At the beginning of World War II, the Navy had several torpedoes, including the air launched Mk 13 and the submarine launched Mk 10 and Mk 14. Early in the war, there were distinct problems with the submarine launched straight running torpedoes. There was a depth control problem and an inertial switch problem with the exploder which resulted in many submarines missing their target and occasionally ending in disaster for the launch submarine.
The National Research Council established research facilities at several academic institutions to provide the United States with technical assistance in the war effort. One of these institutions was the Harvard Underwater Sound Laboratory (HUSL). This laboratory was headed by Dr. Ted Hunt with associate directors, Dr. Eric Walker and Dr. Paul Boner. One of the first research projects that the scientific team solved was the depth control problem of submarine launched torpedoes which caused the torpedo to run under the target instead of impacting. Other problems solved included the exploder mechanism.
Submariners found themselves vulnerable for attack after firing torpedoes that did not hit or fail to explode when they did hit. After the Navy notified the Bureau of Ordinance, the Bureau said their calibration procedures were correct and the submariners were not using the torpedoes correctly. The group of scientists quickly discovered that the calibration tests done on the torpedoes had two basic problems. Calibration of torpedoes in a stationary tank of water did not take into account the pressure reduction due to water flow over the torpedoes’ pressure transducer. This made the torpedo think it was shallower than it was and therefore ran deeper and under the target. Other depth control problems were traced to flight angle differences between lightweight and heavyweight configured torpedoes. The in-water tests were done with a lightweight configuration with the explosive material removed to permit simpler recovery of the torpedo after the test. The depth transducers were calibrated with the lightweight torpedo but the calibration did not account for the sinking factor of heavyweight warshot torpedoes and the flight angle. (This same problem carried over into other torpedo developments.)
Another problem was that the torpedoes that impacted did not explode, especially those hitting at right angles to the target. Those torpedoes that hit the target at grazing or glancing angles had a greater chance of exploding. It was found that the contact exploder mechanism initially required a firing pin movement vertically in the torpedo at right angles to the impact force vector. This force vector caused the firing pin to rise slowly or stick in the vertical tube, especially when the torpedo was at high speed or hitting the target at 90 degrees. These problems were corrected and the scientists went on to other studies.
In January 1942, an important breakthrough came as the result of a captured German torpedo, G7e, with an electric propulsion system. The torpedo could not be readily duplicated because of dimensions and other mechanical problems. The concept of an electric propulsion system to a quieter, wakeless torpedo that could not be easily seen by the target or from the air was adopted and became the Mk 18 torpedo. About 9000 of these were built by Westinghouse. The electric torpedo was slower due to the battery weight, but the wakeless feature plus the relative quietness provided more of a stealth weapon than the thermal alcohol engine of the Mk 14 or the thermal hydrogen peroxide engine of the Mk 16.
Early in 1942, Admiral Louis McKeehan of the Mine Warfare Branch of the Bureau of Ordinance came to Harvard with a secret project to build a homing torpedo for use against submarines.
The concept of an acoustic homing torpedo was pursued by the HUSL scientists and engineers. There were two different projects: (1), passive acoustic homing, and (2) echo ranging (active) homing. The first project, FIDO, reached fruition from concept to production in nine months. It was classified as a mine and was given the name FIDO to confuse German Intelligence and also maintain the work at HUSL. The term FIDO meant dogged determination of the torpedo to engage the target submarine. The second HUSL project resulting in active homing torpedoes will be discussed in a future issue.
Initial testing of the first prototype FIDO, Mk 24 torpedo, was done by HUSL scientists. The first firing occurred on the first anniversary date of Pearl Harbor. On December 7, 1942, FIDO was successfully tested against a simulated target. Initial test of these torpedoes were made off the New England coast in late 1942 with further tests taking place in Key West, Florida. The torpedo had an electric propulsion system with lead acid batteries giving it a speed of 12 knots and a range, or time duration, of 4,000 yards/10 minutes. The homing system consisted of a set of magnetostrictive transducers at each side of the rounded nose and a vacuum tube homing panel which provided steering depending on the incoming angle of the target’s radiated noise. The torpedo was 84 inches in length with a diameter of 19 inches. The torpedo was designed to be an air launched torpedo to combat the German U boat threat operating off the U.S. coast and in the mid Atlantic. Because of weight considerations, FIDO carried a small explosive charge of 92 pounds of HBX-1. The total torpedo weight was 680 pounds. FIDO’s mission was to enter a preset passive circle search and home in on the target submarine’s propeller noise and disable the submarine, causing it to surface where it could be readily attacked by air and surface ships. It was designed as a mission kill torpedo versus a direct torpedo kill.
The first 500 units were tested by HUSL researchers at Key West, Florida in 1942 to 1943. Bell Labs was the prime producer of the 4,000 Mk 24 (FIDO) torpedoes delivered to the Navy. Originally. 10,000 Mic 24 torpedoes were ordered, but because of the high degree of successes against U boats in the Atlantic and the Pacific, the order was cut back to 4,000 torpedoes. As initial production increased, some slight modifications were made to the Mk 24 torpedo including the use of ceramic transducers and relocating the transducers from the nose to the side of the torpedo. A sketch of the Mk 24 torpedo is shown below.
The BUSL test engineers reported that after test firing one of the early prototype test torpedoes, it immediately began homing in the direction of a distant transiting fishing boat and was lost. After the war, a fisherman found it lying on the bottom. Its homing system was still functional after laying on the bottom for three years.
The Mk 24 torpedo was very successful in helping to decimate the German U-Boat fleet. The advent of the long range Liberator bombers provided air cover into the mid Atlantic where German wolf packs were waiting for convoys. The German Submarine Fleet suffered huge losses due to air attacks by torpedoes in 1943.
According to Jolles1 listing of Navy torpedoes, the following statistics reflected the success of this torpedo in the Atlantic. Failures of the torpedo were often the result of improper deployment and tactics.
Mk 24 Torpedo Firings Against German U-Boats
| U.S. Navy | Qthv Allied Forces | TUTAL | |
|---|---|---|---|
| Attacks on U-Boats | 142 | 204 | 346 |
| U-Boats sunk | 31 (22%) | 37 (18%) | 68 |
| U-Boats damaged | 15(120%) | 18 (9%) | 33 |
Adapting the Mk 24 torpedo concept into other torpedoes resulted in other passive homing torpedoes, including the submarine launched Mk 27. Attempts were made to try to adopt the Mk 24’s homing system to the Mk 16 torpedoes, but the self noise from the thermal propulsion engines which burned hydrogen peroxide was too high and affected the homing system. Electric propulsion torpedoes were more readily adaptable to this new passive homing system. The Mk 24 homing system concept was carried into many other torpedoes that were later built for the Navy. The Mark 27 torpedo was an adaptation of the Mk 24 homing system for submarine launch during World War Il. The Mk 27 had fundamentally the same homing system but a longer body carrying a larger warhead. The Mk 27 torpedo will be discussed in future issues.
One of the original FIDO (Mk 24) torpedoes is on display at the Navy Museum at Naval Underwater Weapons Center, Keyport, Washington and a replica is in the lobby at the Applied Research Lab, Penn State. One of the outstanding achievements of this torpedo development was that it went from concept to production in nine months. Our modem torpedoes take between 10 and 15 years to go from concept to production.
After the end of World War II, Harvard requested that all classified work cease and their buildings be vacated. Harvard was expecting a significant increase in enrollment due to the war’s end and the effect of the GI educational bill. Dr. Eric Walker, associate director of HUSL, had accepted a job at Penn state in the Electrical Engineering Department. The Navy, reluctant to lose its scientific technology base, asked Dr. Walker to take about 100 engineers, scientists, and technicians with him. Dr. Walker formed the Ordnance Research Lab (now the Applied Research Lab) in 1945 at Penn State to continue the acoustic torpedo research programs. This laboratory was responsible for conceiving and developing many torpedoes over the years, including the Mk 27 Mod 4, Mk 34-1, Mk 31, Mk 37, Mk 39, Mk 48, Mk 48 ADCAP, Mk 50 etc. Subsequent issues of THE SUBMARINE REVIEW will contain information on the development of these torpedoes.
[Tom Pelick has worked on submarine related issues for over 37 years. After graduating from Penn State, he became a Faculty Research Engineer at the Ordnance Research Lab. His back-ground includes hydrodynamics, optics, acoustics, electronics, and systems. He was one of the design and test engineers that developed the Mk 48 torpedo, and Technical Directors staff with responsibilities for the homing system. He was instrumental in the development of the Mk 48 ADCAP and the Mk 50 torpedoes. He has ridden 14 submarines collecting data for development of acoustic homing systems and served as the Applied Research Lab ‘s representative on numerous intra-lab research teams and committes. He-also serves on a committee of the Underwater Warfare section of the America Defense and Preparedness Association.