Nose Fuze No. 731 Mk I (Service)

Rockets used in

Action

Armed condition

Arming time

Body diameter

Over-all length

Color

Description: This fuze, sometimes called the "Kealy" fuze, has been designed for use with rocket projectiles, particularly where a number of rounds are to be fired in salvos from multiple launchers. It is adapted for automatic setting on the launcher through the medium of electric impulses transmitted from a control point. Increase or decrease of the setting is rapidly applied at will. Setting of individual fuzes by hand may also be effected by means of a time ring on the fuze body, if necessary.

For automatic fuze setting, the launcher is fitted with two pairs of electric magnets for each set of rails, the pole pieces of the magnets extending to within ¼ inch of the outside of the rounds. One pair of magnets on each set of rails serves to increase the fuze set, and the other set to reduce it. The fuze incorporates two soft-iron cruciform armatures, so positioned that, when the round is loaded on the launcher, the ends of each armature are in close proximity to the poles of a magnet. The arrangement is such that an armature will undergo a partial rotation against the action of a spring, when an electric impulse passes through the coils of the proximate electric magnet, and will be re-turned by the spring at the end of the current pulse. Each oscillation causes movement of a ratchet mechanism associated with the armatures, and changes the fuze setting by one-half second.

No mechanical connection exists between the fuze and the launcher. The setting is effected by the magnetic flux passing from the pole pieces of the magnets, through the intervening air grap and fuze casing, to actuate the armatures. To ensure correct posi-tioning of the armatures with respect to the electro-magnet poles when the round is loa-ded on the launcher, an external mark is provided on the fuze, which must be aligned with one of the tail fins on assembly.

Timing is effected by a clockwork train and escapement, which is ordinarily prevented from running by a spring-loaded detent capable of setting back on projection. The rate of retraction of the detent is so controlled by the escapement that an acceleration sus-tained for about 0.4 second must be applied before the fuze will arm. When the detent has completed the setback movement, it allows a spring-loaded shutter to move and un-cover the detonator, and the same time locks the detent in position.

When the fuze is set on SAFE, the detent is locked, the detonator is moved out of the path of the striker, and any automatic setting which may have been applied is can-celled. In addition, with the time ring moved beyond the AUTO position, for manual sett-ing, any automatic setting is cancelled and the automatic gear put out of action.

The percussion element will fire on impact with relatively light structures such as are encountered in aircraft.

Operation: The base of the fuze receives the boss of the gaine, which is held in po-sition with screws. The latter may be loosened to permit alignment of the fuze with the round for automatic setting on the launcher. The base carries the detonator shutter, which is provided with lugs bearing on the surface of the projections on the time ring. When the fuze is set on SAFE, the lower lug bears on the lower portion of the projection. Movement of the ring to the AUTO position causes the lower portion of the projection to clear the lower lug and bring the upper portion into contact with the upper lug, rotating the shutter until the detonator is in line with the axis of the fuze beneath the striker. Si-multaneously, the lobe of the shutter moves out of the path of the arming detent, there-by allowing the fuze to arm on projection.

The base of the fuze supports the timing and release mechanism, which is carried by a number of plates and arranged axially around the hollow striker. The mechanism is en-closed in a brass casing held in position by a ballistic cap of bakelite, which is screwed over the top flange of the internal fuze mechanism. The forward end of the bakelite cap carries the percussion element, consisting of a striker, sleeve, and detonator.

The clockwork train, which times the fuze, is controlled by an escapement, movement of which, in the unarmed position, is impeded by the arming detent, extending into the path of the pallet arm. The arming detent is removed from the pallet on setback, starting the clockwork. Rate of movement of the detent is governed by a gear train, and escape-ment through the medium of a pinion engaging on a rack on the edge of the detent. This control is such that displacement is completed in about 0.4 seconds when the fuze is subjected to a mean acceleration of 60 g.

A safety shutter, carried on a spindle pivoted in the base of the fuze, is normally held under the striker by its arcuate arm, which bears against the side of the detent. When the latter has moved to its full setback positon, the arcuate arm is allowed to move into a slot in the detent, moving the safety shutter, under the action of the shutter spring, out of the path of the striker, and locking the detent in position. A small initial movement of the detent suffices to release the pallet arm and thus allow the clockwork timing train to start with a minimum delay on projection. The fuze will remain safe until full displace-ment of the detent permits movement of the arming shutter.

The striker is held cocked against the action of the striker spring by a catch pivoted on the top plate carried by the upper sleeve, to which is attached the inner end of the time train spring. The catch engages a groove in the striker. A downwardly extending fin-ger of the catch, resting against the edge of the firing plate, prevents movement of the catch under action of the catch spring, and release of the striker, until movement of the timing train brings the finger into a position adjacent to the slot in the firing plate. The time of release of the striker is thus determined by the initial position of the slot in the firing plate relative to the catch finger. Since contact of the finger with the firing plate during the setting operation would introduce undesirable friction, the catch is provided with a stud which serves to lift the finger from the firing plate by initially engaging a ramp carried by a fixed plate of the fuze. The stud rides off the ramp during the early stages of the movement of the timing train.

Automatic setting of the fuze is accomplished by a pair of iron curciform armatures, arranged for limited rotary movement in opposite directions under the influence of mag-netic impulses against the action of the armature springs. The upper armature carries a spindle of an upper pawl, tending to be forced by the pawl spring into contact with the ratchet wheel. The ratchet wheel is carried on the same sleeve that carries the firing plate. Normally, the upper pawl is held out of contact with the ratchet wheel by a dep-ending leg on the pawl, abutting against an arm "A" attached to the lower sleeve, to which is also fixed the lower armature. Movement of the upper armature against the action of its spring will first break the contact of the leg and arm and permit the upper pawl to engage the ratchet wheel. Continued movement will turn the ratchet wheel through a distance of one tooth, against the action of the ratchet-wheel spring. The lower pawl, having two depending legs, which engages the ratchet wheel when the fuze is set on AUTO, retains the ratchet wheel, while the upper armature is allowed to return to its original position. Repetition of the movement of the armature will cause the ratchet wheel to move a distance of one tooth for each oscillation, and, as each tooth corres-pond to half a second, the fuze setting will be reduced from the maximum of 30.5 sec-onds by the corresponding number of oscillations.

Reverse movement of the ratchet wheel in automatic setting is effected by oscillation of the lower armature and its attached sleeve, which carries arm "A" and arm "B". Move-ment of arm "A" releases the upper pawl, which then bears on the ratchet wheel halfway between two teeth, while movement of arm "B" serves to lift the lower pawl away from the ratchet wheel through the medium of the leg. The arms "A" and "B" are so designed that the upper pawl bears on the ratchet wheel before the lower pawl is lifted. The rat-chet wheel is thus allowed to move back the distance of half a tooth under the action of its spring. When the  lower armature is allowed to return, the lower pawl first engages the ratchet and the the upper pawl is raised, thereby permitting the ratchet wheel to move backward a further distance of half a tooth. Thus each oscillation of the lower ar-mature results in backward movement of the ratchet wheel to the extent of one tooth, and consequently results in an increase of half a second in the fuze setting.

It will be noted that when the fuze time ring is moved to the AUTO position for auto-matic setting, the short leg of the lower pawl then rests in a cut-away portion or recess of the cam plate, and the lower pawl bears on the ratchet wheel. However, when the time ring is moved to the SAFE or the manual setting, the short leg will ride in the outer edge of the cam plate, and the lower pawl will be out of contact with the ratchet wheel. The latter will then be free to return under action of its spring, thus cancelling any set-ting givem by automatic means.

Manual setting of the fuze is effected by movement of the time ring, which is gradua-ted in half-seconds from 0 to 30. The time ring carries an internal gear, which meshes with a pinion assembly to rotate a pinion, its associated pinion sleeve, and the cam plate. The latter has an upwardly extending stud, which is adapted to engage a lever mounted for free movement about the axis of the fuze. The ratchet wheel is provided with a similar stud extending downwards into the path of the lever. Movement of the time ring will cause counter-clockwise movement of the cam plate until the short leg of the lower pawl rises from the recess of the cam plate, raising the lower pawl from the ratchet wheel. The ratchet-wheel spring then returns the ratchet wheel to the maximum setting, in which position the pin on the ratchet wheel bears on the stop, and the cam-plate stud forces the lever into contact with the ratchet-wheel stud. Further movement of the timing ring will cause the ratchet wheel and its associated firing plate to move as one with the cam plate.

Functioning of the fuze on impact is caused by operation of a striker and detonator arranged in the nose cap. The striker is held by a metal cup designed to collapse under about 150 lb. pressure. The upper end of the striker bears on the closing plate. On im-pact the plate forces the striker into the detonator, crushing the cap. The flash from the detonator passes down the central tube and through the hollow striker. If the fuze is ar-med, the shuttered detonator will be set off by the blast. If the fuze is not armed, the lower detonator will be sealed off by the safety shutter.