Mk 149 and Mk 155

Rocket uses in

Functioning

Fuzes used with

Arming time

Vane span, inches

Body diameter, inches

Over-all length (without booster), inches

3-1/2

General: The working parts of the fuze are very similar to those of the Nose Fuze Mk 148, which it is replacing. The essential changes are the addition of a second set-back block, a shutter locking pin, a nose cap and clamp, and smaller but less sharply pitched vanes. The fuze body itself is more streamlined. The addition of the spring-loaded weat-her cap over the vanes protects the vanes from icing up during flight of the aircraft at high altitudes. This cap does not spring off until the arming wire is pulled from the clamp when the rocket is fired.

Operation: When the rocket is fired, the arming wire is pulled free from the clamp pin and the compressed weather-cap spring forces the weather cap up, spreading the clamp until the  weather cap is free. The force of inertia causes the two set-back blocks to fall back against the pressure of the set-back spring. This accomplishes two things: First, the vane locking pin is freed from the vane boss and the vanes are free to rotate and screw the striker upward. Second, the lower set-back block forces the shutter locking pin down into the shutter cavity, preventing the shutter from moving over and lining up under the striker as long as the rocket is accelerating (i.e. as long as the rocket motor is burning). After eight vane revolutions, the point of the striker will have risen clear of the shutter; and, upon reaching the end of the threads, the spindle stop as the striker collar rides in the groove in the fuze body. After the rocket propellant has ceased burning, de-celeration occurs and both set-back blocks are forced up by the set-back spring. The shutter locking pin is thus lifted from  the shutter cavity and the shutter moves across the fuze under influence of its spring until stopped by the stop pin. A spring-loaded de-tent in the detonator shutter springs up into a recess in the striker guide, locking the shutter in the armed position. On impact, the striker shears the body threads and is dri-ven into the detonator, setting off the booster lead-in and booster in succession.

Mk 155: The Nose Fuze Mk 155 is used in the F.S. or P.W.P. smoke-filled 3.5-inch Heads Mk 6. The fuze is similar to the Nose Fuze Mk 149, with a burster tube instead of a booster charge.