Tail Fuze No. 850 Mks I and II (Service)

Description: The Bomb Fuze No. 850 Mk I is a hydrostatic-arming, contact-firing bomb fuze specially designed for use in the 250-lb. Buoyancy bombs. The fuze itself con-sists of two portions, the upper portion housing the hydrostatic mechanism, and the lo-wer portion forming the magazine.

The upper portion of the outer casing houses the actuating head. The head is locked to the casing by a ball lock, which is freed on impact with water. The cylinder is of slightly smaller diameter than the outer casing, leaving a small annular space between the cylinder wall and the casing. The lower end of the cylinder is in the form of a disc and is contained in the lower end of the outer casing to which it is soldered. A piston-supporting stop, consisting of a metal disc with protrusions on both faces, clamps a rub-ber washer in position and also supports the cylinder wall and the piston. The upper pro-trusion of the piston-supporting stop is provided with ports, which, together with ports formed in the lower end of the cylinder wall, connect the space below the pistion with that between the cylinder wall and the outer casing.

The piston consists of a metal disc with a central hole into which the lower portion of the piston rod is inserted. This portion of the piston rod ahs a shoulder at its upper end by which the piston rod is raised when the piston is forced up the cylinder by hydrostatic pressure. The piston rod, which is hollow, is made in two sections. The upper section of the rod houses the release spindle, to which it is locked by a ball. The lower portion of the piston rod  is threaded internally to accommodate a screwed ferrule, which houses the upper end of the striker. The upper end of the striker is locked to the ferrule by a ball lock. The ferrule is bevelled at its lower end, and forms part of a lock that prevents the return of the piston after the fuze has been armed. The lock consists of a locking ball, housed in a conical recess formed in the lower portion of the outer casing. A washer and a distance tube are inserted in the lower portion of the piston rod to support the up-per section of the piston rod.

The striker is locked to the screwed ferrule in the lower section of the piston rod by a ball lock, and protrudes through the lower end of the ferrule. The lower end of the striker is housed in a blind hole in the spring-loaded shutter of the magazine, when the fuze is in the safe position. The firing spring, encased in a sheath, is interposed between the lower end of the outer casing and a washer supported by the striker shoulder.

A collar is screwed to the top of the release spindle and forms a bearing surface for the lower end of the actuating head. The thrust spring is interposed between this collar and the upper portion of the piston rod. The actuating head is contained in the upper portion of the outer casing, to which it is locked by a ball lock. The actuating head is bo-red axially to house a hollow inertia bolt, which accommodates a spring-loaded plunger. This plunger is retained in position by the fuze safety clip. which passes through the inertia bolt and the actuating head. A retaining pin prevents ejection of the plunger by its spring when the safety clip is withdrawn.

A light friction spring is fitted to the actuating head and bears on the inertia bolt. This spring engages in a groove in the inertia bolt when the bolt moves downwards.

The magazine, containing two C.E. pellets, is attached to the lower end of the sheath surrounding the firing spring, and is arranged so that the fuze can accommodate itself to any tolerance in the length of the central tube and exploder column in the bomb. A spring-loaded shutter, which carries a detonator, is incorporated in the magazine. The shutter is held in the safe position by the striker and by a transit pin during storage and transit.

The fuze is retained in position in the bomb by a ring nut situated between the flange on the actuating head and the upper portion of the outer casing. A rubber washer around the lower portion of the outer casing forms a water-tight seating, when the fuze is assembled in the fuze pocket of the bomb.

Operation

Bomb Dropped Live – On release from the plane, the safety clip on the fuze is remo-ved by the fuze-setting control link, allowing the plunger to move up under the action of its spring. The plunger ball in the bore of the actuating head is then free to move into the recess in the bottom of the bore.

On impact of the bomb with the water, the nose attachment and tail breaks away. The ??????????????

When the bomb is submerged, water enters the outer casing of the fuze through the holes in the upper portion of the outer casing, and flows through the annular space bet-ween the outer casing and the cylinder wall. It then enters the cylinder through the ports in the cylinder wall and exerts pressure on the lower face of the piston. The piston is thereby raised in the cylinder, taking the piston rod assembly with it. The striker being locked to the release spindle, moves upward with the piston, and the washer on the shoulder of the striker compresses the firing spring. The raising of the striker also frees the spring-loaded shutter in the magazine, which moves over to align the detonator with the striker.

The motion of the piston also raises the release spindle and the actuating head, which, being attached to the contact disc of the bomb, raises the contact disc to the limit of its securing chains. The raising of the piston rod frees the ball lock securing it to the release spindle. The ball in the lower portion of the outer casing moves under the be-velled shoulder on the screwed ferrule, and so prevents return of the piston, once arming of the fuze has been achived.

Having descended to its maximum depth, the bomb ascends contact disc uppermost, at a speed of 3½ ft./sec. under the influence of its buoyancy. Any possiblity of the bomb's firing due to water drag on the contact disc is overcome by the thrust spring in the fuze. On impact with the target during its upward motion, the contact disc is  forced down relative to the bomb causing the actuating  head to move down relative to the outer casing of the  fuze. This, in turn causes the release spindle to move down relative to the piston rod, thus freeing the ball lock by which the striker is the forced into the detonator by its spring and initiates the normal explosive train.

If the bomb fails to encounter any obstruction during its upward motion it will rise to the surface ????????

Bomb Dropped Safe – Provision is made in the fuze for dropping the bomb safe. This is neccessary to provide for jettisoning the bomb in the event of engine trouble during or shortly after the aircraft takes off from a carrier Safe dropping is achieved by a ball, ac-commodated in the bore of the actuating head, which prevents the inertia bolt from moving sufficiently in a downward direction to release the actuating head ball lock.

On release from the plane, the safety clip remains in position on the fuze. On impact with the water, the break-up and removel of the tail may cause the safety clip of the fuze to be withdrawn accidentally. The retardation of the bomb in the water causes the plunger to move downward against the action of its spring, preventing escape of the ball into the recess in the bore of the actuating head. The inertia bolt also moves downward, but it is obstructed by the ball and cannot move sufficiently to release the ball lock se-curing the actuating head to the upper portion of the outer casing of the fuze. The fric-tion spring in the actuating head falls into the groove in the inertia bolt, thus preventing the actuating head from being lifted by wave motions after the bomb has been dropped safe, and ensuring that the bomb remains in the safe condition.

Remarks: The Fuze No. 850 Mk II differs from the Mk I in that the safety clip is re-placed by a cotter pin, which pierces both the plunger and the actuating head.

Figure 161 - Tail Fuze No. 850 Mk I

Figure 162 – Tail Fuze No. 850 Mk I