NOZZLE AND FIN ASSEMBLIES.- The nozzle and fin assembly (fig. 2-7) used with the Mk 4 and Mk 40 motors consists of a nozzle plate, four nozzle inserts, a fin-actuating mechanism, four fins, and a fin retainer contact disc. The nozzle assembly for the Mk 66 consists of the nozzle body, carbon insert, fins, contact band assembly, and weather seal.

The fins are attached by pivot pins to lugs machined on the aft part of the nozzle plate. When folded, the fins lie within the 2.75-inch diameter of the rocket. The fins are notched at the tips to allow attachment of a fin retainer.

The fin-actuating mechanism is a steel cylinder and a piston with a crosshead attached to its aft end. When

the rocket is fired, gas pressure from the motor operates the piston, cylinder, and crosshead. The crosshead is pushed against the heels of the fins, causing the fins to rotate on the fin pivot pins to the open position after the rocket leaves the launcher. After the fins have opened to the final flight position, the crosshead prevents the fins from closing.

There are four nozzle inserts and the detent groove in the aft end of the nozzle plate. They hold the rocket in position after it is loaded in the launcher (fig. 2-8). The major difference between the nozzle and fin assemblies used on the Mk 4 and Mk 40 motors is the type of nozzle inserts used.

Figure 2-8.-Nozzle and fin assemblies. (A) Mk 4 Mods motor (B) Mk 40 Mods motor. 2-6

Figure 2-9.-Mk 16 motor, nozzle, and fin assembly.

The nozzle inserts of the Mk 4 motors are standard or straight cut (fig. 2-8, view A), while the Mk 40 motor nozzle inserts are scarfed or cut at an angle (fig. 2-8, view B). The scarfed nozzles used on the Mk 40 motor impart a stabilizing spin to the rocket when launched at low initial-launch speeds, such as from helicopters and low-speed aircraft. A rocket using the Mk 40 motor is designated a LSFFAR (low-speed, folding-fin, aircraft rocket) and is used only on helicopters or low-speed aircraft.

All Mk 16 motors have a single nozzle with four straight folding fins (fig. 2-9) that attach to the aft end of the motor tube by threads. To protect the propellant grain from moisture and to promote propellant grain ignition, the nozzle opening is closed by a thin metal seal and a foam plug. A light plastic fin retainer holds the fins folded within the diameter of the rocket until propellant ignition. On ignition, the fin retainer is blown off. Gas pressure exerted on the heels of the fins pushes the fins open as they emerge from the launcher tube. The fins are then locked in the flight position by the spring-loaded pawls.

The Mk 16 Mod motor has a modified igniter and a modified nozzle and fin assembly. The nozzle and fin assembly (fig. 2-10) contains four, spring-loaded, wraparound fins inside the motor diameter. The steel nozzle expansion cone has flutes that cause the rocket to spin during free flight. This permits the rocket to be launched from high-speed aircraft, helicopters, and low-speed aircraft.

Figure 2-10.-Mk 71 Mod 0 motor, nozzle, and fin assembly. 2-7

The Mk 71 Mods spring-loaded fin (fig. 2-11) deploy after emerging from the rocket launcher tube. They lock in place (open) by sliding into a locking slot in the flange at the aft end of the fin nozzle assembly. When not actually installed in the launcher, the fins are held in the closed position by a fin retainer band, which must be removed when the rocket is installed into the launcher tube.

Figure 2-11.-Mk 71 Mods motor, nozzle, and fin assembly.