the advent of the Douglas TBD-1 Devastator – whose performance might have seemed sparkling in 1936, but was decidedly unimpressive a few years later. High bomber performance introduced a new problem. A torpedo dropped at high speed from a relatively high altitude typically oscillated as it fell towards the water. If it hit the water at the wrong angle, it would dive and it might also roll. A roll would cause its elevators, which were set to make it rise out of its initial dive, to act as rudders and cause it to hook. Something had to be done to stabilise the torpedo in flight so that it hit the water at the desired angle. The solutions were air tails which guided a torpedo into the water at the right angle, often coupled with breakaway protection for the nose of the torpedo.10 Relatively few wartime photographs show air tails of any type; presumably each user considered the concept secret.
The Regia Aeronautica, the Italian Royal Air Force, developed torpedo tactics before the war, but in line with its view that air power was general-purpose it did not form specialised torpedo bomber squadrons. The primary torpedo bomber was the Savoia-Marchetti SM 79 Sparviero, which could carry two torpedoes, as shown, although in practice they typically carried one. After SM 79s made successful torpedo attacks in 1940, special torpedo attack squadrons, the Aerosiluranti (Siluranti were torpedoes), were formed. They received enormous wartime publicity. (Philip Jarrett)
Throughout the mid-1930s carrier aircraft capable of lifting and carrying torpedoes had to be so large that they could not match dive bomber performance. For a time the US Navy planned to drop them altogether in favour of total reliance on dive bombers (the carrier Ranger was built without torpedo stowage). The fleet recommended not only elimination of torpedoes but restriction to two 500lb bombs for horizontal bombers; in 1932–4 BuAer retained both because it considered the performance penalties involved in carrying a torpedo or a third bomb were negligible. When it sketched a design prior to opening a new bomber competition in 1934, BuAer found that biplane configuration would limit it to 170mph, whereas a monoplane could reach 186mph; it included speed in the specification it released in 1934. This competition in effect saved the aerial torpedo in the US Navy, because it produced a torpedo bomber with high enough performance to have a reasonable chance of surviving enemy fighters: the Douglas Devastator. The rules of the 1934 competition embodied the speed BuAer estimated such an aircraft could reach; the Devastator actually made 206mph, and it proved remarkably free of mechanical defects. As a result, 114 were immediately ordered. Had the Devastator not been developed, further US carriers would probably have been built without torpedo stowage, their attackers limited to dive bombing. It would have been effective against carriers and cruisers, but not, the US Navy thought, against battleships – it took torpedoes to sink the huge Japanese Yamato and Musashi, for example. In addition to its modern design, the Devastator had the US Navy’s first hydraulically-folded wings. Unfortunately aircraft, particularly engine, technology was moving so fast that within a few years it was obsolete. The TBD was powered by a 900hp R-1830 (an abortive version offered in 1940 to the Dutch used a 1200hp R-1820; note the difference in output in an engine of about the same size). According to a post-war BuAer history of attack aircraft development, further development of the TBD suffered because the navy came to emphasise dive bombing instead. Once the fighters and dive bombers exceeded the performance of the Devastator (in 1938), BuAer ran a competition for a faster new torpedo bomber, which became the Grumman Avenger. This time it wanted 50 per cent higher speed and a bomb bay sufficient for a torpedo, three 500lb bombs, or fuel for a range of 3000 miles (during design the bomb bay was enlarged to accommodate an additional 500lb bomb). Like other US (and Japanese) torpedo bombers, the TBD could also be used as a level bomber, typically carrying a single 1000lb or three 500lb bombs. For that purpose it, and the Avenger which succeeded it, had a Norden bombsight. In its case the bombardier’s window was under the belly, normally covered by doors for streamlining (it could not be used if the torpedo was carried). After making a good showing (as a level bomber) in carrier raids on the Mandated Islands in February 1942, and (as a torpedo bomber) at the Coral Sea in May, Devastators were nearly wiped out at Midway. One reason why was that they carried their torpedoes tipped down at a 9° angle. This flight deck photograph suggests the reason why: the torpedoes were carried parallel to the deck, presumably for loading, but the aircraft was tipped back. The Mk 13 torpedo had to be dropped parallel to the water, which required the Devastator to adopt a nose-up attitude which in turn made for slow speed. Typically torpedoes were dropped at an altitude of no more than 100ft and at an air speed of no more than 120mph. These aircraft of VT-6 are shown on board Enterprise shortly before being launched at Midway. Only four of the fourteen launched managed to return to the ship, one being so badly damaged that it had to be thrown overboard. Of a total of forty-three Devastators launched that day, only six made it back to the carriers, two being forced to ditch. The entire squadron launched by Hornet, VT-8, was wiped out. At this time US tactics called for coordinated attacks by dive and torpedo bombers, the idea being that an enemy would be unable to deal with both at the same time. Escorting fighters were to cover the entire attack group. In fact the carrier air groups involved spread out in flight and arrived at the Japanese carriers piecemeal. It happened that the torpedo bombers arrived first, and that they drew the Japanese fighters down to their level to attack them. That left the field open to the dive bombers which soon followed. Had the three types of aircraft arrived together, the Devastator might have emerged from the battle with a far better reputation as the stablemate (with much the same engine) of the Dauntless.
As the British sought to drop torpedoes at higher speeds and from greater altitudes in the 1930s they experimented with means of stabilising them in flight.11 Ultimately they adopted the ‘air rudder’ or air tail they first saw on Norwegian torpedoes. Even then the strength of the torpedo limited drop speed. For example, a bomber approaching at higher than drop altitude had to fly level above the water for a time as it decelerated. By 1942 dive brakes were important because they minimised the level decelerating run above the water. It helped that current British naval torpedo bombers had dive brakes because they had an alternative dive-bombing role (so did the Brigand land-based torpedo bomber then under development). By late 1942 the British wanted their future air-launched torpedo to stand a 300ft drop at 250kts, which was far beyond existing capability.12
The Japanese used a wooden tail frame and special flippers for control in the water. The Germans used a wooden box air tail (three horizontal and two vertical surfaces) and an anti-roll ring using a pair of gyro-controlled fins. The Italian air tail had a longer span and incorporated an anti-roll mechanism using gyro-controlled horizontal rudders (the British monoplane air tail was also gyro-controlled).13
The US Navy seems to have come late to understanding that it needed special fittings. By 1944 its Mk 13 torpedo had a tail ring (shroud ring), a box air tail, and a wooden shroud (the ‘pickle barrel’) around its nose. Given these fittings, torpedoes could be dropped at speeds as great as 400kts and at altitudes above 2000ft.14
Torpedo attack was inherently dangerous because the pilot had to point his aircraft more or less directly at the target, using a torpedo director like that on board a ship to solve the triangle of ship course and speed and torpedo course and speed. In 1934 the Germans demanded that future aerial torpedoes be capable of angling; they may have been the first to do so.15 In 1937 an RAF officer had the same idea, and during the war British aerial torpedoes could angle, the attacker using a Type F sight.16 The Italians seem to have had the same idea. During the war the British initially rejected the US Mk 13 torpedo partly because it lacked this feature.17
The Avenger, the standard US torpedo bomber of the Second World War, arose from a 1939 design competition. Like the Helldiver designed the previous year, the Avenger was designed around the next-generation engine (the 1700hp R-2600). Its most important new feature was that the torpedo was carried internally.
