Analyzing the Arkansas

Commissioned in 1912, the USS Arkansas battleship served in both World Wars before being used as a target in Operation Crossroads-Baker, a Los Alamos Scientific Laboratory–designed test used to study the effects of nuclear weapons on naval vessels. Detonated 90 feet underwater on July 25, 1946, the Baker test device displaced 2.2 million cubic yards of water that shot up into a thick column before crashing back down into the Pacific Ocean.

A well-known photo of the test shows a dark area on the right of the column. For decades, many people assumed this dark spot was the Arkansas—which had been placed approximately 660 feet from ground zero—being swept into the column.

However, by using some basic physics calculations, the unclassified weapons test reports from Operation Crossroads, and critical thinking, it is possible to bust the pervasive myth that the Arkansas was lifted vertically into the column of water.

During the Crossroads test, pressure data was collected on selected ships, and some ships were more instrumented than others. Although pressure data was lost as the Arkansas and other ships close to surface zero sank, the pressure in the water around the blast was recorded. Thus, we know the maximum pressure recorded was 4,800 pounds per square inch (psi) above the blast in the region of the resulting water column.

The Arkansas was 562 feet long, with a 93-foot beam (widest part of the ship) and a 28-foot draft (the depth of the bottom of the ship). She displaced (weighed) around 26,000 tons, so she would not be moved easily in any direction. Not to mention that the Arkansas was anchored to the sea floor by both her bow and stern, which would provide substantial resistance to lifting and tipping movements.

Approximating the bottom of the ship with simple flat plates and doing some fundamental math based on force = mass x acceleration = pressure x area, one arrives at a vertical acceleration of around 400g (400 times the acceleration of gravity). This acceleration lines up well with the maximum recorded accelerations from the other ships in the array during the blast, namely the USS Pensacola, USS New York, and USS Nevada. This also lines up with a supercomputer simulation of the blast that was done using one of the Lab’s hydrodynamic physics codes.

The most interesting fact of this situation is that the shock duration (the time of the pressure pulse that would have caused the ship to accelerate upward) was recorded by many gauges and generally referred to as “less than a millisecond,” which would not produce enough resulting force to lift the entire ship out of the water.

The final discussion point is what the sunken USS Arkansas looks like at the bottom of the lagoon. The description in the reports indicates that the port (left) side hull was largely intact while the starboard (right) side nearest the blast and resulting water column was dished in, deformed, and had many hull plates separated. This indicates that the force was strong enough to deform the ship on the side of the blast but not enough to lift it out of the water.

So what is that dark area on the Crossroads-Baker photograph? The general consensus among many experts is that the soot from the boilers on the Arkansas was shaken loose from a previous test and was pushed out of the stacks as the pressure wave hit the bottom of the ship and traveled up through it, leaving the cloud of soot mixing with the water vapor just above the ship in the photograph.

Laboratory program manager Jeremy Best was the principal investigator for this work, which was also supported by senior historian Alan Carr, scientist Christopher Mauney, and retired scientist Tom Kunkle.