thumb|US World War II Mark IX depth charge. Streamlined and equipped with fins to impart rotation, allowing it to fall in a straight [[trajectory with less chance of drifting off target. This depth charge contained of Torpex.]]
A depth charge is an anti-submarine warfare (ASW) weapon designed to destroy submarines by detonating in the water near the target and subjecting it to a destructive hydraulic shock. Most depth charges use high explosives with a fuze set to detonate the charge, typically at a specific depth from the surface. Depth charges can be dropped by ships (typically fast, agile surface combatants such as destroyers or frigates), patrol aircraft and helicopters.
Depth charges were developed during World War I, and were one of the first viable methods of attacking a submarine underwater. They were widely used in World War I and World War II, and remained part of the anti-submarine arsenals of many navies during the Cold War, during which they were supplemented, and later largely replaced, by anti-submarine homing torpedoes.
thumb|The [[Mk 101 Lulu was a US nuclear depth bomb operational from 1958 to 1972]]
A depth charge fitted with a nuclear warhead is also known as a "nuclear depth bomb". These were designed to be dropped from a patrol plane or deployed by an anti-submarine missile from a surface ship, or another submarine, located a safe distance away. By the late 1990s all nuclear anti-submarine weapons had been withdrawn from service by the United States, the United Kingdom, France, Russia and China. They have been replaced by conventional weapons whose accuracy and range had improved greatly as ASW technology improved.
History
thumb|Depth charges on
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 . 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
thumb|upright|To be effective depth charges had to explode at the correct depth. To ensure this, a pattern of charges set to different depths would be laid atop the submarine's suspected position.
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
thumb|[[USS Agerholm (DD-826)|USS Agerholm (DD-826) 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.