Part Three: WWII Development of conventional Torpedoes 1240-1946
As we have noted earlier, the entry of the United States into WWII led to major changes in the torpedo situation. Huge quantities were required, operational experience exposed problems in service torpedoes and there were needs for new kinds of torpedoes. In this part we consider the new conventional, by which we shall mean non-homing, torpedoes that were developed as part of the WWII research and development effort.
The explosive growth in the number of torpedoes under development, 21 distinct marks, during the four years of U.S. involvement in WWII, was remarkable. The pace was much slower, both before and after; 20 in the entire SO years from 1889 through 1940 and only 13 since 1950. Another change was the involvement of the National Defense Research Committee (NDRC) in torpedo studies, which marked the beginning of the end of the Newport Torpedo Station’s monopoly on torpedo rich and development. University and industrial laboratories became involved through the NDRC. These organizations greatly expanded both the industrial capabilities and the intellectual scope devoted to torpedo research and development and became the primary performers in this realm. Torpedo production was expanded by using manufacturing firms and Government Owned Contractor Operated (GOCO) plants as well as the traditional Navy facilities. Of the roughly 64,0001 torpedoes produced during WWII the Naval Torpedo Stations produced about 46 percent, the GOCOs about 31 percent and the industrial firms about 23 percent.
The Navy, however did not dominate WWII torpedo research and development. Of the new homing torpedoes, which will be discussed in a subsequent part of this series, only one, the Mk 34, was developed entirely by a U.S. Navy activity. Two others were developed in joint Navy/contractor programs. In the realm of conventional (non-homing) electric torpedoes the Navy led the NTS Newport/GE/Exide team that developed the Mk 20 and worked with GE to develop the Mk 36. In addition to the Mt 34. the Navy was solely responsible for the development of the Mt 23, single speed version of the Mt 14. and the Navol torpedoes Mk 16 and 17. A Navy (NTS Newport) monopoly of the torpedo business such as existed with steam torpedoes from 1922 to 1941 disappeared and bu not been re establisbed in the years since WWII. 2 Full scale production of torpedoes at NTS Newport ended in 1946 llld the Goat Island facility was totally closed by 1951. Navy torpedo research and development did continue in the Newport area at a new facility at Coddington Cove.
Conventional (Non-Homing) Torpedoes
In addition to modifications of existing torpedoes, entirely new and significantly changed conventional torpedoes were developed. The two major areas where new developments were made were propulsion and warheads. The major propulsion developments were the use of Navol (a 70 percent solution of hydrogen peroxide, H2O2 in water) to supply the oxygen for combustion in steam torpedoes and the development of successful electric torpedoes. The most important, but often overlooked, warhead development was the conversion from TNT to Torpex with the attendant increase in underwater damage by over SO percent for a fixed weight of hip explosive. Altogether nine of the eleven conventional torpedoes shown in Table 1 were under development during WWII. Tho other two were the last two conventional torpedoes developed by the U.S. Navy and are included to complete the history of conventional torpedoes.
Of these eleven torpedoes only four were issued as service weapons, and of these four only one, the Mk 16 survived after 1950. Further, the Mk 23 was a simplification of the existing Mk 14 torpedo that was made to accelerate production. This does not mean, however, that these torpedoes were unimportant. The wakeless electric Mk 18 sank about a million tons of Japanese shipping in the last years of WWII and the Mk 16, though not used in combat during WWII, was a standard submarine weapon until 1975.
Ever since Robert Whitehead invented the self-propelled torpedo, a key problem has been how to carry enough energy on board to provide the desired range and speed. Burning organic fuels, hydrocarbons or alcohols, represented a huge improvement over compressed air alone, but further progress required improved oxidants. There are two obvious problems in using compressed air as the oxidant, air is only 23 percent oxygen and storing enough air for reasonable range and speed requires air pressures over 2500 psi and consequently a heavy. high performance air flask. Two workable solutions to the oxidant problem were found before the end of wwii, the use of pure oxygen (or a mixture of oxygen and air) and the use of a concentrated solution of hydrogen peroxide in water. Each of these has been tried with varying degrees of success by several navies and high test peroxide (HTP) torpedoes are still being produced, particularly in Sweden. The U.S. Navy experimented with pure oxygen’, but did not go very far with it. Experiments with chemical propulsion, that is, propulsion using energy derived from exothermic reactions, started with internal funding in 1915 at Westinghouse Electric and Manufacturing Co. and continued there with Navy funding from about 1920 until late 1926. The Navy returned to the study of chemical propulsion in 1929 with a program at the Naval Research Laboratory. By 1934 Navol, a concentrated solution of hydrogen peroxide in water, and alcohol became the preferred energy source. This system produced some thermal energy from the exothermic decomposition of the hydrogen peroxide, which also yielded free oxygen. Additional energy was produced by using the oxygen to bum alcohol. The first Navol or chemical torpedo was a converted Mk 10 which was subjected to tank dynamo-meter testing and rqed at Newport. It achieved a range almost three times that of a conventional Mk 10. With this success, a Mk 14 was converted and achieved an almost four fold increase in range. These results led to plans for the production of Mk 17 torpedoes as armament for new destroyers. The program was interrupted shortly after Pearl Harbor by the need to produce standard torpedoes, especially Mk 13 and Mk 14, in an attempt to satisfy urgent fleet requirements. There was no further progress until 1943 when a re-examination of the program determined that the supply of Navol was inadequate. Plans were made (Qr a new production plant, but it was delayed and not finally started until the fall of 1944. Also in 1943 the design of the submarine launched Mk 16 Navol torpedo, with the same envelope as the Mk 14, was begun. Solid knowledge and speculation about the very long range, high speed Japanese 24. Type 93 destroyer launched torpedo’ probably fueled the development of Navol torpedoes. Several hundred each of Mk 16 and Mk 17 torpedoes were completed before the end of WWII, but neither saw use in combat.
The virtues of hydrogen peroxide are that it is a liquid, over 90 percent oxygen by weight as compared to air which 23 percent oxygen, and has a specific volume (volume per pound) about one fifth that of 2800 psi air. In the decomposition of the peroxide, 2H20, … 2H20+ 0 2, over 48 percent of the oxy1en becomes available. Thus about 34 percent of the oxygen in standard Navol (70 percent hydrogen peroxide dissolved in water with stabilizer added) is available for combustion. Navol will provide oxygen to burn about SO percent more fuel than the same weight of air. In addition the decomposition is exothermic and the heat so produced is also useful for propulsion. The water in the Navol and that produced as a decomposition product are converted to steam reducing the amount of fresh water that must be carried. Essentially the entire weight of Navol is used for propulsion. Also, Navol is a liquid and requires only about one pound of steel tank.age to store one pound, whereas 2800 psi air requires about four pounds of air flask per pound of air. When all of these factors are taken into account, Navol can, for a torpedo of fixed range/speed performance and size, dramatically reduce the weight and volume devoted to fuel and oxidant. The same amount of energy as provided by a pound of alcohol, air, water and tank.age can be supplied by about a quarter of a pound of alcohol, Navol, water and tankage and the volumetric saving is even greater. The weight and volume so saved can be used to increase the range and/or provide for a much larger warhead. In addition, there is no inert nitrogen, the principal component of torpedo wakes, in the fuel or oxidant. The combustion products themselves are very soluble in water and so the torpedo is practically wakeless. Unfortunately, there is a risk of uncontrolled decomposition of Navol and the attendant explosive hazard. HMS SIDON was lost in 1955 to just such an accident. The comparison between the Mk 14 and Mt 16 is shown in Table 2.
Both a larger warhead and greater range were provided in the Mk 16 with no sacrifice of speed. Some other components of the Mk 16 differed slightly from those of the Mk 14, in particular, the turbine axis was horizontal rather than vertical and gearing consisted entirely of spur gears. High pressure air, to pressurize expendables containers and power the control, was provided by a five cubic foot, 2800 psi air flask, a little over two feet long. Subsequent Mods of the Mk 16 bad slightly larger warheads, substantially increased range and in some cases a pattern running capability. After wwn the Mk 16 family was extended through Mod 8 and remained in use in submarines until the mid 1970s. Its performance made it a truly formidable weapon. There were occasional problems with spontaneous decomposition of the Navol, and opinions about safety differed with some individuals feeling it was too risky for submarine service. The Mt 17 destroyer torpedo was a larger version of the Mk 16. Both of the Navol torpedoes were good weapons, but their development programs were slow and erratic. One must wonder what impact these would have had if they had been available in 1943 or 1944, especially in view of their larger warheads.
Electric propulsion systems have two apparent advantages: they are wakeless so they do not provide either warning of attack or indication of the location of the attacker’ and they require both less manufacturing effort (estimated for Mk 18 at 70 percent of that required for a comparable steam torpedo, Mk 14) and a lower average manufacturing skill level. These advantages are, however, purchased at the price of significantly shorter range and lower maximum speed; Mk 18 had a range of 4000 y at 29 k.6 U.S. Navy interest in electric torpedoes began in 1915 with a project at Sperry Gyroscope Co. Successor in-house projects, again sporadic, produced designs and development models designated EL and Electric Torpedo Mk 1. Interest was, however, limited by the inferior speed-range characteristics of electric torpedoes. Shortly before U.S. entry in WWII, possibly stimulated by knowledge obtained from British sources that the German Navy was using electric torpedoes, work resumed on electric torpedoes. The resulting design was first designated Electric Torpedo Mk 2 (1941) and later Mk 20 (1943). Twenty of these torpedoes were eventually produced by the General Electric Co. Slow progress on the Mk 20 led to the Mk 18 project which came to be based on the German G7e and was ready for production significantly sooner than the Mt 20.
The major problems in building electric torpedoes are storing enough energy on board to give adequate range and speed and providing, within stringent weight and space constraints, a sufficiently powerful electric motor to achieve the speed. A 21 inch torpedo requires about 100 hp to make 30 k and well over 300 hp to make 45 k. Thus at 30 k a five minute (5000 y) run requires a power plant capable of delivering about 75 kilowatts of power for five minutes-6250 watt-hours. Even with the inevitable losses and taking into account the rapid discharge, batteries that could deliver the required power for four to six minutes could be designed with late 1930s technology, but their weight, about 1500 pounds or roughly half the weight of a Mk 14 torpedo, and volume, over ten feet of a 21 inch torpedo envelope, were serious constraints. These constraints were not significantly lifted until the advent of seawater batteries which enabled U.S. electric torpedo speeds and ranges to exceed 35 k and 5000 y. Severe though the battery problem was, the motor problem was even more difficult. Conventional design of a 100 hp motor might have produced a machine that would fit into a torpedo, but it would have weighted 500 to 1000 pounds. What was required was relaxation of some of the design rules. The critical point was the recognition of the fact that the torpedo motor needed to run only five or so minutes after which it was either lost or, in exercise shots, could be refurbished. Thus severe but short term beating, e.g., 1OO°C in five minutes, and sparking commutators, among other engineering anathemas, could be accepted. With these and other concessions, it became possible to build motors in the 100 hp range that
weighted about 250 pounds, a weight that the 21 inch x 21 feet envelope could accommodate.
The first knowledge of German electric torpedoes came from recovered fragments of the four that sank HMS ROYAL OAK in September 1939. Additional information was obtained from the torpedo that struck SS VOLUNDAM. The first complete German G7e torpedoes were acquired when the German submarine U-570 was captured by the RAF on 27 August 1941. One of these was made available to the U.S. Navy in January 1942 and other G7e torpedoes were found, at about the same time, on the East Coast U.S. beaches. This information stimulated U.S. Navy interest in quickly obtaining electric torpedoes. Following a preliminary meeting on 10 March 1942, Westinghouse was placed under contract to produce an electric torpedo, which, it was quickly agreed, would be an American version of the G7e. The new torpedo was designated Mk 18. This project wu of course competitive with the Mk 2/Mk 20 project and so got little help from NTS Newport. Never the less, in late une of 1942, just 15 weeks after starting work on the project, the first Mk 18 was delivered to Newport for testing. The testing did no go well, Newport was unhelpful if not obstructionist and production was delayed. Again, as a result of pressure from the operating forces action came from CNO/COMINCH Admiral King, who ordered an Inspector General investigation on S April 1943. The much quoted report of that investigation, which was issued in June 1943, says in part:
“The delays encountered were largely the result of the manner in which the project was prosecuted and followed up. These difficulties indicated that the liaison officers at the Bureau of Ordnance failed to follow up and properly advise the Westinghouse Company and Exide Company during the development of the Mark 18 torpedo. … The Torpedo Station had its own electric torpedo, the Mark 2, and the personnel assigned to it appear to have competed and not cooperated with, the development of the Mark 18. .. . Failure to provide experienced and capable submarine officers to the Bureau for submarine torpedo development has been a very serious matter and has contributed largely to the above deficiencies.”
Deliveries of the Mk 18 to the fleet finally began in mid 1943 and they were taken on patrol as early as September 1943. There were, however, continuing difficulties with the new torpedoes, which were not fully resolved until late in the year. About 9000 Mk 18& were produced and they accounted for 30 percent of the torpedoes fired by U.S. submarines in 1944 and 70 percent of those fired in 1945. Though slow and short ranged, the Mk 18 served well in attacking Japanese merchant ships which were the main targets for U.S. submarines during WWII, especially late in the war. Mark 18 accounted for about al million tons out of the 4.8 million ton total of Japanese merchant shipping sunk by submarines during WWII.
The second major development, new warheads, involved the switch from TNT to Torpex as the high explosive. Torpex is a mixture rather than a pure chemical compound as TNT is. The components are TNT 41 percent, RDX (Cyclonite, Hexoaen) 41 percent and aluminum powder 18 percent.’ Torpex is attractive because of the increased explosive energy and higher detonation velocity of RDX as compared to TNT and the prolongation of the pressure wave by the aluminum. On a weight basis, Torpex is conservatively about SO percent more effective than TNT as an underwater explosive against ships. Torpex is, however, more sensitive than TNT and RDX wu expensive and difficult to make safely. The proceu of converting to Torpex torpedo warheads (and depth charge loadings) began with an order for 20 million pounds in early 1942.11 The fint Torpex loaded warheads’° followed late the same year. The 640 pounds of Torpex in a Mk 14 warhead was at least the equivalent of 960 pounds of TNT’1 almost twice the destructive power of the original Mt 14. The reaction of the submariners to Torpex is apparent from an entry for 19 March 1943 in the fourth war patrol report of USS WAHOO: awful wallop.”
This very substantial improvement in warheads is often overlooked in part because the torpedo identification does not automatically identify the warhead and even the warhead Mart doesn’t unequivocally identify the high explosive. Some Mk 14-3A torpedoes were fitted with TNT warheads, most commonly Mt 15, and others with Torpex warheads, most commonly Mk 16. Furthermore torpedo warheads could be easily changed by a tender or depot. The standard COMSUBPAC format for war patrol reports did not require listing torpedo or warhead Marks and Mods until after April 1943.
Several other interesting and important developments were incorporated into WWII conventional torpedo development programs. The most prominent of these were electric controls, seawater batteries and pattern running. Electric controls were standard in homing torpedoes, but the control system dynamics are different for gyroscopic course control. The Mk 18 electric torpedo, as we have noted, used pneumatic controls for several reasons: The German G7e used pneumatic controls; the reliability of pneumatic controls was well established; and there was a risk that using an electric control system might introduce instabilities that would be time consuming to resolve. The Mt 19 torpedo was a Mk 18 with an electrical proportional servomechanism for depth control and solenoid positioned vertical (course control) rudder. The Mk 19 gave way to the Mk 26 which bad similar controls and a seawater battery. About 25 Mk 26 torpedoes were produced but large scale production was deferred in favor of the NTS Newport and General Electric Mk 36 which was also an all electric and seawater battery powered design that was an outgrowth of the Mk 20 program and incorporated a pattern running capability. One or two developmental models of the Mk 36 torpedo may have been built, but it too was deferred in this case in favor of the Mk 42.
The seawater battery was important in that it made possible electric torpedo performance comparable with that of the Mk 14 steam torpedo. Two developmental seawater battery powered torpedoes have been included in Table 2 for comparison purposes. The seawater battery powered Mk 26 was a little slower but longer ranged than the Mk 14 and had the same propulsion figure of merit. The projected Mk 36 represented a substantial improvement over the Mk 14 and had a figure of merit exceeded only by that of the Navol Mk 16.
The basic idea of the seawater battery is to construct a primary battery using seawater as the electrolyte. With this electrolyte a magnesium anode and a silver chloride cathode make a useful 1.SS volt cell. It required some development effort to produce a satisfactory cathode-the principal problem was the high electrical resistance of silver chloride, but these problems were solved. Bell Telephone Laboratories designed and the General Electric Company built the battery for the Mk 26 torpedo. These batteries were evacuated to keep the electrodes dry before use and to provide for rapid filling when the torpedo was launched. They delivered about three times as much energy as the lead acid batteries in the Mk 18 and weighted significantly leas. With this sort of performance seawater battery powered torpedoes became competitive and, though none of those under development during WWII became service weapons, both the Mk 44 and Mk 4S post war service torpedoes used this propulsion scheme. The consumption of expensive silver and the attendant high cost, $6000 to $8000 per unit, was an obvious drawback.
For completeness, we now briefly consider pattern running. The concept is to program a torpedo to make a straight run to a target rich area, for example, the middle of a convoy, and then execute a pattern hoping to hit a target. This is obviously distinct from homing although some homing torpedoes have been programmed to run a straight course and then execute a search pattern for the purpose of acquiring a target on which to home. The pattern running concept has some instinctive appeal in that it would appear to improve the probability of hitting some target. This appeal was enough to induce the German Navy to mount two programs FAT and LUT.13 The U.S. Navy included pattern running in the Mk 36 and Mk 42 development programs, but neither of these entered service. Some Mods of the Mk 16 were equipped with pattern running controls which caused the torpedo to run in circles of 300 yard radius after a straight run of preset length. Pattern running mechanisms in the days of electro magnetical, as opposed to electronic, controls involved complex arrays of cams, gears and levers that were difficult and expensive to design and build. Furthermore pattern running seems to be much less effective than instincts would predict. Roessler sums up the situation in very few words, .. This appears unprofitable.”
The remaining new non-homing torpedoes comprise the Mk 25 which was an improved Mk 13 air launched torpedo and the clearly asset WWII Mb 40 and 42. Mk 25 was a successful design that completed development late in the war. It was not produced in quantity because of hu1e existing stocks of Mk 13 torpedoes. Before these stocks bad been consumed the anti-surface ship mission of air launched torpedoes bad disappeared. the Mk 40 propulsion system was interesting in that it used a multi base solid propellant to produce gas to drive a turbine, which, in turn, drove a pump jet propulser. Such systems became important much later when targets became fast nuclear submarines and will be discussed in more detail in a subsequent part of this series. Mark 42 was an attempt to consolidate into one torpedo all that had been learned about torpedo sub-systems. The program seema to have toppled from its own weight five organizations bad significant involvement in the program, This it was abandoned in favor of a pattern running Mod of the Mk 16. Mark 42 was, however a significant milestone in that it was the last mark assigned to a U.S. Navy non-homing torpedo.
While it does not represent a new torpedo, the large scale research and development program aimed at understanding the dynamics of air launched conventional torpedoes and improving their performance deserves note. This program, carried out mainly at Columbia University and the California Institute of Technology, developed an understanding of the air flight of torpedoes and the problems of water entry. The most visible results were frangible wooden tail extensions and nose drag rings, which were ugly, but stabilized the air flight and reduced the water entry speed. Less visible were the structural changes in the Mk 13 torpedo that were developed to accommodate the large and complex forces associated with water entry.
In the next part of this series we will examine the radically new development of homing torpedoes during WWII.