Inside the Mk 14 Gyroscopic Gunsight

In the first decades of the 20th century, the emerging technology of naval aviation was seen as a revolution in naval warfare. Aircraft could deliver bombs and torpedoes with a speed and lethality that only increased as the aircraft themselves were improved and developed. During the interwar period, the naval planners of many nations recognized this potential and began equipping new ships and retrofitting old ones with antiaircraft armament. Even so, the early years of World War II demonstrated that the need had been badly underestimated, and antiaircraft gunners struggled to shoot down attacking aircraft. It soon became obvious that the U.S. Navy’s inventory of heavy machine guns and 1.1-inch machine cannon lacked the necessary punch for short-range antiaircraft defense. 

Captain William H. P. Blandy, head of the Bureau of Ordnance from 1941 to 1943, addressed this problem by obtaining production rights to manufacture the Oerlikon 20-mm and Bofors 40-mm autocannon and then cramming as many of them on ships as available deck space could accommodate. Effective long-range antiaircraft fire was achieved using 5-inch/38-caliber dual-purpose guns firing shells with radar-controlled proximity fuses. But short-range guns had to deal with any attackers who broke through this first line of defense, relying on their high rate of fire to create a wall of steel. Actually hitting an incoming aircraft depended on the gunner’s ability to lead the target using iron sights aided by tracer rounds. While there were likely to be some good shots in any given crew, with dozens of guns to be manned by less-skilled gunners, an aid to accurate hitting was urgently needed. Enter the gyroscopic gunsight.

Sperry Gyroscope and its consultant Charles Stark Draper of the Massachusetts Institute of Technology had been working on an experimental gunsight that incorporated the rate-of-turn gyroscope that Sperry had patented for use in aircraft instruments. In June 1941, the Bureau of Ordnance asked Sperry to supply prototypes of a compact gyroscopic gunsight that could be mounted on the Oerlikon gun. (See “The Shoebox That Transformed Antiaircraft Fire Control,” Naval History, December 2013, pp. 10–11). 

The Sperry prototype that became the Mk 14 combined a reflector gunsight of the type found in fighter planes with two rate-of-turn gyroscopes—one for traverse and the other for elevation. The reflector gunsight was the original “head-up” display, where the image of an aiming reticle was reflected by an angled mirror onto a piece of coated glass, within the pilot’s line of sight but without obstructing his view. To improve deflection shooting when engaging a turning aircraft, a gyroscope shifted the reticle image so it matched the target’s rate of turn. A pilot chasing another aircraft is more or less in the same plane of reference as his target, and his aircraft experiences the same angular momentum that the target does. The pilot had only to fly his plane so as to keep the reticle on the target and fire his guns without having to mentally compute a lead. 

A gunner standing on a deck, however, has to contend with the motion of his target in both horizontal and vertical planes as well as the pitching and rolling of the ship. Sperry’s innovation was to add another gyroscope, which corrected for motion in the vertical direction.

At the heart of the device is the principle of gyroscopic precession—the tendency of a spinning disc or rotor mounted on a gimbal to cause the gimbal to tilt or pitch when an outside rotation is applied. The traverse gyroscope reacted to the motion of the gun being turned to the right or left, and the elevation gyroscope reacted to the barrel being raised up or down. The small rotation of the gimbals holding the spinning rotors induced by precession was translated by rods connected to the mirrors of the reflector sight, which moved the image of the reticle projected on the glass of the sight accordingly. Combined, the gyroscopes allowed the reticle image of the sight to give the gunner a simple, accurate aiming point that automatically provided a lead angle.

The Mk 14 was housed in a compact water- and dustproof case, 12 inches wide by 11 inches long and 11 inches high, mounted on top of the gun. Small windows on the front and back allowed the gunner to see the reticle image on the reflector sight inside and view the target. A range knob was located on the lower right side, alongside knobs that adjusted the reticle for bore sighting. On the left side was a lever for a polarized light filter and connections for power and the compressed air that spun the gyro rotors.  

The Oerlikon 20-mm cannon was crewed by two men: a gunner and a range setter. The gunner’s job was to smoothly track the motion of his target in the sky, keeping the reticle on it, and then fire his gun. While the gunner would see the target in an apparent direct line of sight, the act of tracking caused the gyroscopes to precess and provide a resultant lead angle, so that the gun was actually firing at a point ahead of the target. The size of the lead was determined by the range setting knob on the side of the case, adjustable in 400-yard increments. Changing the setting rotated gears inside the case that varied how far the gyro linkage rods could move; less movement meant a smaller lead angle. The linkage also accounted for the ballistic drop of the fired projectile. The sailor setting the range received updated information from the air-search radar and adjusted the setting as needed based on whether the target was approaching and closing the range or flying away. 

The Mk 14 was used in combination with the Mk 51 director to remotely control quad 40-mm Bofors guns. A total of 85,000 Mk 14 gunsights were produced for use by the U.S. Navy and the Royal Navy, a considerable feat considering the precise nature of the mechanism. A study analyzing antiaircraft gun performance during the war found that the 20-mm and 40-mm guns accounted for more than 60 percent of all enemy planes shot down. The remarkable Mk 14 gyroscopic gunsight enabled antiaircraft fire accuracy that contributed significantly to that success and helped defeat conventional Japanese air attacks as well as blunt the effects of the late war kamikaze onslaught.