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[[File:USS Cassin Young depth charges.jpg|thumb|Depth charges on {{USS|Cassin Young|DD-793}}]]
The first attempt to fire charges against submerged targets was with aircraft bombs attached to lanyards which triggered them. A similar idea was a {{cvt|16|lb}} [[guncotton]] charge in a lanyarded can. Two of these lashed together became known as the "depth charge Type A".<ref>{{Harvnb|McKee|1993|p=46}}</ref> Problems with the lanyards tangling and failing to function led to the development of a chemical pellet trigger as the "Type B".<ref name="m49">{{Harvnb|McKee|1993|p=49}}</ref> These were effective at a distance of around {{cvt|20|ft|m|sigfig=1}}.<ref name="m49"/>
A 1913 Royal Navy Torpedo School report described a device intended for [[Demining|countermining]], a "dropping mine". At Admiral [[John Jellicoe]]'s request, the standard Mark II mine was fitted with a [[hydrostatic]] pistol (developed in 1914 by Thomas Firth and Sons of Sheffield) preset for {{cvt|45|ft}} firing, to be launched from a stern platform. Weighing {{cvt|1150|lb}}, and effective at {{cvt|100|ft}}, the "cruiser mine" was a potential hazard to the dropping ship.<ref name="m49"/> The design work was carried out by Herbert Taylor at the RN Torpedo and Mine School, [[HMS Vernon (shore establishment)|HMS ''Vernon'']]. The first effective depth charge, the Type D, became available in January 1916. It was a barrel-like casing containing a high [[explosive]] (usually [[trinitrotoluene|TNT]], but [[amatol]] was also used when TNT became scarce).<ref name="m49"/> There were initially two sizes—Type D, with a {{cvt|300|lb}} charge for fast ships, and Type D* with a {{cvt|120|lb}} charge for ships too slow to leave the danger area before the more powerful charge detonated.<ref name="m49"/><ref name="t27">{{harvnb|Tarrant|1989|p=27}}</ref>
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A hydrostatic pistol actuated by water pressure at a pre-selected depth [[detonate]]d the charge.<ref name="t27"/> Initial depth settings were {{cvt|40|or|80|ft}}.<ref name="t27"/> Because production could not keep up with demand,<ref name="m50">{{Harvnb|McKee|1993|p=50}}</ref> anti-submarine vessels initially carried only two depth charges, to be released from a chute at the stern of the ship.<ref name="t27"/> The first success was the sinking of [[SM U-68|''U-68'']] off [[County Kerry]], Ireland, on 22 March 1916, by the [[Q-ship]] [[HMS Farnborough|''Farnborough.'']]<ref name="t27"/> Germany became aware of the depth charge following unsuccessful attacks on [[SM U-67|''U-67'']] on 15 April 1916, and [[SM U-69|''U-69'']] on 20 April 1916.<ref name="t27"/> The only other submarines sunk by depth charge during 1916 were [[SM UC-19|''UC-19'']] and [[SM UB-29|''UB-29'']].<ref name="t27"/>
Numbers of depth charges carried per ship increased to four in June 1917, to six in August, and 30–50 by 1918.<ref name="m50"/> The weight of charges and racks caused ship instability unless heavy guns and torpedo tubes were removed to compensate.<ref name="m50"/> Improved pistols allowed greater depth settings in {{cvt|50|ft}} increments, from {{cvt|50|to|200|ft}}.<ref name="m49"/><ref name="t40">{{harvnb|Tarrant|1989|p=40}}</ref> Even slower ships could safely use the Type D at below {{cvt|100|ft}} and at {{cvt|10|kn|km/h mph}} or more,<ref name="m50"/> so the relatively ineffective Type D* was withdrawn.<ref name="t40"/> Monthly use of depth charges increased from 100 to 300 per month during 1917 to an average of 1745 per month during the last six months of [[World War I]].<ref name="t40"/> The Type D could be detonated as deep as {{cvt|300|ft|m|sigfig=1}} by that date. By the war's end, 74,441 depth charges had been issued by the RN, and 16,451 fired, scoring 38 kills in all, and aiding in 140 more.<ref name="m50"/>
[[File:HMS Ceylon depth charge.jpg|thumb|Depth charge exploding after being released by [[HMS Ceylon (30)|HMS ''Ceylon'']]]]
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The United States requested full working drawings of the device in March 1917. Having received them, Commander Fullinwider of the U.S. Bureau of Naval Ordnance and U.S. Navy engineer Minkler made some modifications and then patented it in the U.S.<ref>{{cite patent |country=US |number=1321428 |title=Horn Mine |inventor1-last=Fullinwider |inventor1-first=Simon P. |inventor2-last=Minkler |inventor2-first=Chester T. |pubdate= 1919-11-17 |fdate=1917-11-11 |assign=[[United States Government]] }}</ref> It has been argued that this was done to avoid paying the original inventor.<ref>{{Citation |title=Museum Discovers Unknown Inventor |publisher=Explosion – Museum of Naval Firepower |url=http://www.explosion.org.uk/index.php?option=com_content&view=category&layout=blog&id=57&Itemid=222&limitstart=42 |access-date=29 September 2012 }}</ref><ref>{{Citation |last=Prudames |first=David |title=Inventor Of The Depth Charge Discovered At Explosion! |date=20 August 2003 |location=Brighton, UK |publisher=Culture24 |access-date=29 September 2012 |url=http://www.culture24.org.uk/history+%26+heritage/war+%26+conflict/art17861 |archive-date=29 September 2012 |archive-url=https://web.archive.org/web/20120929101420/http://www.culture24.org.uk/history+%26+heritage/war+%26+conflict/art17861 |url-status=dead }}</ref>
The Royal Navy Type D depth charge was designated the "Mark VII" in 1939.<ref name="campbell">{{harvnb|Campbell|1985|p=89}}</ref> Initial sinking speed was {{cvt|7|ft/s}} with a terminal velocity of {{cvt|9.9|ft/s}} at a depth of {{cvt|250|ft}} if rolled off the stern, or upon water contact from a depth charge thrower.<ref name="campbell"/> Cast iron weights of {{cvt|150|lb}} were attached to the Mark VII at the end of 1940 to increase sinking velocity to {{cvt|16.8|ft/s}}.<ref name="campbell"/> New hydrostatic pistols increased the maximum detonation depth to {{cvt|900|ft}}.<ref name="campbell"/> The Mark VII's {{cvt|290|lb}} amatol charge was estimated to be capable of splitting a {{cvt|7/8|inch|mm}} submarine pressure hull at a distance of {{cvt|20|ft|m|sigfig=1}}, and forcing the submarine to surface at twice that.<ref name="campbell"/> The change of explosive to [[Torpex]] (or Minol) at the end of 1942 was estimated to increase those distances to {{cvt|26|and|52|ft|m|0}}.<ref name="campbell"/>
The British Mark X depth charge weighed {{cvt|3000|lb}} and was launched from the {{cvt|21|in}} [[torpedo tube]]s of older destroyers to achieve a sinking velocity of {{cvt|21|ft/s}}.<ref name="campbell"/> The launching ship needed to clear the area at 11 knots to avoid damage, and the charge was seldom used.<ref name="campbell"/> Only 32 were actually fired, and they were known to be troublesome.<ref>{{Harvnb|McKee|1993|p=53}}</ref>
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The teardrop-shaped United States Mark 9 depth charge entered service in the spring of 1943.<ref name="c163">{{harvnb|Campbell|1985|p=163}}</ref> The charge was {{cvt|200|lb}} of Torpex with a sinking speed of {{cvt|14.4|ft/s}} and depth settings of up to {{cvt|600|ft}}.<ref name="c163"/> Later versions increased depth to {{cvt|1000|ft}} and sinking speed to {{cvt|22.7|ft/s}} with increased weight and improved streamlining.<ref name="c163"/>
Although the explosions of the standard United States {{cvt|600|lb}} 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 {{cvt|15|ft|m|0}}. 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 [[Unterseeboot 427|''U-427'']], which survived 678 depth charges in April 1945.
==Delivery mechanisms==
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==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 (weapon)|Hedgehog]] and later [[Squid (weapon)|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,
Russia has also developed homing (but unpropelled) depth charges including the [[S3V Zagon]] and the [[RBU-6000|90SG]].<ref name=roe-rpk>{{cite web |title=Anti-submarine rocket launcher system RPK-8 {{!}} |url=http://roe.ru/eng/catalog/naval-systems/shipborne-weapons/rpk-8/ |website=Catalog Rosoboronexport roe.ru}}</ref> China has also produced such weapons.<ref>{{cite web |title=PLANAF conducts live-fire exercise with new guided depth charge |url=https://www.janes.com/defence-news/news-detail/planaf-conducts-live-fire-exercise-with-new-guided-depth-charge |website=Janes.com |date=December 8, 2020|language=en}}</ref>
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==Underwater explosions==
[[File:Nuclear_depth_charge_explodes_near_USS_Agerholm_(DD-826)_on_11_May_1962.jpg|thumb|[[USS Agerholm (DD-826)|USS ''Agerholm'' (DD-826)]] launches an [[ASROC]] anti-submarine rocket, armed with a nuclear depth bomb, during [[Operation Dominic|Dominic Swordfish]] (1962)]]
The high explosive in a depth charge undergoes a rapid chemical reaction at an approximate rate of {{cvt|8000|m/s|order=flip}}. 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.<ref name="proceedings">{{harvnb|Jones|1978|pp=50–55}}</ref>
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.<ref name="proceedings"/>
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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.{{citation needed|date=February 2015}}<!--nearby shock would trigger second depth charge?-->
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 {{cvt|100|kg|lb|order=flip}} of TNT (400 [[megajoule|MJ]]) would normally have a killing radius (resulting in a hull breach) of only {{cvt|3|-|4|m|ft|order=flip|0}} 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 {{cvt|8|-|10|m|order=flip}}. 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==
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