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U.S. Navy’s First Active Acoustic Homing Torpedoes

The Harvard Underwater Sound Laboratory (HUSL) was a scientific base for the development of active and passive homing systems for torpedoes during WWII. The scientists at Harvard and other Labs researched and developed concepts for potential applications in the defense of our country. As reported in the January 1996 issue of THE SUBMARINE REVIEW, by this author and in the April 1996 issue of THE SUBMARINE REVIEW by Dr. Fred Milford, the passive acoustic homing concepts were developed and engineered at HUSL and at Bell Labs. The resulting product was produced by Western Electric, with assistance from General Electric, and became the first U.S. passive homing torpedo Mk 24 (FIDO). There was an independent but cooperative effort between HUSL and Bell Labs. This passive homing system concept was then carried into many other passive homing torpedoes. HUSL also worked with General Electric in the development of the first active homing system for torpedoes.

Historical Background

Dr. Vanevar Bush suggested to President Roosevelt prior to U.S. involvement in WWII that scientists and engineers be utilized to assist in advanced technology applications for the military. In June 1940, President Roosevelt appointed a group of eminent scientists to become part of the National Defense Research Council (NDRC), with Dr. Bush as the chairman. In 1941, NDRC became part of the newly formed Office of Scientific Research and Development (OSRD). When Dr. Bush became director of the OSRD, Dr. Conant, President of Harvard, became the chairman of NDRC. Research laboratories were established at universities, such as Harvard, Columbia and Cal Tech at Pasadena. Top scientists, engineers, and technicians were hired to perform the needed research and development for military applications.

Research and Development

The role of a scientific laboratory to develop concepts, followed by engineering development by other Navy sponsored labs, and finally production by industry is still carried on today. The U.S. Navy has four university laboratories: the Applied Physics Laboratory at Johns Hopkins, founded in 1943; the Applied Research Laboratory at Peen State, founded in 1945 as the Ordnance Research Laboratory (ORL), with the transfer of HUSL personnel; the Applied Research Laboratory at University of Texas founded in 1945 as the Defense Research Laboratory with the transfer of HUSL personnel; and the Applied Physics Laboratory at the University of Washington, founded in 1943. After HUSL closed its doors following the end of WWD, many of the HUSL torpedo scientists, engineers, and torpedo men transferred to Peen State to work at the newly formed Ordnance Research Lab and to the University of Texas to work at the Defense Research Lab. Dr. Eric Walker, Assistant Director at HUSL, moved to Peen State to become the head of the Electrical Engineering Department and at the Navy’s urging, formed the Ordnance Research Lab. Dr. Paul Boner, another Assistant Director at HUSL, returned to Texas and formed the Defense Research Lab. Each of the university labs have a different mission but maintain a cooperative effort since there may be overlapping tasks.

The Navy provides funding to these laboratories to do ongoing research. This is performed as a preventative measure for future application of this accumulated scientific knowledge to answer potential threats posed by unfriendly countries. When there is a threat, the Navy puts out an Operational Requirement to meet the threat with assistance from other Navy laboratories to provide an answer to the threat. After the conceptual system is formulated and prototype tested by the university laboratories, it is then available for contractual bidding by industry. The laboratories assigned to carry on the supervision of developmental engineering are largely the Navy laboratories, such as the Naval Underwater Weapons Centers (NUWC), at Newport, Rhode Island and Keyport, Washington. However,

the university laboratories and the Navy labs will generally have some degree of involvement until after production and Follow-on Test and Evaluation. There is a variable degree of overlap. Navy funding categories for
fundamental research and initial development are 6.0, 6.1 and 6.2. Prototypes were initially funded by 6.2 money, but today prototype development is funded by 6.3 money. Developmental work today is funded by category 6.3 and production is funded by category 6.4. However, as in the mission assignments, there is come overlap.

The feasibility of these concepts is tested in prototype torpedoes as Harvard and Bell Labs had done with the Mk 24 torpedo tests. Torpedo development is one of the missions of the Applied Research Laboratory at Penn State. ARL has been involved in the research and development of most torpedoes in the fleet today with the exception of the Mk 46 torpedo homing system which was developed by the Naval Torpedo Station at Pasadena, CA (ater NOSC, San Diego).

Active Homing Studies

In addition to the passive homing studies at HUSL and Bell Labs under Navy Project N0-94 during WWII, active homing studies were being performed at HUSL and at General Electric under Navy Project N0-181F. This active homing objective was to obtain greater detection range through the use of higher directivity and a reduction of self-noise. Self-noise reduction was a challenging task and required comprehensive studies and experiments. The Mk 18 with electric propulsion was an initial test platform for several homing systems. Other self-noise reduction came about through solving ground loop problems, crosstalk between wires, and harmonics.

HUSL scientist and GE engineers each worked on an echo ranging active homing system. They encountered much difficulty until they learned more about the environment and were able to cope with the resulting acoustic problems. It was difficult for the early active homing systems to distinguish among echoes from the target and the echoes from the bottom, surface, and seaflife. In addition, the vertical direction of the echo is confused by refractive properties resulting from thermal differences in the water and by reflections from the boundaries, surface and bottom. Also, horizontal steering at close-in terminal homing ranges is confused since multiple echoes were received from different sections of the target, such as the bow, stem and sail. At long ranges, the entire target is acoustically ensonified and appears as a point source.
However. as the range gets very short, multiple echoes appear from several sections of the target and this confused the active homing system’s horizontal steering.

Today’s torpedoes are still faced with this problem but have more complex circuitry to provide more accurate horizontal steering.

The problem with the Mk: 18. in addition to learning the environmental effects, was the internal noise level of the torpedo. These active acoustic homing torpedoes may be categorized into first generation consisting of the Mk 32, Mk 35, Mk 37, Mk 43 and the Mk 44 torpedoes. The second generation may include the Mk 46 and Mk 48 torpedoes. The third generation would consist of the Mk 48 ADCAP and the Mk SO torpedoes. Research work at the laboratories leads to improvements in existing torpedoes with advancements in computers and other technologies. For example, some of the transistors used in the Mk 48 are no longer available so new electronic parts replace them as needed.

Mk 32 Torpedo

The first active homing torpedo in the fleet was the Mk 32 torpedo. It was an anti-submarine weapon launched from aircraft and surface ships. It was developed by GE with some combined and competitive effort between HUSL and GE. The Mk 32 torpedo’s homing system was only active and did not have a passive homing capability. The Mk 32 torpedo was about the size of the Mk 24 (FIDO) passive homing torpedo. It was 83 inches long, 19 inches diameter, 700 pound weight, electric propulsion, warhead of 107 pounds HBX, 12 knot speed, and a range of 9600 yards (24 minutes). GE had discarded its crystal transducers in favor of the HUSL magnetostrictive transducers.

Eventually, successful demonstrations of active homing were made by GE during June 1943 in the azimuth plane with the Mk 32 prototype. It would be in early February 1944, before the Mk 32 prototype demonstrated a successful homing attack on a target in three dimensions. Since GE did not have available facilities for production, Leeds and Northrup of Philadelphia was awarded the production contract. However, only 10 torpedoes were produced during WWII and none saw action.

There was some limited active homing work in a developmental torpedo designated the Mk 22. Bell Labs and Westinghouse experimented with active acoustics in the azimuth plane for terminal homing. This work was discontinued in favor of the planned Universal Torpedo to be designated as the Mk 35.

The ORL, in a combined effort with GE, continued post war development of the Mk 32 Mod 2 torpedo. About 3300 torpedoes were produced by the Philco Corporation in Philadelphia and the Naval Ordnance Plant in Forest Park, Illinois. This torpedo saw service from 1950 to 1955, when it was replaced by the Mk 43 torpedo.

The evolution of active homing systems continued at ORL and at GE. ORL pursued the concepts of the Navy Project N0-181 F, designating the work as ORL Project 4 while GE pursued a different approach. These two lines of effort resulted in two distinct types of active homing systems.

Mk 35 Torpedo

As noted earlier in this article, the Navy requested that work begin on a Universal type torpedo with an active homing system. GE was given the contract. The Navy wanted an active homing torpedo capable of being launched from aircraft, surface ship, or submarine. The Mk 35 was the first generation deep diving, long range, acoustic torpedo designed to attack submerged submarines.

The Mk 35 torpedo was based on the acoustic homing system performances of the homing torpedoes Mk 24 and Mk 32. It was originally designed as the Universal Torpedo capable of being launched from any type of platform. During development, the torpedo grew to 162 inches and 1770 pounds eliminating it from aircraft use. It had a 21 inch diameter with an electric propulsion system featuring a seawater battery. It was planned to have an active capability. passive capability. and use a spiral search pattern. It bad a speed of 27 knots and a range of 15 kyds. The Mod 1 version reportedly failed OPEV AL. A Mk 35 Mod 2 torpedo was built with a redesigned homing system based on work at GE and ORL.

Between 1949 and 1952, GE at Pittsfield, Massachusetts built 400 units which saw limited service. It was withdrawn from further development and production in favor of the Mk 37 torpedo. The research, development and testing of this torpedo had cost between $14-lSM.

Homing System Designs

The evolution of the active homing systems from HUSL continued in 1945 at ORL (now ARL) and GE resulted in two distinct active homing system. Both homing systems measured the target echo in terms of the leading edge rise time, amplitude, and echo length relative to the transmitted pulse. However, the HUSL/ORL design had a Doppler gate which separated the echoes based on Doppler of greater than 1.2 knots allowing a greater sensitivity to the amplitude detection of echoes from targets. The Doppler gate provided the first viable Doppler classification method of distinguishing targets from false alarms. This had the adverse effect of not detecting very low Doppler targets, but had the positive effect of significantly reducing the amount of false alarms from reverberation.

One of the designers remembers using a capacitor to slope the front edge of the transmitted pulse to obtain a narrower reverberation spectrum. It was the beginning of what we call today Pulse Weighting or Waveform Shaping. A square pulse would have a wider reverberation spectrum whereas an amplitude modulated pulse would have a much narrower reverberation spectrum. Also, the reverberation spectrum was also dependent on the length of the transmitted pulse. The narrower the pulse, the wider the reverberation spectrum. The wider reverberation spectrum made it difficult to detect Doppler targets. The GE system, without the Doppler gate, could detect the lower Doppler targets, but was subject to a higher false alarm rate.

The ORL transducer design provided a transformer for impedance coupling between the transmitter and transducer which resulted in a greater efficiency, whereas the GE design dumped the power directly into the mis-matched transducer impedance resulting in a loss of transmit power.

In addition, during transmit, the HUSL/ORL design provided the simultaneous driving of all four sectors of the transducer array. During the receive mode, the transducer produced outputs from four quadrants with different phase centers. The phase differences among these signals indicated the three dimensional direction of the arrival of the echo. The input circuit converted these voltages to four in-phase voltages of varying amplitude. The amplitude differences between corresponding pairs gave target angle information simultaneously in the horizontal and vertical planes.

This allowed the torpedo to boresight on the target during the attack. The GE system used a similar transducer with upper and lower halves rather than the four quadrants. Therefore, GE’s system would provide directional steering in the vertical plane, but the horizontal steering was a steeraway technique. The torpedo searched by circling until it received a target detection, then it reversed the turn until the target was lost. This meant that the steeering on the target was held at the side of the horizontal beam rather on boresight.

The processing of signals was a problem with receivers using an amplifier to process each signal from the transducer sectors. It was difficult to maintain the same gain in each amplifer. The HUSL/ORL receiver design addressed this problem by using a single amplifier. The average amplitude of the modulated singal was a measure of the received echo amplitude, the phase of the modulation envolope was an indication of target angle information, and the amplitude of the modulation envelope was a function of both the echo amplitude and the angle between the direction of echo arrival and the transducer axis.

MK 37 Torpedo

The ORL/HUSL active homing system design was selected for the Mk 3 7 torpedo and the contractor was Westinghouse at Sharon, Pennsylvania. ORL’s Nick Abouresk was the Liaison and Project Manager for the technical direction of ORL’s active homing system implementation into the Mk 37 torpedo. The earlier HUSL design was modified by replacing the larger vacuum tubes with miniature vacuum tubes and much attention was given to packaging, stability, and electronic noise reduction. The operating frequency was 60 Khz and the propulsion was a two speed electric motor. This torpedo was the first fleet torpedo to have active and passive homing capabilities throughout the run. It was 135 inches in length, 19 inch diameter, 1430 pounds, warhead of 330 pounds HBX-3, and used a contact exploder. It had a nominal detection and homing range of about 700 yards.This torpedo, which was produced in quantities of over 3300 units at the Naval Ordnance Park at Forest Park, Illinois, served as the U.S. Navy’s primary submarine acoustic torpedo from the mid 1950s until the Mk 48 torpedo replaced it in the early 1970s. Since the Mk 37 torpedo had electric propulsion, it would swim out of the torpedo tube instead of being impulsed, thereby reducing the launch transients and the detectability of the launching submarine. Wire guidance was later added to the torpedo making it 26 inches longer and 230 pounds heavier. The submarine fire control system was also modified to take advantage of the wire guide capabilities.

After replacement of the Mk 37 in the U.S. fleet by the Mk 48, the Mk 37 torpedo was sold to several countries. Today, the Mk 37 torpedo is being used by many countries, including Israel. However, the vacuum tubes in the homing systems of the original versions have been replaced by solid state electronics. U .S firms, such as Alliant Tech and Westinghouse, have contracts to modify and service these torpedoes.

Mk 43 Torpedo

As stated earlier, the Mk 32 torpedo was discontinued in favor of the Mk 43 torpedo. The Mk 43 Mod 0 was developed and produced by GE at Pittsfield, Massachusetts. It was an inexpensive lightweight air-drop torpedo. After 500 of these units were built, they were discontinued in favor of the Mk 43 Mod 1 and Mk 43 Mod 3 torpedoes.

The Naval Ordnance Test Station (NOTS) in Pasadena, California and the Brush Development Co., Cleveland, Ohio, developed the Mk 43 Mod 1 with a 10 inch diameter, 91.5 inch length, 260 pounds weight, warhead of 54 pounds HBX, active homing with a helical search pattern, and a 15 knot speed and a range endurance of about 4500 yards. Brush Electronics and Naval Ordnance Park produced 5000 of these torpedoes until they were replaced by the Mk 44 torpedo. It was the first lightweight torpedo capable of being launched from helicopters, fixed wing aircraft, and surface ships. The Mk 43 torpedo was in the fleet from 1951 to 1957 and was replaced by the Mk 44 torpedo. The Mk 43 torpedoes were sold to the British and perhaps other countries.

Mk 44 Torpedo

The Mk 44 Mod 0 torpedo was a replacement for the Mk 43 torpedo with improvements in speed, warhead size, acoustic homing changes, and pre-launch progrmmable search modes. It
was developed by NOTS and GE. It wns the first air-launched fleet torpedo with a seawater-activited battery to provide power. It was produced at GE and later at the Naval Ordnance Plant. It was in service from 1957 through 1967 on destroyers and aircraft as an ASW weapon until it was replaced by the torpedo Mk 46. The Mk 44 torpedo was sold to foreign governments and also produced in Europe by NATO countries.

Mk 46 Torpedo

The Mk 46 was developed by Aerojet General, Azusa, California and NOTS. It was the first air-launched deep diving, high speed ASW torpedo with active/passive homing and represents the second generation in airborne ASW weaponry. It entered the fleet in 1965 and went through several modifications, from Mod 0 through Mod 5. The Mod 2 version was 102 inches long, a diameter of 12. 75 inches, weight of 508 pounds, speed of 40 knots, range of 12,000 yards. The Mod 4 version is also capable for use in mine systems, such as Captor.

The Mk 46 Mod 5 torpedo was built based on the Near-Term Improvement Program, NEARTIP, and resulted in improvements in acoustic performance in deep and shallow water, countermeasure resistance, guidance and control, and the fire control system. A driving force for this NEARTIP torpedo was to respond to the anechoic coatings on Soviet submarines. The Mk 46 Mod 5 is primarily an ASW weapon and can be launched from surface ships, fixed wing aircraft, helicopters, ASROC, and mine systems. According to Jane’s about 20,000 Mk 46 torpedoes were built for U.S. and foreign use. It is estimated that the U.S. fleet may have at least 13,000 Mk 46 torpedoes in its inventory. More current torpedo modifications will provide significant improvements as the advances in computer and electronic technologies continue. It has not been replaced despite the new advanced Mk 50 torpedo primarily due to cost and reduction of the threat. Jack Slaton, (who worked at ORL, NOSC Alliant Tech, and is now retired) was one of the chief designers of the Mk 46 homing system and was a major contributor to the Mk 50 homing system.

Mk 48 Torpedo and Advanced Torpedoes

As torpedo technology improves because of research at
university and Navy laboratories, these weapons are greatly improved and this in turn provides the submariner with a higher probability of success. The Mk 48 torpedo, which replaced the Mk 37 torpedo in submarines will be discussed in a future issue of THE SUBMARINE REVIEW. Also slated for future publication will be the Mk 48 ADCAP and the Mk 50 torpedoes.

Information Collection and Review

I would like to thank all those who have provided information which helped me to assemble this article on the early active homing system in torpedoes and the events leading to the development of these torpedoes. Reconstruction of history and publication of events can be very rewarding, but it can also serve as a lighting rod attracting a few dissenting and minority opinions as well as many favorable comments. I would appreciate any information you might have on the development of the Mk 48 torpedo that may be included in my next article.

Tom Pelick
609 Berkshire Drive
Stare College, PA 16803

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