M4A2 (Discontinued)

Rocket uses in

Functioning

Arming time

Body diameter, inches

Over-all length, inches

2.51 (without booster)

General: The Fuze M4A2 has been designed to be bore safe for firing from launcher tubes on aircraft or from the ground. There are two different issues of the fuze, indenti-cal except for the delay time, which is slightly less in the rockets fired from aircraft be-cause of the increased velocity of those rockets as compared to rockets fired from ground launchers.

Operation: Before the rocket is loaded on the launcher, the fuze is set for either in-stantaneous (super-quick) action or delay action as desired. This is accomplished by ro-tating the setting pin. For instantaneous action, the pin is rotated so that the function-ing hole (flash channel) in the setting pin mates with the flash hole from the super-quick element. For delay action, the setting pin is rotated 180 degrees, so that the flash hole from the super-quick element is obstructed and the delay firing channel is the only one open. In either position, the setting pin is secured by the spring-loaded locking ball fitting into either of two recesses in the setting pin. The cotter pin is then removed, so that the set-back pin will be free to move back on set-back.

When the rocket is fired, acceleration causes the set-back pin to move rearward, the spring offering sufficient resistance so that the pin reaches its most rearward position only after the rocket has cleared the launcher. This frees the retaining ball to be forced into an escape hole by the spring-loaded delay arming pin as deceleration sets in. As the lower end of the delay arming pin clears the inner end of the detonator slider, the slider is moved over to the armed position by the slider spring. The spring-loaded lock pin rides in a keyway on the underside of the slider and snaps into a recess when the slider rea-ches the armed position. The firing train is now lined up. On impact, the head of the fuze is crushed, the shear wire is sheared, and both strikers are driven inward, initiating both primers. Thus, both the super-quick and the delay elements are ignited, irrespective of the setting of the fuze. If set for instantaneous action, the flash from the super-quick element ignites the detonator before the delay element functions. If set for delay, the flash from the super-quick element will be obstructed by the setting pin and the flash from the delay element will ignite the detonator 0.015 second later if used in an aircraft-launched rocket, 0.1 second later if used in a ground-launched rocket.

Early-designs: The M4A1 has a longer setting pin, with double flash holes, so that the flash from either the super-quick of the delay elements had to pass through the set-ting pin. Hence, if the setting pin was not roatetd to exactly the proper point where the setting-pin flash holes matched with those from the initiating elements, the fuze would not function. In the M4A2, this condition was remedied by the shorter setting pin, so that, even if the pin is not rotated to match with the flash hole from the super-quick element, the delay flash hole will permit firing of the fuze after the slight delay. The M4A1 also had a heavier detonator slider, which occasionally sheared the lock pin and did not stay properly lined up in the armed condition. The M4A2 has a lighter, aluminum slider detonator.

The original M4 had only one issue for both ground- and aircraft-launched rockets with either instantaneous or 0.1 sec. delay settings. The M4A1 had two issues, with eit-her 0.015-second or 0.1-second delay and instantaneous settings. The original M4 also had thicker walls and was conseqzently heavier, and had stronger spring under the set-back pin and delay arming pin, requiring greater acceleration to arm.

Figure 140. Army Nose Fuze (Rocket) M4A2