Description
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, identical except for the delay time, which is slightly less in the rockets fired from aircraft because of the increased velocity of those rockets as compared to rockets fired from ground launchers.
The M4A1 had a longer setting pin, with double flash holes, so that the flash from either the super-quick or the delay elements had to pass through the setting pin. Hence, if the setting pin was not rotated 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 superquick 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 M1A1 had two issues, with either 0.015 second or 0.1-second delays and instantaneous settings. The original M4 also had thicker walls and was consequently heavier, and had stronger springs under the setback pin and delay arming pin, requiring greater acceleration to arm.
Functioning
Before the rocket is loaded on the launcher, the fuze is set for either instantaneous (super-quick) action or delay action as desired. This is accomplished by rotating the setting pin. For instantaneous action, the pin is rotated so that the functioning 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 setback 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 reaches 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 superquick and the delay elements are ignited, irrespective of the setting of the fuze. If set for instantaneous action, the flash from the superquick 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 clement 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.
Hazardous Components
No information about hazardous components.
See Also
Rocket, 4.5 inch HE, M8 Series
Source(s)
TM 9-1385-51, Ammunition (Conventional) for Explosive Ordnance Disposal (1967)
OP 1664, Volume 1 - US Explosive Ordnance (1947)
