Part Six: Post-WWII Air-Launched/Lightweight Torpedoes
Post-WWII lightweight and air launched torpedo development was subject to constraints similar to those that affected the development of submarine launched-heavyweight torpedoes. Changed defense economics and a changed world political scene led to changed policy and changed requirements. In addition, however, an objective review of the use of air launched torpedoes during WWII showed that fewer than 1500 torpedoes bad been launched by U.S. Navy aircraft against surface vessels. Increased and more effective antiaircraft armament bad made attacking surface warships with air-launched torpedoes a risky business. About 350 torpedoes were, however, dropped in October 1944, primarily during the Battle of Leyte Gulf, with considerable success. Furthermore, new aircraft were not tolerant of large heavy external loads and moved in the direction of smaller fuselage cross sections that did not readily accommodate torpedoes with the Mk 13 envelope. A few hundred homing torpedoes with the 22.5 inch x 161 inch Mk 13 dimensions were, however, produced after WWII. These new torpedoes were not used to any appreciable extent as service weapons and there were, in any case, large stocks of the venerable Mk 13. Most heavy air-launched programs were terminated at the end of World War II. The air launch requirement for the Mk 35 was eliminated and only 200 Mk 41(a 1327 pound simplified Mk 35) air-launched 21 inch torpedoes were produced for evaluation.
With the added stimulus of the major U.S. Navy focus on antisubmarine warfare, post-war air launched torpedo development focused on lightweight, generally less than 1000 pounds, acoustic homing, anti-submarine torpedoes. Although modem lightweight and heavyweight torpedoes had their respective origins in the very similar Mk 24 and Mk 27 Mod 0 torpedoes of WWII, they have become very different not only in size and weight, but also in their attack paradigm. A heavyweight torpedo is launched thousands of yards from its target, well beyond the acquisition range of the homing system. It must then travel to the vicinity of the target, acquire the target and then attack. A lightweight torpedo is normally delivered to the near vicinity of its target by the launch platform (aircraft, surface vessel, CAPTOR mine or ASROC). In early weapons, no provision was made for a straight run; helical search began on water entry or after a large angle dive to an initial search depth. Current torpedoes, however, typically provide for a preset straight run up to about 1000 yards. A concomitant of the smaller size and weight as compared to heavyweight weapons is that the endurance at maximum speed is typically smaller, less then 10 minutes for most U.S. lightweight torpedoes.
The passive homing Mk 24, which has been discussed in an earlier part of this series, was available at the end of WWII and continued in use into the 1950s. It deserves to be called the first lightweight homing torpedo. Development of a modestly improved, heavier version, Mk 34, continued. Mk 32, the successful GE active homer with the same envelope as the Mk 24, was resurrected, improved and put into production as an air or surface vessel launched ASW torpedo. Other active homers Mks 43, 44 and 46 followed. The last of these still serves with U.S. forces and the Mk 44 survives in many navies and possibly as reserve stock for the U.S. Navy. Mks 50 and 51 were competitors to replace Mk 46. In the swim-off Mk 50 was chosen but only very limited quantities have so far been procured. Production ended in 1996.
Development of an improved air launched passive homing torpedo began in 1944 as the Mine Mk 44 project at Mine Warfare Test Station, Solomons, Maryland. The torpedo that was developed and designated Mk 34 was 19 inches in diameter and 123 inches long. It weighed 11120 pounds, not really lightweight, and carried a 116 pound HBX warhead. It was thus substantially larger than Mk 24. The homing system of the first version of the Mk 34, Mod 0 (1945), had a passive acoustic homing system which was essentially identical to that used on the Mk 24. Propulsion was improved with a larger battery, but the same motor, which increased the maximum speed to 17 knots. The new propulsion system also provided a two speed capability, with the switch to high speed triggered by the acoustic signal. These improvements were accommodated by using two Mk 24 cylindrical sections joined by a mating ring. Both sections contained lead acid storage batteries for propulsion. The forward section contained the hydrophones and the after section the control panel. Since there was no control panel in the forward cylinder, a significant volume was available for an auxiliary high explosive charge. In retrospect, it would have been easy, and possibly more appropriate, to designate this torpedo Mk 24 Mod X. The Mk 34 Mod O was produced in limited quantities. Its development was completed too late for it to be used in WWII. In 1948, with growing U.S. Navy concern about the Soviet submarine threat, a program was initiated to improve the Mk 34 and issue it as at least an interim ASW weapon. With major contributions from the Penn State Ordnance Research Laboratory, the design was improved and prepared for production. Over 4000 Mk 34 Mod 1 torpedoes were produced by American Machine and Foundry; the Naval Ordnance Plant, Forest Park, Illinois; and the Naval Mine Depot, Yorktown, Virginia. Beginning in 1948 the Mk 34 supplemented the Mk 24. It remained in service until about 1958, when the first post-WWII lightweight torpedo, the Mk 43, began to see service with the fleet.
Beginning in 1950, the truly lightweight Mk 43 was developed against a weight limit of 350 pounds. Two competing versions of this torpedo were developed, the 12.75 inch Mod 0 General Electric design, of which 500 were produced for evaluation, and the 10 inch Mod 1 (and later Mod 3) which was selected for full scale production. The Mod 1 torpedo drew on the experience of Brush Development in developing the 10 inch Mk 30, which, as noted earlier, was the successful, but not procured, backup for Mk 24. The Mk 43 Mod 1 torpedo weighed only 260 pounds so that even the small helicopters of the 1950s could carry one or two of them. There wu no need for specialized torpedo bombers to carry torpedoes of this size. Active acoustic homing wu used. The 54 pound warhead could accomplish mission kills, but actual sinkings would probably have been fortuitous. Two other limitations of the Mod 1 were its 15 knot maximum speed and 650 foot maximum depth. Both of these were remedied in the Mod 3 which had a maximum speed of 21 knots and a maximum operating depth of 1000 feet. Mk 43 Mod 3 also had fins that did not extend beyond the body. so it could be launched from Mk 32 tubes on surface vessels. Inserts were, however, required to accommodate the 10 inch diameter. There wu little overlap between the Mk 43 torpedo and the Mk 32 tubes in the U.S. Navy, so this launch mode wu not of major importance. The Mk 43 entered fleet service in 1951 and Mod 3 remained in service together with the Mk 34 until 1957 when the Mk 44 began to rather quickly replace both.
Post-war evaluation of the German Type XXI submarine had made it abundantly clear that anti-submarine weapons, including lightweight ASW torpedoes, would have to deal with submarines capable of 20 knot submerged speeds and diving depths close to 1000 feet. These requirements were beyond the capabilities of the Mk 43 torpedoes and posed significant technological challenges. Consequently, about 1952, very soon after the Mk 43 entered production, projects designated EX-2 were started to deal with these requirements. The weight limit was relaxed to 450 pounds and somewhat larger, but much more capable torpedoes resulted. The EX-2A developed at NOTS Pasadena was a 12 inch diameter by 98.5 inch, 415 pound, passive homing torpedo. The General Electric EX-28, which was selected for full scale development and production as the Mk 44 Mod 1 lightweight torpedo, was a pure active homer and marked a return to 12.75 inch diameter that GE had used in the Mk 43 Mod 0. The increased diameter and a slightly greater length, 101.3 inches, accommodated a 75 pound HBX-3 warhead and a greatly improved propulsion system consisting of a seawater activated battery, a 30 hp motor and contra-rotating propellers. This propulsion system gave a range of 6000 yards at 30 knots. Major improvements were also made in the homing system and search programming. The acquisition range was increased to a maximum of 1000 yards, about 25 percent better than the Mk 43. Mk 44 Mod 0 had only a helical search pattern, but the initial search depth could be preset to any one of six levels between 50 feet and 900 feet. Similarly, the search floor could be set to any one of five depths between 150 feet and 900 feet. Later Mods had preset gyro controlled runout up to about 1000 yards. After selection of the EX-2B, final development was undertaken at the Ordnance Systems Division of the General Electric Company at Pittsfield, Massachusetts. NOTS Pasadena was responsible for technical direction. Production began in 1956 and the Mk 44 began to replace the Mk 43 in 1957. Eventually over 10,000 Mk 44 torpedoes were produced at GE, NOP Forest Park and American machine and Foundry for U.S. and foreign navies. Additional torpedoes were produced in the United Kingdom, Canada, France, Italy and Japan. Mk 44 became the NATO standard lightweight torpedo. It could be launched from helicopters, fixed wing aircraft, or Mk 32 torpedo tubes on surface vessels and it was the payload for which ASROC was designed.
Successful as it was, the Mk 44 clearly lost effectiveness as target speeds increased towards 30 knots, and, of course, only rare good fortune would enable it to deal with nuclear submarines capable of 30 knots submerged speed. In response to this threat, the RETORC I program led to the Mk 46 lightweight torpedo much as RETROC II later led to the Mk 48. The final Development Characteristic issued in June 1956 specified, in addition to the ability to attack a 30 knot submarine (roughly 45 knot torpedo speed), an operating depth range from 50 to 1000 feet and a size that would fit the external racks and bomb bays of existing ASW aircraft, i.e., about the same size as the Mk 44 torpedo, 12.75 inches by 100 inches. A competitive bidding process that involved an initial set of 14 competitors resulted in a development contract being awarded to Aerojet General Corporation in May 1958. Under this contract the Mk 46 Mod 0 was developed.
Mk 46 Mod 0 was 12.75 inches by 102 inches and weighed 568 pounds. It was powered by a solid propellant which produced hot gasses to drive an 84 hp axial piston swash plate engine. Unofficial reports credit this torpedo with a range of 9500 yards at 45 knots and 50 foot depth (7000 yards at 40 knots and 1500 foot depth). The warhead carried 96 pounds of H-6 high explosive.
The solid propellant propulsion system of the Mod 0 apparently suffered from maintenance problems which were solved by replacing it with a new, but conceptually similarly system. The new engine was also of the axial piston configuration, but used a sinusoidal cam, rather than a simple swash plate, to double the number of power strokes per revolution. The solid propellant was replaced by a liquid mono-propellant called Otto fuel. This fuel is burned in an external combustion chamber and the combustion products used to power the engine. The shroud ring steering of Mod 0 gave way to conventional fins and the first major production version, Mk 46 Mod 1, emerged. The first production contracts for Mod 1 were awarded in 1965. The propulsion system has remained essentially the same for a series of upgrades and modifications (through Mods SA(S) and 6) which may extend the life of the Mk 46 in U.S. service to SO years. These improvements have mainly affected the control system, but it may be noted that the high explosive was changed to PBNX-103, which is about 25 percent more powerful than H-6 in underwater use. Also, Mods 4 and 6 were developed for use in CAPTOR mines.
Less visible were enormous improvements in the sonar and control systems. Transistorized electronics reduced size and weight reduced power requirements and improved reliability to the point that the Mk 46 could be designed with elaborate control logic and an enhanced acquisition range within the size and weight limits. Even Mod 0 had sufficient acquisition range to make wide vertical apertures possible. These wide apertures made helical search modes unnecessary and greatly reduced search time. A simple circular search at 750 feet was sufficient. In later Mods such a search will, with high probability, acquire a submarine target in a cylindrical volume 3000 yards in diameter and extending from 50 to 1500 feet below the surface. Alternatively for attacks on distant targets a snake search can be set. Sonar characteristics can be set by onboard logic, which also provides attack mode control and re.attack capability. Some aspects of the improvement can be seen in Table I which compares Mk 46 with its predecessor Mk 44 and its successor Mk 50. The Mk 46 Mod 5, NEARTIP (Near-Term Improvement Program) was an almost completely new and greatly improved lightweight torpedo that has taken advantage of further developments in electronics to improve onboard logic, signal processing and the seeker. Propulsion was also modified to provide a second speed for slow, quiet, long endurance search. Over 20,000 Mk 46 torpedoes have been built and delivered to the U.S. and over 20 foreign navies. It is the current NATO standard lightweight torpedo.
The final U.S. lightweight torpedo developments through 1996 are: the Mk 50, named Barracuda by Honeywell; and the Light Hybrid Torpedo (LHT). The Mk 50, for which some characteristics are given in Table I, originated as a response to the high speed, deep diving threat presaged by the Soviet Alpha submarine. Development of the Advanced Lightweight Torpedo (AL WT), as it was finally called, began in 1972 shortly after the first Alpha was completed. A convoluted acquisition process began with a six year technical assessment phase. Four teams participated in a concept development phase. Two of the four were selected to produce prototypes, Ex-50 and Ex-51, for a swim-off. The Honey-well-Garrett Ex-50 was selected in 1981 and became the Mk 50. Full scale development began in 1983, but it did not go smoothly. In the FY89 SecDef Annual Report dated 18 February 1988 (p. 198) we read dnesign difficulties, however, have dictated a restructuring of the Mk 50 torpedo program. We now anticipate cost increases and a 21 month delay in the Mk SO’s full scale development program. The restructured program will ensure that the torpedo is reliably designed and vigorously tested prior to entering production.” The program did, however, survive, but only a few hundred, rather than the initial goal of almost 8000, were produced. Production for the U.S. Navy ended in 1996 and there are currently no torpedoes of any kind in production for the U.S. Navy.
Table I: Post WWII Lightweight Torpedoes
|Mk 44 Mod 1||Mk 46 Mod 1||Mk 50|
|Threat||Poll WWII Dieasel SS||SSN||Aplba/Mike|
|First Procurmnet||1957||1965||FY 87|
|Diameter/Length||12.75″ x 101.3″||12.75″ x 102″||12.15″ x 111.5″|
|Weight||433 lbt||580 lbt||770 lbt|
|Range/Speed/Depth||6000yd @ 30kt depth Independent 500-1000 ft||12,200 yd @ 45 kt @ 50 ft: 10,100 yd @ 45 kt @ 1500||20,000 yd @ 55-60 depth Independent|
|Propulsion||Electric, Seawater battery||Auto fule, five cylender axial||SCEP-Turbin|
|Aequlaition Range||800 yd (eat.)||1300 yd Average|
|Wadbead||75 lb HBX||96 lb H-6 later PBXN-103||Shaped charge|
Acquisition problems not withstanding, the little detail that has been released about the Mk SO reveal some novel and interesting technology. The Stored Chemical Energy Power System (SCEPS) was one of the most innovative. The basic idea, which is to use an exothermic chemical reaction as a source for heat to drive a thermal power plant, was explored and discarded by the U.S. Navy in the 1920s. The SCEPS, which was developed at the Penn State Applied Research Laboratory, was an entirely new start on this
mode of propulsion. It uses the exothermic reaction of lithium with sulphur hexaflouride as a source of heat to generate steam. The steam drives a tmbine equipped with a condenser so the water is recycled. A unique feature of this cycle is that the combustion products from the combination of lithium and sulphur hexaflouride are solids, rather than gasses, that occupy less volume than the original components. Thus there is no overboard discharge and no decrease in performance at large operating depths (Note that the Mk 46 Mod 1 has 15 to 20 percent less range at 1500 feet as compared to 50 feet). Also interesting is the apparent use of a shaped charge and follow-through warheads in an attempt to disable double hulled submarines.
The LHT is a combination of elements from the Mk 50, the Mk 46, the Mk 48 and ADCAP with improved electronics to make a high performance torpedo with minimal development costs. Reversion to the Mk 46 axial cylinder reciprocating engine combined with the new fuel control valve developed for the Mk 48 rather than continued development of SCEPS is interesting and might reflect cost or maintenance problems with the latter or an effort to establish commonality with the Mk 48/ADCAP torpedo” LHT is expected to enter production around 20017 , this, however, may now be wishful thinking.
Torpedoes for Surface Vessels
As we have noted in other parts of this series, no U.S. Navy torpedoes have been developed and issued to the fleet specifically for surface vessels since the Mk 17 Navol torpedo. Quintuple mounts disappeared from U.S. destroyers as the anti-surface vessel role of destroyers diminished and it became necessary to reduce topside weight and provide space for increased anti-aircraft defense. There were, however, attempts to reintroduce heavy weight ASW torpedoes in destroyers. Fixed 21 inch tubes, usually Mk 25, were mounted on the 0-1 level, in deck-houses and in the transoms of various destroyer type vessels in attempts to adapt Mk 37 and Mk 48 torpedoes to such platforms. The Mk 2 launcher could also launch the Mk 37 torpedo, but this combination was little used. None of these attempts was a great success and heavyweight torpedoes are not, at the present time, part of the standard armament of U.S. Navy cruisers, destroyers or other surface vessels. Lightweight torpedoes have, however, become standard armament for frigates, destroyers and cruisers, but as ASW weapons replacing depth charges rather than WWII types of torpedo armament. The first move in this direction was to use Mk 32 torpedoes launched by Mk 2 launchers which tossed the torpedo over the side much as the WWil PT boat launchers did with Mk 13 torpedoes. Since the late 1950s Mk 32 torpedo tubes, most commonly in trainable triple mounts with Mk 43, Mk 44, or later, Mic 46 torpedoes have been standard ASW armament for surface vessels.
Whither the Torpedo?
We have now looked at essentially all U.S. Navy automobile torpedoes from the 1871 Newport fish torpedo through the Mk 50 and Mk 51. It is perhaps worthwhile adding a few words of speculation about the future of the torpedo as a U.S. Navy weapon. First of all torpedoes have a future, if for no other reason, because they and mines are for all practical purposes the only ASW weapons in service and probably the only ones even on the horizon. Certainly torpedoes that are in some sense better than the existing Mk 46, Mk 48/ADCAP and Mk 50 could be designed and built and improvements in existing torpedoes are possible. One obvious and frequently mentioned improvement would be quieting and efforts in this direction appear to be underway. Shallow water performance of torpedoes has been substantially improved and it will probably be further improved. With current austere budgets, it seems likely that such improvements as are made will be made by modifying existing torpedoes rather than by entirely new designs. Mks 46 and 48 may well have 50 year service lives.