In 1953, the United States Department of Defense was conducting a series of nuclear weapons tests called Operation Upshot-Knothole. During this operation, eleven nuclear devices of varying yields were exploded at the Nevada Test Site, about sixty-five miles north of Las Vegas. Among those atomic explosions was a test called Shot Grable, where the U.S. Army tried out its new toy, nicknamed “Atomic Annie.”
Annie was a 280 millimeter portable artillery piece, able to fire her 800 pound shells a distance of about seven miles. But the thing that made her really special was the fact that her shells packed a nuclear payload with the equivalent destructive power of 15,000 metric tons of TNT.
The test area had been prepared in advance by plugging trees into large holes drilled in the ground, and by scattering vehicles, buildings, railroad cars, bridges and other equipment at varying distances from the blast site in order to study the amount of damage that occurred. Seventeen days before Atomic Annie was tested, a twenty-seven kiloton nuclear device codenamed Shot Encore had been exploded over the same general vicinity, at a height of about 2800 feet.
History’s first atomic artillery shell explosion, Shot Grable, occurred on May 25, 1953. Annie’s 15-kiloton shell burst with precision accuracy over the designated target area, about 500 feet above the ground. Surprisingly, even though the yield was only about half that of Encore, it did a great deal more damage. This is because the shell caused a very abnormal destructive waveform called a “precursor.”
A precursor is a very strong dynamic wind caused by the shell’s oblique angle of approach, and its high horizontal speed. The nuclear explosion essentially inherits the shell’s forward momentum, which sweeps across the landscape causing extensive drag damage in addition to the typical destruction. For instance, a jeep which had been left virtually untouched by the much more powerful Encore device was completely torn apart by the artillery blast, and thrown a distance of about 500 feet.
Twenty of these artillery cannons were manufactured in the early 1950s as well as eighty of its artillery warheads, but it was later replaced by smaller, lower-yield alternatives. In 1991, the U.S. withdrew all of its nuclear artillery shells from service, and Russia did the same in 1992. Shot Grable was the only nuclear artillery shell ever actually fired by the United States.