Depth charge

The first attempt to fire charges against submerged targets was with aircraft bombs attached to lanyards which triggered them. A similar idea was a guncotton charge in a lanyarded can. Two of these lashed together became known as the "depth charge Type A". Problems with the lanyards tangling and failing to function led to the development of a chemical pellet trigger as the "Type B". These were effective at a distance of around . A hydrostatic pistol actuated by water pressure at a selected depth detonated the charge. anti-submarine vessels initially carried only two depth charges, to be released from a chute at the stern of the ship. Even slower ships could safely use the Type D at below and at or more, It has been argued that this was done to avoid paying the original inventor. ==Depth charges==

Mark VII The Royal Navy Type D depth charge was designated the "Mark VII" in 1939. Initial sinking speed was with a terminal velocity of at a depth of if rolled off the stern, or upon water contact from a depth charge thrower. US Mk 9 The teardrop-shaped United States Mark 9 depth charge entered service in the spring of 1943. The charge was of Torpex with a sinking speed of and depth settings of up to . Later versions increased depth to and sinking speed to with increased weight and improved streamlining. Although the explosions of the standard United States Mark 4 and Mark 7 depth charge used in World War II were nerve-wracking to the target, a U-boat's pressure hull would not rupture unless the charge detonated within about . Getting the weapon within this range was a matter of luck and quite unlikely as the target took evasive action. Most U-boats sunk by depth charges were destroyed by damage accumulated from an extended barrage rather than by a single charge, and many survived hundreds of depth charges over a period of many hours, such as , which survived 678 depth charges in April 1945. ==Delivery mechanisms==

The first delivery mechanism was to simply roll the "ashcans" off racks at the stern of the moving attacking vessel. Originally depth charges were simply placed at the top of a ramp and allowed to roll. Improved racks, which could hold several depth charges and release them remotely with a trigger, were developed towards the end of the First World War. These racks remained in use throughout World War II because they were simple and easy to reload. Some Royal Navy trawlers used for anti-submarine work during 1917 and 1918 had a thrower on the forecastle for a single depth charge, but there do not seem to be any records of it being used in action. 1277 were issued, 174 installed in auxiliaries during 1917 and 1918. The bombs they launched were too light to be truly effective; only one U-boat is known to have been sunk by them. Later depth charges for dedicated aerial use were developed. These are still useful today and remain in use, particularly for shallow-water situations where a homing torpedo may not be effective. Depth charges are especially useful for "flushing the prey" in the event of a diesel submarine hiding on the bottom. ==Effectiveness==

The effective use of depth charges required the combined resources and skills of many individuals during an attack. Sonar, helm, depth charge crews and the movement of other ships had to be carefully coordinated. Aircraft depth charge tactics depended on the aircraft using its speed to rapidly appear from over the horizon and surprising the submarine on the surface (where it spent most of its time) during the day or night (at night using radar to detect the target and a Leigh light to illuminate it immediately before attacking), then quickly attacking once it had been located, as the submarine would normally crash dive to escape attack. As the Battle of the Atlantic wore on, British and Commonwealth forces became particularly adept at depth charge tactics, and formed some of the first hunter-killer groups to actively seek out and destroy German U-boats. Surface ships usually used ASDIC (sonar) to detect submerged submarines. However, to deliver its depth charges a ship had to pass over the contact to drop them over the stern; sonar contact would be lost just before attack, rendering the hunter blind at the crucial moment. This gave a skilful submarine commander an opportunity to take evasive action. In 1942 the forward-throwing "hedgehog" mortar, which fired a spread salvo of bombs with contact fuzes at a "stand-off" distance while still in sonar contact, was introduced, and proved to be effective. Pacific theater and the May Incident In the Pacific Theater during World War II, Japanese depth charge attacks were initially unsuccessful because they were unaware that the latest United States Navy submarines could dive so deep. Unless caught in shallow water, an American submarine could dive below the Japanese depth charge attack. The Japanese had used attack patterns based on the older United States S-class submarines (1918–1925) that had a test depth of ; while the WWII Balao-class submarines (1943) could reach . This changed in June 1943 when U.S. Congressman Andrew J. May of the House Military Affairs Committee caused The May Incident. The congressman, who had just returned from the Pacific theater where he had received confidential intelligence and operational briefings from the US Navy, revealed at a press conference that there were deficiencies in Japanese depth-charge tactics. After various press associations reported the depth issue, the Japanese Imperial Navy began setting their depth charges to explode at a more effective average depth of . Vice Admiral Charles A. Lockwood, commander of the U.S. submarine fleet in the Pacific, later estimated that May's ill-advised comments cost the US Navy as many as ten submarines and 800 seamen killed in action. ==Later developments==

For the reasons expressed above, the depth charge was generally replaced as an anti-submarine weapon. Initially, this was by ahead-throwing weapons such as the British-developed Hedgehog and later Squid mortars. These weapons threw a pattern of warheads ahead of the attacking vessel to bracket a submerged contact. The Hedgehog was contact fuzed, while the Squid fired a pattern of three large, depth charges with clockwork detonators. Later developments included the Mark 24 "Fido" acoustic homing torpedo (and later such weapons), and the SUBROC, which was armed with a nuclear depth charge. The USSR, United States and United Kingdom developed nuclear depth bombs. , the Royal Navy retains a depth charge labelled as Mk11 Mod 3, which can be deployed from its AgustaWestland Wildcat and Merlin HM.2 helicopters. Russia has also developed homing (but unpropelled) depth charges including the S3V Zagon and the 90SG. China has also produced such weapons. Signaling During the Cold War when it was necessary to inform submarines of the other side that they had been detected but without actually launching an attack, low-power "signalling depth charges" (also called "practice depth charges") were sometimes used, powerful enough to be detected when no other means of communication was possible, but not destructive. ==Underwater explosions==

launches an ASROC anti-submarine rocket, armed with a nuclear depth bomb, during Dominic Swordfish (1962) The high explosive in a depth charge undergoes a rapid chemical reaction at an approximate rate of . The gaseous products of that reaction momentarily occupy the volume previously occupied by the solid explosive, but at very high pressure. This pressure is the source of the damage and is proportional to the explosive density and the square of the detonation velocity. A depth charge gas bubble expands to equalize with the pressure of the surrounding water. This gas expansion propagates a shock wave. The density difference of the expanding gas bubble from the surrounding water causes the bubble to rise toward the surface. Unless the explosion is shallow enough to vent the gas bubble to the atmosphere during its initial expansion, the momentum of water moving away from the gas bubble will create a gaseous void of lower pressure than the surrounding water. Surrounding water pressure then collapses the gas bubble with inward momentum causing excess pressure within the gas bubble. Re-expansion of the gas bubble then propagates another potentially damaging shock wave. Cyclical expansion and contraction can continue for several seconds until the gas bubble vents to the atmosphere. Consequently, explosions where the depth charge is detonated at a shallow depth and the gas bubble vents into the atmosphere very soon after the detonation are quite ineffective, even though they are more dramatic and therefore preferred in movies. A sign of an effective detonation depth is that the surface just slightly rises and only after a while vents into a water burst. Very large depth charges, including nuclear weapons, may be detonated at sufficient depth to create multiple damaging shock waves. Such depth charges can also cause damage at longer distances, if reflected shock waves from the ocean floor or surface converge to amplify radial shock waves. Submarines or surface ships may be damaged if operating in the convergence zones of their own depth charge detonations. The damage that an underwater explosion inflicts on a submarine comes from a primary and a secondary shock wave. The primary shock wave is the initial shock wave of the depth charge, and will cause damage to personnel and equipment inside the submarine if detonated close enough. The secondary shock wave is a result of the cyclical expansion and contraction of the gas bubble and will bend the submarine back and forth and cause catastrophic hull breach, in a way that can be likened to bending a plastic ruler rapidly back and forth until it snaps. Up to sixteen cycles of secondary shock waves have been recorded in tests. The effect of the secondary shock wave can be reinforced if another depth charge detonates on the other side of the hull in close time proximity to the first detonation, which is why depth charges are normally launched in pairs with different pre-set detonation depths. The killing radius of a depth charge depends on the depth of detonation, the payload of the depth charge and the size and strength of the submarine hull. A depth charge of approximately of TNT (400 MJ) would normally have a killing radius (resulting in a hull breach) of only against a conventional 1000-ton submarine, while the disablement radius (where the submarine is not sunk but is put out of commission) would be approximately . A larger payload increases the radius only slightly because the effect of an underwater explosion decreases as the cube of the distance to the target. ==See also==