The Mark 24 torpedo, nicknamed “Fido” for its ability to sniff out enemy submarines, was one of the first “smart” weapons developed during World War II. An official definition for smart weapons does not exist, but such devices are characterized by a set of sensors—electromagnetic, acoustic, heat, etc.—that enable the weapon to precisely home in on its target. Those smart weapons that are not directed from the launch platform via a wire, radio control, laser illumination, or radar signals are said to be autonomous, or fire-and-forget weapons; once launched the platform is free to take evasive action or continue its mission. The Fido fell into this category.
The idea for an acoustically directed torpedo originated in Division 6 of the National Defense Research Committee (NDRC) established by President Franklin D. Roosevelt in June 1940 to coordinate, supervise, and conduct scientific research on the problems underlying the development, production, and use of mechanisms and devices of warfare.
After the NDRC was established, Secretary of the Navy Frank Knox asked that the committee form a panel to advise him and Rear Admiral Harold G. Bowen, the director of the Naval Research Laboratory, on innovative ways to defeat enemy submarines. The panel—which would shortly evolve into Division 6 of the NDRC—would be responsible for handling all the scientific research related to the submarine-detection problem. Its tasks included finding personnel to conduct research, issuing contracts for the research, and organizing the findings.
Senior researchers assigned to Division 6 had their first meeting with representatives of the various commands interested in developing weapons for antisubmarine warfare (ASW) in November 1941. It appears likely one of those present was Commander Louis W. McKeehan, U.S. Naval Reserve, the former director of the Yale University physics laboratories who was then assigned to the Bureau of Ordnance (BuOrd). In late November, he issued a memorandum addressed to the NDRC asking if it was “feasible to devise acoustic equipment for homing control of a self-propelled torpedo-like body.”
McKeehan knew the severe limitations of the depth charge, the principle ASW weapon in the Navy’s arsenal, which a diving submarine easily could evade by changing depth and direction. Once submerged, the submarine was difficult, if not impossible, to locate precisely enough to effectively employ depth charges. A homing torpedo that could seek out an enemy submarine moving in the ocean’s three-dimensional depths was the obvious solution.
McKeehan solicited the help of Frederick V. Hunt, the director of Harvard University’s Cruft Laboratory, which would later become known as the Harvard Underwater Sound Laboratory (HUSL). Hunt in turn contacted Alfred B. Focke, a civilian scientist in BuOrd, who agreed to supply Hunt’s group with the external torpedo casing together with the propulsion and rudder. Working with technical representatives from the Navy, Hunt’s team formulated the initial specifications for an acoustically controlled, air-launched homing torpedo initially designated the Mark 24 “mine.” According to Harvey Brooks, a physicist who worked on the project, it was called a mine because McKeehan believed this was the only way to get the project moving. Reclassification as a mine removed it from the purview of the torpedo specialists, who had too many other projects and problems to worry about. Other sources claim it was so labeled for security reasons.
The initial specifications for the Mark 24 called for an antisubmarine weapon that could be dropped by an aircraft from 200 to 300 feet at a speed of about 120 knots. Its size and weight would allow it to fit into a 1,000-pound bomb rack. Speed would be around 12 knots, which would afford the torpedo a 4-knot margin over the maximum speed of a submerged submarine.
When the Committee on Sub-Surface Warfare met on 13 January 1942 to review the proposal for what was now named Project Fido, four separate entities were involved with the design of the new torpedo: HUSL, working on the acoustic problem; Bell Laboratories, on guidance and control; General Electric, on the electric propulsion motor; and the David Taylor Model Basin, which was responsible for the shell design. By then all parties agreed that the project to develop a homing torpedo was of the utmost importance.
By October 1942, the Fido engineering team had finalized and tested the Mark 24 design, and a production contract for 5,200 “mines” was issued by BuOrd to the Western Electric Company. The entire design process from initial concept to production lasted just ten months.
The Mark 24 torpedo that emerged from the process was 19 inches in diameter, 7 feet long, and weighed 680 pounds, with a 92-pound HBX warhead.1 A 5½-horsepower General Electric motor propelled the weapon, using an Exide lead acid storage battery for power. Target detection was accomplished by four hydrophones symmetrically arranged around the circumference of the forward end of the torpedo casing. Once in the water, the Fido could detect the sound of a submarine’s propellers when within about 1,500 yards of a boat. If no propeller sounds were detected upon entering the water, the Mark 24 was programmed to begin a circular search, which it could maintain for 10 to 15 minutes.
The first production units began to enter service in early May 1943. The initial submarine kill was achieved by a Royal Air Force Liberator of No. 86 Squadron on 12 May when the Fido it launched struck U-436, causing her to sink. The U.S. Navy scored its first kill two days later when a PBY Catalina from Patrol Squadron 24 sank U-640.
Over the next two years, U.S. and Allied aircraft launched 204 Mark 24 torpedoes in attacks against enemy submarines, sinking 37 and damaging 18. Fido sinkings were responsible for approximately 15 percent of all German submarines sunk by aircraft during World War II; not bad considering the weapon was only deployed in the last two years of the war.
The Fido’s success fueled the U.S. Navy’s interest in adapting the Mark 24’s control system for a submarine-launched torpedo. This idea was first broached at a 23 November 1943 conference; the NDRC was asked “to give the most urgent consideration to means for modifying the Mark 24 to permit its use by submarines against small surface craft.” It was initially thought this torpedo could be ejected from a submarine’s torpedo tube using compressed air, but the Navy quickly decided it would be preferable from a tactical standpoint if the torpedo “swam” out of the tube under its own power. In less than a month, Bell Labs had converted the Mark 24 into a prototype, which was successfully launched under its own power from a torpedo tube on a test barge at Solomons, Maryland.
The new weapon, code-named “Cutie,” was designated the Mark 27 torpedo. The Mark 27 Mod 0 was a modified version of the Fido, having a slightly longer body equipped with a set of wooden rails that enabled it to fit a standard 21-inch torpedo tube. It had a floor switch to keep it from attacking the launching submarine and various arming, warm-up, and starting controls needed for its swim-out launch mode. At 12 knots, the Mark 27 had a maximum range of 5,000 yards. Eleven hundred Cuties were built by Western Electric and delivered between June 1944 and April 1945.
1. HBX is a high explosive made from TNT, RDX, aluminum, lecitin, and wax. It was developed to replace TORPEX, which was shock sensitive.