IJNAF and IJAAF Aircraft Ordinance
Part I Aircraft Torpedoes
by Bryan Wilburn
This article on torpedoes starts a multi-part series on Japanese Aircraft ordnance of WWII. Source data has been compiled from records in the various archives in the Washington, D. C. area. Where doubt exists a "?" will be used. While it has taken an inordinate amount of time to "Bring it all together", hopefully it will clear up some questions and help shed light an a dark corner of WWII history. The majority of information on torpedoes has been gleaned from the 0.S. Naval Technical Mission to Japan, 1945‑46, and photographs.
Much of the credit for the success of Japanese torpedoes must go to Rear Adm. Naruse for his determination in evolving a Potent weapon. Unlike U.S. torpedo technology, the Japanese conducted a vigorous upgrade and testing program. The Type 91 aircraft torpedo was first built in 1931, as opposed to the WWI vintage U.S. Mark 13 unit. Contrary to popular belief the Type 91 series did not use oxygen though.
A problem that plagues all aircraft torpedoes is stability on water entry. The U.S. never fully got the hang of it and experienced problems well into 1944. The major problem is spinning on entry. This causes the torpedo to "Fish Hook'' or turn sharply as the tall fins enter time water.
The Japanese approached the problem on two fronts. First they tested two types of detachable tail frames. The "box' and "X", see diagrams(click to see diagrams), were both used during WWII. While the Box was the most effective, it was impractical for confined bomb bays. Box units were used for under-fuselage and wing mounts.
X units, on the other hand, were more commonly used m all types of installations. Why they were called X & Box types is a mystery to me. The X looks more like a box and vice versa.
Secondly, to futher combat "Fish Hooking" two small anti-spin flippers were first installed on Type 91, Mod 2 units, 1942, and all subsequent models. The pair of flippers were gyro controlled and located just forward of the tail fins. The Flippers in turn had detachable wooden fins of bath small and large designs. Small units were used more often. Both tail frames and "Flipper Slippers" slipped on and broke away on water entry.
Another problem was the drop envelope, how fast, how high, etc. The ideal condition with mast early units was at 180Kts and 350 feet with a 170 to 200 bow angle at 1000m. With, I might add, Lord knows haw many anti-aircraft guns blasting away at you. Rather unsettling thought, isn't it? An improper drop would cause the torpedo to dive to the bottom or porpoise and snap in half. A 4mm latex rubber sheath, extending back 24" from the nose aided in cushioning water impact shock. The sheath would shatter on impact. (Click to see specifications)
While the technical mission report stated that no other type of exploders than the Type 90, Mk2 pocket unit were used, torpedoes sporting 17" "Whisker Pistols", Type ?, Mk 3, were found by U.S. intelligence personnel. This modification enabled the torpedo to be used as a bomb. There were reports of pilots attempting to get bomb type hits with their torpedoes. U. S. observers thought the pilots were nuts, if they only knew.
The sketch of the training warhead shows prongs on the tip, in all likelihood to facilitate recovery not detonation.
Rear Adm. Naruse, in his postwar interviews also indicated that the Japanese tested an 8 finned torpedo and a ring propeller shroud. Finding both unsatisfactory, they were discarded. An interesting side note, Pacific Wrecks by Charles Darby, Kookabura 1979, shows an 8 finned afterbody photographed at Bonis AFB, Bougainville Is., p. 19.  Possibly an operational test model? Test units were Type 91, Mod 3.
Undoubtedly the Type 91 torpedo aided in the sweeping initial victories in the Pacific. Towards the end of WWII however, they played an ever decreasing role as U.S. momentum, hordes of F6F Hellcats and a drastic drop in I.J.N.A.F. pilot proficiency after both the Midway & Guadalcanal disasters changed the course of the war. These, along with many more factors rendered the torpedo to be almost useless during the final cataclysmic battles of WWII.
Coloration: Naval Operational torpedoes were left in a bare metal condition and coated with a light film of oil. The casings were forged steel. The latex sheath looks like it is creamy white. Props were Sk or Vg steel with the Mod 3 (1942) onward. Pre Mod 3 units had bronze props. Training torpedoes appear to be a light grey in photographs. Plywood fins were unpainted, and occasionally the Box type had two sets of straps, acting as struts. 1 have not found evidence that the warheads were painted red or other than I have indicated.
Army Peggy units: Body, tail units and stabilizers are the same as their naval counterparts. The warhead appears to be a lamp black similar to Army bombs.
The moveable parts of the torpedo tail units and anti-spin flippers have been darkened only to show proportion.

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