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.
* * *
|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.|
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.
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.
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
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.
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.