11.75-inch A.R. "Tiny Tim"

Over-all length, inches

Total weight, pounds

Head length, inches

Head weight, pounds

Motor length, inches

TNT filling, pounds

Fuzes

Mk 1 Mod 0

Mk 1 Mod 1

Mk 2 Mod 0

Mk 157 Mod 2
Mk 163 Mod 0
Mk 162 Mod 0

Heads: The Mk 1 Mod 0 consists of a standard 500-lb. S.A.P. Bomb AN-M58A1 modi-fied for this particular use. The changes include the removal of the suspension lugs, an increase in the number of threads securing the base plate, the use of a new base plug to take the Fuze Mk 157 Mod 1, and the use of an adapter ring welded around the after end as a means for attaching the rocket motor. The motor gases are sealed from the high explosive in the body by coating the threads of the base plate with a luting compound, and also by a gasket under the fuze body flange.

The Mk 1 Mod 1 is similar to the Mk 1 Mod 0, except that a projcetile-type gas seal is added around the head of the Fuze Mk 157 Mod 2.

The Mk 2 Mod 0 was developed from the Mk 1 Mod 1. This head differs in that it has a solid-nosed "Common" head and a base plate modified to take three Base Fuzes Mk 157 Mod 2. The projectile type gas seal is used around all fuzes and also between the base plate and the forged steel body. The Fuze Mk 157 Mod 2 is used with one Auxiliary Boos-ter Mk 1 Mod 0, or the Fuze Mk 163 Mod 0 with one Auxiliary Booster Mk 19 Mod 0.

The Head Mk 3 Mod 1 was issued for practice to simulate the Head Mk 2 Mod 0.

The Head Mk 4 Mod 0 is slightly lighter and shorter tang previous heads. The head weight is 578 pounds, inclusing 152 pounds of TNT. The head lenght is 46 inches. Three Fuzes Mk 163 Mod 0 or Mk 162 Mod 0 are used in the base. The penetrative character-istics of this head are the same as those of present types, but slightly better over-all performance may be expected, because of the small increase in velocity resulting from reduction in total weight.

The Practice Head Mk 5 dummies the Mk 4. It is 44.75 inches long and weighs 569 pounds. It has a cavity for a smoke puff.

The head are shipped loaded and fuzed. A cuff protects the threads on the adapter ring and the fuze durign shipment.

Motors: The Mk 1 Mod 0 consists of a steel tube, the after end of which is threaded to receive a plate having twenty-five nozzles. The motor tube contains four propellant grains of solventless extruded ballistite of cruciform cross-section, weighing 147 pounds. The grains are shielded from each other by an X-shaped partition which extends longitu-dinally for almost the full length of the motor tube. The grains and the partition are sup-ported by the grid and are strapped together by aluminum bands.

In normal operation, the gases from the burning powder do not pass through the cen-tral nozzle, which is closed by a copper blowout disc. Only when the pressure in the mo-tor exceeds approximately 22,500 pounds per square inch is this disc expelled, bringing the central nozzle into operation. The use of a blowout disc allows the rocket motor to perform satisfactorily over a greater temperature range. It has one disadvantage, how-ever, in that at motor temperatures of about 100° F., where the normal operating pres-sure is just enough to shear the disc, it is impossible to predict whether it will blow out or not. If it does, the burning time is lengthened and the gravity drop is increased, so that the rocket may miss the target.

Four black powder charges of about 0.5 pound each, contained in plastic cases at the front ends of the grains, provide the ignition for the propellant. They are set off by two small electric squibs in each case, which are connected to receptacles in the nozzle plate. The burning of the propellant is markedly affected by its moisture content. Conse-quently, the motors are sealed at both ends. Each of the 24 peripheral nozzles is sealed with a thin steel cup. The front end is sealed with a thin steel disc having in its center a small blowout window. This window is blown out by the motor pressure, allowing the pro-pellant gases free access to the base fuze. These closure should not be removed.

The igniter Mk 19 Mod 0 has recently been developed for use in 11.75-inch A.R. mo-tors. Known as a tin-plate case igniter, it consists of a single metal case 3.38 inches in diameter and 1.8 inches deep, with a wall thickness of 0.01 inch. Four clips are soldered to the base of the case, attachment to the motor charge support. The case contains 230 grams of F.F.F.G. black powder. Contained in the case are two electric squibs con-nected in parallel to the ignter lead wires.

Motors must never be fired above the rated temperature stencilled on the motor, be-cause they are likely to burst. Below the lower rated temperature, occasional ignition failures and interrupted burning may be experienced.

The Motor Mk 1 Mod 1 is identical to the Mk 1 Mod 0, except that the motor tube is of higher tensile strength and the pigtail connection has been replaced by two receptac-les built into the base plate.

The Mk 1 Mod 2 is a design in which the dead space between the forward motor clo-sure and the base of the rocket head has been eliminated. This motor is similar in other respects to the Mk 1 Mod 0, except that the over-all lenght has been reduced to 75.75 inches and the pigtail connection has been replaced by two receptacles built into the base plate.

The Motor Mk 1 Mod 3 is the Bureau of Ordnance production of the Mk 1 Mod 2.

The Motor Mk 2 Mod 2 weighs 600 pounds and is 72.04 inches in length. Length and weight reduction have been accomplished by improvements in design detail, so that the total amount of propellant in the motor has not been changed. Because of design and construction changes, the Motor Mk 2 Mod 2 may be used only with the Head Mk 4 Mod 0.

Figure 139. 11.75-inch A.R. "Tiny Tim"