Acoustic torpedo

An acoustic torpedo is a torpedo that aims itself by listening for characteristic sounds of its target or by searching for it using sonar. Acoustic torpedoes are usually designed for medium-range use, and often fired from a submarine.

The first passive acoustic torpedo was the G7es T-5 Zaunkönig torpedo deployed in late World War II by the German U-Boat fleet. This weapon was developed to attack escort vessels and merchant ships in convoys. In July 1944, Russian commando frogmen discovered T-5 torpedoes aboard a sunken German submarine, U-250. Torpedoes were safely delivered to surface ships. Key components of the G7es T-5 Zaunkönig torpedo were later ordered by Joseph Stalin to be given to British naval specialists. However, after a protracted journey to Kronstadt the two Royal Navy officers were not allowed access to the submarine and returned home empty handed. The capture of U-505 marked the second time that allied forces gained access to this technology. The T-5 was countered by the introduction by the Allies of the Foxer noise maker.

Since its introduction, the acoustic torpedo has proven to be an effective weapon against surface ships as well as serving as an anti-submarine weapon. Today, acoustic torpedoes are mostly used against submarines.

Before a torpedo is launched, the target must be 'boxed in'. A fire control system on the firing platform will set an initial search depth range which is passed to the weapon's microprocessor. The search parameters cover the expected depth of the target.

Acoustic homing torpedoes are equipped with a pattern of acoustic transducers on the nose of the weapon. By a process of phase delaying the signals from these transducers a series of "acoustic beams" (i.e. a variation of acoustic signal sensitivity dependent on the incident angle of the noise energy). In early homing torpedoes the "beam patterns" were fixed whereas in more modern weapons the patterns were modifiable under on-board computer control. These sensor systems are capable of either detecting sound originating from the target itself i.e. engine and machinery noise, propellor cavitation, etc., known as passive sonar or responding to noise energy reflections as a result of "illuminating" the target with sonar pulses,known as active sonar. Acoustic torpedoes can be compared to modern fire-and-forget guided missiles. What this means is the enemy (most likely a submarine) will be detected by sonar in any direction it goes. The torpedo will start with passive sonar, simply trying to detect the submarine. Once the torpedo's passive sonar has detected something, it will switch over to an active sonar and will begin to track the target. At this point, the submarine has probably started evasive maneuvers and may have even deployed a noisemaker. The torpedo's logic circuitry, if not fooled by the noise maker, will home in on the noise signature of the target submarine.

Military examples
United States:
 * RUR-5 ASROC Ship Launched anti submarine Missile
 * MK 48 ADCAP Submersion Launch Torpedo
 * MK 24 Passive Homing Surface / Submersible Fire Torpedo
 * MK 32 Active Homing Surface / Submersible / Air Fire Torpedo
 * MK 15 Surface to Surface Torpedo