Aparatus A.D. Type J Mks I, IA and II (Service)

General: This rocket is designed to erect a lethal wire vertically over a ship in the path of low-level or dive-bombing aircraft. The wire acts both to bring down the aircraft if it strikes the wire, and as a visible deterent fto force the aircraft to abandon the at-tack.

Description: The head canister is a light cylindrical metal container, into which is packed a 62-inch parachute, called the top parachute, the cords of which are firmly se-cured to the base of the head canister. A thermal fuze is fitted into the head canister and consists of a pellet of L.D.N.R., a length of safety fuse, and 100 grain of G.12 gun-powder, which acts as an ejection charge.

The Tails, Propelling, 2-in., Mks IVA and VA are standard 2-inch rocket motors, with a rigid stirrup wekled to their after end. No fins are fitted to the motor. Contained within the motor body is a tubular cordite grain, into the head of which is fitted an electrical ig-niter. The after end of the motor is sealed off by a light metal closing disc, through which pass the ends of the igniter leads terminating in a two-pin plug. The proposed new nomenclature for the rocket motors is Motor, Rocket, 2-in., No. 2 Mks I and II.

The main container is a cylindrical-shaped metal container about 14 inches in diame-ter, with a removable lid. Inside the container are two smaller concentric tubes, the ou-ter of which is cone-shaped, while the inner is cylindrical. Also encased within the main container are 825 feet of wire, a fabric bag into which is packed a 62-inch diameter pa-rachute called the lower parachute, and a 20-inch diameter parachute called the trail parachute.

The wire is coiled down between the cone-shaped cylinder and the main container, and swivels to the top and bottom ends of the wire. The bottom swivel is shackled to the ring at the foot of the cords of the lower parachute, with the ring just protruding through the fabric bag. Cords which keep the bag closed are connected to the trail pa-rachute by a weak cord connection and a rip cord. The trail parachute and fabric bag are packed into the small cylinder, and have the swivel passing through a special slot. The cylinder is then sealed with a light metal lid.

Operation: The rocket is fired when the attacking plane is within about 800 yard of the ship. The wire is fully run out 3½ seconds after firing, and pulls the lower parachute bundle and the trail parachute up out of the inner cylinder, the lid of which is ripped off by the bottom swivel as it is pulled up. Meanwhile the heat from the burning cordite grain ignites the L.D.N.R. pellet in the thermal fuze, and this in turn ignites the safety fuse. The safety fuse burn for about 6½ seconds and then initiates the gunpowder ejection charge, blowing off the top part of the head container and allowing the top parachute to open. This occurs at an altitude of about 1,100 feet. The lower parachute, still wrapped in its bag, is suspended at the bottom of the wire, and the trail parachute, which is now open, is attached to the bundle by the weak cord connection and the rip cord. After the top parachute opens, the wire begins to fall at about 42 ft./sec. Twelve seconds after firing, the bottom parachute enters the water. As soon as this occurs, the wire ceases to be lethal, but it remains a visible deterrent for an additional 12 seconds, by which time the top parachute has fallen to an ineffective height.

If the plane strikes the wire during its lethal stages, the pull on the wire breaks the weak cord connection between the lower parachute and the trail parachute. This allows the trail parachute to pull the rip cord on the bundle, opening the fabric bag and freeing the lower parachute. The sudden jerk caused by the wind operating on the lower and top parachutes is sufficient to throw the plane momentarily out of control, or in some cases to cause it to break up.

Remarks: The round must not be fired outside of the temperature range of 0 to 120° F., except motors marked "S.U./K." which may be fired within the range of -5 to 130° F.

The Apparatus Mk I differs from the Mk II in that the ceilling height of the rocket is 800 ft. and the wire is only 600 feet long.