Mark 24 mine

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Mark 24 mine
Mark24 mine.jpg
Mark 24 acoustic torpedo
Type Acoustic torpedo
Place of originUnited States, Canada, United Kingdom
Service history
In service1942–1948 [1]
Used by United States Navy
Royal Navy
Fleet Air Arm
Royal Canadian Navy
Wars World War II
Production history
Designer Western Electric Company
Bell Telephone Laboratories
Harvard University Underwater Sound Laboratory [1]
Designed1942 [1]
Manufacturer General Electric Company [1]
Western Electric Company
No. built4000 [2]
Mass680 pounds [1]
Length84 inches [1]
Diameter19 inches [1]

Effective firing range4000 yards
(10 minutes search duration) [1]
Warhead HBX [1]
Warhead weight92 pounds [1]
Mk 142 fuze, contact exploder [1]

Engine Electric, secondary battery [1]
Speed12 knots [1]
preset circle search, passive acoustic [1]

The Mark 24 mine (also known as FIDO or Fido) is an air-dropped passive acoustic homing anti-submarine torpedo used by the United States during the Second World War. It entered service in March 1943 and remained in use with the US Navy until 1948. Approximately 4,000 torpedoes were produced, with 204 ultimately being deployed during the war. As a result of its use, 37 Axis submarines were sunk and a further 18 were damaged. The torpedo was also supplied to the British and Canadian forces. The deceptive name of "Mark 24 Mine" was deliberately chosen for security purposes, to conceal the true nature of the weapon.

Submarine Watercraft capable of independent operation underwater

A submarine is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. It is also sometimes used historically or colloquially to refer to remotely operated vehicles and robots, as well as medium-sized or smaller vessels, such as the midget submarine and the wet sub.

Torpedo self-propelled underwater weapon

A modern torpedo is a self-propelled weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with its target or in proximity to it.

Axis powers Alliance of countries defeated in World War II

The Axis powers, also known as "Rome–Berlin–Tokyo Axis", were the nations that fought in World War II against the Allies. The Axis powers agreed on their opposition to the Allies, but did not completely coordinate their activity.



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 highly 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 which had started development as far back as 1933. The Falke T-4 and Zaunkoenig 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.

G7e torpedo torpedo

The G7e or more appropriately the G7e/T2, G7e/T3, and G7e/T4 Falke torpedoes were, with the exception of the T4 model, the standard torpedoes for Germany during World War II. All of the G7e models shared standardized dimensions for all German torpedoes designed for use by U-boats during World War II, they measured 53.3 cm (21 inches) in diameter, 7.16 m in length, and carried a Schießwolle 36 warhead of 280 kg. All were powered by 100 hp (75 kW) electric motors and lead-acid batteries which needed constant maintenance to maintain their reliability. Additionally, the batteries of these torpedoes needed to be preheated to a temperature of 30 °C (85 °F) to operate with maximum speed and range, though generally this was a non-issue as U-boats had the element of surprise and often had the advantage of firing the first shot.

The G7es (T5) "Zaunkönig" ("wren") was an acoustic torpedo employed by German U-boats during World War II. It was called the GNAT by the British.

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.


Mark 24 mine diagram Mark 24 mine diagram.jpg
Mark 24 mine diagram

The US Navy began studies into an air-dropped anti-submarine torpedo in the autumn of 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 Office of Scientific Research and Development project 61 (FIDO).

Bell Labs research and scientific development company

Nokia Bell Labs is an industrial research and scientific development company owned by Finnish company Nokia. Its headquarters are located in Murray Hill, New Jersey. Other laboratories are located around the world. Bell Labs has its origins in the complex past of the Bell System.

Both Bell Labs and HUSL proceeded with parallel development of torpedoes, with a complete exchange of information between them. Western Electric were to develop a lightweight, shock resistant, 48 volt lead-acid battery capable of providing 110 amps for 15 minutes. General Electric were 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.

Western Electric Company was an American electrical engineering and manufacturing company that served as the primary supplier to AT&T from 1881 to 1996, and to the local Bell Operating Companies until 1984. The company was responsible for many technological innovations and seminal developments in industrial management. It also served as the purchasing agent for the member companies of the Bell System.

General Electric American industrial company

General Electric Company (GE) is an American multinational conglomerate incorporated in New York City and headquartered in Boston. As of 2018, the company operates through the following segments: aviation, healthcare, power, renewable energy, digital industry, additive manufacturing, venture capital and finance, lighting, and oil and gas.

David Taylor Model Basin

The David Taylor Model Basin (DTMB) is one of the largest ship model basins—test facilities for the development of ship design—in the world. DTMB is a field activity of the Carderock Division of the Naval Surface Warfare Center.

The guidance system consisted of four hydrophones placed around the midsection of the torpedo, connected to a vacuum tube-based sound processing array. A Bell Labs proportional navigation and HUSL non-proportional steering system had been demonstrated by July 1942.

Vacuum tube Device that controls electric current between electrodes in an evacuated container

In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

Proportional navigation

Proportional navigation is a guidance law used in some form or another by most homing air target missiles. It is based on the fact that two vehicles are on a collision course when their direct Line-of-Sight does not change direction as the range closes. PN dictates that the missile velocity vector should rotate at a rate proportional to the rotation rate of the line of sight, and in the same direction.

An existing Mark 13 torpedo 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.

Mark 13 torpedo air-launched 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.

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 FIDO proved so effective that the order was reduced to 4,000. The torpedoes ended up costing $1,800 each.


Upon water entry, 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 a target at a depth of 50 feet (15 m), this was later changed to 150 feet (45 m). To prevent the torpedo 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 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 U-657 [3] or U-640. [4] On 13 May an RAF Coastal Command Liberator B/86 had attacked a U-boat with a FIDO, but this vessel, U-456, [5] was only damaged, sinking the following day from damage received. One of these vessels was the first U-boat sinking achieved using FIDO. During its career, the torpedo sank a total of 37 submarines, achieving an effectiveness of about 22%, compared with about 9% for depth charges.

from US Navy OEG Study No. 289, 12 August 1946 provides the following data related to Mark 24 effectiveness:

Number of attacks in which Mark 24s were launched264
Total number of Mark 24 torpedoes launched - all targets340
Number of Mark 24s launched against submarines204
Number of Mark 24 attacks on submarines by US aircraft142
Number of Mark 24 attacks by Allied (primarily British) aircraft62
Number of German U-boats sunk by FIDO31
Number of German U-boats damaged by FIDO15
Number of Japanese submarines sunk by FIDO6
Number of Japanese submarines damaged by FIDO3
Total number of submarines sunk by FIDO (German & Japanese)37
Total number of submarines damaged18

General characteristics



  1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 "Torpedo History: Mark 24 mine" . Retrieved 11 June 2013.
  2. Jolie, E.W. (15 September 1978). "A Brief History of US Navy Torpedo Development" . Retrieved 19 June 2013.
  3. Kemp p117
  4. Neistle p78
  5. Kemp p116
  6. Blair, Clay, Jr. Silent Victory (Bantam, 1976), p.788.


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