Aerial torpedo

An aerial torpedo, airborne torpedo or air-dropped torpedo is a naval weapon, the torpedo, designed to be dropped into water from an aircraft (fixed-wing aircraft or helicopter) after which it propels itself to the target. First used in World War I, air-dropped torpedoes were used extensively in World War II, and remain in limited use today. Aerial torpedoes are generally smaller and lighter than submarine- and surface-launched torpedoes.

Historically, the term "aerial torpedo" was used to describe flying bombs and pilotless drone aircraft intended as weapons, the precursor to modern cruise missiles. Today, the term refers primarily to water-borne torpedoes launched from the air.

Design
A successful aerial launched torpedo design needs to account for
 * The distance it travels through the air before entering the water
 * The heavy impact with the water

The Japanese Type 91 torpedo used aerodynamic tail stabilizers in the air. These stabilizers (introduced in 1936) were shed off when it entered the water. And a new control system (introduced in 1941) stabilized the rolling motion by countersteering both in the air and the water. The Type 91 torpedo could be released at speed of 180 knots (333 km/h) from 20 m (66 ft) into shallow water but also at 204 knots (the Nakajima B5N2's maximum speed) into choppy waves of a rather heavy sea.

Tactics and usage
The idea of dropping lightweight torpedoes from aircraft was conceived in the early 1910s by Bradley A. Fiske, an officer in the United States Navy. Awarded a patent in 1912, Fiske worked out the mechanics of carrying and releasing the torpedo from a bomber, and defined tactics that included a night-time approach so that the target ship would be less able to defend itself. Fiske determined that the notional torpedo bomber should descend rapidly in a sharp spiral to evade enemy guns, then when about 10 to 20 ft above the water the aircraft would straighten its flight long enough to line up with the torpedo's intended path. The aircraft would release the torpedo at a distance of 1500 to 2000 yd from the target. Fiske reported in 1915 that, using this method, enemy fleets could be attacked within their own harbors if there was enough room for the torpedo track.

World War I
In July 1914, the first British aerial torpedo was dropped in trials performed in a Short "Folder" by Lieutenant (later Air Chief Marshall Sir) Arthur Longmore. In November 1914, Germans were reportedly experimenting at Lake Constance with the tactic of dropping torpedoes from a Zeppelin. In December 1914, Squadron Commander Cecil L'Estrange Malone commented following his participation in the Cuxhaven Raid that "One can well imagine what might have been done had our seaplanes, or those which were sent out to attack us, carried torpedoes or light guns."

On August 12, 1915, a Short Type 184 piloted by Flight Commander Charles H. K. Edmonds from HMS Ben-my-Chree operating in the Aegean Sea took off with a 14 in, 810 lb torpedo to fly over land and sank a Turkish supply ship in the Sea of Marmara. Five days later, a Turkish steamship was sunk by a torpedo aimed again by Edmonds. His formation mate, Flight Lieutenant G. B. Dacre, sank a Turkish tugboat after being forced to land on the water with engine trouble. Dacre taxied toward the tugboat, released his torpedo and was then able to take off and return to Ben-My-Chree. A limitation to using the Short more widely as a torpedo bomber was that it could only take off carrying a torpedo in conditions of perfect flying weather and calm seas, and, with that load, could only fly for a little more than 45 minutes before running out of fuel.

On May 1, 1917, a German seaplane loosed a torpedo and sank the 2784 LT British steamship Gena off Suffolk. A second German seaplane was downed by gunfire from the sinking Gena. German torpedo bomber squadrons were subsequently assembled at Ostend and Zeebrugge for further action in the North Sea. Later in 1917, the U.S. Navy began to perform trials using a 400 lb dummy torpedo which, in its first air drop, porpoised from the water back up in the air and almost hit the aircraft. Several British torpedo bombers were built, including the Sopwith Cuckoo, the Short Shirl and the Blackburn Blackburd, but a squadron was assembled so late in the war that it achieved no successes.

Interwar years
The United States bought its first 10 torpedo bombers in 1921, variants of the Martin MB-1. The squadron of U.S. Navy and Marine fliers was based at Naval Weapons Station Yorktown. General Billy Mitchell suggested arming the torpedo bombers with live warheads as part of Project B (the anti-ship bombing demonstration) but the Navy was only curious about aerial bomb damage effects. Instead, a trial using dummy heads on the torpedoes was carried out against a foursome of battleships steaming at 17 knots. The torpedo bombers scored well.

In 1931, the Japanese Navy developed the Type 91 torpedo, intended to be dropped by a torpedo bomber from a height of 330 ft and a speed of 100 kn. In 1936, the torpedo was given wooden attachments to the tail to increase its aerodynamic qualities—these attachments were shed upon hitting the water. By 1937, with the addition of a breakaway wooden damper at the nose, the torpedo could be dropped from 660 ft and a speed of 120 kn. Tactical doctrine determined in 1938 that the Type 91 aerial torpedo should be released at a distance of 3300 ft from the target. As well, the Japanese Navy developed night attack and massed day attack doctrine, and coordinated aerial torpedo attacks between land- and carrier-based torpedo bombers.

The Japanese divided their bomber squadrons into two groups so as to attack an enemy battleship from both frontal quarters and make it extremely unlikely for it to be able to avoid the torpedoes by maneuvering, and more difficult for it to direct anti-aircraft fire at the bombers. Even so, Japanese tactical experts predicted that, against a battleship, the attacking force would be able to score hits at a rate of only one-third that observed during peacetime exercises.

Beginning in 1925, the United States began designing a special torpedo for purely aerial operations. The project was discontinued and revived several times, and finally resulted in the Mark 13 torpedo which went into service in 1935. The Mark 13 differed from aerial torpedoes used by other nations in that it was wider and shorter. It was slower than its competitors but it had longer range. The weapon was released by an aircraft traveling lower and slower (50 ft high, 110 kn than its Japanese contemporary.

World War II
On the night of November 11–12, 1940, Swordfish biplane torpedo bombers of the British Fleet Air Arm sank three Italian battleships at the Battle of Taranto using a combination of torpedoes and bombs. In the course of the chase of the German battleship Bismarck torpedo strikes were attempted in very bad seas, and one of these damaged her rudder allowing the British fleet to catch her. The standard British airborne torpedo for the first half of World War II was the Mark XII, an 18 in model weighing 1548 lb with an explosive charge of 388 lb of trinitrotoluene (TNT).

German aerial torpedo development lagged behind other belligerents—a continuation of neglect of the category during the 1930s. At the beginning of World War II, Germany was making only five aerial torpedoes per month, and half were failing in air-drop exercises. Instead, Italian aerial torpedoes made by Fiume were purchased, with 1,000 eventually delivered.

In August 1941, Japanese aviators were practicing dropping torpedoes in the shallow waters of Kagoshima Bay, testing improvements in the Type 91 torpedo and developing tactics for the attack of ships in harbor. They discovered that the Nakajima B5N torpedo bomber could fly 160 kn, faster than expected, without the torpedoes striking the bottom of the bay 100 ft down. On December 7, 1941, the leading wave—40 B5N torpedo bombers—used the tactic to score more than 15 hits during the attack on Pearl Harbor.

In April 1942, Adolf Hitler made the production of aerial torpedoes a German priority, and the Luftwaffe took the task over from the Kriegsmarine. The quantity of available aerial torpedoes outstripped usage within a year, and an excess of aerial torpedoes were on hand at the end of the war. From 1942 to late 1944, about 4,000 aerial torpedoes were used, but some 10,000 were manufactured during the whole war. Torpedo bombers were modified Heinkel He 111 and Junkers Ju 88 aircraft, but the Focke-Wulf Fw 190 fighter aircraft was successfully tested as a delivery system.

The Mark 13 torpedo was the main American aerial torpedo, yet it was not perfected until after 1943 when tests showed that it performed satisfactorily in only 33 of 105 drops made from aircraft traveling faster than 150 kn. Like the Japanese Type 91, the Mark 13 was subsequently fitted with a wooden nose covering and a wooden tail ring, both of which sheared off when it struck the water. The wooden shrouds slowed it and helped it retain its targeting direction through the duration of the air drop. The nose covering absorbed enough of the kinetic energy from the torpedo hitting the water that recommended aircraft height and speed were greatly increased to 2400 ft high at 410 kn.

In 1941, development began in the United States on the FIDO, an electric-powered air-dropped acoustic homing torpedo intended for anti-submarine use. In the United Kingdom, the standard airborne torpedo was strengthened for higher aircraft speeds to become the Mark XV, followed by the Mark XVII. For carrier aircraft, the explosive charge remained 388 lb of TNT until later in the war when it was increased to 432.5 lb of the more powerful Torpex.

During World War II, U.S. carrier-based torpedo bombers made 1,287 attacks against ships, 65% against warships, and scored hits 40% of the time. However, the low, slow approach required for torpedo bombing made the bombers easy targets for defended ships; during the Battle of Midway, for example, virtually all of the American torpedo bombers were shot down.

Korean War
After World War II, anti-aircraft defenses were sufficiently improved to render aerial torpedo attacks suicidal. Lightweight aerial torpedoes were disposed or adapted to small attack boat usage. The only significant employment of aerial torpedoes was in anti-submarine warfare.

During the Korean War the United States Navy successfully disabled the Hwacheon Dam with aerial torpedoes launched from A-1 Skyraiders.

Modern weapons
Since the advent of practical anti-ship missiles technology, aerial torpedoes have largely been reduced to use in anti-submarine warfare. Missiles are generally much faster, with longer range and do not have the same launch altitude limitation of aerial torpedoes. Some modern aerial anti-submarine torpedoes do have the necessary guidance capability to engage surface vessels, though given the widespread availability of missiles on aircraft and the small, specialized warhead on anti-submarine aerial torpedoes, this is not an option normally considered.

At the peak of the Falklands war, however, the Argentine Air Force, in collaboration with the Navy, outfitted an FMA IA 58 Pucará prototype, AX-04, with pylons to mount Mark 13 torpedoes. The aim was the possible production of Pucaras as torpedo-carrying aircraft to enhance the anti-ship capabilities of the Argentine air forces. Several trials were performed off Puerto Madryn, but the war was over before the technicians could evaluate the feasibility of the project.

As a result of the loss of the role of anti-shipping aerial torpedo in modern naval doctrine, true torpedo bomber units no longer exist in modern armed forces. The most common platform for aerial torpedoes today is the ship-borne anti-submarine helicopter, followed by fixed-wing anti-submarine aircraft such as the American P-3 Orion.

The caveat to the above are torpedoes delivered by missiles/rocket systems, designed for anti-submarine warfare. Some designs are a straightforward mating of a rocket-propulsion system to the torpedo with a purely ballistic attack profile, such as the American ASROC. More complex, aerial drone-based system with autopilot have also been deployed, such as the Australian Ikara. Most such systems are designed to be deployed from surface ships, though exceptions exist such as the Soviet navy RPK-2 Viyuga which can be launched from both surface ships and submarines.

Given the relatively soft nature of submarines, modern anti-submarine aerial torpedoes are much smaller than anti-ship aerial torpedoes of the past, and often classified as light weight torpedoes. They are also often of cross-platform design, able to be deployed from both aircraft and surface ships. Examples include the American Mark 46, Mark 50 and Mark 54 torpedoes. There are few if any aerial torpedo designs that are also used by submarines, owing to the significantly reduced size and thus capability of aerial torpedoes compared with their full-sized submarine counterparts such as the American Mark 48 torpedo.