In its modern meaning, a torpedo is a self-propelled weapon that travels underwater to its target, which can be a surface ship or an underwater vessels. It can be launched from an aircraft, ship, or submarine. As long as the final attack is underwater, hybrid devices, such as weapons launched from a submarine, which travel to the surface to fire a rocket motor that carries a torpedo to a distant point where it re-enters the water.
The term "torpedo", prior to the Whitehead design, had been used for non-self-propelled weapons that today would be called naval mines. Another usage, such as used on the American Civil War human-powered submarine, CSS Hunley, sank USS Housatonic with a device that is another obsolete term, an explosive charge at the end of a long pole mounted on the bow of the submarine. The Hunley pressed the explosive against the hull of the Housatonic, detonated the charge, and escaped, although the submarine later sank with all hands; the reason remains unproven.
To be called a torpedo, the device must have enough independent control to keep itself on a straight course, so the firer can fire a course predicted to intercept that of a moving target. Beyond that, modern torpedoes use on-board sensors, or commands sent through a trailing wire, to alter its course to stay aimed at a moving target.
Basic torpedo capabilities
The basic stabilized design came from Robert Whitehead, in 1866. The first Whitehead torpedo, using compressed air to turn its propeller, was stable in azimuth, but was unsatisfactory in not being able to control its depth. Whitehead's attention was drawn to the problem in 1864, when Giovanni Luppis, a retired Austrian naval officer, sought his assistance in making Luppis' attempts at a torpedo work.
Where Luppis had not been able to make his device travel at all, it was a design concept of Luppis that Whitehead adapted to solve the depth control problem. Whitehead and Luppis had a contract, but it gave effective control to Whitehead. The combined concept was impressive enough, in demonstrations, to be bought by the Austrian Navy in 1869 and the (British) Royal Navy in 1870.
Early torpedo tactics
The first operational torpedoes were fired from launchers on the decks of warships, whose primary weapons remained guns. Before long, however, small, fast torpedo boats emerged, which used torpedoes as its main weapons, and were protected only by speed and maneuverability: "eggshells armed with sledgehammers". These were the first example of what is now known as fast attack craft, which may now be armed with guided missiles. Since battleships and cruisers were too cumbersome to chase and kill torpedo boats, a new type of oceanworthy but nimble ship emerged, first called the torpedo boat destroyer.
Torpedo boat destroyers became increasingly more flexible escorts, and, ironically, were themselves armed with torpedoes. The typical torpedo boat was limited to coastal waters and could not accompany an ocean fleet, but the new class, now called destroyer, now had both offensive and defensive capabilities.
In 1912, the first attempt was made to deliver a torpedo from the air, but much technical refinement was needed.
There were significant torpedo actions, using straight-running torpedoes, in the Second World War. The most powerful torpedo was the Japanese "Long Lance". U.S. submarine torpedoes were almost completely useless until inadequately tested features were removed. U.S. and German submarines were principally targeted against commerce, respectively coming close to starving Japan and the U.K. Major surface actions, including the Battle of Surigao Strait, the last battleship-to-battleship action, were quite effective.
The next generation, however, was to be the guided torpedo, with air- and submarine-launched versions first developed, respectively, to kill submerged submarines, or to sink antisubmarine escort vessels.
Guided torpedoes entered service in the latter part of the Second World War, using passive acoustic sensors. Modern guided torpedoes use combinations of active and passive onboard sonar, computers, and wire guidance.