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History of Force Measurement in the US – Part 23

Dr. Briggs proceeded slowly and cautiously when it came to the atomic issue, but one week before the attack on Pearl Harbor, there was an official recommendation made to commit to the production of an atomic bomb. Less than 2 weeks after Pearl Harbor, a timeline was established that would result in the production of a finished bomb by January 1945. Five promising approaches to bomb production were identified and all were to be explored through the pilot plant stage. The Bureau’s role at this point became “the development of analytical procedures for controlling the purity of critical materials in the reactors and in the bomb.” In this capacity, the Bureau received almost 9,000 material samples on which they performed almost 30,000 analyses.

Work on the bomb progressed through research performed in the military, governmental and educational sectors to the point of the assemblage of a dream team of theoretical and experimental physicists, mathematicians, armament experts, specialists in radium chemistry and in metallurgy, specialists in explosives and in precision measurement at Los Alamos, New Mexico. The Bureau contributed a group from its proximity fuze program and a team dedicated to the purification of U235 scrap so it could be used again. Finally, in July 1945, the bomb was tested successfully. https://www.youtube.com/watch?v=Ru2PWmGIoB8

Though overshadowed by the immensity of the development of the atomic bomb, the WWII era yielded two other amazing advances – namely, the airburst proximity fuze and radar. The airburst proximity fuze allowed for bombs to be detonated in the air prior to impact with the ground which greatly enhanced their destructive power. Using this technology, detonation occurs when radio waves emitted are reflected back to the device with sufficient intensity to indicate close proximity to a large object triggering an electronic switch to initiate the detonation.

The Bureau became involved in work on this type of fuze after the NDRC (National Defense Research Committee) assigned the research to the Department of Terrestrial Magnetism at the Carnegie Institution of Washington in 1940. Within the Bureau, the work fell to the team that had previously constructed the radiosonde and radiotelemeter. Within 6 months, they determined that different types of radio would be necessary for rotating projectiles (used by the Navy in antiaircraft guns) and nonrotating (for the Army and Air Force to use with bombs, rockets and mortars). Only the work on the nonrotating element fell to the Bureau, which focused on the potentials of either an acoustic fuze or a photoelectric fuze. After eliminating acoustic and other methods, testing began using the Doppler effect of reflected radio waves. By early 1941 they had achieved proof of concept but it took almost 2 more years to develop to the point of being used by the military in combat operations.

Testing and development continued and the program outgrew its laboratory space at the Bureau in 1942. In December of that year, with requests for additional fuze types and other related projects, the Bureau consolidated the various projects into the ordnance development administration. Since the face of the war was constantly changing, the fuze projects were as well. A project established to meet one threat might have to be retooled as that threat gave way to another. Fuze designs had to be tweaked to accommodate different types of exploding weapons. For example, the differences presented by the dry battery used in the bomb fuze as opposed to the power source for the rocket fuze. The dry battery was far more temperature sensitive and had a very short shelf life. These limitations meant an alternate power source had to be found – ultimately resulting in a small generator being fitted to the spinning vane of the conventional bomb fuse. This solution all but eliminated the problems of shelf life and temperature sensitivity, and also made the bomb safer to handle, since it wouldn’t detonate unless sufficient wind passed the vane (as in a drop) to produce enough power to trip the fuze.

All of the advances in fuze and detonation technologies dramatically increased the destructive power of exploding weapons used by the U.S. during WWII. In fact, the technology was so powerful, that use of the fuzes was forbidden in circumstances where the enemy might be able to recover a fuze for later analysis or identify its nature simply by observation. For example, bombs incorporating the proximity fuze were not used for D-Day for fears that a fuze might be recovered from the beach at Normandy. As the war neared its end, fuze plants were “monopolizing 25 percent of the total facilities of the electronic industry and 75 percent of all molding plastics firms.”

**The information presented here is drawn from “Measures For Progress: A History of The National Bureau of Standards” (Rexmond C. Cochrane)

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