U.S. NAVY TORPEDOES

Part Five: Post WWII Submarine Launched/ Heavy weight Torpedoes

While it is not our purpose here to discuss defense economics or national security policy, it is important to remember that the end of WWII dramatically changed the requirements, the associated force structure and the budget of the U.S. Navy. In 1946 the total number of ships in the U.S. Navy was about one-fifth what it had been in 1945, there were fewer than half as many destroyers, one-fourth as many submarines and one-tenth as many destroyers escorts. Aircraft are more difficult to count, but there were probably one-fifth as many serviceable naval aircraft in 1946 as there were in 1945. Annual expenditures for the Navy in 1946 were a third of what they had been in 1945 and fell to one-fourth the 1945 level by 1947. Total obligational authority dropped to one-tenth the 1945 level by 1948. Torpedo acquisition had to be pursued within this austere environment.

The end of WWII also brought an end to the ambivalence reflected in the hold hands with the devil description of U . S-U. S.S.R. relationships during the war. It was not until 1948 that a formal national security policy towards the Soviet Union was issued, but for naval planning and weapons acquisition purposes the hypothetical enemy was the U.S.S.R. even in the early post war years. In 1946 the Soviet Navy consisted of about 130 ocean going submarines, IO large surface combatants, 68 destroyers, 68 minesweepers and numerous coastal vessels including small submarines. Whether as a result of astute analysis, or the need to have a credible mission to survive1, the U.S. Submarine Force, given the structure of the Soviet Navy, seized on anti-submarine warfare as one of its most important missions. This decision had a profound affect on post WWII torpedo programs. No torpedo of any kind without the capability to attack submerged submarines has entered service with the fleet since 1945, whereas the only submarine launched torpedo with that capability that even reached prototype stage before 1945 was the Mk 33 of which only 30 models were built.

In 1946 the U.S. Navy found itself with huge stocks of a variety of operational torpedoes and numerous torpedo projects in various stages of completion. Post war funding could not support all of the development projects, so they were pruned down to those that could quickly produce useful interim ASW weapons and those that had major longer term potential. Subsequent development projects not only incorporated increasingly sophisticated refinements of concepts that were originated during WWII, but also introduced entirely new concepts. Three of the more important new concepts were wire guidance, discrimination and onboard attack logic. Such refinements greatly enhanced the effectiveness of torpedoes and were made possible in large measure by the continued rapid development of electronics in the post-war era. These and other improvements, however, raised the unit cost. A modem submarine launched torpedo carries a 1997 price tag that easily exceeds $IM. On the other hand, if one torpedo destroys a $1B enemy SSN, the exchange ratio is very favorable.

Post-WWII torpedoes fall naturally into two groups: heavy weight submarine and surface vessel launched torpedoes and light weight air and surface vessel launched torpedoes.2 Interestingly, there have been no torpedoes developed in the post war years exclusively for surface ships. All post-WWII surface launched torpedoes have been adaptations or dual use versions of air or submarine launched weapons. Accordingly, after a few comments on the continued use of WWil torpedoes, we consider the development of heavy weight torpedoes since 1945. Light weight torpedoes will be considered in the next part of this series.

Post-War Use of WWII Torpedoes

The straight running steam, electric and Naval torpedoes, Mks 13, 14, 15, 16, 17, 18 and 23, which were operational3 at the end of the war continued as service weapons. By 1950 only Mks 14, 15 and 16 remained in service” and the Mk 15 disappeared as trainable 21″ torpedo tubes were removed from destroyers. Mk 16 (Naval) remained n service until 1975 and the venerable Mk 14 (steam) was not finally withdrawn from service until 6 March 1980.5 The three homing torpedoes that had entered service during WWII, Mk 24 (air launched ASW), Mk 27 Mods 0 (submarine launched anti-escort) and Mk 28 (submarine launched anti-surface vessel) continued in service until they were replaced by improved weapons, the Mk 28 remaining in service until 1960. Many torpedo projects were discontinued, in some cases after reaching the prototype or pre-production stage. As noted below, some WWII projects were continued or reactivated during the immediate post war years. Thus much of the U.S. Navy torpedo programs from 1945 to 1950 represented refinement and adjustment of WWD programs to new peacetime requirements.

Heavy Weight Torpedoes

Mark 35. The value of homing torpedoes as anti-submarine weapons had been well demonstrated by the Mk 24 torpedo, and in 1943 a program was begun to develop a submarine launched homing torpedo with both anti-submarine and anti-surface vessel capabilities. This torpedo development, designated Mk 33, was discontinued in 1945 after 30 test and evaluation units had been produced. The concept was, however, retained in a new program, the Mk 35, with the same contractor, General Electric, beginning in 1945. This was an ambitious program that originally envisioned passive acoustic search, active homing, a seawater battery and launch from submarines, surface vessels or aircraft. Development was slow and cancellati<.>n was a real possibility on several occasions. The air drop capability was eliminated in 1947 and the first of approximately 500 production torpedoes appeared in 1949. Fleet use was, however, limited and the Mk 35 was withdrawn from service around 1960. Among the unique features of the 21″ x 162″, 1770 pound Mk 35 were: gyro controlled run out, active/passive guidance, a seawater battery to give a range of 15,000 yards at 27 Jcriots and a deep, by late 1940s standards, diving capability.

Mk 27 Mod 4. With the Mk 35 program experiencing difficulties and the engineering development program for the Mk 37 torpedo, which is discussed below, just beginning, the U.S. submarine service found itself in 1946 with an ASW mission, but without a weapon capable of attacking submerged submarines. Further, neither the Mk 35 nor the Mk 37 could reasonably be expected to be available quickly. This situation and the sizable Soviet submarine force were probably the driving forces in the initiation of the Mk 27 Mod 4 project at the Penn State Ordnance Research Laboratory (ORL) in early 1948.6 The Mk 27 Mod 0 torpedo had been a useful anti-escort weapon during the last 11 months of WWII. Several improved models had been developed including Mod 3 which, like the other improved Mods, had been lengthened to a little over ten feet to accommodate a larger warhead 1 and an improved battery. Mod 3 was unique in having a gyroscopic control for initial runout making a standoff offensive rather than purely defensive anti-escort weapon. When the Bell Telephone Laboratories withdrew from the torpedo program at the end of WWII. six Mk 27 Mod 3 torpedoes had been completed and three were ready for field testing. 8 About 100 additional Mk 27 Mod 1 torpedoes were available for conversion to Mod 3 and some may have been fairly far along in the conversion process. Apparently work on the Mk 27 Mod 3 continued at a low level through 1947, possibly at Navy laboratories. In 1948, with increasingly ominous intelligence estimates of the Soviet submarine fleet as backdrop, the Navy and ORL negotiated an urgent development program for the Mk 27 Mod 4 torpedo. With the existing Mk 27 torpedo developments as background and several years of post-war electronics development to draw on, the Mk 27 Mod 4 was expeditiously developed by ORL engineers. What emerged was a 19″ x 125.75″, 1175 pound torpedo with a 128 pound warhead, a 15.9 knot speed and a range of 6200 yards (12 minutes). The acoustic control system consisted of four body mounted hydrophones, amplifiers and servo systems very similar to those of the Mk 24 and earlier Mk 27s. 9 Gyroscopic control provided for a preset initial straight enabling run on a predicted intercept course. After enabling, a circular search was initiated and continued until a target was acquired by the acoustic system. The acoustic signals guided the torpedo on a pursuit course to the target. If acoustic contact was lost, the circular search mode was re-established. Electrical fire control settings were used. These features were similar to those in the Mk 27 Mod 3, but the implementation had been greatly refined and many important additions and improvements were made by the ORL project team. The most important addition was the selectable capability to attack either submerged submarines or surface vessels. Mk 27, Mod 4 was not, however, fast enough to make a successful attack on an alerted 17 knot Type XXI submarine. With that proviso, the Mk 27 Mod 4 was an available, high performance anti-submarine/anti-surface vessel weapon for U.S. submarines. This was the first submarine launched torpedo capable of attacking submerged submarines adopted for U.S. fleet use. About 3000 were procured from AVCO Corporation and Naval Ordnance Plant (NOP), Forest Park, between 1949 and 1954. Mk 27 Mod 4 was gradually replaced by Mk 37 Mod 0 between 1956 and 1960.

Mk 37. Even before the Mk 35 became operational the development of another superficially similar torpedo, the Mk 37, began. In retrospect, the Mk 37, which is frequently described as the first modern ASW torpedo, is clearly a major milestone in torpedo development. Engineering development of the Mk 37 began in 1946, but its origins are found in WWII projects at Harvard Underwater Sound Laboratory (HUSL) and ORL. The active homing systems pioneered by these laboratories had many sophisticated and useful features. One of these was Doppler enabling which rejected echoes from stationary targets and so avoided homing on reverberations or other false targets. Another important feature was conical scanning, using four quadrant transducers, during reception. This system used a single amplifier to generate both azimuthal and depth steering signals. The ORL system10, which was a significant improvement on the original HUSL system, had been tested in modified Mk 28 torpedoes. Beginning in 1946 Westinghouse and ORL combined this active homing system with a passive homing system, appropriate logic circuits, a new propulsion system and a new torpedo body to make the Mk 37.

The Westinghouse-ORL team produced 30 torpedoes for development testing in 1955-56. Large scale production was undertaken at NOP, Forest Park and the Mk 37 began its long career as the primary U.S. submarine launched ASW torpedo. The Mk 37 Mod 0 was 19″ in diameter by 135″ long; weighed 1430 pounds; used two speed, 26 knots (10,000 yards) and 17 knots (23,000 yards), electric propulsion; and carried a 3300 pound warhead. The guidance was a preset straight gyro controlled enabling run on a predicted intercept course followed by passive acoustic search using snake or circular search pattern. After target acquisition, the torpedo was guided by the passive acoustic system until, at a range of about 700 yards. the echo strength in the active system became sufficient for active homing and attack. The active homing mode was. as previously noted, Doppler enabled to prevent attacks on stationary false targets.

The Mk 37 Mod 0 torpedo was a very sophisticated weapon, but the initial straight enabling run, which could take up to 15 minutes, was preset and not alterable until it was completed. During that time the target could, either incidentally or for deliberate evasive purposes, maneuver and compromise the homing phase of the attack. To obviate this problem and old idea, wire guidance, was resurrected. The first effort in this direction was the Mk 39 which was a Mk 27 Mod 4 modified by the addition of a wire dispenser, appropriate controls and improved propulsion. The modifications were developed by ORL and Vitro Corporation. One hundred twenty torpedoes were converted by Philco and used, beginning around 1956, for fleet familiarization and evaluation, mainly in the seven SSK conversions of WWII fleet boats. In addition to the torpedo modifications, it was necessary to modify the fire control system to provide appropriate control signals and the torpedo tubes to accommodate the wire. In operation the Mk 39 became a bearing rider, that is it was manual I y steered to keep it on the line of bearing from the launching submarine to the target. This form of guidance is not particularly efficient and it has other limitations among which we note: 1) only one wire guided torpedo at a time can be launched and controlled, 2) for the run time of the torpedo the maneuverability of the firing submarine is limited, 3) torpedo noise masks the acoustic signature of the target, and 4) the torpedo on the bearing line indicates the direction to the firing submarine. In spite of these limitations. the Mk 39 program clearly demonstrated the improved effectiveness of wire guidance against a maneuvering target.

The success of the Mk 39 led to the development by Vitro Corporation and ORL of the Mk 37 Mod 1. a wire guided version of the Mk 37. which began its long service with the fleet in 1960. The guidance system was generally similar to that of the Mk 39 with the incorporation of corrected intercept guidance in addition to the bearing rider mode. Command enabling and new search modes were also introduced. The Mk 37 Mod 1 was longer, slower and heavier than the Mod o. but it offered greater target acquisition effectiveness and was more effective against agile submarines.

Mk 37 Mod 0 torpedoes were withdrawn from service and refurbished and reissued as Mod 3; Mod 1 torpedoes were similarly converted to Mod 2 with deliveries beginning in 1967. The refurbishing involved many changes, one of note being the switch from magnetostrictive to ceramic piezoelectric transducers. This change enhanced the acquisition range to about 1000 yards and avoided loss of sensitivity with depth.

The Mk 37 was an excellent anti-submarine weapon until the submerged speeds reached the 20 plus knot13 range and diving depths began to exceed 1000 feet. The probability of sinking or seriously damaging a submarine capable of over 20 knots with a 24 knot torpedo is unacceptably low (unofficial figures given 10 percent for the Mk 37) and meeting such threats required new weapons. Significant upgrades of the Mk 37 have been made and its progeny remain in service with many navies as the NT37C, D, E and F which are much faster. operate deeper and boast modern solid state control systems. The U.S.. Navy, probably wisely, developed new torpedoes to address the new threats.

Mk 45. Two solutions to the high speed, deep diving subma-rine problem were implemented. The first was the nuclear warhead incorporated in the Mk 45 (ASTOR). The torpedo itself was relatively conventional except for the use of a seawater activated battery to power a 160 hp electric motor. This propulsion package gave a speed of 40 knots and a range from 11,000 to 15,000 yards. Guidance was by a gyro, depth gear, wire combination using the attacking submarine’s sonar to track the target. There was no homing capability. The warhead was detonated only by a signal sent along the wire; there was no contact or influence exploder in the torpedo. The wire guidance and command detonation were not only important in getting the torpedo to the target, they also satisfied the requirement for positive control of the nuclear warhead. Development of the Mk 45 was completed in FY60, it was approved for service use in FY61 and production deliveries began in FY63. 14 It was withdrawn from service in 1976 when the Mk 48 had demonstrated its capability and the advisability of using tactical nuclear weapons for ASW purposes became questionable.

The basic Mk 45 torpedo was modified by Westinghouse to make a conventional torpedo for foreign military sales, the so-called Freedom torpedo. A few demonstration models were built but none were sold.

Mk 48 and Mk 48 ADCAP. The non-nuclear approach to the high speed, deep diving submarine was a very fast, deep diving torpedo with a high performance guidance system, that is, a much improved Mk 37 that would take full advantage of post WWII technology. Consideration of such weapons, both submarine launched and air launched, began in November 1956 as part of the RETORC (Research Torpedo Re-Configuration) program. By 1960 a specific heavy weight torpedo project had emerged and was designated first EX 10 and later Mk 48. Development characteristics for the new torpedo included a range of 35,000 yards at a speed greater than 55 knots and a 2500 foot depth limit. After a bidder qualification exercise and competition between the qualified bidders, a project definition contract was awarded to Westing-house. A parallel contract was awarded to Clevite for the development of an alternative acoustic system. The Westinghouse contract was subsequently extended to include the development of the turbine powered Mk 48 Mod 0 which had only an ASW capability. Some Mod Os were produced for evaluation, but by 1967 it had been decided that an anti-surface vessel capability was also needed. Some feeling persists that this was more a ploy to keep Clevite in the running than a significant operational requirement. A competition between the Mk 48 Mod 1, which had emerged in rudimentary form from the Clevite contract, and Mk 48 Mod 2, a redesign of the Westinghouse Mod 0 followed. The Westinghouse torpedo used a Sunstrand turbine, as used in the Mod 0, for propulsion while Clevite used Otto fuel in an external combustion, axial piston engine. One of several selection factors was apparently the better efficiency of the piston engine, especially when running deep, as opposed to the quieter. but less efficient turbine. The acoustic systems were also somewhat different. In 1971 after competitive evaluation a full scale production contract was awarded to Gould 15 (formerly Clevite). The first Mk 48 Mod 1 torpedoes were delivered to the fleet in 1927, 12 years after the development characteristics had been approved.

The Mk 48 Mod 1 torpedo was 21′ by 230′, weighted 3440 pounds and carried a warhead with 650 pounds of PBXN-103. Frequently published, but unofficial, data indicate that it was capable of 55 knots for 35,000 yards and could operate as deep as 2500 feet, but not at maximum speed. Its acoustic homing system is reported to have an acquisition range of 4000 yards, about four times that of the Mk 37. This performance is impressive and generally adequate for dealing with 30+ knot, deep diving targets.

The Mk 48 torpedo is divided into five functional sections (groups). 16 These groups and their contents are briefly:

• the nose, containing the acoustic system and the homing control logic (HCL)
• the warhead, containing the high explosive, exploder and the Mk 12 electronic assembly, which is presumably a proximity fuzing device
• the control group, comprising the command, gyro and power control units
• the fuel tank containing not only the fuel but also the guidance wire dispenser
• the afterbody/tailcone group comprising the engine, control surfaces and actuators, combustion chamber and the alternator.

Most of the electronics was designed as functional item replacement (FIR) units (the approximate equivalent of aircraft line replaceable units) to reduce maintenance time and simplify the process. This concept also facilitates upgrading by installing new FIRs. The command control unit Mk 154, for example, was replaced by Mk 168 to accommodate the change to two-way communication in the wire guidance system for Mk 48 Mod 3.

The combination of substantial onboard capability (HCL) to control search, homing and re-attack maneuvers and wire guidance provides a formidable weapon. The addition of two-way communication (TELCON) in the Mod 3 provided data from the torpedo sonar and actual torpedo operating data (course, speed, depth etc.) To the submarine fire control system, thus substantially enhancing performance. Mod 4 added envelope expansion features, including increased speed and deeper diving, and a fire and forget capability. Existing torpedoes were upgraded by kits and Mod 4s were production torpedoes from 1980 on. Mod 5 was an interim upgrade of existing torpedoes pending the availability of ADCAP. The Mk 48 torpedo had teething problems, but it is a very sophisticated, high performance weapon. Published photographs of the destruction of targets attest to its effectiveness. The main technical criticism of the Mk 48 seems to be that it is very noisy.

Prior to the mid 1960s Soviet submarines had diving depths of 650 to 1000 feet and submerged speeds under 30 knots. Early Mk 48 capabilities were clearly capable of attacking such targets. The advent of the Soviet Alpha submarine17 with its non-magnetic titanium hull, 2500 foot diving depth and submerged speed in excess of 40 knots apparently produced a validated threat against which the Chief of Naval Operations issued a new operational requirement in 1975. Two approaches to satisfying this requirement were initiated. The first was the Mk 48 envelope expansion program, mentioned above, which exploited the capabilities of the existing torpedo. The second was essentially a new torpedo 18, ADCAP. The major changes in ADCAP involved entirely new digital electronics, inertial guidance (replacing the gyro system), a major reduction in volume devoted to electronics, a corresponding major increase in fuel capacity, a strengthened shell and, of course, inclusion of the Mk 48 envelope expansion features. The Mk 48 piston engine was retained but with a greater fuel flow rate to yield an estimated 63 knot speed. Much of the change was made possible by the introduction of integrated circuits, including microprocessors, whose small size made it possible to move many of the functions of the control group into the nose. The guidance wire spool was moved to a position aft of the enlarged fuel tank and other layout changes were made. FY94 saw the final buy of ADCAPs. Improvements in ADCAP are to be made by modification of the existing inventory. The first of these is known as MOD ADCAP and entered production in FY95. Research on quieting the ADCAP has been underway since 1986, but the justification for quieting has been recently questioned by GAO.

ADCAP is externally essentially identical with the Mk 48, but it requires a modified fire control system. Appropriate modifica-tions have been made or incorporated in new construction and the ADCAP is the principal torpedo for attack submarines. Trident SSBNs, however, continued to carry Mk 48 torpedoes, though the appropriate fire control modifications may be being implemented.

Several other heavy weight torpedo projects were initiated after WWII. Two were discontinued because of the success of other projects, Mk 38 because of the success of Mk 37, and Mk 47 because of the success of Mk 48. The Mk 42 pattern running development was simply overtaken by events, more capable torpedoes preempted its mission. As noted, Mk 49 was intended for the ADCAP but not used.

Summary

The main trends in post WWII U.S . Navy torpedo development are relatively easy to identify. Soon after the end of WWII, the principal target became the submarine with surface vessels really secondary targets at best. Two types developed, heavy torpedoes for submarines and light weight torpedoes primarily for aircraft but also deployed on surface ships. Traditional steam torpedoes were phased out, though the Mk 14 lingered for a long time, in favor of electric propulsion. Electric propulsion gave way to advanced external combustion piston engines as the submerged speed of submarines increased to around 30 knots. The appearance of the Soviet Alpha presented an apparent threat that required even higher speeds and further propulsion improvements yielded adequate torpedo speed. The most striking evolution, however, has been in guidance and control. The rudimentary homing systems of WWII evolved into sensitive, high power, long range systems operating in both active and passive modes. Wire guidance was added to heavy weight torpedoes to provide mid-course guidance based on the attacking submarine’s sonar and fire control system. As the size and weight of electronics decreased, onboard signal processing and command logic were added. Modem U .S. Navy torpedoes are sophisticated guided weapons capable of following instructions delivered by wire or operating autonomously to attack and, if necessary, re-attack their targets.