America's use was mainly limited to earlier Porpoise-, -, and -class boats. Until the appearance of the class, most American submarines only carried 4 bow and either 2 or 4 stern tubes, something many American submarine officers felt provided inadequate firepower. This problem was compounded by the notorious unreliability of the Mark 14 torpedo. Late in World War II, the U.S. adopted a homing torpedo (known as "Cutie") for use against escorts. It was basically a modified Mark 24 Mine with wooden rails to allow firing from a torpedo tube. Air launch Aerial torpedoes may be carried by fixed-wing aircraft, helicopters, or missiles. They are launched from the first two at prescribed speeds and altitudes, dropped from bomb-bays or underwing hardpoints. Handling equipment Although lightweight torpedoes are fairly easily handled, the transport and handling of heavyweight torpedoes is difficult, especially in the tight spaces in a submarine. After the Second World War, some Type XXI submarines were obtained from Germany by the United States and Britain. One of the main novel developments seen was a mechanical handling system for torpedoes. Such systems were widely adopted as a result of this discovery. Classes and diameters Torpedoes are launched in several ways: From a torpedo tube mounted either in a trainable deck mount (common in destroyers), or fixed above or below the waterline of a surface vessel (as in cruisers, battleships, and armed merchant cruisers) or submarine. Early submarines and some torpedo boats (such as the U.S. World War II PT boats, which used the Mark 13 aircraft torpedo) used deck-mounted "drop collars", which simply relied on gravity. From shackles aboard low-flying aircraft or helicopters. As the final stage of a compound rocket or ramjet powered munition (sometimes called an assisted torpedo). Many navies have two weights of torpedoes: A light torpedo used primarily as a close attack weapon, particularly by aircraft. A heavy torpedo used primarily as a standoff weapon, particularly by submerged submarines. In the case of deck or tube launched torpedoes, the diameter of the torpedo is a key factor in determining the suitability of a particular torpedo to a tube or launcher, similar to the caliber of the gun. The size is not quite as critical as for a gun, but the diameter has become the most common way of classifying torpedoes. Length, weight, and other factors also contribute to compatibility. In the case of aircraft launched torpedoes,