The Problematic Magnetic Exploder
The U.S. Navy innovation was supposed to result in the ultimate submarine weapon—a torpedo that would explode under a ship. That sort of attack would snap a small ship in half and bypass the layers of side compartments that were supposed to protect a larger ship—such as a battleship or carrier—against torpedo hits. The magnetic exploder, which responded to a sharp increase in the earth’s magnetic field caused by a passing ship, was such a closely held secret that it had a cover name: the Index Device. The usual demonstration for submarine officers was to pass a metal cigarette case near a magnetic exploder. A click would be heard. If something as small as the case could trigger the exploder, surely any warship also would do so.
As usual, there was a rub. When the U.S. Navy tried torpedoes armed with the magnetic exploder in Pacific war combat, too often they detonated prematurely or not at all. The device’s sensitivity was limited to avoid misfires. Although in theory an underbottom explosion can be effective even if it occurs well under a ship, the magnetic exploder had to come within about ten feet of the target’s bottom to be triggered. It took some time for the Bureau of Ordnance (BuOrd) to accept that its torpedoes were running too deep.
Once that problem was solved, torpedoes still exploded short of their targets, particularly when they were fired at major warships such as carriers. These explosions advertised the submarine’s presence; worse, the larger targets typically were the best escorted ones. Probably because so much had been expected of the magnetic exploder, the backup contact exploder also was poorly designed. In mid-1943 the Pacific Fleet ordered all magnetic exploders disabled and conducted tests that forced BuOrd to redesign the contact exploder. Once that device had been fixed, the U.S. submarine force wiped out the Japan’s merchant marine and also much of the Imperial Japanese Fleet. Had the problem not existed, presumably the U.S. submariners would have been far more effective much earlier, and the war might have been noticeably shortened.
So what went wrong with the miracle magnetic torpedo exploder? BuOrd had become interested in a magnetic exploder about 1921. The British had tried magnetic mines late in World War I, and U.S. mine developers produced an equivalent (the Mk VII mine) postwar. BuOrd asked whether the same mechanism might be applied to a torpedo. It may have been inspired in part by the German wartime development of a magnetic exploder. The Germans believed that this device solved a serious torpedo fire-control problem: setting a torpedo to run at a specific depth so it would hit its target.
As it happened, the German exploder was considered both unsafe and unreliable, hence not worth simply copying. Instead, in June 1922 BuOrd created two parallel projects to develop a magnetic exploder: G-53, using the electric current generated in a coil moving through the earth’s magnetic field; and G-67, using existing mine mechanisms. G-53 succeeded. It was intended to be effective against a typical submarine when passing 25 feet beneath her keel. The project included measurements of the magnetic fields of various ships in dockyards.
On 8 May 1926, the new Mk 6 exploder was tested against the decommissioned submarine L-8. The first shot failed, but the second blew the submarine in half. These were the Mk 6’s only live shots, but a hundred instrumented (non-warhead) test shots were fired against the cruiser Indianapolis. These demonstrated that the magnetic exploder worked—when the torpedo had an exercise head. The fact that no further live shots were fired before war broke out is generally attributed to lack of funds, but it also might be argued that no one wanted to risk leaking the existence of so important a weapon. The magnetic exploder was considered so secret that its handbook was not printed for distribution.
During the war, even after the running-depth problem was solved, the issue of torpedoes exploding prematurely remained. It turns out that the torpedo developers misunderstood ships’ magnetic fields. They thought the fields extended only directly below a ship. We now know that a ship’s magnetic field (of sufficient intensity to activate the exploder) is more pancake-shaped. The larger the ship, the farther it extends beyond the ship’s hull. That explains why the premature explosion problem was especially bad when attacking big ships. In addition, a torpedo bouncing up and down in a rough sea might sense a rapid-enough change in the earth’s magnetic field to activate its exploder.
The question submariners asked then and later was whether a much more aggressive test program using live torpedoes would have uncovered the problems and given them the miracle weapon BuOrd had promised. Unquestionably it would have uncovered the depth-control problem. However, the large-ship problem might not have been discovered. Very few large surplus warships were available as targets during the interwar period, however generous Congress and the naval leadership might have been.
BuOrd was not alone in seeing the magnetic (proximity) exploder as a miracle weapon. Both the British and Germans also tried it (the Japanese did not, perhaps because they lacked a sufficiently sophisticated electronics industry). Like the Americans, the British were satisfied with a single successful test shot against a relatively small warship, a destroyer leader. The Germans seem not to have tested their exploder at all. During World War II, both found the performance of their magnetic exploders extremely embarrassing and disabled them—well before the U.S. Navy did so.
The Germans were alone in persevering with further development. They produced an active (rather than passive) magnetic exploder. It created its own magnetic field and responded to the distortion in that field created by a ship. Although full details of the German device were captured in 1945, neither the U.S. Navy nor the Royal Navy seems to have been interested. In both cases the magnetic exploder had failed too badly.
Wartime torpedo problems led U.S. submariners to demand extensive live-fire testing. That is why so many decommissioned ships have been sunk by modern Mk 48 and ADCAP torpedoes. Unfortunately, the failure of the magnetic exploder also convinced the U.S. Navy that such mechanisms were fundamentally impractical. Surely no one would waste further effort on them. Hence the only torpedoes that might attack U.S. warships would hit their sides. To deal with this threat, U.S. aircraft carriers were given elaborate side protection systems consisting of layers of filled and void compartments. The ships still faced the threat of underbottom attack by large ground mines, but protection against contact hits had a substantial effect on carrier design.
Sometime in the early 1960s, the Soviets published a book on torpedoes apparently intended for teenagers contemplating naval service. It included a description of the only torpedo exploder the Soviet Navy then used—a German-style active magnetic device. The authors clearly thought the information was unclassified. If all Soviet torpedoes used such exploders, the huge investment the U.S. Navy had made in side protective systems was largely devalued.
The shock of this revelation seems to have driven the U.S. Navy back into using a magnetic exploder. That the Mk 48 torpedo uses a proximity fuze of some type is evident in the many photos of ships blown in half by the weapon (a contact hit will not do the same kind of damage). Moreover, it appears that virtually all current torpedoes in other navies also use some type of proximity fuze, either magnetic or acoustic. Published plans of the new British Queen Elizabeth II–class carrier show no side protective system at all—an indication that the torpedoes that may someday be fired against these ships would explode under them. In this sense the Germans and later the Soviets vindicated BuOrd’s judgment that a viable magnetic exploder really is worthwhile.
Dr. Friedman is the author of many books about warships, aircraft, and their weaponry, including multiple editions of The U.S. Naval Institute Guide to World Naval Weapon Systems and most recently Fighters over the Fleet: Naval Air Defence from Biplanes to the Cold War (Naval Institute Press, 2016).