I.B. 25-lb. Mks I and II (Obsolete)

Description: The Bomb Mk I consists of a cylindrical steel tube secured to a hollow, sharp-pointed nose by a nose adapter (built up externally to streamlined from by a card-board fairing), a column of seven closed containers, termed fire-pots, accommodated in the body tube, a flanged base adapter which affords a mounting for a cap holder, a tail cone with a cylindrical tail vane secured to it by four vane supports, and a tail tube which accommodates a striker pellet, with a shear wire and creep spring, and a safety rod.

Each of the seven fire-pots consists of a cylindrical container made of magnesium al-loy and formed with a central tube, which communicates, through a fire hole, with a re-cess in a central boss on the base of the container. The container is closed by a magne-sium alloy lid, which has a shallow central recess in its outer side. Holes through the lid place this recess in communication with the interior of the container. The ignition cap contains a layer of gunpowder and a layer of detonating composition.

The central tube of each of the fire-pots houses a delay charge and is surrounded by a delay charge and a priming charge for the thermite and magnesium filling, which occu-pies the remainder of the annular chamber in the fire-pot. A loop of quickmatch extends into the bore of the central tube and has its two ends carried out through the slots in the tube and through two of the holes in the lid. An ejection charge of gunpowder is re-tained in the recess in a central boss of each fire-pot by a paper disc, which is secured in position by shellac.

The tail cone fits over the flange of the base adapter and is held assembled to the bomb body by a tail nut, which screws onto the rear end of the tail tube against a tail adapter. The tail adapter fits into the rear end of the tail cone.

The striker pellet has a sharp point at the forward end and is held in the safe position by a shear wire passed through holes in the striker pellet and teil tube. The ends of the shear wire are soldered to prevent removal. The rear end of the striker pellet receives the forward threaded end of the safety rod. The creep spring is housed in the forward end of the tail tube and bears against the cap holder and the striker pellet. The safety rod passes through the tail tube and is screwed into the threaded bore of the striker pel-let.

The Bomb Mk II has only a slight difference in the tail plate.

Parachute Attachment: Either a Parachute Attachment No. 1 Mk I or a No. 1 Mk II may be used with this bomb. The Attachment No. 1 Mk I consists of a small parachute housed in a container of sheet metal, consisting of a cylindrical housing plate and a co-ver. It fits loosely in the cylindrical strut of the bomb tail. The chute is 15 in. in diameter and vented at the peak. The Attachment No. 1 Mk II is similar, except that it is housed in a collapsible cardboard container between cardboard packing discs.

Striker Attachment: There are two striker attachments, the No. 1 Mk I, and the No. 1 Mk II. No. Mk I consists of a coupling fork, snatch rod, striker, coupling socket, and a retaining sleeve, which houses a spring. The No. 1 Mk II is similar, except that the coup-ling fork is fitted with a quick-release pin.

Functioning: When the bomb using the Parachute and Striker Attachments No. 1 Mk I is released, the cover pulls the chute out. The chute opens and pulls the snatch rod against the action of the spring until the striker rod retaining balls escape into the annu-lar recess in the retaining sleeve. The striker rod then moves down against the striker pellet. On impact, the inertia of the rod and pellet combine to shear the shear wire, overcome the creep spring, and piece the ignition cap, igniting the blowing charge and blowing the base adapter and complete tail and parachute assembly from the body tube. The quickmatch, which passes through the lid and into the central tube of the rearmost magnesium alloy fire-pot, is also ignited, and, in turn, the quickmatch ignites the delay charges in and around the central tube of the fire-pot. While the central tube delay charge is burning through, the outer delay charge initiates the surrounding priming charge, which, in turn, ignites the incendiary composition around it. Meanwhile the cen-tral delay charge burns through and fires the ejection charge in the base of the fire-pot. This has the effect of ejecting the activated fire-pot from the bomb and also igniting the quickmatch of the second fire-pot etc. Each ejected fire-pot will continue to burn for approximtaly 10 minutes; the magnesium alloy container and lid being consummed. Final-ly, the quickmatch in the lid closing the nose of the bomb is ignited by the cast ejection charge and initiates the priming charge, which, in turn, ignites the incendiary composition of the nose, and provides an additional fire source.

When used with the Parachute and Striker Attachments No. 1 Mk II, the parachute is blown out of the cylindrical shroud on the tail and the chute opens, freeing the striker mechanism. Functioning after this is same as that with Attachments No. 1 Mk I.

Suspension: The bombs may be carried in a Small Bomb Container, or from a bomb carrier by a lug on a suspension band.

Incendiary Filling: 1 lb. Thermite, and 5 lb. 4 oz. magnesium

Figure 73 - I.B. 25-lb. Mk I