World War II Database

Country of Origin	United States
Type	Torpedo
Diameter	480.000 mm
Length	2.100 m
Weight	308.000 kg
Range	5.556 km
Machinery	Electric Motor - Acoustic Homing
Explosive Charge	42kg HBX
Speed	22.00 km/h

Contributor: David Stubblebine

ww2dbaseOfficially designated as the Mark 24 Mine, the United States' Mark XXIV was in no way a mine but was in every way an air-dropped anti-submarine acoustic homing torpedo - and a very successful one.

Conceived even before the United States entered World War II, the idea for an acoustically homing anti-submarine torpedo was among the earliest projects for Vannevar Bush's team at the Office of Scientific Research and Development (OSRD). The OSRD called it Project 61 with a codename of FIDO (perhaps because it would sniff out its targets). Early Navy documents referred to the concept as an acoustic mine and for security reasons, as well as to keep it away from the meddling Bureau of Ordnance and their delay-ridden torpedo approval process, the official designation as a mine was kept. Both names, FIDO and Mine, stuck to the weapon throughout its operational lifetime.

As was Bush's way, development of the constituent parts was delegated to the teams most qualified to get those pieces done. The overall development was led jointly by Harvard University's Underwater Sound Lab and Bell-Telephone Laboratories. Harvard and Bell Labs were tasked to separately but cooperatively develop a passive acoustic homing and control system. Western Electric was to develop a suitable lead-acid battery, General Electric was to design and fabricate the electric propulsion and steering motors while also investigating an active acoustic homing system, and the Navy's David Taylor Model Basin would provide assistance with hydrodynamics and propulsion.

The final acoustic design consisted of four crystal hydrophones arranged around the girth of the torpedo body, one each on the top, bottom, right side, and left side. In this way, sound coming from the torpedo's right side, for example, would be shielded on the left side by the torpedo body, and so on for the other three. The steering mechanisms would then simply direct the torpedo toward the loudest noise. The device would be dropped from an aircraft so there would be no issue of the weapon homing in on the vessel that launched it. Even so, the final design included a 30-foot "depth ceiling" so that the torpedo would not threaten any friendly surface ships that could be nearby. As an air-launched weapon, the Mark 24 was designed to fit in the same space as a standard 1,000-pound bomb and fitted with shackles so it could be hung from the same bomb rack. The Mark 24 would dive to a depth of 50-feet (later 150-feet) and circle while searching for a sound source within specified frequencies that it would then home in on. Its batteries could maintain 12 knots for about 15 minutes for a maximum travel of 5,000 meters. The slow speed of the FIDO was one of its most closely guarded secrets since the speed was enough for it to catch most submerged submarines but almost every submarine could outrun it on the surface.

Development of other United States Navy torpedoes took place at the Naval Torpedo Factory in Newport, Rhode Island by the Navy's Bureau of Ordnance (BuOrd). Hindsight clarifies what was only suspected at the time that BuOrd's torpedo development tended to be overly lengthy and yielded under-engineered and faulty weapons. Bush's OSRD format avoided all that with the work given to results-driven private sector engineers and an overall authority only one step removed from Franklin Roosevelt. Development of the Mark 24 was remarkably fast, especially when contrasted against BuOrd. The first serious conversation about the concept of an acoustic torpedo occurred very in early Dec 1941, before the Pearl Harbor Attack. Design plans were put on paper later that same month and 1942 saw prototype development and field testing. As early as Jun 1942, the first production order was placed for 10,000 devices. The first operational torpedoes were delivered to the Navy in Mar 1943 and to the front-line units in May 1943. The weapon arrived in the North Atlantic just in time for what the Germans would call "Black May."

On 12 May 1943 in the mid-Atlantic, German submarine U-456 was caught on the surface by a Liberator bomber from RAF Coastal Command's No. 86 Squadron operating out of Northern Ireland. The submarine dove but the bomber was able to drop a Mark 24 FIDO near U-456's position. The torpedo successfully homed in on the submarine, exploded, and caused considerable damage. The bomber had to break off the attack but later that same day as destroyer HMS Opportune began a depth charge attack, U-456 was forced to dive again and the earlier damage proved too much; the submarine sank with all 49 hands. This first successful deployment of the Mark 24 was just seventeen months after its first planning meeting.

The United States Navy's first operational success came two days later in the North Atlantic about midway between Iceland and Greenland. Lieutenant (junior grade) Phillip Bodinet flying a PBY-5A Catalina with Patrol Squadron VP-84 operating out of Reykjavik, Iceland launched one FIDO torpedo against submarine U-640 resulting in the loss of the boat and all 49 on board.

Over the next few weeks, the Mark 24's contribution to Black May was significant. For security reasons, the weapon was listed in the official reports as the "Mark 24 mine" if it was identified at all. Many more successes quickly followed and immediately the FIDO success rate was about twice that of air-dropped depth charges. That success rate showed how effective the weapon was and caused the Navy to reduce the Mark 24 production order from 10,000 to 4,000.

The Mark 24 torpedo is officially credited with sinking 31 German and 6 Japanese submarines. Also, fifteen German and three Japanese submarines were damaged by the FIDO but survived. Throughout the remainder of World War II, the Mark 24 "Mine" remained top-secret and wartime photographs of them are scarce. The device remained classified until well after the war and saw service with the United States Navy until 1948.

Sources:
United States Navy
UBoat.net
Naval Weapons of the World (navweaps.com)
U-Boat Archive
The Pacific War Online Encyclopedia
Japanese Naval History (combinedfleet.com)
Ron Larham, Hart Plain Institute for Studies
Wikipedia ww2dbase

Last Major Revision: Oct 2021

Mark XXIV Torpedo Interactive Map

Photographs

See all 11 photographs of Mark XXIV Torpedo

Mark XXIV Timeline

22 Dec 1941	Barely two weeks following the first conceptual meeting of scientists at Vannevar Bush's Office of Scientific Research and Development about the feasibility of an acoustically homing anti-submarine torpedo, plans for such a device were put on paper (codenamed FIDO).
12 May 1943	Just 17 months after the first conceptual meeting for an acoustic homing torpedo, the Mark 24 FIDO aerial torpedo had its first operational victory when a Liberator bomber of RAF Coastal Command’s 86 Squadron caught German submarine U-456 on the surface in the mid-Atlantic. As the submarine dove, the bomber dropped one Mark 24 torpedo that guided itself to the submerged submarine and exploded, causing major damage. Later that same day when U-456 was forced to dive deep to avoid a depth charge attack by destroyer HMS Opportune, the previous damage proved too great and the submarine sank with all 49 hands.
14 May 1943	The United States Navy achieved its first success with the Mark 24 FIDO acoustic homing torpedo when Lt(jg) Phillip Bodinet of Patrol Squadron VP-84 flying a PBY-5A Catalina dropped one Mark 24 torpedo against German submarine U-640 about midway between Iceland and Greenland resulting in the loss of the boat and all 49 on board.
26 May 1943	The United States Navy’s Patrol Squadron VP-84 scored another success with the Mark 24 FIDO acoustic homing torpedo south of Iceland when Lt Robert Millard dropped one Mark 24 torpedo from his PBY-5A Catalina against German submarine U-467. The U-Boat was lost with all 46 hands.
24 Jun 1943	PBY Catalina aircraft flying with Patrol Squadron VP-84 located German submarine U-194 in the mid-Atlantic and launched one Mark 24 FIDO acoustic homing torpedo. U-194 was lost with all 54 hands.
14 Jul 1943	TBF-1 Avenger aircraft flown by Lt(jg) John Ballantine flying from USS Santee in the Atlantic south of the Azores attacked the surfaced German submarine U-160 using Mark 24 FIDO acoustic homing torpedoes. U-160 was lost with all 57 hands.
15 Jul 1943	Different groups of TBF-1 Avenger aircraft and F4F Wildcats flying from USS Santee in the mid-Atlantic made two separate attacks on surfaced German submarines using Mark 24 FIDO acoustic homing torpedoes each with underwater explosions reported. Only U-509 was reported lost in this area on this date.
30 Jul 1943	TBF-1 Avenger aircraft from USS Santee in the Atlantic southwest of the Azores attacked the surfaced German submarine U-43 using Mark 24 FIDO acoustic homing torpedoes. U-43 was lost with all 55 hands.
3 Aug 1943	US Navy Lt(jg) Zeke Cormier flying a TBF Avenger from escort carrier USS Card dropped two depth charges and one Mark 24 FIDO on what was believed to be a German submarine in the mid-Atlantic. One submarine was reported sunk but this was never confirmed.
7 Aug 1943	In a coordinated attack by six TBF-1 Avenger torpedo bombers and four F4F-4 Wildcat fighters with Composite Squadron VC-1 flying from USS Card, German supply submarine U-117 was sunk with Mark 24 FIDO acoustic homing torpedoes. U-117 had been supplying U-66 in the mid-Atlantic when both were caught on the surface by Lt(jg) A.H. Sallenger in his Avenger. U-117 was sunk and U-66 was likely damaged.
8 Aug 1943	In poor weather with heavy swell south of Greenland, German submarine U-262 (Kapitänleutnant Rudolf Heinz Franke) was awaiting refueling from U-664 whilst U-760 was being supplied. At 1010 hours, a TBF Avenger aircraft and F4F Wildcat aircraft from escort carrier USS Card located the submarines and attacked. The Wildcat strafed the decks of U-262 while the Avenger approached with depth charges. The gunners aboard U-262 hit both attackers. The Wildcat crashed, killing Ensign John F. Sprague. The Avenger’s bomb bay was hit, jamming the release mechanism. The Avenger pilot, Lt(jg) Asbury H. Sallenger pulled away, and was hit again in the starboard wing. The crew manually released two depth charges (damaging U-262 with a near-miss) and jettisoned a Mark 24 FIDO torpedo, and Sallenger made a water landing. Sallenger and the gunner survived, but the radio operator went down with the aircraft. The two survivors were spotted by another aircraft from USS Card and were picked up by destroyer USS Barry in the afternoon. TBF Avenger pilots Lt(jg) C.R. Stapler and Lt(jg) “Zeke” Cormier dropped additional Mark 24 FIDO torpedoes on a moving oil slick that was likely from the damaged U-262, but results were unobserved. U-262, though damaged, made her way back to base. In a separate attack, companion German submarine U-664 launched three torpedoes at USS Card under the cover of darkness. There were no explosions and USS Card reports make no mention of this, indicating Card was unaware of the attack. Planes from Card would sink U-664 the next day.
11 Aug 1943	TBF-1 Avenger aircraft and F4F-4 Wildcats flying from USS Card in the mid-Atlantic attacked the surfaced German submarine U-525 using two depth charges and one Mark 24 FIDO acoustic homing torpedo. U-525 was lost with all 54 hands.
27 Aug 1943	TBF-1 Avenger aircraft flying from USS Card in the mid-Atlantic launched two separate attacks on two separate German submarines using Mark 24 FIDO acoustic homing torpedoes. U-508 was damaged but escaped and U-847 was sunk with all 62 hands.
4 Oct 1943	A TBF Avenger patrol aircraft with Composite Squadron VC-9 from Hunter-Killer escort carrier USS Card discovered German submarines U-264, U-422, and U-455 refueling from “Milchkau” U-460 on the surface of the Atlantic 440 miles north of the Azores. Attacking with aerial depth charges and one Mark 24 FIDO acoustic homing torpedo, U-422 was sunk immediately while the other three submarines submerged. As more aircraft and escort ships arrived in the area, a hunt for the other submarines ensued resulting in U-460 being sunk by aerial depth charges about seven miles away. U-264 and U-455 got away but U-264 was damaged. By breaking up this gathering of submarines, Convoy UGS-19 and its 102 merchant ships was able to safely pass through this area and complete its crossing from the United States to North Africa.
12 Oct 1943	Lt(jg) Letson “Sam” Balliett piloting a TBF Avenger flying from USS Card attacked a refueling operation between German submarines U-488 (“Milchkau”) and U-402 in the mid-Atlantic using one Mark 24 FIDO acoustic homing torpedo. Although this attack claimed one sinking, both submarines escaped with only minimal or no damage. Later, another TBF Avenger from Card flown by Lt(jg) Doty attacked and damaged U-731.
13 Oct 1943	TBF Avengers flying from USS Card attacked and sank German Type VIIC submarine U-402 in the mid-Atlantic using the Mark 24 FIDO acoustic homing torpedo.
31 Oct 1943	TBF-1 Avenger aircraft flying from USS Card in the mid-Atlantic flown by Lt(jg) W.S. Fowler and Lt(jg) Letson “Sam” Balliett launched a coordinated attack on German submarine U-584 using Mark 24 FIDO acoustic homing torpedoes. U-584 was lost with all 53 hands. U-91 was also attacked at the same rendezvous point but escaped unharmed.
12 Dec 1943	TBF Avengers from USS Bogue attacked the surfaced German submarine U-172 in the eastern Atlantic using Mark 24 FIDO acoustic homing torpedoes. U-172 evaded the attack but a 27-hour running battle followed.
20 Dec 1943	TBF Avengers from USS Bogue attacked the surfaced German submarine U-850 in the eastern Atlantic with Mark 24 FIDO acoustic homing torpedoes. The U-850 was lost with all 66 hands.
24 Jun 1944	Acting on intelligence intercepts, Hunter-Killer carrier USS Bogue attempted to intercept the meeting between German submarine U-530 and Japanese submarine I-52 in the mid-Atlantic as I-52 was transiting to Germany with 21,000kg of precious metals and other intelligence cargo. A TBM Avenger from Bogue located I-52 on the surface but not U-530. Launching Mark 24 FIDO acoustic homing torpedoes against the submarine, I-52 was sunk with all 109 aboard.
2 Jul 1944	Acting on intelligence intercepts, escort carrier USS Wake Island attempted to intercept a German submarine making her way home from an unsuccessful patrol in the Gulf of Guinea. At 2145 hours local time, the TBM-1C Avenger aircraft flown by Ensign Frederick Moore sighted the surfaced U-543 off the coast of Africa between the Canary and the Cape Verde Islands. U-543 fired on the airplane and landed three hits from her 20mm guns. Ensign Moore attacked with two depth charges and one Mark 24 FIDO acoustic homing torpedo, sinking the submarine with all 58 hands.
16 Jul 1944	Just as anti-submarine aircraft from escort carrier USS Card developed a submerged target southwest of Mona Island by means of sonobuoys, the main bearings of the carrier’s single turbine burned out leaving the ship dead in the water. The aircraft launched two Mark 24 FIDO homing torpedoes without result. Contact was lost and the airborne aircraft were diverted to Borinquen Field, Puerto Rico.
31 Jul 1944	TBM Avengers from escort carrier USS Card dropped sonobuoys on an oil slick in the mid-Atlantic and detected propeller cavitation noises. Two Mark 24 FIDO homing torpedoes were dropped with one confirmed underwater explosion. No confirmation of a submarine was obtained.
6 Aug 1944	TBM Avengers from escort carrier USS Card detected submarine noises from sonobuoys in the mid-Atlantic and launched two Mark 24 FIDO homing torpedoes with one confirmed underwater explosion. No confirmation of a submarine was obtained.
28 Sep 1944	While bound from Bordeaux, France for Penang, Malaya, German submarine U-219 was attacked on the surface in the Central Atlantic by TBM Avenger and FM-2 Wildcat aircraft from Composite Squadron VC-6 off carrier USS Tripoli. The air attack consisted of strafing, rockets, depth charges and Mark 24 FIDO acoustic homing torpedoes but U-219 escaped, but not before fighting back and destroying one Avenger.
1 Oct 1944	60 miles west of Palau, US Navy PBM-3D Mariner from Patrol-Bombing Squadron VPB-16 flown by Lt Floyd Wardlow launched a Mark 24 FIDO acoustic homing torpedo on the diving Japanese submarine I-177. The submarine was damaged severely and was sunk 2 days later in a Hedgehog attack from destroyer escort USS Samuel S. Miles.
21 Jan 1945	West of Ulithi, Japanese submarine I-48 with its deck loaded with four Kaitens was spotted on the surface by US Navy Lt Frank Yourek flying a PBM-3D Mariner. The patrol aircraft released two depth charges and one Mark 24 FIDO acoustic homing torpedo that severely damaged I-48. Two days later north of Yap, destroyer escorts USS Conklin and Corbesier made a Hedgehog attack on I-48 and the submarine was sunk with all 118 hands plus the four Kaiten pilots.
26 Feb 1945	45 miles west of Iwo Jima, Japanese submarine I-368 was spotted on the surface by US Navy Lt(jg) F.M. Fay flying a TBM-1C Avenger from USS Anzio. The Avenger released a Mark 24 FIDO acoustic homing torpedo and I-368 was lost with all 86 hands.
31 Mar 1945	In the Philippine Sea, Japanese submarine I-361 with its deck loaded with five Kaitens was spotted on the surface by US Navy Lt(jg) Sam Stovall flying a TBM-3E Avenger from USS Anzio. The Avenger released a Mark 24 FIDO acoustic homing torpedo and I-361 was lost with all 76 hands plus the five Kaiten pilots.
29 Apr 1945	Between Okinawa and Iwo Jima, Lt(jg) Donald Davis flying a TBM-3 Avenger with Composite Squadron VC-92 from USS Tulagi attacked Japanese submarine I-44 running on the surface with six Kaitens on her deck. Davis dropped one depth charge and one Mark 24 FIDO acoustic homing torpedo that exploded against the crash-diving submarine’s hull. I-44 was lost with all 130 hands plus four Kaiten pilots.

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