Meanwhile, major moves were made in terms of the propulsion units. By the end of the Great War the standard British torpedo engine was a wet-heater four-cylinder radial made of bronze, with integral cylinder barrels and heads as in contemporary automobile practice. Because of the increasing weight of the air flask, required to withstand ever higher pressures, experiments were made with hydrogen peroxide, which needed a lighter containment vessel, to produce oxygen via a catalyst. These developments were shelved by the British, but taken up by the Germans, the Japanese and the Americans in the latter part of the Second World War.
To obtain more power from existing pressure vessels, thought was given to using air enriched with oxygen — up to 57 per cent by weight, or even pure oxygen. The British 21in Mark VII of 1928 was the Royal Navy’s first enriched air torpedo, carried by the London-class heavy cruisers, and this led to the 24.5in Mark 1 installed in the Nelson and Rodney. Before the outbreak of the Second World War, however, due to corrosion problems with the air vessels, both enriched air models were changed to run on normal compressed air.
At around the same time, the British were perfecting the burner-cycle reciprocating engine, which retained the classic four-cylinder radial layout. The bore/stroke ratio of these compact radial units bore little resemblance to the contemporary automobile long-stroke inline reciprocating engine, with its inherent disadvantages of piston friction and heavy, out of balance, reciprocating weights. The radial engine could thus continue to rival the American preference for the turbine engine. The first British torpedo to use the burner-cycle engine was the long-lived Mark VIII for submarine use. It was a modernised model of the Mark VIII which would be fired against the General Belgrano fifty-five years later. The corresponding torpedo for surface ships was the Mark IX.
The Brotherhood burner-cycle engine was fed with compressed air at around 840psi. A small amount of paraffin was atomised in the air and burned. The resultant gas was fed into the cylinders at a temperature of 1000 °C, and additional fuel was injected just before the piston reached top dead-centre. The compression caused the fuel mixture to detonate, driving the piston down as in a diesel engine. The exhaust gas was evacuated through ports in the cylinder, but there were also two auxiliary exhaust ports in the piston crown. By 1945 this impressive power unit would have been tuned to produce up to 465bhp, sufficient to propel a 21in torpedo at 50 knots. Plans to run a version of this engine on nitric acid promised to produce 750bhp, but the outbreak of war meant that it was never built.
In 1923 German experiments with electric torpedoes continued in secret in Sweden, and the design was finalised six years later. Since the Versailles treaty banned Germany from possessing submarines, the electric torpedo was held in readiness until Hitler came to power and began repudiating the terms of Versailles.
Magnetic influence exploders had been developed during the Great War, and the Duplex exploder was fitted to Royal Navy torpedoes from 1938. But the old problem of insufficient live-firing tests cropped up, and was to seriously affect their performances. The only navy to carry out large-scale live torpedo firing was the Imperial Japanese navy, which had expended many obsolete warships in tests in the lead-up to the Second World War. The Japanese saw in the torpedo the weapon they needed to give them an edge over the numerically superior US fleet in the Pacific. Japanese war strategy involved wearing down the US Navy in a series of actions across the Pacific Ocean until parity had been reached with the Japanese battle line, when the dreadnoughts would move in for the decisive final battle. A major part of this strategy depended on the torpedo: they set to work to produce the best in the world, and in this they succeeded.
Japanese development of an electric torpedo for submarines began in 1921, inspired by the model the Germans had introduced to their U-boats just prior to the Armistice. The design was finalised by 1925. The 21in torpedo was powered by two 54-cell lead-acid batteries feeding a 95ehp motor. It could run at 28 to 30 knots out to 7000m (7660yds), carrying a 300kg (660lbs) warhead. It became the Type 92 in 1934, but manufacture was suspended, ready for mass production in the event of war.
The Imperial Japanese navy studied other German late war developments, including the 600cm 23.6in torpedo. They had previously tried out the 27.5in Fiume torpedo produced in around 1900, and in 1905 they had ordered 24in torpedoes from Fiume for coastal defence. Now they decided to produce a heavyweight torpedo of their own for the anticipated conflict with the Americans. The result was the 24in Year 8 torpedo of 1919, capable of 38 knots over 10,000m (11,000yds) and carrying a 345kg (759lbs) warhead. Ten years later the 24in Type 90 appeared, capable of 46 knots over 7000m (7660yds) with an explosive charge of 375kg (825lbs).
They had briefly tested oxygen-enriched torpedoes in 1917. Future Admiral Oyagi, during the two years (1926–27) he spent at the Whitehead factory in England, heard rumours that the Royal Navy was fitting oxygen-fuelled 24.5in torpedoes in Rodney and Nelson. In fact the 24.5in Mark 1 originally ran on oxygen-enriched air, but on his return to Japan, Oyagi headed up a team to work on producing a version of the Japanese 24in torpedo to run on 100 per cent oxygen.
There were severe problems to overcome. The oxygen had to be prevented from coming into contact with any of the lubricants in the torpedo mechanism, to avoid the risk of explosion. More serious were the explosions which occurred in the engine combustion chambers as soon as the oxygen and kerosene fuel were injected. The design team overcame this hazard by starting the torpedo on compressed air, stored in a ‘first air bottle’ and only then gradually changing over to pure oxygen. They succeeded in producing a working torpedo, which was designated the Type 93, from the year 2593 in the Japanese calendar when the design was finalised.
The Type 93 oxygen-fuelled engine produced 520 horsepower at 1200rpm, compared with the 240hp of the British 24.5in and the 320hp of the initial Mark VIII. It could run at 49 knots for 20,000m (22,000yds), an exceptional performance. At 36 knots it would reach out to a phenomenal 40,000m (44,000yds). It carried a 490kg (1078lbs) warhead, capable of inflicting devastating damage. And it was practically wakeless. To profit fully from its deadly characteristics the IJN introduced power-reloading gear to their large fleet destroyers, following the provision of multiple reloads on board their cruisers.
Since the Japanese had previously had difficulty in forging the air vessels required for their licence-built Whiteheads, they constructed a special 4000-ton press to forge the body and after end of the flasks for the Type 93 out of steel billets. The air flask forward end cover was fixed with a large copper washer, the internal pressure keeping the joint gas-tight, and the arrangement proved extremely satisfactory.
The Type 93 was too large to be carried in submarines, so a smaller 21in oxygen torpedo was designed for them in 1935, the Type 95. It could run at 49 knots for 9000m (9840yds) and at 45 knots it reached out to 12,000m (13,000yds). The Type 95 carried a 405kg (891lbs) warhead. In 1943 the Model 2 would carry a warhead of 550kg (1210lbs). The Type 95 first air bottles often leaked, and while in the tubes on cruisers and destroyers it was a simple matter to verify the pressure of their oxygen-fuelled torpedoes at regular intervals and, if necessary, top it up, in a submarine this was not so easy, a factor leading to the electric Type 92 being resuscitated.