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"Japanese 250-kg HE and Incendiary Bomb" from Tactical and Technical Trends

A report on Japanese high-explosive, incendiary bomb in WWII, from Tactical and Technical Trends, No. 36, October 21, 1943.

[DISCLAIMER: The following text is taken from the U.S. War Department publication Tactical and Technical Trends. As with all wartime intelligence information, data may be incomplete or inaccurate. No attempt has been made to update or correct the text. Any views or opinions expressed do not necessarily represent those of the website.]


The high-explosive, incendiary bomb is one of the most effective instruments for the crippling and destruction of military and economic objectives. The following details and diagrams relative to the Japanese 250-kg high-explosive and incendiary bomb are based on a field report made by trained bomb disposal officers in New Guinea.

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a. Details

Overall length  9 ft 9 in
Diameter of body  12 in
Wall thickness  7/32 in
Total weight  550 lb (approx)
Main filling  HE, and 750 (approx) incendiary cylinders.
Color  Light grey
Markings  Red band on tail. 6 in silver band on end of nose.

b. Use

Due to "Air bursts", and propulsion by the HE charge, incendiary cylinders are showered over a comparatively large area. Likely targets would be grounded airplanes, airfield installations, munition and supply dumps, and buildings.

The known radius of action of the incendiary cylinders is 176 yards.

c. Description

The body is made up in three separate parts -- the barrel, a cast steel nose, and a conical-shaped tail-unit.

(1) There are ten layers of incendiary cylinders in the barrel, stacked on end and separated by perforated celluloid trays, the whole being retained in the barrel by front and rear perforated steel closing disks.

(2) The nose has a threaded opening at the front end for the fuze. The rear end is machined down to form a spigot over which the barrel is fitted and the joint welded externally. The front perforated steel closing disk fits into an internal machined recess. Two trays of cylinders extend into the nose-piece. An HE charge surrounding the gain of the nose fuze is housed in front of the closing disk.

(3) The tail unit is made of 3/16 inch steel and is connected to the barrel by a welded cast-steel insert. A tail fuze-adaptor is welded to the apex of the cone. Four tail fins 1/16 inch in thickness with a cylindrical stabilizing band and a perforated steel closing plate are welded to the sides of the cone. Four smaller subsidiary tail fins are interspaced between the normal fins and positioned closer to the barrel of the bomb.

(4) The incendiary filling consists of approximately 750 incendiary cylinders. These are pieces of 1 1/8 inch external diameter m.s. tubing of 1/8 inch wall thickness, 2 3/4 inches in length and filled with what is thought to be a mixture of rubber impregnated with phosphorus.

(5) A tail-fuze is screwed into the threaded adaptor welded to the tail cone. This fuze incorporates a clockwork mechanism, and is designed to achieve an air burst 150 feet to 200 feet above the ground. It has a delicately balanced firing pin, which would respond to a slight jar in the case of an unexploded bomb. Arming vanes are incorporated in the fuze.

[Japanese 250 KG H.E. Incendiary Bomb, WWII]

There is a nose-fuze which, when fitted with an instantaneous-type mechanism is thought to detonate the bomb on impact should the tail-fuze fail to function as intended.

d. Action

Normally these bombs burst at a height of 150 to 200 feet above the ground, so that presumably the tail-fuze is capable of being timed to operate in accordance with the proposed height of release of the bombs. When the tail-fuze functions as above, the HE filling detonates, bursting the bomb and showering the area beneath with incendiary cylinders.

The incendiary cylinders are propelled with great force; some have penetrated hard ground to a depth of four inches. They burn fiercely for about 65 seconds, the steel attaining a red heat and charring wood four minutes after ignition has taken place. Very few cylinders were found unburnt. Some were found as far as 176 yards from the point over which the bomb had burst.

Of four bombs released from an altitude of 20,000 to 26,000 feet, in one area, three burst at heights of 150 to 200 feet above the target, but one penetrated the ground to a depth of three inches before detonating. Apparently this bomb was one in which the tail-fuze failed to operate as intended, detonation being caused by the nose-fuze functioning on impact with the ground.

e. Disposal

Where a bomb fails to explode, the tail-fuze, of the clockwork type and incorporating a delicately balanced firing pin, will most likely be in an extremely sensitive condition and readily susceptible to a slight jar or movement of the bomb.

The fuze can be identified by a graduated setting scale, 0 to 50 seconds horizontal, on the upper portion of the body. The bomb should not be moved until a solidifying substance is injected into the rotor through the side inspection hole.


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