FIDO- The First US Homing Torpedo

By Tom Pelick

Early in 1942, the Mine Warfare Branch of the Bureau of Ordinance asked the engineers at the government sponsored research Harvard Underwater Sound Lab, HUSL, to develop a homing torpedo. The homing torpedo became the Mk 24 torpedo, but was known as FIDO to confuse German Intelligence and also to maintain the work at HUSL. The first firing and successful test occurred off the New England Coast against a simulated target on December 7, 1942 on the anniversary of the Pearl Harbor attack.

The FIDO was an air-dropped torpedo with a passive homing capability using four magnetostrictive transducers and a vacuum tube homing panel for steering in the vertical and azimuth plane. The torpedo was 84 inches long and 19 inches in diameter, weighed 680#, with an electric motor with lead acid batteries capable of producing 12 knots, and carried a warhead of 92# of HBX-1. The torpedo would be dropped near a submerged submarine, go to a pre-determined depth and circle while looking for radiated noise from the target. Its mission was to home on the source of the radiated noise, primarily the propellers, and disable the submarine and cause it to surface where the aircraft and surface ships could then attack the surfaced submarine. FIDO was designed as a mission kill torpedo, not a direct kill. That is, the torpedo disabled the submarine and the aircraft bombing finished it off.

According to the development engineers, about 50 units were hand built at HUSL. Some of these were reportedly taken by the Navy for immediate use against enemy submarines. Bell Labs and Western Electric were the prime producers of the Mk 24, FIDO and worked closely with the HUSL engineers. The basic difference between HUSL and Bell Labs version was non-proportional steering and proportional steering and location of the hydrophones.

HUSL technicians tested the first 500 units produced at Key West. The USS TORSK was one of the targets at Key West in 1943 for testing FIDO. Admiral, Ret. Corwin Mendenhall also writes in his book entitled “Submarine Diary” about testing homing torpedoes at Key West in 1943. According to published records, there were about 345 firings of FIDO against German U-boats. The October 1999 National Geographic magazine relates how the Japanese submarine I-52 was attacked and sunk by FIDO.

Originally, 10,000 units were ordered by the Navy, but because of the high degree of success against U-Boats in the Atlantic and Pacific, the order was cut back to 4,000 units. FIDO was modified later to use ceramic transducers and fitted with side rails to use in submarine launch tubes and was designated the Mk 27 torpedo.

Other work at HUSL included the active homing concepts which were later included in the Mk 37 during its development at the Ordnance Research Lab (Now the Applied Research Lab). Following the end of World War II, HUSL closed and the Navy asked the engineers at that Laboratory to continue their work at the Ordnance Research Laboratory. Further development at ORL resulted in developing the guidance and control for following torpedoes: Mk27-4, Mk 34-1, Mk 37, Mk 39, Mk 48, Mk 48 ADCAP. and the Mk 50.