The Mk 23 Katie

I've previously mentioned that during the 1950s the Iowas were fitted with a 16" nuclear shell, the Mk 23 "Katie".¹ This is a fascinating device, one of a plethora of nuclear weapons that were developed in the mid-50s. But how did the Katie come about, and what modifications were made to the battleships to allow them to use it?

Me with the Mk 23 at the National Atomic Museum²

The Katie has its origins in the Mk 9, the world's first artillery-fired atomic projectile (AFAP). The Mk 9 was the Army's first nuclear weapon, part of its bid to keep itself relevant in an era when atomic weapons were expected to dominate the battlefield of the future.³ The designers faced serious challenges. An artillery shell is an incredibly difficult environment to put a complicated device like a nuclear warhead. It must withstand normal handling, thousands of Gs of acceleration as it's fired, and the centrifuge of a shell spinning at 10,000 rpm or more. It must be incredibly reliable, as a nuclear shell that doesn't go off is not only a waste of money but might give the target valuable materials or insight. And it needs to be compact, as big artillery pieces are heavy, expensive, and difficult to move. Ideally, a nuclear shell would be for an existing weapon, probably the 240 mm M1 howitzer, the biggest gun in the Army's inventory.

280mm nuclear shell at the National Atomic Museum

Unfortunately, when development began in 1949, the state of the art in nuclear weapons technology wasn't really up to solving all of these problems very well. Most nuclear weapons use an implosion system, where carefully-shaped explosives compress a core of fissile material until it's dense enough to support a chain reaction. It's efficient and powerful. However, the smallest implosion bomb of the time, the Mk 7, was 30" in diameter. This would require a gun of impractical size for any sort of tactical use. So the designers turned to gun-type devices. These assembled their fissile material by firing one section into another using what was essentially a gun. Little Boy, the bomb dropped on Hiroshima, was of this type. It was much less efficient than an implosion device,⁴ and required 145 lb of uranium instead of plutonium, but it was inherently more durable and could be made smaller. The final yield was around 15 kt, the same as Little Boy, and slightly lower than the implosion-type weapon dropped on Nagasaki.

280mm Atomic Cannon at the National Atomic Museum

Even using a gun-type device, the designers were unable to squeeze it into a 240 mm shell, and a specialized weapon, the M65 280mm "Atomic Cannon", was developed to fire the nuclear shell. The M65 was a high-profile weapon, with the prototype taking part in Eisenhower's inaugural parade. However, while it was impressively mobile for such a large weapon, taking only 15 minutes to switch between moving and firing, it was large, expensive, and had an annoyingly short range of only 26,300 yards. Only 20 were ever built, and they lasted a mere decade in service before missiles and nuclear shells for smaller artillery pieces rendered them obsolete.

However, for a few years the M65 was one of the most important pieces of equipment in the Army inventory, and they set about trying to fix at least one of its flaws, namely the rather short range. The easiest way to do this was to make the shell lighter, and an improved version of the Mk 9, the Mk 19, was promptly designed. The Mk 19 shells weighed only 600 lb, as opposed to the 850 lb of the Mk 9, at the cost of using even more precious uranium. This increased range to 32,700 yards.

Iowa in the late 50s

Into this came the Navy and, more importantly, the Marine Corps. The Navy already had atomic bombs for land attack and hunting submarines, but the Marines were nervous about the Navy's ability (and/or willingness) to provide nuclear fire support during an amphibious invasion. A nuclear 16" shell would calm these worries nicely.⁵ It would be relatively easy to build, drawing on the existing knowledge of nuclear artillery shells, and provide shore bombardment capability without relying on airplanes and bombs that might be diverted elsewhere. The project officially began in late 1953, and the Army offered to simply adapt the internals of their shells to fit a 16" gun. When it entered the stockpile in October of 1956, the Mk 23 used the same nuclear components as the Mk 19, and had the same yield.⁶ The resulting shell was ballistically identical to the standard 1,900 lb HC shells, allowing them to use the same cams in the fire control computer.

A Mk 23 at the National Atomic Museum⁷

Ultimately, 50 Mk 23s were manufactured, and Iowa, New Jersey and Wisconsin⁸ were modified to carry ten nuclear and nine Mk 24 practice projectiles in the magazine of Turret II, as well as a single drill projectile.⁹ If one of the other turrets was to fire the Mk 23s, they would have been transferred via the rail system installed on Broadway. Unfortunately, the exact nature of these modifications has never been published, but at the time, nuclear components were almost always stored separately from the rest of the weapon. It's likely that there was a secure locker for the uranium, and an area where specially-trained crew members could assemble the projectiles. The locker would probably have been guarded 24/7 by the ship's Marine detachment, as were the Tomahawks in the 80s.

The US Navy refuses to confirm or deny the deployment of nuclear weapons at sea, with the exception of the ballistic missile submarine force. However, Sumrall indicates that the full compliment of nuclear shells did go to sea aboard Iowa and Wisconsin, and that the later ship did fire a number of the practice projectiles. New Jersey apparently only carried a single drill projectile.¹⁰ Only 18 months after the Mk 23 was introduced into service, the last battleship was retired. Despite this, the nuclear shells remained in the US stockpile until 1962. One projectile was apparently modified for a peaceful nuclear test under Project Plowshare in 1961, although I'm not sure if it was ever used.¹¹

Grable Shot, Operation Upshot-Knothole

The only live test of any of these shells was when a Mk 9 was fired from one of the 280 mm cannons during the Grable shot of Operation Upshot-Knothole at the Nevada Test Site in 1953.¹² This was only the second gun-type device ever set off, and the only live firing of an atomic artillery shell. The test was a complete success, and the weapon had the same yield as Little Boy had had eight years earlier, despite an elevenfold reduction in weight and a fourteenfold reduction in volume.

Later nuclear artillery shells fit into 8" and 155 mm shells, and most used more efficient implosion designs. There was even some speculation that nuclear shells would be developed for the Iowas when they were reactivated, although I've never seen any suggestion that this was seriously pursued. The 280mm projectiles almost had a second encounter with the battleships, too. During New Jersey's deployment to Vietnam, the Navy looked into using some of the leftover 11" shells,¹³ fitted with sabots, to extend her range inland.¹⁴ This never came to fruition, nor did similar suggestions for reuse during the 80s.

Me examining the Mk 23

The Mk 23 is a fascinating artifact of the early Cold War. While it was only in service for a few years and never came anywhere close to use, it retains the distinction of being the only nuclear projectile ever created for a naval gun. And the mere fact of its existence offers an interesting window into the mindset of US thinkers during the height of the nuclear arms race against the Soviets.