The B6N Tenzan (‘Jill’) was the planned replacement for the ‘Kate’ which carried out Japanese carrier torpedo attacks at the outset of the Pacific War. It was designed to a 1939 (14-Shi: FY40) specification calling for a maximum speed of 250kts (200kts cruising speed) and a range of 1000nm with the standard 800kg load (torpedo or bombs). The speed was comparable with that of the US Avenger designed at about the same time, and the range was chosen to support the preferred tactic of attacking beyond the enemy’s attack range. The Tenzan was designed at a time of steeply rising engine performance, its engine being about 80 per cent more powerful than that of its predecessor. Although the prototypes were complete in the spring of 1941, the Tenzan had serious teething troubles, and it did not enter production until 1943. It fared poorly against US naval fighters in its first major engagement, the battle of the Philippine Sea in June 1944. (Philip Jarrett)
In the Mediterranean the British found that barrage fire could force Italian torpedo bombers to release their weapons at greater ranges, so that after 1940 they were relatively ineffective against fleet units (but not convoys).20 In addition to conventional torpedoes, the Italians had air-launched circling torpedoes specifically for use against convoys, roughly as the Germans used pattern-running torpedoes launched by submarines. Circular runs were more efficient than the German zigzag inside a convoy, and the aircraft did not have to fear a torpedo coming back at it.21 The Germans later adopted the larger of the two Italian circlers.
Because a torpedo bomber approached almost horizontally, it was susceptible to the same sort of smokescreen which would frustrate an attacking ship. Most wartime radars were not good enough to permit blind attacks. Several navies found smokescreens effective against torpedo bombing.22
Torpedo bombing was the dominant form of wartime night anti-ship attack because of its relative simplicity and because a ship presented so large a target. In 1943–4 the Imperial Japanese Navy used land-based long-range G4M (‘Betty’) torpedo bombers at night (the US Navy did not yet have many carrier-based night fighters). For example, one torpedoed the US carrier Intrepid in her rudder in November 1943. Initial attacks were conducted by single aircraft. The target ship might be unaware of the attack until the torpedo struck, hence would not take evasive action. In order to line up its target visually, the bomber generally laid a path of floating flares the movement of which indicated windage. It then turned back and flew down the path to drop its torpedo. This type of attack made the development of blind-firing anti-aircraft weapons more urgent.
Aichi’s B7A Ryusei torpedo bomber (‘Grace’) was part of the 16-Shi (1942) programme. It was intended to extend the reach of Japanese carriers and thus to minimise the problem of carrier air defence: if the Japanese fleet could outreach the US fleet, and if its aircraft could penetrate US defences, then it could strike without being struck. The Japanese consistently managed to outrange the US fleet, but the combination of effective fighter control and effective anti-aircraft fire made that outreach useless. The Ryusei was intended to replace both the standard attack aircraft: the B6N torpedo bomber and the D4Y dive bomber. Given enough engine power, an airframe stressed to dive-bomb could lift a torpedo. That was the case with both the US SB2C Helldiver (although it was not used as a torpedo bomber) and the British Barracuda (a torpedo bomber used exclusively as a dive bomber). Manoeuvrability was to be equal to that of a Zero (A6M) fighter, to give the Ryusei reasonable immunity from interception. Normal range was to be 1000nm (maximum 1800nm). The prototype was completed in May 1942. Note that the operational concept considerably predated Midway. Production seems to have been hampered by slow engine development, as it did not begin until April 1944. (Philip Jarrett)
As the US fleet came closer to Japanese land bases, night torpedo attacks became more common, using shore-based single-engined aircraft. Typically a reconnaissance aircraft spotted the fleet at dusk and provided a rough location to the strike force. Night tactics employed off Palau in the spring of 1944 were considered typical. Enemy aircraft milled around outside gun range, then formed up for a sudden run in at low altitude (200–300ft) and medium speed (150–180kts), low altitude having been chosen to avoid US radar. They approached in a rough line abreast. During each such attack a single aircraft stood off about 20–30nm away, probably co-ordinating the attack.
Maximum strike range from base was about 150nm.23 This distance was probably related to the uncertainty of future target position due to the time interval between report and attack. The strike force spread out into a wide arc once it got to within 15–20nm of the fleet. ‘Betty’ and ‘Jill’ both carried what the British called Air to Surface Vessel (ASV) radar, which was equivalent to the early-war British metric-wavelength set using dipole antennas along the body of the aircraft. It could not look dead ahead. At any height above 1000ft it could detect a battleship at a range of 45nm. It appeared that the strike force spread out either because homing on search radars became confused (the fleet had so many of them) or because ASV was inaccurate looking right ahead. The attack then spread out, aircraft dropping to 500ft and forming smaller attack groups.
The standard British Monoplane Air Tail, under a Beaufort. (Dr Raymond Cheung)
An Italian torpedo with air tail, under an SM 79.
Japanese doctrine was to place the target between the striking force and the moon (the preferred time was when the moon was between 10° and 50° above the horizon). The target was illuminated either by the shadowing aircraft calling in the strike or by two specially-assigned strike aircraft. In either case the shadowing aircraft dropped course lights behind the target when the strike force was about 20nm away. As soon as they could be seen by the strike force, it altered course to line up with these flares. When in the desired position, the strike force gave the signal to illuminate, at which time flares were dropped on the side of the target away from the strike force.
Strike forces could vary from three to as many as forty aircraft; by late 1944 carrier strikes on Japanese airfields were limiting typical numbers to no more than twenty night torpedo bombers. Aircraft flew most of the way to the target at 3000–4000ft, and sometimes not more than 1000ft, which would limit long-range radar detection. New Japanese tactics (as of late 1944) included a higher approach so that the attackers could use their own radar effectively. Striking forces sometimes homed on US air-search radars.
The Italians developed Motobomba, a circling electric torpedo which could be dropped in the path of a moving force. In that way it functioned as a cross between a torpedo and a mine, since it covered a larger area than a mine but did not have to be aimed like a torpedo. Motobomba was adopted by the Germans in the Mediterranean. (Dr Raymond Cheung)
By 1944 the Japanese were well aware of ‘Window’ (chaff: strips of aluminium foil or other radar reflectors), which had been used against their German allies the previous year. During the invasion of Saipan (June 1944), Japanese attackers made frequent and varying use of it, in US eyes demonstrating a good understanding of its potential. Often a group of aircraft dropped ‘Window’ while orbiting, then left the infected area to attack from a different direction. It was intended to leave