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    Mars Bluff ''Broken Arrow''
     
    On the afternoon of March 11, 1958, the children of the Gregg family were in their playhouse in the woods behind their house in Mars Bluff, South Carolina. About four o’clock they tired of the playhouse and moved 200 feet to the side yard. This kept them from becoming the first Americans killed by a nuclear weapon released on U.S. territory. |Read More|


    B-47 Stratojet
    U.S. Air Force B-47E medium bomber serial number 53-1876A dropped its nuclear weapon in the woods behind the Greggs’ house at 4:19 P.M. The high-explosive trigger in the bomb blew up on contact with the ground, leaving a crater 50 feet across and 35 feet deep and injuring the three girls. All that remained of the playhouse were a few twisted shards of the corrugated metal that had been its roof.

    At 8:00 that morning Capt Earl Koehler and his crew arrived at Hunter Air Force Base just outside Savannah, Georgia, to fly their B-47 in Operation Snow Flurry that afternoon. Snow Flurry was not routine training but rather part of a “Unit Simulated Combat Mission and Special [i.e., nuclear] Weapons Exercise.”

    Briefings for the mission had begun ten days before takeoff, and two generals had appeared to emphasize the exercise’s importance. Aircraft 53-1876A, accompanied by three other B-47s from the 375th Bombardment Squadron, was to carry a nuclear weapon to Bruntingthorpe Air Base, England, conducting a midair refueling en route off the east coast of Canada. Before landing, the crew was to make a practice bomb run over England, transmitting an electronic signal to simulate the bomb release. Computers on the ground were to determine the accuracy of the “drop” and award points accordingly. Had the mission been completed, the crew would have had a tense, exhausting 18-hour day.


    The bomb

    Nuclear weapons then, as now, contained a high-explosive trigger to compress a uranium/plutonium core to a critical mass and initiate the chain reaction of a nuclear explosion. Perhaps the single greatest trick in nuclear-weapons design is to focus the immense force of the trigger explosion onto the core with precisely the proper geometry, strength, and timing to compress it to the exact degree required by the laws of physics to start a chain reaction, rather than simply blow the bomb apart. There are two types of high explosives that could be used in the trigger. One could be set off by concussion, such as a bullet or contact with the ground; the other—the type the military today invariably insists upon—could take great physical abuse without going off. Unfortunately, the triggers used in nuclear weapons in 1958 contained the former.


    The SAC

    Curtis LeMay
    Gen. Curtis LeMay
    The Strategic Air Command (SAC) was founded in 1946 to provide a force capable of nuclear retaliation against the Soviet Union. In October 1948, Curds LeMay took over as commander. During World War II he had earned the unaffectionate title Iron Ass for what his airmen considered a fanatic devotion to training. In his bombing campaign against Germany, LeMay had developed a scheme of flying bombers in tight formations over enemy territory, thus permitting them to protect one another from enemy fighters. To improve bombing accuracy, he came up with the idea of “lead” navigation. Selected crews became highly familiar with the approach routes to specific targets, and the entire tightly grouped formation would drop its bombs when the lead plane did. LeMay’s revolutionary tactics became standard U.S. practice; but flying large bombers in tight formations for hours was not a natural act, and it took a great deal of work to develop the concentration to do it routinely.

    Not surprisingly, LeMay brought the deep faith in training he developed during World War II to his new job at SAC. Virtually every aspect of the routines he devised was quantified, with points awarded for performance counting heavily in promotions. The results of particularly important activities, such as simulated combat missions like “Snow Flurry,” were relayed electrically to SAC headquarters, where senior officers, including the commander himself, reviewed them aggressively. If a mission did not go well, the local commander might receive a direct message from LeMay demanding a personal briefing on the reasons. Local commanders were, to say the least, anxious to avoid this.


    The plane

    The six-engine B-47 was the first modern jet bomber. The initial production model came out in March 1950; at more than 600 miles an hour, it was faster than any operational jet fighter in the world. It had a three-man crew with room for one passenger and was capable—barely—of crossing the Atlantic without refueling. On takeoff a fully loaded B-47 was virtually a flying gas tank, being just over 52 percent fuel by weight. But it could not dump its internal fuel tanks in an emergency, as could civilian airliners, and it wasn’t structurally strong enough to land safely with a large fuel load.

    When a fully fueled B-47 left the ground on takeoff, the choice was either to fly for several hours in order to burn off fuel or to crash. Nuclear weapons could add another 5,000 to 20,000 pounds, making the aircraft even harder to fly. Perhaps feeling some guilt over its failure to provide the B-47 with adequate safety features, the Air Force stipulated that on takeoff and landing the crew should be able to drop its nuclear weapon(s) promptly in an emergency, at least marginally improving the pilot’s ability to control the aircraft.

    Nuclear weapons were held in a B-47 by two systems: a complicated but reliable pneumatically powered catch that could be operated by the crew and a manually inserted steel locking pin. With the locking pin in place, it was impossible to drop the weapon. With the pin out, the crew could jettison the weapon almost immediately. By regulation, the locking pin was to be out for landing and takeoff.


    Losing a “device”

    Mk-6 fission bomb
    The nuclear weapon that landed on Mars Bluff was a Mark 6 30-kiloton bomb, an older fission (non- hydrogen) design considered highly reliable. It weighed 7,600 pounds, was 10 feet 8 inches long, and had a maximum diameter of 61 inches. Starting at 08:00 March 11, a specialized two-man loading crew took one hour and seven minutes to put the bomb into Air Force 531876A. When the loading team had trouble engaging the steel locking pin, they called the weapons release systems supervisor for assistance. He took the weight of the weapon off the plane’s bomb-shackle mechanism, put it onto a sling, and then “jiggled” the pin with a hammer until it seated.

    The bomb was put back on the shackle, and preflight checks continued. But neither the bomb-loading crew nor the aircrew ran the locking pin through its engage/disengage cycle with the bomb’s weight on the shackle. For the crew to receive maximum points for its unit under the ground rules, all preflight checks had to be finished by 10:30. It is difficult not to suspect that institutional pressure to gain points led to omission of this step.

    After the bomb had been loaded and the preflight checks completed, the crew went to briefings on weather and operations, had lunch, and returned to the plane about 2:40. At 3:42 Captain Koehler started his engines. At 3:51, as required by regulations, copilot Woodruff rotated his seat to face aft and pulled the lever to disengage the locking pin from the nuclear weapon. It could now be dropped instantly in case of an emergency. At 3:53 the plane took off to join three other B-47s for a formation flight to Europe. When the B-47 reached an altitude of 5,000 feet, Woodruff again rotated his seat, this time to re-engage the locking pin. He worked the locking lever unsuccessfully for five minutes as the B-47 climbed to 15,000 feet to join the three other aircraft. At this point, the crew knew it had a problem.

    The pilot told the bombardier, Captain Kulka, to go into the bomb bay to try to seat the locking pin by hand. This was not a trivial decision; the bomb bay was not pressurized, so the entire plane had to be depressurized. Because the plane was at 15,000 feet, the crew had to go on oxygen. Further complicating matters, the entrance to the bomb bay was so narrow that a parachute could not be worn into it. The task was doomed from the start; later testimony indicated Kulka had no idea where to find the locking pin in the large and complicated bomb-release mechanism.

    After a tense 12 minutes searching for the pin, the bombardier decided, correctly, that it must be high up in the bomb bay and invisible because of the curvature of the bomb. A short man, he jumped to pull himself up to get a look at where he thought the locking pin should be. Unfortunately, he evidently chose the emergency bomb-release mechanism for his handhold. The weapon dropped from its shackle and rested momentarily on the closed bomb-bay doors with Captain Kulka splayed across. Kulka grabbed at a bag that had providentially been stored in the bomb bay, while the more-than-three-ton bomb broke open the bomb-bay doors and fell earthward. The bag Kulka was holding came loose, and he found himself sliding after the bomb without his parachute. He managed to grab something—he wasn’t sure what—and haul himself to safety. Moments later the plane was rocked by the shock wave of the blast when the bomb hit the ground.

    In case of an unscheduled bomb drop, Air Force regulations required the crew to immediately notify its base by a special coded message. Because the procedure had never been used, the operations center at Hunter Air Force Base did not recognize the strange incoming message. As a final indignity, the pilot was reduced to radioing an open, uncoded message to the civilian tower at the Florence airport six miles west of Mars Bluff asking them to advise Hunter by telephone that aircraft 53-1876A had lost a “device”.

    The plane then turned back to photograph the site with its aerial camera. This was not difficult; the plume of smoke was easily visible from nearly three miles up. Because the plane could not dump fuel, it descended to the denser air at 6,000 feet, where it circled for 2 hours and 26 minutes before landing uneventfully.


    On the ground

    The reception of the B-47 crew back at their base was perhaps more difficult than their flight. By regulation, all crew members on missions carried loaded pistols. As the crew clambered out of the aircraft, they were met by armed air police, who relieved them of their weapons and took them to a room in the base operations center. They were told they would be obliged to stay here “at least overnight”; they were not allowed to contact their families or anyone else. The fear seems to have been that they had dropped the bomb deliberately. Later that evening, General LeMay, who was by then vice chief of staff of the Air Force, called Captain Koehler directly to get a telephone briefing on what had happened, understood Koehler’s explanation, and the crew was released.

    The accident was featured prominently in the national and international press. Overall, what is surprising is that coverage essentially disappeared after three days, and despite conflicting statements from the Air Force, the press did not investigate whether radioactive material had been released into the atmosphere. As it turned out, classified Air Force radiation studies released in 1997 indicate that radiation after the Mars Bluff explosion was barely above background levels. As was the rule in peacetime, the fissionable nuclear core of the weapon was stored elsewhere in the aircraft in what was called “the birdcage.” In a war situation, the crew would have transferred the nuclear core from the birdcage to the bomb bay and inserted it into the bomb.

    The incident obliged the Air Force to make significant changes. The composition of the high explosive used in nuclear-weapon triggers was promptly reformulated. No longer would it be possible for the explosive trigger in a nuclear weapon to be set off by concussion; the new design required a specific electrical impulse. While the Air Force and the Department of Energy do not discuss such matters, it seems likely that the changes cost hundreds of millions of 1958 dollars. Also, within days of the accident, a regulation was published requiring that locking pins be inserted in nuclear-weapon bomb shackles at all times, including takeoffs and landings.


    Source article: Americanheritage.com



    1958 Newsreel - "Dead' A-Bomb Hits U.S. Town"


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    Posted on Friday, August 08 @ 11:35:15 MDT by sonicbom
     
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