3.5-inch and 5.0-inch A.R. with 3.25-inch Motor

General: The 3.5-inch rockets were designed to be used against smaller targets, such as submarines and tanks. For larger targets, the 5.0-inch rocket was developed from the 5-inch anti-aircraft shell. The 3.5-inch Solid Head Mk 8 and the 3.5-inch F.S. and P.W.P. Smoke Filled Heads Mk 6 are the only ones now being issued. The 3.5-icnh H.E. heads were replaced by the 5.0-inch heads. The former were never issued, because of the small load TNT carried, as compared to the 5.0- inch heads.

Heads

3.5-inch Mks 1 and 2: The head is of solid steel and contains no high explosive or fuze. The shape of the round gives a releatively long underwater travel at shallow depth-of-entry angles (about 20 degrees), and it is used as a semi-armor-piercing projectile against submarines or tanks. The Mk 1 was the California Institute of Technology pro-duction which was adopted by Bureau of Ordnance and designated the Mk 2.

3.5-inch Mks 3 and 5: The head is filled with TNT and fitted with an adapter in the nose to take the Fuze Mk 149. With a second adapter, the diameter is reduced to 1.5 in-ches to take the Fuze Mk 148. These rounds were not issued and were replaced by the 5.0-inch heads, which contain a greater load of high explosive.

3.5-inch Mk 4: The head has a semi-armor piercing nose and is filled with TNT. This round was not issued, because of the small load of high explosive, and was replaced by the 5.0-inch heads.

3.5-inch Mks 6 and 9: The head is filled with F.S. or P.W.P. smoke. The Mk 9, the initial California Institute of Technology production, was not issued. The Bureau of Ord-nance, in adopting this head, increased the length 1-½ inches and issued the round as the Mk 6.

Figure 127. 3.5-inch A.R. (Head Mk 6)

3.5-inch Mk 8: The head is of solid steel and contains no high explosive or fuze. The round was developed to give better underwater travel, and replaces the 3.5-inch Head Mk 2.

Figure 128. 3.5-inch A.R. (Head Mk 8)

5.0-inch Mk 1 Mod 0: The head is filled with TNT and weighs 46.5 pounds when fit-ted with a Fuze Mk 143. The same adapter rings are used as on the 3.5-inch Head Mk 5. The head is issued with a nose plug. The nose fuze must always be assembled in the head before firing. Fire with the fuze on "safe" if delay is desired. The head is shipped with the base fuze sealed in place. This base fuze must not be removed.

5.0-inch Mk 1 Mod 1: This head differs from the 5.0-inch Head Mk 1 Mod 0 only in that the nose is especially cavitated to take the Fuze Mk 172 Mod 0, which is larger than the Mk 149 or other nose fuzes and therefore is not interchangeable. with them.

Motor

The 3.25-inch Motor Mk 7 is used with the 3.5- and 5.0-inch heads described above. At the forward end of the motor are a black-powder iginiter and an electric squib. Two electric leads extend through the motor and out the after end to a cable and plug con-nection. At the after end of the motor, there are a nozzle and a bag of silica gel which acts as a dehydrating agent in keeping moisture from the ballistite grain. The grain used in the cruciform type with inhibitors, 33 inches long, 2.75 inches in diameter, and weigh-ing 8.5 pounds.

The tail consists of four sheet-metal fins set 90° apart and welded to a central cylin-der. The tail is slipped over the after end of the motor and is secured by a tail locking ring, which screws on.

Remarks: The 3.5-inch (H.E. and F.S.) have a maxium velocity of 1,200 ft./sec. ex-clusive of plane speed, as compared of 800 ft./sec. for the 5.0-inch H.E.

The 3.5-inch Heads Mk 11, incendiary, and Mk 12, gas, were never loaded.

Figure 129. 5.0-inch A.R. with 3.25-inch Motor