Mark 24 mine

The Mark 24 mine (also known as FIDO or Fido) is an air-dropped anti-submarine weapon (ASW) incorporating passive acoustic homing system and torpedo integration. It was used by the United States, the British and Canadian forces during the Second World War 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 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. 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, the 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. However, it still managed to sink some Japanese merchant ships. 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. 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.