oxygen-enriched air torpedoes with the 21 in. Mk. VII of the 1920s designed for the s although once again these were converted to run on normal air at the start of World War II. Around this time too the Royal Navy were perfecting the Brotherhood burner cycle engine which offered a performance as good as the oxygen-enriched air engine but without the issues arising from the oxygen equipment and which was first used in the extremely successful and long-lived 21 in. Mk. VIII torpedo of 1925. This torpedo served throughout WW II (with 3,732 being fired by September 1944) and is still in limited service in the 21st Century. The improved Mark VIII** was used in two particularly notable incidents; on 6 February 1945 the only intentional wartime sinking of one submarine by another while both were submerged took place when HMS Venturer sank the German submarine U-864 with four Mark VIII** torpedoes and on 2 May 1982 when the Royal Navy submarine sank the Argentine cruiser with two Mark VIII** torpedoes during the Falklands War. This is the only sinking of a surface ship by a nuclear-powered submarine in wartime and the second (of three) sinkings of a surface ship by any submarine since the end of World War II). The other two sinkings were of the Indian frigate and the South Korean corvette ROKS Cheonan. Many classes of surface ships, submarines, and aircraft were armed with torpedoes. Naval strategy at the time was to use torpedoes, launched from submarines or warships, against enemy warships in a fleet action on the high seas. There were concerns torpedoes would be ineffective against warships' heavy armor; an answer to this was to detonate torpedoes underneath a ship, badly damaging its keel and the other structural members in the hull, commonly called "breaking its back". This was demonstrated by magnetic influence mines in World War I. The torpedo would be set to run at a depth just beneath the ship, relying on a magnetic exploder to activate at the appropriate time. Germany, Britain, and the U.S. independently devised ways to do this; German and American torpedoes, however, suffered problems with their depth-keeping mechanisms, coupled with faults in magnetic pistols shared by all designs. Inadequate testing had failed to reveal the effect of the Earth's magnetic field on ships and exploder mechanisms, which resulted in premature detonation. The Kriegsmarine and Royal Navy promptly