Heavyweight Torpedoes 1918-1945
Mr. Milford is the author of an eight part series on USN torpedoes in THE SUBMARINE REVIEW in 1996 through 1998. He followed with an article on Soviet and Russian post-WWII torpedoes in October 1998. Part I of this Japanese subject appeared in January 2000.
As we have noted, by the end of the First World War the Imperial Japanese Navy (UN) had developed 45 cm torpedoes for submarines and 21 inch torpedoes for surface vessels. These torpedoes were competitive with those of other navies. Other navies had, primarily as a result of WWI, accumulated more current experience in the combat use of torpedoes both as submarine and surface launched weapons. UN participation in WWI did not involve torpedo warfare. Most of the other navies also had made more progress in switching to larger 21 inch torpedoes. Further, Japan lagged in the development of a submarine force. Only sixteen submarines had been completed for the UN by the end of 1918. Major navies, in contrast, had each completed 80 to 100 submarines by that date. Japanese submarine torpedo armament consisted of 45 cm torpedoes and only three (of the sixteen) submarines carried more than two torpedoes. Surface vessels also carried 45 cm torpedoes. The first ship in the UN fitted with 21 inch torpedo tubes was the destroyer URIKAZE, which was completed at Yarrow in 1915, but not delivered to Japan until 1919. The cruisers TENYU and TATSUTA, which completed in 1919, also carried 21 inch torpedoes. Some 21 torpedoes were acquired from Whitehead-Fiume and Jure produced some before WWI. Major production of 21 inch torpedoes began, however, with a· four year run, 1918-1921, during which the Mitsubishi Nagasaki torpedo plant produced over 500 21 inch Type 44 torpedoes. In 1922 this plant began production of21 inch Six Year Type torpedoes with a run of 250. By the time production of this type ended over 3500 had been manufactured.
Thus in the years immediately following WWI the UN torpedo establishment found itself technically competitive with the other major navies, but somewhat behind in torpedo deployment. The subsequent deployment of 21 inch torpedoes on surface vessels was relatively rapid and included both new construction and rearmament of existing vessels. Twenty-seven more submarines fitted with 45 cm torpedo tubes were, however, laid down between 1919 and 1923. The transition to 21 inch tubes began with submarines laid down in 1920-21, somewhat later than in the USN or the RN. The only new Japanese submarines completed after 1923 that carried 45 cm tubes were one experimental submarine (no. 71) and midgets, smaller that 100 t submerged. The universal technical objectives for improved torpedoes included more effective (destructive) warheads, higher speed, greater range and improved accuracy together with good reliability, availability, maintainability, and durability. There are two principal approaches to these objectives, increased size and new technology. The Japanese torpedo establishment pursued both, following rather different lines than those followed by other navies.
The Development of Conventional Heavy (21 inch and 24 inch) Torpedoes
The starting point for the post WWI development of UN heavy torpedoes was the 21 inch Sixth Year Type. As previously described this was a conventional steam torpedo with a four cylinder radial engine that was competitive with torpedoes in service in other navies at the time. There were three key events in the subsequent development of Japanese heavyweight torpedoes: the introduction of 24 inch torpedoes for surface vessels; the use of horizontal double acting engines following a Whitehead design; and the development of oxygen torpedoes. In this section we will consider the first two events, which involve conventional steam torpedoes, saving the fantastic story of the development of oxygen torpedoes for the next section. The specifications of the torpedoes we will be discussing are given in the table.
In torpedo design there is a trade-off among speed, range, and warhead weight. For example, for a given size and propulsion system, warhead weight can be increased at the expense of reduced fuel and oxidant and concomitant reduced range. Providing a larger engine can increase speed, though this may require a modified or new engine design, but range and/or warhead weight must be reduced if the more powerful engine is larger or heavier. One way to mitigating these restrictions is to increase the size of the torpedo. The IJN followed this course by developing the 24 inch Eighth Year Type, a scaled up version of the 21 inch Sixth Year Type, which boasted no innovations other than size. These huge weapons were mounted on cruisers beginning with the Nagara class, laid down in 1920-21, and on destroyers beginning with the Mutsuki class, laid down in 1923-26.
Both the 24 inch Eighth Year Type and the 21 inch Sixth Year Type were conventional designs for their time. Propulsion was provided by an external combustion system burning kerosene with air as oxidant and a four-cylinder radial engine of the White-head/Schwartzkopf type. Fresh water was carried and injected into the combustion chamber for cooling and the thermodynamic advantage of the steam cycle. Performance of the 21 inch torpedo was competitive, but not spectacular. The engine of the 22 inch version had about 50 percent more swept volume and so about 50 percent more horsepower. About 60 percent more internal volume was available for the larger engine, a larger warhead and more fuel and compressed air. Drag, which for a torpedo is roughly proportional to the wetted surface area times the square of the speed, increased by about 40 percent. Thus the maximum speed of the torpedo was slightly increased. The additional volume was used to increase the range by a third and the warhead weight by almost 75 percent to 345 kg (750 lbs). The Eighth Year Type 24 inch torpedo was a formidable weapon. Except for the very limited production of24.5 inch torpedoes designed for HMS RODNEY and NELSON torpedoes this large were not seen until the Soviet Navy introduced 65 cm torpedoes in the 1970s.
The small increase in speed obtained by going from 21 inch to 24 inch diameter reflected a fundamental limitation on the displacement (swept volume) of single row radial engines of constrained diameter. With this constraint, significantly higher power required higher mean effective pressure and/or higher rotational speeds. Both of these possibilities posed serious practical problems so torpedo designers examined alternative engine designs. Several possibilities emerged-the Royal Navy burner cycle engine, two row radial engines in Italy and turbines in the U.S. are examples. A fourth possibility was a two-cylinder, horizontal, double acting engine designed by Whitehead-Fiume around 1909. Before WWI torpedoes using this horizontal engine were sold to several navies and examples were consigned to other Whitehead sites. Among the navies that purchased torpedoes with horizontal engines was the UN, which purchased ten 45 cm torpedoes of this type in 1914. This development was shelved during WWI, but clearly knowledge of it and details of the design were widely disseminated among the numerous Whitehead companies and to at least five navies.2 The major advantage of this engine was that because of its configuration, particularly its horizontal orientation, it could accommodate both larger diameter cylinders and longer stroke. Since it was a double acting engine, it was equivalent to a four cylinder single acting engine. The resulting larger displacement made it possible to fit torpedoes of a given diameter with engines that were more powerful than the four cylinder radials and so increased the maximum speed to about 45 knots. Torpedoes using this engine design were developed for several navies after WWI and in the early 1920s Whitehead-Weymouth was offering to build torpedoes using this new engine configuration for export.
In 1926 the UN ordered the smallest acceptable quantity, twenty, of these new torpedoes from Whitehead-Weymouth.3 The price was 30,000 Yen (about $14,000) per torpedo including exercise heads. In addition Vickers-Annstrong, by then the parent company of Whitehead-Weymouth, was paid a lump sum of 150,000 Yen (about $70,000) for full instruction in all aspects of torpedo design and manufacture. This presumably included a license to manufacture torpedoes of this design in Japan. Whitehead-Weymouth began work on the torpedoes in 1927. During the construction a team of eight Japanese naval officers was in residence in Weymouth and had essentially unlimited access to the Whitehead ,plant. The report produced by this team reportedly ran to sixty bound volumes and must have been a veritable bible of late 1920s torpedo technology.
The Weymouth torpedoes were completed in the fall of 1929 and shipped to Japan. The first service torpedoes with horizontal engines were designated 21 inch Type 89. 4 It is not clear whether or not these torpedoes were 1) simply Weymouth torpedoes fitted with warheads, 2) the result of a production program based on the 1914 acquisition from Fiume, or 3) developed in a program parallel to and based on the Weymouth torpedoes. Type 89 torpedoes were issued to the fleet beginning in 1931. The 21 inch Type 89 torpedo was 125 kg heavier than the Sixth Year Type, but most of that weight, 95 kg, went into a larger warhead. Engine power was
approximately doubled yielding a maximum speed was 45 knots, an increase of25 percent. The range was 5500 meters, somewhat less than that of its predecessor. The two cylinder engine became very popular with the Japanese torpedo establishment and engines of this design powered all subsequent heavy surface and submarine launched torpedoes of the UN except the Type 92 electric.
The two cylinder engine design was quickly enlarged to serve 24 inch torpedoes and the 24 inch Type 90 emerged to replace the Eighth Year Type. This torpedo with its 400 kg warhead and range of 7000 meters at 46 kt was the final development of conventional, heavy, UN steam torpedoes. In addition to steam torpedoes a relatively unspectacular 21 inch Type 92 electric torpedo was designed, but the design was shelved. Ten years later, in 1942, the Type 92 electric torpedo was put into production and about 650 were produced to supplement the Types 95 and 96 submarine launched oxygen torpedoes. At the beginning of the 1930s, Japanese torpedo performance was as good as that of any in the world and the UN enjoyed the substantial, but relatively unknown, advantage of 24 inch torpedoes for surface vessels. This advantage was increased in the next generation of torpedoes, which consisted of oxygen and enriched air torpedoes as discussed in the next section.
Oxygen Torpedoes
Almost as soon as steam torpedoes were developed, it was widely recognized that the energy stored in the compressed air was small compared to that stored in the hydrocarbon or alcohol fuel. The primary function of the compressed air in a steam torpedo was to provide the oxidant for the combustion of the fuel. In particular, the nitrogen in the compressed air contributed very little to the performance of the torpedo, but added considerably to the weight, occupied valuable volume and was largely responsible for the distinctive wake left by steam torpedoes. It occurred to many individuals in the torpedo development community that replacing the compressed air with pure compressed oxygen or finding an alternative source of oxygen, for example, hydrogen peroxide, would be very advantageous. Among others, the Royal Navy and the Japanese Navy began experiments using enriched air, a mixture of air and pure oxygen, in place of air in steam torpedoes. The Japanese experiments began in 1916, but they were abandoned after a few years apparently because of explosions, which occurred at high enrichments, and other hazards. The Royal Navy began experimenting with enriched air torpedoes in the early 1920s. Two working torpedoes of this type, the 24.5 Mk. I and the 21 Mk. VII were developed and issued.
The team of Japanese officers at the Whitehead-Weymouth plant from 1927 to 1929 correctly concluded, on the basis of unofficial information and observation, that large diameter enriched air or oxygen torpedoes were fitted in RODNEY and NELSON. This conclusion was reported back to Japan in the summer of 1928. By the end of the year work on oxygen torpedoes had been resumed at the Torpedo Testing Department of the Kure Naval Arsenal. The first experiments involved modified 24 inch Eighth Year Type torpedoes, converted to use a mixture of 50 percent oxygen and 50 percent air. These torpedoes, which were designated Special Torpedo B, were successfully test fired in 1932. In parallel with these experiments a 24 inch pure oxygen torpedo, Special Torpedo A, was being designed. One of the biggest problems in oxygen torpedoes is that they tend to suffer oil-oxygen explosions in the vicinity of the starting valve when the engine is started. Starting the propulsion system using air and then switching to pure oxygen circumvented this problem in Model One and Two torpedoes. In the Model 3 carbon tetrachloride, a well-known fire suppressant was injected during start up. The amount injected was enough to prevent explosions, but not enough to prevent combustion. The first design was completed toward the end of 1932 and the construction of two trial production torpedoes was initiated. After successful test firing in 1933, Special Torpedo A was temporarily designated 24 inch Torpedo Type 935 (1933 was Year of the Empire 2593) and this designation eventually became permanent. This enormous torpedo came to be known in post WWII years as the Long Lance.6 The Type 93 torpedo was officially adopted for service in 1935 and beginning in 1938 the slightly modified Type 93-1-2 (Type 93 Model 1 Modification 2) was issued to four cruisers, the Myoko/Nachi class. These vessels were fitted with new Type 92 torpedo rubes to fire these 24 inch oxygen torpedoes. Type 93-1-2 torpedoes carried 299 kg (658 lb) of pure oxygen, the equivalent of 1495 kg (3289 lb) of air. Eliminating over a ton of nitrogen made possible a 490 kg warhead and a range of 20,000 meters at 49 knots. Several other versions of the Type 93 torpedo were developed including the experimental high speed, 56 knot, Type 93-2 and the Type 93-3 with a larger, 780 kg warhead and shorter range. Significant production was, however, limited to the Type 93-1-2 and the Type 93-3. The latter arrived rather late in WWII. Only a few were fired in combat and some were diverted to Kaiten or human torpedoes. The Kaiten Type l was based on the Type 93 torpedo. Over 300 were produced using some of the production of Type 93 Model 3 torpedoes. The number of these weapons actually used during WWII was quite small. Successes were claimed for about 50 Kaiten attacks, but Allied records indicate that they sank only two ships, MISSISSINEW A, (AO 59) and UNDERHILL (DE 682).
The other heavy oxygen torpedoes of the UN were the 21 inch Types 94, 95, and 96. The 21 inch Type 94 was really a light air-launched oxygen torpedo and is better discussed in that context. Types 9 and 96 were submarine launched torpedoes that replaced the Type 89. Their general specifications are given in the Table. The Type 95 torpedo was a smaller version of the Type 93. The Type 96 torpedo was essentially the Type 95-1 with less fuel and air enriched to 38 percent oxygen instead of pure oxygen. The range was halved to 4500 meters but this was entirely adequate and further accepted because it alleviated operators’ concerns over the safety of pure oxygen torpedoes in submarines. About 300 Type 96 torpedoes were produced. Compared to the submarine launched torpedoes of other navies, the IJN torpedoes were slightly faster, significantly longer ranged and carried a greater weight of high explosive. The higher speed represented a small, but real, advantage in that it increased the hit probability for a given accuracy of fire control inputs and solutions. Hit probability against a single ship target at 4000 yards is, however, so low, even with relatively good fire control, that the principal utility of long range capability is in browning shots. 7 In most cases better high explosives, Torpex, for example, in U.S. and British torpedoes, more than compensated for the increased warhead weight of Japanese torpedoes.
Japanese torpedoes were not failure free. At the Battle of the Java Sea, 27 February 1942, ten of forty-three Type 93 torpedoes detonated prematurely and none hit their targets.9 A rather large number of submarine launched torpedoes suffered detonator failures as a result of depth instabilities at shallow depths. The accelerations caused by these instabilities were large enough to trigger the ball type detonator and cause prematures. Other problems occurred, but except for the propulsion system, Japanese torpedoes were generally simple, rugged and reliable.
Torpedo supply may, however, have been a problem a full load-out of surface vessels with 24 inch torpedo tubes would have required about 2250 torpedoes including onboard reloads. This plus 360 allocated to Kaitens would account for all of the largest number, 2600, I have seen quoted for Type 93 production. It appears that in some surface engagements older, 24 inch steam torpedoes were fired. In any case, it is clear that Type 93 torpedoes were not in long supply. Japanese submarines were probably short of torpedoes. In 1944, for example, the average number of submarines in commission was 60. The production reported for that year, which, according to the USNTMJ was 960 21 inch torpedoes (16 per boat), was the largest for any year of the war. This was worse than the appalling U.S. situation for 1942 when about 20 torpedoes were produced per boat in commission, but the smaller Japanese submarine force on 1 January 1944 made one patrol per boat and either fired all of its torpedoes or was lost, and the number consumed significantly exceeded the number produced. This rather meager supply of torpedoes was adequate for the way IJN surface and submarine forces operated in the later years of the war, but it is difficult to be sure what was cause and what was effect.
Deployment and Combat Use of Torpedoes by the IJN
We have already commented on the transition from 45 cm torpedoes to 21 inch torpedoes in submarines and surface vessels. Except for MUSASHI and YAMATO, which had no torpedo armament, Japanese battleships were armed with torpedo tubes until the 1930s. Torpedo rubes were removed from the battleships during the 1930s in the course of various reconstructions. All of the eighteen heavy, or A-class, cruisers were built with 24 inch torpedo tubes. Those that were not originally equipped with rubes capable of firing Type 93 oxygen torpedoes were converted before Pearl Harbor. During WWII the torpedo armament was twelve or sixteen 24 inch tubes and usually twenty-four Type 93 torpedoes including reloads.
Fifteen of the light cruisers that were commissioned in the Japanese Navy after WWI were built with 24 inch tubes. Several others were converted to 24 inch tubes. All of these cruisers probably were capable of firing Type 93 torpedoes, but some may not have been issued oxygen torpedoes umil some time after Pearl Harbor if at all. The two oldest light cruisers, three vessels built as training cruisers and two ex-Chinese cruisers, were not rearmed with 24 inch tubes. OYODO had no torpedo rubes. KITAKAMI and OI were remarkable in that they were converted to torpedo cruisers with forty 24 inch tubes in ten quadruple mounts and forty Type 93, oxygen torpedoes. This configuration, however, was not extremely useful and the two ships were further modified. KITAKAMI eventually landed all of her tubes.
Japanese destroyers from the Mutsuki class (1925-27) on carried from four to fifteen 24 inch torpedo tubes. Of the 111 destroyers with which the UN entered WWII, 81 were armed with 24 inch torpedo rubes. The 32 fleet destroyers and 32 smaller destroyers (DE equivalents) that were added during WWII all carried 24 inch torpedo tubes. Twenty-four inch torpedoes were the dominant torpedo armament of Japanese destroyer type vessels. The total 24 inch torpedo load-out, including onboard reloads, of IJN destroyer type vessels that served in WWII was an astonishing 1640!
The IJN developed two major types of torpedo tactics for surface forces, “long range concealed attack” and “close-in strike home” attacks. In long range attacks, which seem to have been abandoned by 1943, large numbers (plans called for as many as 100) of torpedoes were fired from ranges in excess of 20,000 yd. in “close-in” attacks torpedoes were fired at about 4000 yd before opening gunfire. Both of these tactics depended on stealth and that was lost when radar became standard equipment and its proper use was understood. Japanese training, especially for close-in attacks, was ruthless. Significant casualties and damage to materiel were accepted as costs of proficiency. This proficiency was amply demonstrated in the year years of WWII in the Pacific.
The effectiveness of Japanese surface launched torpedoes and tactics is not easy to evaluate. The U.S. Navy lost about 75 DE and larger surface combatants as a result of enemy action in the Pacific. Japanese surface launched torpedoes, in some cases in combination with gunfire, sank sixteen of these: seven and nine destroyers. Six other Allied ships, five cruisers and one destroyer, were sunk by Japanese surface launched torpedoes or torpedoes and gunfire. Eleven cruisers and six of the destroyers were sunk in 1942, one cruiser and three destroyers were sunk in 1943 and one destroyer in 1944. Through the end of 1942 the UN maintained a favorable or at least balanced exchange ratio, i.e., they lost the same number or fewer ships than the U.S. for every class of surface combatants DE and larger except light fleet carriers (CVLs).11 Japanese surface launched torpedoes played a large role in the successes through the end of 1942, but from then on the U.S. lost only four destroyers and one cruiser to these weapons. One of the 1943 destroyer losses, STRONG, was caused by a Type 93 torpedo fired from 22,000 yards, a counterexample illustrating occasional startling success at very long range. U.S. destroyer-launched torpedoes12 sank or contributed to the sinking of two battleships, one cruiser and twelve destroyers. All but one of these sinkings occurred after July 1943.
While, as we have noted, a significant number of Japanese submarines were completed after WWI with 45 cm torpedo tubes, all medium and large submarines, except one, completed after 1923 mounted 21 inch tubes. Most of the large I-series submarines had six or eight torpedo tubes and carried twelve to twenty torpedoes. The medium RO-series usually had four tubes and carried eight or ten torpedoes. TQe 21 inch torpedoes were Sixth Year Type, Type 89, Type 94 and Type 96 as discussed earlier. Altogether about 180 I-and RO-series Japanese submarines were involved in WWII. Rohwer 13 lists about 400 Japanese submarine attacks in which the targets were believed to have been sunk or damaged. Actual sinkings were 1 ~1 merchant vessels, 17 naval vessels (DE and Larger and submarines), three small naval vessels and five naval auxiliaries. Of these, five merchantmen were sunk by gunfire alone. Th!s record, about one sinking per submarine, does not compare favorably with U.S. , British or German results, which ranged from 2- 1/2 to 4 sinkings per boat. These are admittedly crude comparisons, but they are so striking that it seems unlikely that any refinement would lead to a grossly different evaluation. The problem, however, was not torpedo performance or the submarines or the officers and crews. It was, as Morison observed many years ago, doctrine and possible constraints arising from torpedo supply. UN submarines were diverted to supply, recon-naissance and other missions at the expense of anti-shipping work. There were other problems, including command and control, but we say again, torpedo failures, though there were some, were not an important contributor to the comparatively poor record.
Conclusions
Battles and wars are won by a combination of weapons, doctrine, manpower, training and tactics. All of these components are essential. Weapons are unique in that in modem warfare they are complex and their development requires a substantial infrastructure. R&D, manufacturing. and test and evaluation facilities in particular. The young Imperial Japanese Navy was faced with the problem of acquiring weapons when it had no infrastructure. The solution was to import weapons, manufacrure them under license, modify the foreign designs and manufacture modified weapons, and finally design and manufacrure indigenous designs. This efficient and effective strategy was followed in the acquisition of torpedoes for the Imperial Japanese Navy with outstanding success culminat-ing in the development of very large, 24 inch, torpedoes and oxygen propulsion systems. Production, however, was not adequate to sustain WWII operations. The production shortfall was exacerbated by shortages of strategic materials, particularly high performance metals and alloys. Torpedoes are only the weapons. The other ingredients, doctrine, manpower, training and tactics, received appropriate attention leading, by 1941, to an outstanding Japanese capability in surface torpedo warfare. Japanese victories in surface actions in the first fifteen months of WWII in the Pacific were in no small measure due to this capability. The initial advantage was lost because of the rapid growth and acquisition of new technology and operational experience by the U.S. fleet. Japanese submarine doctrine, both strategic and tactical, was defective and torpedoes were in relatively short supply. These factors were in large measure the causes of the poor performance of the Japanese Navy’s submarine force.
