Nose Fuze No. 28B Mks II and III (Obsolescent)

Description: The No. 28B is a pyrotechnic delay, aerial burst fuze, with possible time settings from 0 - 22 seconds, although 7 seconds is generally the minimum employed. The fuze consists of a body, an igniting mechanism, a fixed ring and setting ring, each containing a pyrotechnic delay element, a valve mechanism to prevent premature functi-oning of the fuze, and a magazine containing a gunpowder charge for igniting the photo-flash or flare unit.

The fuze body has a flange, on the upper surface of which the fixed ring and the set-ting ring rest. Below the flange the body is externally threaded for insertion into the flash or flare. A securing ring screws onto these threads and locks the fuze in position.

A spring-loaded detonator is housed in a chamber in the fuze body, opposite a fixed striker, and is held in position by the firing rod, movement of which is impeded by a spring-loaded ball engaging a groove in the rod. A flash channel extends from the deto-nator chamber to the delay element in the fixed ring.

The movable setting ring rests on a flange on the fuze body, and is placed below the fixed ring, which is secured by a screw to the fuze body. The delay elements are placed on the under surface of each ring in a circular groove, broken by a metal bridge. Commu-nicaton between the grooves is effected by a hole in the top of the setting ring. An igni-ter pellet is placed in the upper end of this hole, and, when the fuze is set on safe, this pellet is masked by the bridge on the fixed ring. When the fuze is set live, the setting ring is moved to a position where its delay powder is above a flash pellet, located in a hole in the body. This hole communicates with a horizontal flash channel, which opens into a vertical flash channel. Freedom of movement of the setting ring is restricted by a tensioning ring, which is locked in position by fixing screws.

A spring-loaded valve is located in the vertical flash channel, and is supported in a valve guide carried by a threaded arming-vane spindle. To the upper end of the spindle an arming-vane collar, with vanes attached, is secured by a split pin. Rotation of the spindle is prevented by the firing rod, part of which is loacted in a groove in the arming-vane collar. A retaining plate prevents the spindle and the arming vane from being sepa-rated from the fuze. To prevent the valve from binding if the vane should be rotated downwards, a stop screw in the vane collar can engage with a similar screw in the retai-ning plate.

Two safety pins connected to a spring clip prevent movement of the spindle and the firing rod when they are in place.

Operation: When the firing rod is withdrawn upon release from the plane, the spring-loaded detonator is released and forced against the fixed striker. The flash from the de-tonator passes through the flash hole and ignites the delay powder in the fixed ring. The delay powder burns for an interval of time determined by the position of the setting ring. At the end of this interval the igniter pellet in the setting ring is fired and ignites the de-lay powder in the setting ring. The delay train in the setting ring burns until it reaches the flash pellet, which is then ignited.

While the fuze powder has been burning, the arming van spindle has been rotating un-til the valve has been raised from its seating, so that, when the flash pellet is ignited, the flash can pass through the horizontal and vertical channels and ignite the magazine pellet. The pellet fires the magazine charge and ignites the flare or photoflash into which the fuze is inserted.

Remarks: This fuze is obsolescent, and is similar to early Fuzes No. 28 which are now obsolete. Although the fuzes are calibrated from 0 - 22 seconds, generally the numerals from 0 to 4 are painted out, and in some cases the numbers start at 4. In actual prac-tice, no setting below 7 is authorized to be used, on account of the possibility of "blinds" or misfire.

Figure 146 - Nose Fuze No. 28B

Figure 147 – Nose Fuze No. 28B Mk III