the M121 was a suitable weapon, the rate at which M121s were being used as a clearing device for helicopter landing zones was rapidly depleting the limited supply of bombs, which had long since been out of production. At the peak of their use, 20 M121s were being requested every month. Ten M121s were dropped just between 12 and 20 December 1968. The possibility of restarting production of the weapons was turned down. However, while the replacement weapon, then known as the CD-1, was in development, M121s continued to be used. A comparative test between the M121 and the CD-1 was conducted on 11 May 1969, sealing the weapon's fate. However, M121s were used until the supply was entirely exhausted, with the last weapon being dropped on 8 August 1970. At that time, the Commando Vault mission fell entirely to the replacement BLU-82/B.
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