Feb 11

The Antiaircraft Fire Control 'Shoebox'

Tuesday, February 11, 2020 12:01 AM

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The prototype of the Mark 14 gunsight was produced by Charles Draper’s confidential instrument laboratory at MIT in order to provide a proof of concept for skeptical Navy leaders. (Charles Stark Draper Laboratory)

In the spring of 1941, the staff of the fire-control section in the newly reorganized Bureau of Ordnance’s Research and Development Division was struggling with the problem of how to provide fire control for the heavier antiaircraft machine guns that were just entering production, such as the 40-mm Bofors and the 1.1-in machine cannon. Although a series of development contracts had been awarded to the traditional suppliers of fire-control directors, none of the devices submitted to date lent themselves to quantity production, none had proven to work, and all were deemed too difficult to maintain afloat.

Unbeknown to anyone in the Bureau of Ordnance, Charles Stark Draper, a professor at the Massachusetts Institute of Technology (MIT), was already working on a secret design for a gyroscopically controlled, lead-computing gunsight that would be a godsend to the Navy. The idea for the lead-computing sight had come to him a year earlier, in May 1940, while he was working with the Sperry Gyroscope Company on a new rate-of-turn indicator for aircraft.

Dr. Draper, or “Doc” as he was known to friends and colleagues, was having lunch with the pilots and engineers who had conducted the tests on the new instrument when someone brought a newspaper to the table that contained headlines about German tanks moving into France. The word circulated by the press was that French gunners could be successful if the target was standing still, but they couldn’t hit anything that was moving fast. Draper immediately saw the possibility of taking one of his gyroscopically controlled turn indicators and converting it for use as a lead-computing sight. He correctly deduced that the gyroscope could be used to offset the line of sight just enough to compensate for a tank’s movement during a shell’s time of flight, leading to a certain hit.

Draper suggested the idea to Sperry, which worked out an arrangement with MIT for a cooperative project to investigate the possibility of transforming his rate-of-turn indicator into a lead-computing sight. That summer, Doc borrowed all the books he could find on ballistics and fire control to study the knowledge and technology available.

An Oerlikon gun crew practices with Draper’s Mark 14 lead-computing gunsight. Most of the 85,000 Mark 14s produced under license by the Sperry Gyroscope Company were installed on the Oerlikon antiaircraft guns that dotted the decks of British and U.S. warships. (U.S. Navy)

By September he had worked out the theoretical basis for the sight and had begun to experiment with hardware. Within six months he had constructed a prototype the size of a shoebox that was affixed to a .22-caliber rifle for testing. The “shoebox” contained a small gyroscope that was used to offset an illuminated reticle that acted as the sight’s aiming point, providing just the right amount of lead for a moving target. The shoebox was a little too heavy for the gun to be handled easily, so Draper mounted the rifle on a pipe stand for support. He then began shooting at a towel on a moving rope, and tested and adjusted the sight until he could hit the target effortlessly.

The British Admiralty became interested in the device after Sir Henry Fowler tested a prototype at the MIT shooting range in December 1940. Fowler wanted to buy three manufacturing prototypes for testing in England, but the Instrumentation Laboratory was just a working laboratory, and besides, Sperry now owned the rights to it. The company was given a contract to manufacture three of the MIT sights for the Admiralty instead. (Sperry must have botched the job, because the British showed no further interest in the device.)

The operator looked through Draper’s sight and used the handlebars to control the movement of the Mark 51 director, which controlled the 40-mm guns to which it was connected. (U.S. Navy)

While Sperry was busy attempting to build a sight of its own design for the Admiralty, the Bureau of Ordnance became aware of Draper’s work through the efforts of a young lieutenant assigned to the Bureau, Horatio Rivero. Rivero had investigated the problem of antiaircraft fire control while studying for his master’s degree in electrical engineering at MIT during the 1939–40 academic year. Although he had studied under Doc, his old professor was noncommittal when Rivero tried to discuss the project with him. While Draper did not discourage Rivero, he was evasive when asked how to solve a bearing-friction problem with the gyroscope.

Rivero, then head of the Bureau of Ordnance’s radar desk, probably found out about Draper’s secret project during one of his visits to MIT in conjunction with the radar work being done at the radiation laboratory. Regardless of how it came about, Rivero arranged to visit Draper at MIT with Lieutenant Commander Marion E. Murphy, head of the Fire Control Section, on 28 May 1941. They were the first U.S. Navy representatives to see Draper’s lead-computing gyroscopic gunsight.

Other representatives from the Navy soon witnessed a demonstration of a laboratory prototype on the rifle range at the Watertown Arsenal. Discussions followed with Draper and the engineers at Sperry concerning arrangements to test the sight at the Naval Proving Ground.

On the day after the test, Murphy telephoned Sperry and asked its people to provide descriptive specifications and an approximate price for 50 sights. The response was 30 units for the 20-mm Oerlikon and 20 units for a remote-control gun director. The subsequent order for 12 experimental gunsights placed on 4 August 1941 was the first of 85,000 Mark 14 gunsights built and installed on American and British warships during World War II. The gyroscopically controlled Mark 14 gunsight automatically calculated an aircraft’s future position, taking into account gravity and distance, making it easier for the gunner to track the fast-moving target.

Murphy, along with others in the bureau, immediately recognized that the quickest solution to the heavy antiaircraft fire-control problem plaguing the bureau could be solved by mounting Draper’s lead-computing sight on a stand-alone director that could be used for remote control. Using a computing device much less complicated and easier to manufacture than what the rangekeepers previously used on the Navy’s antiaircraft directors would alleviate the problem of gun vibration.

Thus, when the Navy ordered the first 12 experimental Draper sights from MIT on 4 August 1941, it stipulated that four pedestal-mounted directors be supplied as well. These too would be designed by Draper and his assistants at MIT. Draper’s original design, which quickly became known as the “barber chair,” turned out to be unsatisfactory when tested in December 1941. After conferring with Sperry engineers, it was decided to abandon the barber-chair design, which involved the objectionable feature of a seat for the operator, and start anew on a “handlebar” design officially designated as the Gun Director Mark 51. Murphy, in conjunction with Lieutenant Commander Ernest E. Herrmann of the antiaircraft production section, undertook direct responsibility for the new design. A production order for 1,000 Mark 51 directors was authorized in January 1942.

The Mark 14 sight was quickly adapted to provide remote-control direction to the new 40-mm Bofors antiaircraft gun, which was ultimately responsible for nearly half of all the kamikazes shot down by shipboard antiaircraft fire. (National Archives)

At a time when the application of 20-mm fire control was of utmost concern within the bureau, Murphy and Herrmann continued to promote the importance of the Draper sight for use as a heavy machine-gun director. Thanks to a concerted effort by the two men, the first Mark 51 was shipped to Dahlgren for testing between 4 and 7 May 1942, where it received Proving Ground approval. Within a month, the first production Mark 51 directors were being delivered to the Fleet. Fourteen thousand had been produced by the time the war ended in 1945.

The importance of Draper’s sight cannot be overemphasized, as it played an essential part in providing the shipboard air-defense system needed to defend the Fleet. This was especially true in the later months of the war when the kamikaze threat was at its greatest.

The added effectiveness of Draper’s lead-computing sight is difficult to assess. Nevertheless, it should be noted that during the period of the campaign in the Philippines when the dreaded kamikaze first appeared, 20-mm and 40-mm guns under the control of Draper’s sight accounted for 78.6 percent of all suicide planes brought down by shipboard antiaircraft fire.

The Mark 14 gunsight and the Mark 51 director were the first in a series of directors developed by Draper’s lab during and immediately after the war. The reputation garnered by Draper and his laboratory through this work was instrumental in leading to his selection to develop the inertial guidance system for the Polaris missile and every fleet ballistic missile thereafter. Today, the Stark Draper Laboratory continues to be the sole source supplier of the inertial navigation system employed by the Navy’s Trident II missile.