During WWII, the U.S. Army determined that there was a need for a mobile long range heavy artillery weapon to attack strategic targets behind enemy lines - communication centers, fortified positions, enemy long range artillery and other high value targets. After the war, it was determined that a heavy artillery weapon that could fire an atomic warhead would be even more desirable. This kicked off More..a dual development effort - designing and building the actual weapon and designing a small enough atomic warhead to be used as a shell for the weapon.
In 1952, the cannon was complete. The 280-mm Atomic Cannon weighted 88 tons and was transported by two vehicles capable of moving at speeds up to 40 miles per hour. The cannon could be set up and ready to fire in 15 minutes or less.
But it wasn't until 1953 that it could be tested with an atomic shell.
On May 25, 1953, during the Operation Upshot-Knothole test series conducted at the Nevada Atomic Test Site, the first and last atomic shell was fired by Atomic Annie as part of the Grable test. The shell was fired a distance of seven miles, was exploded in the sir and had a yield of 15 kilotons (15,000 tons of dynamite/TNT.)
The test was considered a success and a total of twenty atomic cannon were built. But over the years, they were slowly retired as their strategic value was greatly diminished. This happened for three reasons. The first was that the world was adverse to atomic weapons used in future conflicts. The second was more practical. Due to the weight of the cannon - 88 tons, it could only travel on high quality roads - something that generally did not exist in the conflicts of the 50s and 60s. The third reason was simple progress. Smaller cannons - 8 inch and 155 mm - were developed that were much more mobile and of more pracical value in the conflicts of the 50s and 60s.
Today, there are only four 280-mm cannons still in existence. They are all retired and located at various locations in the United States. Atomic Annie still exists. It is located at Fort Sill. The other three that exist are at the National Atomic Museum in Albuquerque, NM; Fort Riley, Kansas and Aberdeen Proving Ground, MD.
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