5-inch/38 and the Kamikaze
Thomas Wildenberg
Reading Trent Hone’s article “Countering the Kamikaze” (October, pp. 28–35) leads one to conclude that the combination of the 5-inch/38-caliber gun firing a proximity-fused (VT) shell was the most effective antiaircraft weapon used against the kamikazes. This was true only when the 5-inch gun was able to place the shell within the lethal kill radius of the VT fuse. Unfortunately, this was rarely achievable, for several reasons.
When Japan began employing suicide planes, all major U.S. combatants—carriers, battleships, cruisers, and destroyers—were equipped with air search radars and frequently received advance warning from combat air patrols; however, the average range at which these ships opened fire on attacking kamikazes was only 6,400 yards, according to a postwar study conducted by the Center for Naval Analyses. This was considerably less than the 17,000-yard maximum range of the 5-inch guns carried by the majority of these ships and provided insufficient time for a proper fire-control solution to be computed. Thus, relatively few kamikazes were destroyed by 5-inch guns despite the greater destructive power of their shells, which was considerably more powerful than the 20-mm and 40-mm projectiles that claimed the vast majority of suicide bombers destroyed. A wartime study by the headquarters of the Commander in Chief, U.S. Fleet, concluded that 20-mm and 40-mm guns accounted for almost 77 percent of the suicide bombers destroyed by shipboard gunfire, while 5-inch guns accounted for only 19 percent (“Antiaircraft Action Summary, Suicide Attacks, April 1945,” Cominch P-009).
The 5-inch/38 relied on the Mk 37 Director for fire control. The mechanical “brain” of the Mk 37, its Mk 1 computer, was designed around the same mathematical algorithms embodied in the Mk 1 Rangekeeper, which was designed on the assumption that a target’s course and speed would remain constant. During the initial setup of the Mk 37, an estimate of the target angle and horizontal speed had to be manually entered into the Mk 1 computer. Range, too, had to be entered, either manually from the value obtained from the optical range finder or from the Mk 37’s radar.
The amount of time required for the computer to converge on a solution—the so-called time constant—varied according to range. The time constant was so large at long range (14 seconds at 30,000 yards) that a solution could not be reached in time to open fire. At short ranges the time constant was so small (2 seconds at 2,000 yards) that it caused unstable oscillations in the output. This problem remained the Achilles’ heel of the Mk 37 throughout the war. This shortcoming was an accepted fact in 1944 when the National Defense Research Committee (NDRC) was asked to investigate the performance of the Mk 37. In addition to documenting the slow convergence time, NDRC Division 7’s theoretical group also uncovered errors in corrections for ship roll and pitch equations introduced when the Mk 1 Rangekeeper was modified for antiaircraft use. The errors were particularly large on a heavily rolling ship, such as a destroyer, and at the high elevation angles needed to counter dive-bombing attacks.
These shortcomings, along with the sheer complexity of its operation, made the Mk 37 impractical for use against the tactics employed by kamikazes, which relied on a high-speed descending approach that was converted into a steep dive 2,000 yards ahead of the target. An alternate tactic used by kamikazes was to approach at wave-hopping altitude before popping up at the last minute for a diving attack. The Mk 37 was ill-suited for either tactic.
Mr. Hone responds:
I appreciate Thomas Wildenberg’s clarification regarding the capabilities of the 5-inch/38-caliber gun and the associated Mk 37 Director. It certainly is true that automatic antiaircraft weapons (20-mm and 40-mm guns) accounted for far more kamikaze aircraft shootdowns than the 5-inch/38. The point I endeavored to stress in “Countering the Kamikaze” was that, despite these results, the Navy was quick to recognize the significantly greater potential of weapons firing the VT fuse and rapidly moved to adopt them.
A postwar analysis from the office of the Commander in Chief, U.S. Fleet, makes the reasons for this clear. Since the 40-mm shell had a contact fuse, to hit an enemy twin-engine bomber approaching head-on, the shell would have to be placed in an area of about 90 square feet. For a 5-inch VT, the same target was roughly 3,900 square feet, more than 40 times larger (“Antiaircraft Action Summary, World War II,” Information Bulletin No. 29, 8 October 1945). This is one of the reasons why the 3-inch gun with VT fuse was a natural replacement for the 40-mm, despite the latter’s relative success against Japanese planes.
As Wildenberg points out, the much larger target area created by the VT fuse would make little difference if the guns could not be brought onto the target in time, and he is correct to highlight the limitations of the Mk 37. However, skilled users of the system found ways around those limitations. In his oral history, Vice Admiral Lloyd Mustin described using the Mk 37 in a “rate control mode” that could generate a fire control solution very quickly. He suggested that this approach was never sanctioned by the Bureau of Ordnance, but that it spread in the combat theater, especially among officers of the fast battleships. Later, as part of Task Force 69, Mustin ran a series of experiments and determined that the Mk 37 system could take a “surprise target” under fire in six seconds, including the time it took to swing the director from a centerline position and acquire a target approaching from amidships. This was far less time than other contemporary analyses.
Mustin emphasized that the most important limiting factor was not the director or the equipment, but the human element—the people and how they did things. It is essential to keep this in mind as we analyze the U.S. Navy’s capabilities during World War II, as there was often significant variation from ship to ship and crew to crew.
A Japanese Perspective
Kunio Hosoi
Regarding your end of World War II coverage in the August issue, my parents grew up in Tokyo. By August 1945, their homes already had burned down. My father, a 16-year-old high school boy, was forced to work for the Imperial Japanese Navy Aircraft Works in Yokosuka with his classmates. He assisted in making catapults for I-400–class submarines. He told me, “We had enough food and drink; we were lucky to have the naval work.” My mother, a 12-year-old elementary school girl, was in Tochigi Prefecture, north of Tokyo, with her teacher and classmates to avoid the air campaign against the capital. She returned to Tokyo in March 1946.
