Mark 24 mine
The Mark 24 mine was not a mine at all. It was an acoustic torpedo, one of the most effective anti-submarine weapons of World War II, and its misleading name was entirely deliberate. The United States Navy called it a mine to conceal what it could actually do: hunt a submerged submarine by the sound of its own propellers.
Approximately 4,000 were built. Of the 340 launched in combat, 204 were fired specifically at submarines. They sank 37 and damaged 18. That record would raise an obvious question: how did engineers in the early 1940s build a weapon smart enough to chase a target through three dimensions of dark ocean water, dropped from a moving aircraft, with no guidance from the crew once it hit the surface?
The answers reach back to a secret project, a web of university labs and corporate partners, and a crash development program that went from concept to combat in roughly a year. The torpedo's nickname was FIDO, and its story runs from a December 1941 engineering brief to a May 1943 kill.
Acoustic torpedoes were not a new idea. The concept existed by World War I, but the science of underwater sound was not yet good enough to make them work. Insufficient understanding of underwater acoustics kept the idea on the shelf for decades.
Germany did not wait. By 1933, German engineers were developing electrically driven acoustic torpedoes. The resulting G7e and G7es designs were anti-ship weapons that ran at a preset depth. They entered submarine service in 1943. The United States received its own comparable anti-ship acoustic torpedo, the Mark 28, in 1944.
But a weapon aimed at submarines faces a harder problem than one aimed at surface ships. Anti-submarine torpedoes must detect and respond to far greater changes in depth and azimuth. A submarine can dive, climb, and turn in three dimensions while a surface ship is essentially pinned to one plane. FIDO had to solve that problem while also being small enough and rugged enough to survive being dropped from an aircraft into the ocean.
USN studies for FIDO began in late 1941. By December of that year, requirements had been issued under the designation Project 61 to four separate teams: Bell Telephone Laboratories, the Harvard Underwater Sound Lab, General Electric, and Columbia University's Special Studies Group.
Bell and Harvard worked in parallel with complete information sharing between them. Western Electric took on the battery problem, developing a lightweight, shock-resistant 48-volt lead-acid unit capable of delivering 110 amps for 15 minutes. General Electric was responsible for the propulsion and steering motors, as well as investigating active acoustic homing. David Taylor Model Basin handled hydrodynamics and propulsion research.
The guidance system that emerged used four hydrophones mounted on the torpedo's midsection, feeding a vacuum tube signal processing array. Both a Bell proportional navigation system and a Harvard non-proportional steering system were demonstrated by July 1942. Bell's proportional approach was ultimately selected.
The torpedo body itself started as a modification of the Mark 13. Engineers reduced its length, diameter, and weight. The nose was hemispherical to hold the explosive charge. A conical tail section carried four stabilizing fins, rudders, and a single propeller. The result was, by the designers' own description, relatively short and fat.
In June 1942, the Navy decided to begin production even though air-drop testing had not yet been completed. Pre-production prototypes were still being tested as late as December 1942. The first production models reached the Navy in March 1943.
When FIDO hit the water, it did not immediately home in on anything. It began a circular search pattern at a predetermined depth, that depth controlled by a bellows and pendulum system. The torpedo waited for sound.
The passive acoustic system was tuned to detect a 24 kHz signal, the frequency produced by a submarine's propellers. Once the signal from the hydrophones exceeded a preset threshold, the proportional homing system took control and steered toward the source. The four piezoelectric hydrophones worked together to triangulate direction.
Initially the search depth was set at 50 feet, or about 15 meters. That was later changed to 150 feet, roughly 45 meters, to better match where submerged submarines actually ran. A safety cutoff prevented attacks above 40 feet, keeping the weapon from turning on surface ships.
FIDO's speed was one of its secrets. At 12 knots with an endurance of 10 minutes, it had a range of about 4,000 yards. That pace was slow enough that a surfaced U-boat could outrun it. A submerged submarine, however, could not. The Navy kept that limitation classified.
The propulsion system was deliberately ordinary. The motor driving the single propeller was a 7.5 horsepower General Electric washing machine motor, an off-the-shelf commercial component adapted for combat use. The warhead carried 92 pounds of HBX high explosive. The entire weapon weighed 680 pounds and measured 84 inches in length with a 19-inch diameter.
Two engagements in May 1943 compete for the distinction of FIDO's first kill. On the 13th of May, an RAF Coastal Command Liberator designated B/86 damaged a submarine; it sank the following day. On the 14th of May, a USN Catalina sank another. The exact order remains contested.
By the war's end, FIDO had compiled a statistical record detailed in US Navy Office of Evaluation Group Study No. 289, dated the 12th of August 1946. The weapon was launched in 264 separate attacks. Of the 204 torpedoes fired at submarines, 31 sank German U-boats and 15 damaged them. Against Japanese submarines, FIDO sank 6 and damaged 3. The combined total was 37 submarines sunk and 18 damaged.
The effectiveness rate worked out to approximately 18 percent, compared with 9.5 percent for aircraft-launched depth charges. FIDO was roughly twice as likely to produce a kill as the weapon it supplemented. Those numbers were significant enough that the initial production order of 10,000 units was eventually cut to 4,000. The weapon performed so well that fewer were needed.
Each torpedo cost $1,800 to produce. The Navy used FIDO until 1948, three years after the war ended.
FIDO's design principles did not stop with the weapon itself. A variant called the Mark 27, nicknamed Cutie, was developed for a different mission: submarine-launched attacks on surface vessels. Where FIDO was dropped from aircraft hunting submarines below, Cutie was fired from below hunting ships above.
The Mark 27 entered service in the Pacific theater in the summer of 1944. Its first confirmed combat success came in November of that year. Lieutenant Commander Carter L. Bennett, commanding the submarine Sea Owl, damaged a Japanese patrol vessel in the Yellow Sea.
Common questions
What was the Mark 24 mine and why was it called a mine?
The Mark 24 mine was actually an air-dropped acoustic homing torpedo, not a mine. The United States Navy called it a mine deliberately to conceal its true capabilities as an anti-submarine weapon.
How effective was the Mark 24 FIDO torpedo in World War II?
FIDO sank 37 Axis submarines and damaged 18 during the war, achieving an effectiveness rate of approximately 18 percent. This compared favorably to the 9.5 percent effectiveness rate of aircraft-launched depth charges.
How did the Mark 24 FIDO torpedo find its targets?
FIDO used four piezoelectric hydrophones operating at 24 kHz to passively detect the sound of a submarine's propellers. Once the signal exceeded a preset threshold, a proportional homing system steered the torpedo toward the source.
Who developed the Mark 24 FIDO torpedo?
Development was shared among four teams: Bell Telephone Laboratories, the Harvard Underwater Sound Lab, General Electric, and Columbia University's Special Studies Group. The project was designated Project 61 under the Office of Scientific Research and Development.
When did the Mark 24 torpedo enter service and how many were produced?
The Mark 24 entered Allied service in March 1943. Approximately 4,000 were produced, reduced from an initial order of 10,000 because the weapon's performance exceeded expectations.
What was the Mark 27 Cutie torpedo and how did it differ from FIDO?
The Mark 27, nicknamed Cutie, was a variant developed for submarine use against surface vessels, inverting FIDO's role. It entered service in the Pacific in the summer of 1944, and its first combat success came when the submarine Sea Owl damaged a Japanese patrol vessel in the Yellow Sea in November 1944.
All sources
3 references cited across the entry
- 2A Brief History of US Navy Torpedo DevelopmentE.W. Jolie — 15 September 1978
- 3ReportSummary Technical Report of the National Defense Research Committee — A Survey of Subsurface Warfare in World War IIEric Walker — Office of Scientific Research and Development — 1946