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The Mark 24 mine (also known as FIDO or Fido) is an air-dropped anti-
submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...
warfare weapon (ASW) incorporating passive acoustic homing system and
torpedo A modern torpedo is an underwater ranged weapon launched above or below the water surface, self-propelled towards a target, and with an explosive warhead designed to detonate either on contact with or in proximity to the target. Historically, ...
integration. It was used by the United States, the British and Canadian forces during the
Second World War World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposi ...
and entered service in March 1943 and remained in use with the US Navy until 1948. Approximately 4,000 torpedoes were produced, with 340 ultimately being deployed during the war. Two-hundred and four torpedoes were launched against submarine targets, with 37
Axis An axis (plural ''axes'') is an imaginary line around which an object rotates or is symmetrical. Axis may also refer to: Mathematics * Axis of rotation: see rotation around a fixed axis * Axis (mathematics), a designator for a Cartesian-coordinat ...
submarines being sunk and a further 18 damaged. The deceptive name of "Mark 24 mine" was deliberately chosen for security purposes, to conceal the true nature of the weapon.


Background

The concept of a torpedo which would "home" on its target had been studied by torpedo designers as far back as the First World War. While the concept was interesting, implementation had to await a better understanding of the physics of sound generation and transmission in the sea and the development of the technology from which such a torpedo could be designed and constructed. During World War II, German submarines were equipped with electrically driven acoustic homing torpedoes for which development had started as far back as 1933. The Falke T-4 and Zaunkönig T-5 torpedoes entering service in 1943 were designed to attack surface ships and ran at a preset depth. A similar torpedo (MK28) entered US submarine service in 1944. While effective against surface ships, the MK28 was of limited use against submarines, due to its inability to track and adjust to changes in both depth and azimuth. The design of Fido enabled it to meet the size, weight, and aerial launch specifications associated with air-drop water entry, in addition to addressing the shortcomings of earlier torpedo tracking and control systems.


Development

The US Navy began studies into an air-dropped anti-submarine torpedo in late 1941. Based on a formal set of requirements, Harvard Underwater Sound Lab (HUSL) and Bell Telephone Labs began development in December 1941. These projects later became the Office of Scientific Research and Development project 61 (FIDO). Both Bell Labs and HUSL proceeded with parallel development of torpedoes, with a complete exchange of information between them.
Western Electric The Western Electric Company was an American electrical engineering and manufacturing company officially founded in 1869. A wholly owned subsidiary of American Telephone & Telegraph for most of its lifespan, it served as the primary equipment ma ...
was to develop a lightweight, shock resistant, 48 volt lead-acid battery capable of providing 110 amps for 15 minutes.
General Electric General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable ene ...
was to design and fabricate propulsion and steering motors and to investigate an active acoustic homing system. David Taylor Model Basin was to assist with hydrodynamics and propulsion. The guidance system consisted of four
hydrophones A hydrophone ( grc, ὕδωρ + φωνή, , water + sound) is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potenti ...
placed around the midsection of the torpedo, connected to a
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as ...
-based sound processing array. A Bell Labs proportional navigation and HUSL non-proportional steering system had been demonstrated by July 1942. An existing
Mark 13 torpedo The Mark 13 torpedo was the U.S. Navy's most common aerial torpedo of World War II. It was the first American torpedo to be originally designed for launching from aircraft only. They were also used on PT boats. Design Originating in a 1925 des ...
provided the body of the torpedo, it was modified by shortening the hull, reducing the diameter, reducing the weight, and designing a hemispherical nose section to carry the explosive charge, and a conical tail section with four stabilizing fins and rudders and a single propeller. The effect of these modifications was to produce a relatively short, "fat" torpedo. In June 1942, the US Navy decided to take the torpedo into production, even though there was still major testing work remaining on the project, including air-drop testing. The Bell Labs version of the guidance system was selected for production, with proportional homing. Testing of the pre-production prototypes continued on into December 1942, and the US Navy received the first production models in March 1943. Initially 10,000 torpedoes were ordered, but the FIDO proved so effective that the order was reduced to 4,000. The torpedoes ended up costing $1,800 each.


Description

Upon water entry, the FIDO performed a circular search at a predetermined depth controlled by a bellows and pendulum system. This continued until the potential target's 24 kHz acoustic signal detected by the hydrophones exceeded a predetermined threshold level, at which point control was then shifted to the passive acoustic proportional homing system. Initially the torpedoes were set to search for targets at depths of 50 feet (15 m), this was later changed to 150 feet (45 m). To prevent the torpedo from accidentally attacking surface ships, it resumed its circling search if it rose above a depth of 40 feet (12 m). The torpedo's relatively low speed was kept secret because, although
U-boats U-boats were naval submarines operated by Germany, particularly in the First and Second World Wars. Although at times they were efficient fleet weapons against enemy naval warships, they were most effectively used in an economic warfare rol ...
could not outrun the torpedo when submerged, they could outrun it on the surface.


Combat history

On 14 May 1943 a Catalina of the US Navy attacked and destroyed a U-boat; this was either or . On 13 May an RAF Coastal Command Liberator B/86 had attacked a U-boat with a FIDO, but this vessel, , was only damaged, sinking the following day from damage received. One of these vessels was the first U-boat sinking achieved using a FIDO. During its lifetime, the torpedo sank a total of 37 submarines, achieving an effectiveness of about 18%, compared with 9.5% for aircraft-launched depth charges. ''US Navy OEG Study No. 289, 12 August 1946 provides the following data related to Mark 24 effectiveness'':


General characteristics

*Diameter: 19 inches (48 cm). *Length: 84 inches (2.13 m). *Weight: 680 lb (308 kg). * Warhead: 92 lb (41.7 kg)
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high explosive An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An ...
. * Propulsion: 5 hp (3.7 kW) electric motor driving a single propeller, powered by a 48 volt lead acid battery. * Speed and endurance: for 10 minutes, giving a range of about 4,000 yards (3,700 m) * Homing system: 4
piezoelectric Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress. The word '' ...
hydrophones operating at 24 kHz and vacuum tube signal processing system with proportional steering. * Maximum drop altitude: 200 to 300 ft (60 m to 90 m) * Maximum aircraft launch speed: 120 knots (220 km/h).


Variants

* The ''
Mark 27 torpedo The Mark 27 torpedo was the first of the United States Navy 19-inch (48-cm) submarine-launched torpedoes.Kurak, September 1966, p.145 This electrically-propelled torpedo was 125 inches (3.175 m) long and weighed 1174 pounds (534 kg). The tor ...
'' (''Cutie'') was developed for submarine use against surface vessels. It saw service in the Pacific war from the summer of 1944. Lieutenant Commander Carter L. Bennett's ''Sea Owl'' achieved the Mark 27's first combat success, damaging a Japanese patrol vessel in the Yellow Sea in November.Blair, Clay, Jr. ''Silent Victory'' (Bantam, 1976), p.788.


Notes


Sources

* http://uboat.net/allies/technical/fido.htm * https://web.archive.org/web/20060224092332/http://www.navytorpedo.com/html/legacy/USNT4.htm * Blair, Clay, Jr. ''Hitler's U-Boat War: The Hunters, 1939-1942''. * Blair, Clay, Jr. ''Silent Victory''. Bantam, 1976. * Paul Kemp : ''U-Boats Destroyed'' ( 1997) * Axel Niestle : ''German U-Boat Losses during World War II'' (1998) {{Use dmy dates, date=June 2017 Torpedoes Torpedoes of the United States World War II naval weapons Aerial torpedoes World War II weapons of the United States Weapons and ammunition introduced in 1942