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Type 89 Combustion Time Fuze
This fuze was used on H.E. projectiles for the Japanese 75mm and 105mm Anti-Aircarft Guns.

The fuze consists of a stack four of circular rings, each with an internal channel containing a compressed black powder delay train.
The overall length of the train (delay time) is adjusted by rotating these rings.
The first (top) and third rings are fixed. The second and fourth rotate together.
Time increments are marked at the bottom, from 0 to 30 seconds

The origin of this design can be traced back to the Bormann Time Fuze invented about 1840. It used single layer delay "ring", providing up to a five second delay and was used extensively during the American Civil War.

The concept was modified, improved on and evolved through WWI and on to WWII by various countries. See my page about the WWI British No.80 combustion time fuze, for a detailed description of how these stacked delay rings work.

The Japanese Type 89 is among the last combustion time fuze designs. Advances in manufacturing technology for mechanical time fuzes made the concept obsolete.

The Type 89 has the ignition pellet located at the base of the fuze.
When the shell is fired, that pellet is forced down onto the firing pin, igniting the primer and sending a flash up the central tube where it is directed into the topmost lead-in, lighting the first delay train.

The fuse elements burn in succession, moving from top to bottom, until reaching the lower black powder tray. The powder produces flame that issues from a center port in the base plug, which initiates the detonation sequence.

Associated with the Type 89 is an Auxiliary Detonating Fuze placed directly below. The flash from the base of the Type 89 is directed through a small flash hole and communicated via a series of arming and safety stages to a detonator and booster.

One observation about these two fuzes, the one at left dated July 1944, the other May 1939.
The earlier (right) has two compression discs, visible under the first ring. I'm assuming the purpose of those is to provide tension to the stack, keeping the timing rings tight to prevent unintended rotation.
The later has a helical spring, internally (shown in the parts photo above), essentially providing the same function, although I can rotate the rings with much less pressure.
Design change? Something added/deleted along the way after the war? The helical spring is not documented. Just pointing it out for those who might notice the difference.

Another small detail.... that little nose cap. Present on one fuze and not the other. My guess is it blows out when the round is fired, providing a vent hole for escaping combustion gases (?).

Japanese 75mm Type 88 Anti-Aircraft Battery, Sumay, Guam, Mariana Islands, 5 Oct 1944

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