acoustic, though there have been other target sensor types used. A ship's acoustic signature is not the only emission a torpedo can home in on; to engage U.S. supercarriers, the Soviet Union developed the 53–65 wake-homing torpedo. As standard acoustic lures can't distract a wake homing torpedo, the US Navy has installed the Surface Ship Torpedo Defense on aircraft carriers that use a Countermeasure Anti-Torpedo to home in on and destroy the attacking torpedo. Warhead and fuzing The warhead is generally some form of aluminized explosive, because the sustained explosive pulse produced by the powdered aluminum is particularly destructive against underwater targets. Torpex was popular until the 1950s, but has been superseded by PBX compositions. Nuclear torpedoes have also been developed, e.g. the Mark 45 torpedo. In lightweight antisubmarine torpedoes designed to penetrate submarine hulls, a shaped charge can be used. Detonation can be triggered by direct contact with the target or by a proximity fuze incorporating sonar and/or magnetic sensors. Contact detonation When a torpedo with a contact fuze strikes the side of the target hull, the resulting explosion creates a bubble of expanding gas, the walls of which move faster than the speed of sound in water, thus creating a shock wave. The side of the bubble which is against the hull rips away the external plating creating a large breach. The bubble then collapses in on itself, forcing a high-speed stream of water into the breach which can destroy bulkheads and machinery in its path. Proximity detonation A torpedo fitted with a proximity fuze can be detonated directly under the keel of a target ship. The explosion creates a gas bubble which may damage the keel or underside plating of the target. However, the most destructive part of the explosion is the upthrust of the gas bubble, which will bodily lift the hull in the water. The structure of the hull is designed to resist downward rather than upward pressure, causing severe strain in this phase of the explosion. When the gas bubble collapses, the hull will tend to fall into the void in the water, creating a sagging effect. Finally, the weakened hull will be hit by the uprush of water caused by the collapsing gas bubble, causing structural failure. On vessels up to the size of a modern frigate, this can result in the ship breaking in two and sinking. This effect is likely to