LUT (‘Lagenunabhangigertorpedo’ or ‘torpedo independent of target inclination’) was a more sophisticated version of the FAT, introduced in early 1944, with more variable patterns, but was only used operationally towards the end of the war. Some sources mention problems with input spindles refusing to detach on launch, leading to hot running in the tube.
The introduction of a homing torpedo had been expected by the Allies, but nevertheless its effect was a great shock. The G7e electric torpedo was virtually silent compared with the thermal-engined type, and it was this feature which permitted the successful development of passive acoustic homing. Experiments in Germany had begun in 1935, and the first homing torpedo, T4 ‘Falke’ (‘falcon’) was issued to the Kriegsmarine in January 1943. Of around a hundred produced, some thirty were used. It was soon replaced by the superior T5 ‘Zaunkönig’ (‘wren’), known to the Allies as the GNAT (German naval acoustic torpedo). The T5 was designed for use against escort vessels moving at between 12 and 19 knots, the preferred speed for an Asdic search. The T5 carried four hydrophones in a flat-nosed version, or two behind a round-nosed Bakelite cap. The cap was filled with glycerine and ethylene glycol to ensure good acoustic transmissions to the hydrophones. After launching, the U-boat usually went deep and silent, to avoid attracting its own T5. Although two U-boats were suspected of having been sunk by their T5s, post-war analysis revealed that in fact they had been sunk by Allied surface and air forces.
It is believed that a total of 640 T5s were fired during the Second World War, achieving a hit rate of 6 per cent, owing to the widespread introduction of countermeasures by the escorts. Towards the end of the war an improved version, the Zaunkönig 2 was introduced, with variable settings intended to bypass escorts and strike at slower merchant ships, plus resistance to countermeasures. Only one boat was equipped before the war ended, and none were fired.
During the war the German navy was involved in over fifty torpedo research projects, which represented a huge investment in scarce time and resources. Among the projects which never went into production, but which would form the basis for many post-war developments were the following.
‘Lerche’ was a system of torpedo control using hydrophones in the torpedo that relayed the sounds via wires to a controller in the launch submarine. He could differentiate between the propeller sounds of a target and the noise produced by a Foxer decoy, and steer his torpedo accordingly.
‘Boie’ was a system using active sonar pinging instead of passive hydrophones. The Germans conducted a great deal of research into differentiating between different echoes, such as those from the surface. Its successor was ‘Geier’ (‘vulture’), with active sonar, ranging to 280yds (256m).
‘Ibis’ was a wake-follower homing torpedo, as was ‘Fasen’ (‘pheasant’) which detected the echoes from a ship’s wake but then went into a preset search pattern. ‘Marchen’ was a magnetic homing torpedo. Finally, in 1944 the Germans built and tested a tandem body torpedo propelled, not by a conventional propeller, but by a ‘flapping wing’ carried between the two hulls. It worked well in trials, but obviously could not be launched from a U-boat, although air-dropping was a possibility.
In a different class altogether was the ‘Ingolin’ range of torpedoes using hydrogen peroxide as a source of oxygen, based on research by Dr Walther. The ‘Steinwall’ powered by a turbine engine reached the long range of 24,000yds (22,000m) and was intended for use with anti-convoy LUT programming.
The largest German torpedo design was the Junkers M5, using a closed-circuit oxygen power source. It was 29.6in diameter with a length of 36ft. The engine produced around 600bhp, but every test model sank, and the project was abandoned.
In the Atlantic, the balance had swung in completely the opposite direction to the Pacific: in the latter part of the Second World War, the surface escorts and patrol aircraft had come to dominate the German submarines. Despite their homing torpedoes, they had been driven from the Atlantic by superior Allied ASW technology, supported by carrier- and shore-based aircraft. The true submarine boats, the fast schnörkel-equipped Type XXIs, were coming into service at the very end of the war in Europe, when it was far too late to affect the outcome.
CHAPTER 7
Aerial Torpedoes
Under the impetus of total war, by the end of the conflict of 1914–1918 the early, primitive, flimsy aircraft had developed out of all recognition. The types of torpedo bomber will be described in the following part, but here we will examine the actual torpedoes they carried into action.
The first requirement was for light weight, so the Short seaplanes sent to the Dardanelles by the British in 1915 could at first only carry the RGF Mark X 14in model. These had been made in 1897, and they were of the older, non-heater type, but still adequate for aircraft dropping. Even so, the planes found it difficult to take off from water carrying a torpedo, and the observer/rear gunner was often left behind. With more powerful engines in later models, the 18in RGF Mark IX could be carried aloft, and its much larger warhead did more damage; 18in was to remain the standard aerial torpedo size through to the end of the Second World War.
For their part the Germans experimented with torpedo-dropping planes, and designed special stand-off weapons to be carried by Zeppelins (see Part III).
In 1919 the US Navy began experimenting with torpedo planes, the first trials using the 18in Bliss-Leavitt Mark 7 Type D short torpedo designed to fit the tubes on older submarines. In May 1920 they experimented with the normal length Mark 7 Mod 5 torpedoes, dropped at 50–55 knots. One torpedo dropped from a height of 18ft (5m) was undamaged while another dropped from 30ft (9m) suffered severe damage. The Mark 7s were accordingly strengthened to survive the shock of water entry. They were also fitted with an exploder safety pin that was pulled out on dropping.
Mark 7 Mod 1A torpedoes were used in the mass torpedo practice run conducted against the battleship Arkansas on 22 September 1922. Attacking from both sides over a twenty-five-minute period, the aircraft dropped seventeen torpedoes at ranges between 500 and 1000yds (450 to 900m), and scored eight hits. In 1924, trials showed that the Mark 7 could be successfully dropped at an airspeed of 95 knots and a height of 32ft (10m).
The following year the US Navy began development of a torpedo which would gain fame in the Second World War, the 22.4in Mark 13. Designed specifically for use from aircraft, because of its increased diameter the Mark 13 could not be launched from the standard 21in torpedo tube. Later in the Second World War, however, it would be fitted to PT-boats, launching from a roll-off rack (see Part III).
On the outbreak of war in 1939, the 18in torpedo was standard in the air forces of Britain (Mark XII), Germany (LF5), Italy (Fiume model), Norway, and Japan (Type 91). The French alone opted for a smaller model, the 15.7in aerial torpedo Model 1926.
Similar to the Japanese, and to a lesser extent the British, the Germans were bedevilled by disruptive inter-service rivalries. The most serious was that between the Luftwaffe and the Kriegsmarine. The nascent German navy had no requirement for torpedoes smaller than 21in, but the Luftwaffe did. The latter were obliged to turn to the Norwegians for the design of the 18in torpedo they would use in the coming conflict. In 1934 they purchased torpedoes from the Hørten factory, and set up a modest production facility for them. They also sought Italian aircraft torpedoes, and in 1942 Hitler gave priority to the production of aircraft torpedoes. By 1945 some ten thousand had been produced, but in 1939 only the Norwegian 18in design was in service.