Armaments & Innovations - The Revolutionary Rangekeeper

By Thomas Wildenberg

As David Mindell explains in his groundbreaking study Between Human and Machine (Johns Hopkins University Press, 2002): “When the navy began to implement director firing, Elmer Sperry saw an opportunity. . . . The same devices that transmitted compass readings from the master gyro to repeaters could also transmit fire control information from the director to the plotting room and the turrets. Building on this idea, in 1914 Sperry introduced a set of data transmitters for fire control based on the gyrocompass repeater.” Sperry’s repeaters replaced the notoriously unreliable voice tubes that had previously been used to send target-bearing and turret-train settings to and from the plotting room.

In 1916 Sperry introduced a fire-control system that included an analog computer invented by Ford that recorded on a rolling paper chart the battle as it evolved. Ford’s battle tracer combined inputs from Sperry’s data transmitters to provide a plot of the firing ship’s course along with that of the target ship. The chart allowed the gunnery officer in the ship’s plotting room continuously to see the present range and bearing of the target. According to the product bulletin on the Sperry Fire Control System, the recording furnished “practically a bird’s eye view of the maneuvers involved.”

As David Mindell points out, the record produced by the battle tracer allowed the Navy’s senior officers to monitor the performance of the plotting team, revealing carelessness or incompetence. It was an essential piece of Sperry’s system for director fire that was designed to concentrate the control of all gunfire at one point, causing the entire battery of the ship to operate as a single unit. But missing from the Sperry Fire Control System was an adequate method of determining the change in range due to target motion, which was still being supplied by the Vickers Range Clock, introduced in the U. S. Navy in 1908.

Ford resigned from Sperry in May 1914 to set up the Ford Marine Appliance Corporation. His intention was to manufacture a competing gyrocompass, but he abandoned that idea when the Navy requested proposals for an improved replacement for the Vickers Range Clock, which had the fatal flaw of requiring a manual adjustment whenever the range rate changed. Toward this end he established the Ford Instrument Company in the latter part of 1915 to manufacture precision instruments of his own design for fire control.

In response to the Navy’s proposal, Ford created a state-of-the art mechanical computer—the Ford Rangekeeper—that predicted a target’s future position based on a number of data inputs that were inserted through various knobs and hand cranks. Because the rangekeeper was similar to the Argo Clock developed by British inventor Arthur H. Pollen, some historians claim that the former was pirated. Mindell, who has made an extensive study of this issue, thinks Ford’s design was probably a combination of his own experience; memos by Reginald Gillmor, the head of Sperry’s London office; and the specifications in the Navy’s proposal request. When evaluated against Sperry’s new range predictor in the spring of 1916, the Navy judged the Ford Rangekeeper far superior to any existing method of tracking target range.

Navy Bureau of Ordnance orders followed shortly thereafter, and the Ford Instrument Company began manufacturing the Rangekeeper Mark 1. The first one was installed in the battleship Texas in 1917.

Ford’s rangekeeper solved the fire-control problem by calculating the target’s future range and bearing, taking into account a shell’s time of flight (which at maximum range was about one and a half minutes). The latter was an important aspect of modern gunnery because a vessel moving at 20 knots, the typical speed of the latest battleships, would progress 675 yards (approximately three ship lengths) along her course in one minute. To provide the firing range to the target, Ford incorporated a mechanical integrator of his own design that calculated the range from the range rate generated by other analog devices within the rangekeeper.

The rangekeeper’s operation was based on the firing ship’s course, received from a repeating gyrocompass; the ship’s own speed, which was entered manually; measurements of the target’s range and bearing taken by the ship’s rangefinder, transmitted to the plotting room, and entered manually; and estimates of the target speed and course provided by the ship’s gunnery officer.

The Mark I and the various improved models that followed were the key element in all main-battery fire-control systems developed for the U.S. Navy in the interwar years. The Ford Instrument Company also developed a small-ship version, the Mark II, known as the “Baby Ford,” for use in destroyers and for the secondary batteries of larger ships. The effectiveness of the Mark I rangekeeper was improved in 1926 with the addition of target-bearing output. In that same year the company delivered the Navy’s first dual-purpose director, the Mark 19, designed for use against aircraft and surface targets.

Like its predecessors, the Mark 19 relied on a Ford rangekeeper to predict the future target position. Since the rangekeeper’s solution was based on the assumption that the target maintained a steady course and speed, the Mark 19 was only effective against horizontal bombers, as these aircraft needed to maintain a constant altitude and speed for accurate bombing. It was the first in a series of directors developed by the Ford Instrument Company during the following years (i.e., the Mark 28, Mark 33, and Mark 37) to provide fire control for the dual-purpose 5-inch guns that were considered the mainstay of the U.S. Fleet’s shipboard air defense. By then Ford had become so ensconced in the Navy’s fire-control business that one observer called his company the secret Fire Control Design Section of the U. S. Navy.

At the start of World War II, Ford’s latest rangekeeper, the Mark 8, which first saw service in the Portland -class cruisers, was the most advanced fire-control system in the world. Ford had improved on the accuracy of earlier models by adding corrections for deck and trunnion tilt, wind speed, and drift. When the Mark 8’s successor was introduced during the latter stages of the war, the Navy, in keeping with the changes in technology taking place, designated it as the Computer Mark 1.

Mr. Wildenberg is an independent historian and scholar who specializes in technical innovation in the Navy. He is the author of five books on naval history, including Billy Mitchell’s War with the Navy: The Interwar Rivalry over Air Power (Naval Institute Press, 2014).



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