US NAVY TORPEDOESby Frederick J Milford
Part Five: Post WW-II Submarine Launched/ Heavyweight Torpedoes
Reproduced with permission from the October 1997 issue of THE SUBMARINE REVIEW,
a quarterly publication of the Naval Submarine League, P.O. Box 1146, Annandale, VA 2200
While it is not our purpose here to discuss defense economics or national security policy, it is important to remember that the end of WW II dramatically changed the requirements, the associated force structure and the budget of the US Navy. In 1946 the total number of ships in the US 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 destroyer escorts. Aircraft are more difficult to count, but there were probably only 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 WW II also brought an end to the ambivalence reflected in the "hold hands with the devil" description of US-USSR 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 USSR even in the early post-war years. In 1946 the Soviet Navy consisted of about 130 ocean going submarines, ten 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-WW II 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 US 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 WW II, but also introduced entirely new concepts.
1 Frank Andrews in "Submarine Development Group Two" (The Submarine Review, April 1983, p.5) says "In 1946 it was evident that there would be no budget bucks for submarines unless they could be put to a meaningful use."
Post-WW II torpedoes fall naturally into two groups, heavy weight submarine and surface vessel launched torpedoes and light weight air and surface vessel launched torpedoes2. Interestingly, there have been no torpedoes developed in the post war years exclusively for surface ships. All post-WW II 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 WW II 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 WW II TORPEDOES
The straight running steam, electric and Navol 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 service4 and the Mk.15 disappeared as trainable 21" torpedo tubes were removed from destroyers. Mark 16 (Navol) remained in service until 1975 and the venerable Mk.14 (steam) was not finally withdrawn from service until 6 March 19805. The three homing torpedoes that had entered service during WW-II, 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.
2 The split is at about 1000 lb. Some air-launched torpedoes exceed that limit. We use "light weight" as synonymous with "air launchable" in accord with contemporary usage.
3Mks.16 and 17 were the two USN Navol (hydrogen peroxide) torpedoes. Both were in production at the end of the war, but neither was used in combat.
4Mk.18 electric torpedoes were, however, occasionally found in after torpedo rooms even in the early sixties.
5NAVSEA letter to CNO 63Z222:AB 8510 Ser 142 dated 6 March 1980. The Mk.14 was declared obsolete around 1960, but this designation was officially withdrawn in 1969 and it continued in service as above.
As noted below, some WW-II projects were continued or reactivated during the immediate post-war years. Thus much of the US Navy torpedo program from 1945 to 1950 represented refinement and adjustment of WW II programs to new peace-time requirements.
The value of homing torpedoes as antisubmarine 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 antisubmarine and anti-surface vessel capabilities. This torpedo development, designated Mk.33, was discontinued in 1946 after thirty 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 cancellation was a real possibility on several occasions. The air drop capability was eliminated in 1947 and the first of approximately 400 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 lb Mk.35 were: gyro controlled run out, active/passive guidance, a seawater battery to give a range of 15,000 yards at 27 knots and a deep, by late 1940's standards, diving capability.
With the Mk.35 program experiencing difficulties and the engineering development program for the Mk.37 torpedo, which is discussed below, just beginning, the US 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 19486. The Mk.27 Mod.0 torpedo had been a useful anti-escort weapon during the last eleven months of WW II. 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 and an improved battery7.
6 See Thomas J Pelick "Post-WW II Torpedoes 1945-1950" Submarine Review July 1996, pp.94-99. A January 1948 intelligence report crediting the USSR with 229 confirmed submarines is cited in Norman Polmar and Jurrien Noot "Submarines of the Russian and Soviet Navy, 1718-1990" Annapolis: US Naval Institute Press, 1991. It seems unlikely that the Korean War which was unexpected and began in June 1950 had any impact on the decision to begin the Mk.27 Mod.4 development.
7 Mods.1 and 2 were also a little over ten feet long and had larger warheads.
Mod.3 was unique in having a gyroscopic control for initial runout making a stand-off offensive rather than purely defensive anti-escort weapon. When the Bell Telephone Laboratories withdrew from the torpedo program at the end of WW II, six Mk.27 Mod.3 torpedoes had been completed and three were ready for field testing8. 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 very expeditiously developed by ORL engineers. What emerged was a 19" x 125.75", 1175 lb torpedo with a 128 lb 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.27ís9. 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. Mark 27 Mod.4 was not, however, fast enough to make a successful attack an alerted 17 knot Type XXI submarine. With that proviso, the Mk.27 Mod.4 was an available, high performance antisubmarine/anti-surface vessel weapon for US submarines. This was the first submarine launched torpedo capable of attacking submerged submarines adopted for US fleet use. About three thousand were procured from AVCO Corp. and Naval Ordnance Plant Forest Park between 1949 and 1954. Mk.27 Mod.4 was gradually replaced by Mk.37 Mod.0 between 1956 and 1960.
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 WW II projects at Harvard Underwater Sound Laboratory (HUSL) and the Penn State Ordnance Research Laboratory (ORL).
8 Bell Telephone Laboratories "Torpedo Mark 27", Report 6.1-sr1294-2338 to NDRC/OSRD dated 17 August 1945.
9 Illustrations in the Ordnance Pamphlet for the Mk.27 Mod.4 torpedo, OP 699, show body mounted hydrophones. There may have been experimental modes with nose mounted transducers.
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.18 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 thirty torpedoes for development testing in 1955-56. Large scale production was undertaken at NOP Forest Park IL and the Mk.37 began its long career as the primary US submarine launched ASW torpedo. The Mk.37 Mod.0 was 19" in diameter by 135" long; weighed 1430 lb; used two speed, 26 knots (10,000 yards) and 17 knots (23,000 yards), electric propulsion; and carried a 330 lb 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 fifteen 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 an old idea11, 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 WW II fleet boats.
10 The active homing system as developed by ORL is often called the "project 4 panel". The designation "panel" arose because torpedo electronics were arranged on circular panels in the Mk.24 and the name simply stuck.
11 Wire guidance was used in the nineteenth century Nordenfeldt and Sims-Edison torpedoes. The idea had been pursued, though not in conjunction with acoustic guidance, by the German torpedo establishment during WW II and a wire guided shore based German torpedo, called SPINNE (T10), was developed. This torpedo carried over 5000 yards of wire and was built in small quantities. After the war the Royal Navy experimented with wire guidance for torpedoes using SPINNE wire dispensers, but prototypes of useful service weapons were not produced until 1955.
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 manually 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 submarine12. 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.0, but it offered greater target acquisition effectiveness was 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 antisubmarine weapon until the submerged speeds reached the 20+ knot13 range and diving depths began to exceed 1000 ft. The probability of sinking or seriously damaging a submarine capable of over twenty knots with a twenty-four knot torpedo is unacceptably low (unofficial figures give 10% 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 US Navy, probably wisely, developed new torpedoes to address the new threats.
12 † The last of these is relatively unimportant for a quiet torpedo, but for a high speed, noisy torpedo it would be a distinct disadvantage. Later guidance paradigms avoid this particular problem. The other aspect of the argument is that a faster torpedo requires submarine maneuvering limitations for a shorter time.
13 Nautilus, SSN 571, was commissioned in 1954 and was capable of submerged speeds in excess of 20 k. The first Soviet nuclear powered submarine was laid down in 1954 and completed in 1958. By 1962 the Soviet navy had completed perhaps as many as twenty-three, ten (of thirteen) NOVEMBER, eight HOTEL and five ECHO I, nuclear powered submarines capable of submerged speeds greater than twenty knots and the large Echo II class was on the way. Initial estimates of the speed of the NOVEMBER class were low. It was eventually learned that these submarines were capable of 28-30 knots submerged.
† Two solutions to the high speed, deep diving submarine 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 k 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 FY6314. 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-WW II 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 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 ft depth limit. After a bidder qualification exercise and competition between the qualified bidders, a project definition contract was awarded to Westinghouse. 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.0's 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.
14 These dates are from the unclassified versions of SecDef reports for the appropriate fiscal years.
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 Gould15 (formerly Clevite). The first Mk.48 Mod.1 torpedoes were delivered to the fleet in 1972, twelve years after the development characteristics had been approved.
† The Mk.48 Mod.1 torpedo was 21" by 230", weighed 3440 lb and carried a warhead with 650 lb 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) href="#fn16">16. These groups and their contents are briefly: 1) the nose, containing the acoustic system and the homing control logic (HCL); 2) the warhead, containing the high explosive, exploder and the Mk.12 electronic assembly, which is presumably a proximity fuzing device; 3) the control group, comprising the command, gyro and power control units; 4) the fuel tank containing not only the fuel but also the guidance wire dispenser; and 5) 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.
15 Both the bureaucratic process and the contractor base have convoluted histories. The former occurred during the early McNamara years and rivals the TFX (F-111) in complexity and political undercurrents. Among the contractors, in 1969 Clevite and Gould merged with Gould being the surviving name. To further confuse the situation Westinghouse bought the Gould torpedo business in 1988. In March 1996 the Westinghouse defense and electronics business was sold to Northrop-Grumman. Gould produced the bulk of the Mk.48 torpedoes and Hughes and Westinghouse produced the ADCAPs.
16 This description is based primarily on Jane's Weapon Systems" 1986-87 and 1987-88 editions.
The combination of substantial on-board 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 ft and submerged speeds under 30 knots. Early Mk.48 capabilities were clearly capable of attacking such targets. The advent of the Soviet ALPHA submarine 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, 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.
17 Construction of the first ALPHA submarine began in 1965 and was completed in 1971, but it suffered manifold problems. The second was completed in 1979 and followed by five more. Unclassified photographs of ALPHA appeared in 1978-79 with rudimentary legends. Unclassified Congressional testimony in 1982 indicates that the Navy was aware of the ALPHA program in 1976-77. The ALPHA submarines may have been viewed as precursors to large scale serial production of submarines with similar characteristics, however, the SIERRA class has a reported diving depth of 2100 feet and submerged speed of 34 knots. The later AKULA class is reported to have a diving depth of 1300 ft, a submerged speed of 35+ knots and, for the improved AKULA, a greatly reduced acoustic signature. Assuming that these reports are reasonably accurate, the high speed, deep diving threat that materialized was not as severe as that presaged by the ALPHA.
15 According to Friedman (WNWS 1991-92, p.713) the designation EX 49 was assigned to the new torpedo in 1977 followed by Mk.49 in 1984. Mk.49 was, however, never used and the torpedo is known only as Mk.48 ADCAP or simply ADCAP, derived from advanced capability
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 modifications have been made or incorporated in new construction and the ADCAP is the principal torpedo for attack submarines. Trident SSBNís, however, continued to carry Mk.48 torpedoes, though the appropriate fire control modifications may be being implemented.
† Several other heavyweight torpedo projects were initiated after WW II. 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-WW II US Navy torpedo development are relatively easy to identify. Soon after the end of WW II, the principal target became the submarine with surface vessels really secondary targets at best. Two types developed, heavy torpedoes for submarines and lightweight 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 thirty 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 WW II evolved into sensitive, high power, long range systems operating in both active and passive modes. Wire guidance was added to heavyweight 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, on-board signal processing and command logic were added. Modern USN torpedoes are sophisticated guided weapons capable of following instructions delivered by wire or operating autonomously to attack and, if necessary, re-attack their targets.