Mk 156 and Mk 158

Rocket uses in

Functioning

Arming time

Vane span, inches

Maximum body diameter, inches

Over-all length (without booster), inches

6.9

General: The two fuzes are identical, except that the Nose Fuze Mk 158 Mod 0 has a shear wire in the set-back collar. These fuzes were designed to replace the Nose Fuzes Mk 131 and Mk 136 respectively.

Operation: The safety pin is removed when the rocket is loaded on the launcher, leaving the vanes secured by the set-back collar, which has a leaf spring holding it up by pressure against the locking pins. On firing, the set-back collar moves back (breaking the shear wire in the Nose Fuze Mk 158), freeing the locking pins from the groove in the col-lar. On impact with water, the force on the vanes causes a torque sufficient to shear the radial shear wire holding the vane cup to the neck of the fuze. The vanes are free to rotate, unthreading the spindle through the neck of the fuze body. As the spindle draws back from the inertia weight, the spring under the firing sleeve forces the sleeve and inertia weight up. Since the firing pin is locked to the firing sleeve by three locking balls, the firing pin and its spring, cocked against a collar on the firing pin, rise with the sleeve and inertia weight. As the sleeve clears the four spring-loaded sleeve stops in the fuze body just above the firing pin guide, they spring out under the sleeve. After approximat-ely four vane rotations, the firing pin will have been raised by the sleeve sufficiently to clear the shutter cavity, allowing the spring-loaded detonator shutter to move over into the armed position with the detonator lined up with the firing pin and booster lead-in. The detonator shutter is locked in position by a spring-loaded detent. The spindle conti-nues to rise until the washer to which it is keyed engages a groove in the underside of the fuze neck. The inertia weight, sleeve, and fiing pin cease rising when the sleeve en-gages the reatining ring. On contact with a submarine or other underwater obstruction, inertia causes the weight to move forward on normal impact or laterally on oblique im-pact, thereby allowing the three locking balls to jump out and release the spring-loaded firing pin.

Remarks: The sleeve stops in the fuze body which spring out under the sleeve are provided in order to allow the spindle to be screwed back down from the armed position without danger of forcing the striker into the detonator. In view of the fact that the inertia piece may be insecurely lodged over the striker spindle, this procedure is not re-commended on any rocket that has once been fired.

Do not remove these fuze from a projectile with any tool other than the span-ner which is issued with the fuze. Separation of the two parts of the fuze body arms the fuze.

Figure 157. Navy Nose Fuzes (Rocket) Mk 156 and Mk 158