Fighting the U-boats

Weapons and Technologies

Torpedo MK 24 - "FIDO"

The First American ASW Acoustic Homing Torpedo

Historical 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 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 W.W. 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. However, such a torpedo, if designed as an air launched anti-submarine weapon, would need to be capable of tracking the target in depth as well as azimuth, fit within size and weight limitations imposed by the launch aircraft, and be able to withstand the shock loads incurred from an air-drop water entry.

FIDO Studies & Requirements

In the fall of 1941, the US Navy asked the National Defense Research Committee (NDRC) to consider the feasibility of a small, anti-submarine, acoustic homing torpedo which could be air-launched. In response to the Navy's request, a meeting was convened at Harvard University's Underwater Sound Lab (HUSL) on 10 December 1941, attended by members of the scientific and industrial community, to discuss the possibilities. At a second meeting a short time later at the David Taylor Model Basin (DTMB), the following preliminary requirements were promulgated:

Within a few days, both HUSL and Bell Telephone Labs submitted formal proposals for development to the Office of Scientific Research and Development (OSRD) which were accepted and eventually became OSRD Project 61 (FIDO).

MK24 Development

Late in Dec. 1941, a directive was issued to both HUSL and Bell Laboratories to proceed with development. The initial development assignments were arranged as follows:

One of the first of many key problems to be solved during the torpedo's development was the question of whether the sound baffling effect of the hull could be used to provide the degree of hydrophone array directivity required for steering; i.e., would the hull shield, for example, the left hand hydrophone from signals arriving from the right side, and vice versa, such that a sufficient signal amplitude difference between the hydrophone outputs would be achieved? This question was investigated within the first few weeks of the program during a series of in-water tank tests with a hydrohone element mounted in a model hull section. Directionality measurements of the hull baffling effect and signal-to- noise ratio calculations were made in the frequency range from approximately 15 kHz to 25 kHz. The results of these investigations indicated the basic array concept was feasible.

The hydrophone array which evolved consisted of four hydrophones symmetrically located flush in the hull at mid-body, spaced around the hull circumferentialy at the 0, 90, 180, and 270 degree positions (where 0 represents the topmost hull position at the centerline.) The signals from the hydrophone pair located at the 90 and 270 degree positions provided steering information in the horizontal plane ("left", "right" steering). The hydrophone signals were compared and processed to provide steering commands to the rudders which would turn the torpedo in the direction of the hydrophone receiving the strongest signal. When both hydrophone signals were of equal amplitude the torpedo would steer straight ahead. In this fashion, one hydrophone pair provided the signals necessary for "turn left", "go straight ahead", or "turn right" steering.

In a similar fashion, the hyrophone signals from the pair at 0 and 180 degrees were processed to provide the steering commands to the elevators for "down", "straight ahead", or "up", steering in the vertical plane.

The combined net effect of both the rudder and elevator commands was to steer the torpedo in the direction from which the target's noise signals were arriving. The 12 knot speed for 15 minutes was estimated to provide sufficient speed advantage over a submerged submarine and the endurance necessary to enable the torpedo to steer itself into an eventual collision with the target, causing the warhead to detonate on contact.

The homing system pursued by HUSL employed magnetostrictive hydrophone elements and provided for "non-proportional" steering; i.e., the rudders, in response to the amplitude of the acoustic signals being received, were positioned either in the straight ahead position or hard over in the full "turn left" or "turn right" position. The BTL system used piezoelectric hydrophone elements and was based on "proportional steering"; i.e., the rudder deflections were proportional to the difference in amplitude of the paired hydrophone signals. However, in both systems, irrespective of the signal processing and steering technique employed, the torpedo was turned in the direction of the hydrophone producing the strongest signal. By the end of July 1942, following extensive in-water testing and evaluation runs, both the HUSL and BTL guidance systems had demonstrated sound control in both vertical and horizontal planes.

DTMB was given the task of producing a protoype hull in Jan 1942. An existing Mk13 torpedo body 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 4 stabilizing fins and rudders and a single propeller. An initial propeller design was produced and several modifications and variations tested and evaluated during the development program.

Upon water entry, FIDO was designed to enter a circle search pattern at a predetermined depth controlled with a conventional bellows/pendulum system. Circle search was held until the target's acoustic signal exceeded a predetermined threshold level when control was then shifted to the acoustic homing system. The first FIDOs were set to search at 50 ft. depth which after more experience was gained in operational use was later changed to 150 ft.

Mk24 Production

Experimental torpedo designs were developed and tested by both HUSL and BTL with the test results mutually shared between the two organizations. Influenced largely by the successful steering and guidance system developments achieved to that time, the Navy in June1942 decided to proceed to production. However, at that time the air drop testing program had not been completed and there was still much significant testing and design work to be accomplished. In October of 1942, convinced a successful torpedo could be built, the Navy "froze" the design for production. The BTL version, which incorporated many HUSL concepts and developments, was selected for production which proceeded immediately. However, air-drop and in-water testing of pre-production prototypes continued on into December 1942. The first production models were delivered to the Navy in March 1943.

The first production model MK24 FIDO had the following features:

Size: 19" dia. x 84" long
Weight: 680 lbs.
Propulsion: Single propeller driven by a 5 HP electric motor-
48 volt lead acid battery
Speed and Endurance: 12 kts for approx. 15 min.
Warhead: 92 lbs HE
Homing System: 4 piezoelectric hydrophones operating at 24 kHz providing signals to a "vacuum tube" signal processing system with proportional steering. The hydrophone positions were moved slightly forward of mid-body about 8 inches behind the joint ring between the hemispherical nose section and cylindrical body section.
Max. Drop altitude: 200 -300 ft
Aircraft Launch Speed: 120 kts.
Search Pattern: Initial circle search approximately 150 ft. dia. at 50 ft depth; shift to acoustic homing upon target detection. Initial search depth was later changed to 150 ft.

Production of MK24 was placed with Western Electric, the manufacturing facility for the Bell Telephone system. With a couple of exceptions, the production team was made up primarily of those contractors who had participated in the torpedo's development. Battery production, after development by Western Electric, was given to Electric Storage Battery Co. and hull production was turned over to a manufacturer of bathtubs whose name at this time is unknown.

MK24 FIDO Production Team

Major Contractors Type of Contract Item Produced
Western Electric Prime Contractor Torpedo MK24 Production
General Electric Subcontractor Propulsion and steering motors
Electric Storage Battery Co. Subcontractor Batteries
(Unknown) Subcontractor Hulls

The initial production order was for 10,000 torpedoes but FIDO proved so effective in combat the order was reduced to a little over 4,000 units. The final production lot cost was $1,800 ea.

The Mk24 development/production timeline indicates the remarkable effort and speed with which this weapon was designed, produced, and introduced into the fleet:

Dec. 1941 First planning meeting convened
Dec. 1941 Harvard Underwater Sound Lab proposal submitted
Jan. 1942 Bell Telephone Labs proposal submitted.
Jun. 1942 Navy places 1st production order.
Oct. 1942 Mk24 design freeze.
Mar. 1943 First production units delivered to Navy.
May 1943 First 500 units produced.
May 1943 First U-boat sunk by a "FIDO".

The total elapsed time from the first planning meeting to the sinking of a submarine by an operational production torpedo - 17 mo.

The MK24 FIDO Record in Combat

The first confirmed FIDO sinking is believed, at this time, to have occurred 14 May 1943 when a PBY Catalina flying boat from US Navy VP-84 attacked and sank U-640 with a MK24 torpedo [more probable is that the first victim was the U-657 on May 17 - Editor]. Most US Navy composite squadrons flying from the ASW escort carriers operating in the Atlantic from mid 1943 on were equipped with FIDO as were the land based patrol squadrons. The torpedo was also supplied to the British and Canadian forces.

US Navy OEG Study No. 289, 12 August 1946 provides the following estimates of MK24 usage and results achieved:

Number of attacks in which Mk24s were launched 264
Total Number of Mk24 torpedoes launched - all targets 340
Number of MK24s launched against submarines 204
Number of Mk24 attacks on submarines by US aircraft 142
Number of submarines sunk by FIDO 31
Number of submarines damaged by FIDO 15
Number of MK24 attacks on subs by Allies (primarily British) 62
Number of submarines sunk by FIDO 6
Number of submarines damaged by FIDO 3
Total number of submarines sunk by FIDO (German & Japanese) 37*
Total number of submarines damaged 18
*Note: Includes five Japanese submarines sunk;
1 in the Atlantic
4 in the Pacific

A later submarine launched version of FIDO (MK27 Cutie) was developed for use against surface vessels and saw service use in the Pacific war beginning in the summer of 1944.

Torpedo MK24 FIDO continued in service with the US Navy until 1948.

Books dealing with this subject include

Torpyedy VMF SSSR. Korshunov, Yu.L., and Strokov, A.A., 1994.

Weapons and Technologies