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Mark 24 Mine

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Mark 24 Acoustic Torpedo

The Mark 24 acoustic torpedo, a device known to the initiated as FIDO or Fido, was a rather clever bit of aerial ordnance developed by the United States during the tumultuous years of World War II. Its designation as a "mine" was a deliberate piece of misdirection, a rather transparent attempt to shield its true anti-submarine capabilities from prying eyes. This torpedo, a product of collaboration between the United States, Canada, and the United Kingdom, entered Allied service in March 1943. The United States Navy continued to employ it until 1948, a testament to its enduring, if somewhat dated, utility. Of the approximately 4,000 produced, a mere 340 were deployed in combat scenarios. The results, however, were hardly negligible: 204 fired, sinking a respectable 37 Axis submarines and leaving another 18 sufficiently damaged to be considered out of the fight.

Background

The theoretical underpinnings of the acoustic torpedo had been kicking around since the era of World War I, but the technology was, frankly, insufficient. The understanding of underwater acoustics was rudimentary at best, rendering such concepts largely impractical. By 1933, Germany was already dabbling in electrically powered acoustic torpedoes, eventually leading to the development of the G7e and G7es anti-ship torpedoes, which were deployed on submarines by 1943. These torpedoes operated at a preset depth, a rather blunt instrument by later standards. The USN itself would eventually field a similar anti-ship torpedo, the Mark 28, in 1944.

However, the demands placed upon an anti-submarine weapon were considerably more stringent. ASW torpedoes needed to be acutely sensitive to greater variations in both depth and azimuth than their anti-ship counterparts. FIDO, burdened with the additional constraint of being air-dropped, represented a significant engineering challenge. It wasn't simply a matter of making a torpedo; it was about making a torpedo that could survive the indignity of being hurled from the sky and still possess the intelligence to hunt beneath the waves.

Development

The genesis of the Mark 24 mine can be traced back to late 1941, when the USN initiated studies for what would become the FIDO. The task of conceptualizing and developing this elusive weapon fell to the capable hands at Bell Telephone Labs and the Harvard University Underwater Sound Lab (HUSL). Development commenced in December of that year, under the auspices of the Office of Scientific Research and Development's Project 61, a rather understated codename for a project of such potential impact: "FIDO."

Bell and HUSL, rather refreshingly, operated in a state of complete information sharing, a rarity in the cutthroat world of wartime research. Western Electric was tasked with the creation of a lightweight, shock-resistant, 48-volt lead-acid battery. This was no trivial undertaking; it needed to deliver a substantial 110 amps for a respectable 15 minutes. Simultaneously, General Electric was enlisted to design and fabricate the propulsion and steering motors, and to explore the feasibility of an active acoustic homing system. Even the David Taylor Model Basin lent its expertise to the project, assisting with the crucial aspects of hydrodynamics and propulsion.

The heart of FIDO's intelligence lay in its guidance system. It employed four hydrophones strategically placed on the torpedo's midsection, feeding data into a sophisticated vacuum tube-based sound processing array. By July 1942, both Bell's proportional navigation and HUSL's non-proportional steering systems had been successfully demonstrated. The choice was made to proceed with Bell's proportional homing guidance system.

The physical form of the torpedo was a modified version of the Mark 13 torpedo, albeit significantly scaled down. Its length, diameter, and overall weight were reduced to meet the requirements of aerial deployment. The nose section was a simple hemispherical housing for the explosive charge, while the conical tail section housed four stabilizing fins, rudders, and a single propeller. In essence, FIDO was a relatively short and rather "fat" torpedo, not built for elegance, but for a singular purpose.

In a move that underscored the urgency of the war, the USN decided in June 1942 to initiate production, even though critical testing, including extensive air-drop trials, remained incomplete. Pre-production prototypes underwent rigorous testing well into December 1942, and the first production models were finally delivered to the USN in March 1943. The initial order for 10,000 FIDO torpedoes was subsequently revised downward to 4,000, a consequence of the weapon's unexpectedly superior performance. Each unit carried a price tag of $1,800, a considerable sum for the era.

Description

Upon its baptism by water, FIDO embarked on a programmed circular search pattern. This maneuver, executed at a predetermined depth, was governed by a rather ingenious bellows and pendulum system. The passive acoustic proportional homing system then sprang to life once the target's distinct 24 kHz acoustic signature, detected by the hydrophones, surpassed a predetermined threshold. Initially, these torpedoes were programmed to search for targets at depths of 50 feet (15 m). This was later adjusted to a more effective 150 feet (45 m). To prevent accidental engagements with surface vessels, the torpedo would revert to its circular search pattern if it ascended above depths of 40 feet (12 m).

FIDO's relatively modest speed was a closely guarded secret. While it could outmaneuver submerged U-boats, it was no match for a surfaced submarine. This was a calculated trade-off: prioritize agility and homing capability over raw speed, a sensible decision when dealing with the elusive prey of the underwater world.

Combat History

The distinction of the first U-boat sunk by FIDO is a matter of historical debate, with two engagements in May 1943 laying claim. On May 14th, a USN Catalina aircraft sank U-657 or, according to some accounts, U-640. A day earlier, on May 13th, an RAF Coastal Command Liberator B/86 managed to damage U-456; the submarine met its end the following day. In total, FIDO accounted for the destruction of 37 submarines, achieving an effectiveness rate of approximately 18%. This figure stands in stark contrast to the 9.5% effectiveness of aircraft-launched depth charges, highlighting FIDO's significant contribution to the anti-submarine war.

The U.S. Navy's Office of Evaluation and Games (OEG) Study No. 289, dated August 12, 1946, provides a more granular breakdown of the Mark 24's performance:

  • Number of attacks in which Mark 24s were launched: 264
  • Total number of Mark 24 torpedoes launched (all targets): 340
  • Number of Mark 24s launched against submarines: 204
  • Number of Mark 24 attacks on submarines by US aircraft: 142
  • Number of Mark 24 attacks by Allied (primarily British) aircraft: 62
  • Number of German U-boats sunk by FIDO: 31
  • Number of German U-boats damaged by FIDO: 15
  • Number of Japanese submarines sunk by FIDO: 6
  • Number of Japanese submarines damaged by FIDO: 3
  • Total number of submarines sunk by FIDO (German & Japanese): 37
  • Total number of submarines damaged: 18

General Characteristics

  • Diameter: 19 inches (48 cm). A rather portly dimension, but necessary for housing the components.
  • Length: 84 inches (2.13 m). Not exactly a torpedo built for speed or distance.
  • Weight: 680 lb (308 kg). Manageable for aerial deployment, but not insignificant.
  • Warhead: 92 lb (41.7 kg) HBX high explosive. Sufficiently potent to ensure a decisive outcome.
  • Propulsion: A standard 7.5 hp (5.57 kW) General Electric washing machine electric motor, repurposed for naval warfare, drove a single propeller. Power was supplied by a 48-volt lead-acid battery. Ingenuity born of necessity, I suppose.
  • Speed and endurance: 12 knots (22 km/h) for 10 minutes, yielding an operational range of approximately 4,000 yards (3,700 m). Adequate for its intended purpose, if not exactly thrilling.
  • Homing system: Four piezoelectric hydrophones operating at 24 kHz, coupled with a vacuum tube signal processing system featuring proportional steering. The brains of the operation, such as they were.
  • Maximum drop altitude: 200 to 300 ft (60 m to 90 m). Low enough to avoid undue attention, high enough for a reasonable trajectory.
  • Maximum aircraft launch speed: 120 knots (220 km/h). A necessary parameter for safe release.

Variants

The Mark 27 torpedo, affectionately nicknamed "Cutie," emerged as a derivative of the Mark 24, specifically designed for submarine deployment against surface vessels. It saw action in the Pacific theater from the summer of 1944. Lieutenant Commander Carter L. Bennett, aboard the Sea Owl, is credited with achieving the Mark 27's first combat success, damaging a Japanese patrol vessel in the Yellow Sea in November of that year. It's amusing to think of these underwater weapons being adapted for different roles, like repurposing a hunting dog for herding sheep. Different skills, same basic chassis.