ROCKET USED IN

FUNCTIONING

ARMED CONDITION

ARMING TIME

MAX. BODY DIAMETER

OVERALL LENGTH

5-1/2"

GENERAL:

The Mk 157-0 is essentially similar to the Mk 146 fuze, with the following differen-ces: (1) a .02 second delay detonator replace the non delay detonator of the Mk 146; (2) the firing pin and the firing pin body are pinned together by a thin lock wire. The Mk 157-0 fuze has been developed to afford greater penetration of the target than was possible with the non-delay detonator of the Mk 146. The Mk 157-0 is being replaced by the Mk 165-0 which consists of the Mk 157-0 with a motor adapter and an improvi-sed detonator shutter locking arrangement.

The Mk 157 Mod 1 differs from the Mk 157-0 in that the material for the fuze body has been considerably strengthened, and the number of external threads has been ap-proximately doubled. In all other respects the fuzes are identical.

OPERATION:

When the rocket is fired, gas under considerable pressure from the rocket motor passes through the inlet screen underneath the inlet screw and enters the pressure chamber. As the gas pressure builds up the diaphragm bears against the arming plun-ger, breaking the shear wire and forcing the arming plunger down. The locking ball, which preventing the  upward movement of the firing pin body, moves into the narrow protion of the arming plunger. The firing pin body is then forced upward by its compres-sed spring, carrying the firing pin out of the detonator shutter. The shutter, however, is not yet allowed to move over into has armed position. The force of set-back, which continues in effect while the rocket is accelerating throughout the bruning period, has thrust the shutter upward engaging the shutter locking pin with the hole in the bottom of the  firing pin guide.

At the end of the bruning period of the rocket motor, deceleration begins and the shutter rides forward, disengaging the locking pin from the guide. The shutter spring moves the shutter over in the armed position where it is locked by a spring loaded de-tent which is housed in the firing pin guide and engages a recess in the shutter.

On impact the firing pin body and firing pin overcomes the striker spring and ride down against the detonator, at the same time shearing the thin copper lock-wire which holds the firing pin and firing pin body together. The purpose of the lock-wire is to pre-vent crushing of the delay detonator by the combined weight of the firing pin body and the firing pin, which is blunt.

REMARKS:

1. The sensitivity of the Mk 157-0 fuze is somewhat less than that of the Mk 146 fuze since the percussion type primers used in delay explosive trains are inherently less sensitive than the 'stab' type primer caps used in instantaneous detonators.

2. The Mk 157-0 and Mk 157-1 fuzes are always shipped to the field installed in the base of the rocket.

3. Not attempt should ever by made to remove this fuze from the assembled round for any purpose prior to firing, e.g., to clean the fuze or substitube a base plug for the fuze. Anything less than a perfect seal between the fuze and the adapter in the base of the rocket body will allow the gases from the rocket motor to seep into the body and contact the H.E. filling. Premature explosion of the rocket is then highly probable.

Mk 159:

The fuze Mk 159 is similar to the Mk 157, except that the delay time has been chan-ged to 0.015 seconds. The fuze is used in the base of the 5.0" Rocket (5.0" Motor - Mks 5 and 6 heads). Other monor structural changes have been made as follows: (a) a slightly heavier shear wire; (b) the inlet screen and inlet washer have been replaced by a brass washer having one side flat and the other radially serrated. The radially serra-ted side faces the fuze head so that the motor gases can enter the fuze diaphragm chamber; (c) the number of external threads on the body has been increased and 'run out' just below the flange to afford a snug fit for the sealing washer; (d) the fuze has been completely waterproofed. The Mk 159 is shipped to the field installed in the base of the rocket head.