DATA:

BOMBS USED IN

Mk 231-0

Mk 240-0

FUNCTIONING

ARMED CONDITION

ARMING TIME

OVERALL LENGTH

MAX. BODY DIAMETER

VANE SPAN

MATERIAL

Steel, aluminum alloy, and brass.

GENERAL:

The Mk 231-0 and Mk 240-0 are tail hydrostatix fuzes in the general design scheme of the Mk 229 and Mk 230. However, they are considerably simpler in construction and easier to manufacture. The Mk 240-0 is four inches longer than the Mk 231-0 to assure proper arming is larger bombs, is otherwise identical with the Mk 231-0. A single depth setting is provided and the fuzes are expected to function in a depth range of 25-30 ft.

To provide positive action against fuze function on reverse impacts (partioularly ri-cochete), these fuzes have an inertia ring which adds its weight to that of the piston assembly to counteract the force of the two counterbalances exerting a contray pres-sure in such impacts. On normal impacts, the inertia ring merely rests on the firing pin housing.

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Upon release of the bomb from the plane, the arming wire is withdrawn permitting rotation of the vane and the vane shaft.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 60 revolutions the stem has risen sufficiently to allow the two arming balls to fall inward freeing the hydrostatic 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 described above. After sub-mersion, water flows around the 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 in reverse impact. Hydrostatic pressure now extends the bellows, thus forcing the hydrostatic piston downward and compressing the firing spring. After the piston has moved 9/32 in., the six retaining balls locking the firing plunger to the plunger housing junp into the annular recess in the pistion. The compressed firing spring thrusts the plunger against the firing point initiating the explosive train.

REMARKS:

(1) The lower end of the arming stem is grooved for about .2 in. 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. However, arming would be stopped 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.

(2) 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.

(3) When the Mk 240-0 fuze is used in AN-M66 bombs, it is to be equipped with a special 26.5° pitch vane, to assure proper arming; the standard 20° pitch vane of the Mk 231-0 will be used when the Mk 240-0 is installed in AN-M65 bombs. The special pitch vane will be painted. (No color stated as yet).

(4) Production of the Mk 231-0 fuze has been stopped because of adequate supplies of the AN-Mk 230-5 and AN-Mk 230-6 fuzes. The Mk 240-0 will not be produced at all.