Mk 231 Mod 0 and Mk 240 Mod (Tail Hydrostatic)

Bombs

Mk 231 Mod 0

Mk 240 Mod 0

Functioning

Armed condition

Arming time

Over-all length, inches

Body diameter, inches

Material

Steel, aluminum alloy, and brass

General: The Tail Hydrostatic Fuzes Mk 231 Mod 0 and Mk 240 Mod 0 are tail hydros-tatic fuzes in the general design scheme of the Mk 229 and Mk 230. They are consider-ably simpler in construction, however, and easier to manufacture. The Mk 240 Mod 0 is four inches longer than the Mk 231 Mod 0, to assure proper arming in larger bombs, but is otherwise identical to the Mk 231 Mod 0. A single depth setting is provided, and the fu-zes are expected to function in a depth range of 25 to 30 feet.

To provide positive action against fuze function on reverse impacts (particularly rico-chezs), these fuzes have an inertia ring which adds its weight to that of the piston as-sembly to counteract the force of the two counterbalances exerting a contrary pressure in such impacts. On normal impacts, the inertia ring merely rests on the firing-pin hous-ing.

Operation: Upon release of the bomb from the plane, the arming wire is withdrawn, permitting rotation of the vane and the vane shaft. Rotating with the vane and threading out of the arming-stem guide is the arming stem; after 40 to 50 vane revolutions the stem has risen sufficiently to allow the two arming balls to fall inward, freeing the hydro-static piston and arming the fuze.

At normal impact with water, the inertia counterbalances prevent the fuze from firing; and, on reverse impact, the inertia ring operates as previously mentioned. After submer-sion, water flows around th baffle ring and into the fuze through the two ports. The baffle ring prevents the fuze from firing when subjected to sudden surges of water such as might occur on reverse impact. However pressure now extends the bellows, thus forc-ing the hydrostatic piston downward and compressing the firing spring. After the piston has moved 9/32 inch, the six retaining balls locking the firing plunger to the plunger housing jump into the annular recess in the piston. The compressed firing spring thrusts the plunger against the firing point, initiating the explosive train.

Remarks: The lower end of the arming stem is grooved for about 0.2 inch of its length. The purpose of this feature is to arrest arming of the fuze, should the bomb be accidentally released in water without the arming wire, as in crash landings. In this event, water might rotate the vane, initiating arming action. Arming would be stopped, however, once the grooved section faced the balls, since hydrostatic pressure on the piston assembly would force the balls into the grooves, jamming the arming stem in place and preventing further rotation of the vane.

The vane shaft is connected to the arming stem by a slip joint, to remove the danger of arming the fuze by a blow damaging or carrying away part of the arming mechanism. In this connection, it will be noted that the arming-mechanism tube has a circumferential groove near its base which is provided to cause shearing at that point, should the force of impact break off part of the arming mechanism.

When the Tail Hydrostatic Fuze Mk 240 Mod 0 is used in the 2,000-lb. G.P. Bomb AN-M66, it is to be equipped with a special 26.5° pitch vane, to assure proper arming; the standard 20° pitch vane of the Mk 231 Mod 0 will be used when the Fuze Mk 240 Mod 0 is installed in the 1,000-pound G.P. bomb AN-M65. The special-pitch vane will be paint-ed. (No color has been specified at the time of writing.)

Production of the Tail Hydrostatic Fuze Mk 231 Mod 0 has been stopped because of adequate supplies of the Tail Hydrostatic Fuzes AN-Mk 230 Mods 4, 5, and 6. The Mk 240 Mod 0 will not be produced at all.

Figure 368. Tail Hydrostatic Fuze Mk 231 Mod 0