Base Fuzes No. 865 Mk I, No. 878 Mk I, and No. 297 Mk I (Service)

Rockets used in

Action

Armed condition

Body diameter

Over-all length

Color

Description: These fuzes are identical except that the No. 865 and the No. 297 in-corporate a delay element, whereas the No. 878 is instantaneous. The No. 865 and the No. 297 differ only in length of delay used. These fuzes are base detonating fuzes, armed by gas pressure from the thermal unit, which is ignited by the rocket motor.

The fuze consists of a cylindrical brass housing, closed at one end by a perforated brass plug, and at the other end by a magzine cap containing a booster charge C.E. Im-mediately beneath the perforated brass cup is located a metal diaphragm, to which is at-tached a long arming rod. This arming rod runs through a channel drilled off-center through the fuze body. In a central chamber in the fuze body are located a pointed strik-er and creep spring. In the unarmed position, this striker is held away from the detonator by a retaining ball, which engages an annular groove in the striker and a hole in the fuze body. The ball is held engaged in the striker by the arming rod.

Beneath the striker is located a detonator containing about two grams of Composition A or A1 mixture, a delay pellet of G.20 gunpowder (in the No. 865 and No. 297), a se-cond detonator of lead azide, and a C.E. detonator lead-out. Beneath the lead-out is lo-cated a shutter containing a booster lead-in of C.E. In the unarmed position, this lead-in is offset, and the explosive channel is sealed off by the metal body of the shutter. In the armed position, however, the stemmed C.E. channel in the shutter is aligned with the rest of the explosive train, forming a direct line to the booster charge in the magazine. The shutter is held offset in the unarmed position by the lower part of the arming rod. Two notches are cut in the arming rod. allowing passage to the shutter and clearnce for the retaining ball when the fuze is armed.

The fuze is armed by the pressure of gases generated by the combustion of gunpow-der in the thermal initiator, which is used in conjunction with the fuze. The initiator is po-sitioned in the base of the shell adapter; and when the shell is threaded into the shell ring of the motor, the initiator is brought closely against the fron obtruator of the motor.

The initiator consists of a thin steel base, adjacent to which is a septum containing L.D.N.R. composition. Above the septum is a chamber containing a charge of G.20 gun-powder confined by a cardboard disc, which is held in position by a metal disc with a central hole. The heat from the burning propellant is conducted through the head obtru-ator in the motor and the base of the initiator to the L.D.N.R. composition, which is igni-ted by the rise in temperature. The L.D.N.R. composition in turn initiates the G.20 gun-powder, and the pressure produced reverses the diaphragm of the fuze and arms the fuze.

Operation: Depression of the diaphragm by the explosion of the thermal initiator mo-ves the arming rod forward until the two notches in the rod are adjacent to the retaining ball and the pin on the lead-in shutter. The shutter is then aligned by its spring with the rest of the explosive train, and the retaining ball is allowed clearance to move out of en-gagement with the striker.

On impact, the striker rides forward against its spring and strikes the detonator, which fires the delay pellet (in the No. 865), which in turn fires the secondary detonator, lead-in, and booster.

Remark: The explosive train in the No. 878 is identical to that of the No. 865, except that the delay pellet is removed. The Fuze No. 297, used in the shells for the 3-in. Lilo assault rocket, is identical to the Fuze No. 865, except for the length of the delay time.