OP 1664 - U.S. Explosive Ordnance (Vol. 2); Part 6 - Bombs and Bomb Fuzes; Chapter 20 - Bomb Fuzes; Section 3 - Navy-Designed Bomb Fuzes: Mk 243 Mod 0 and Mk 244 Mods 0 and 1 (Nose Mechanical Impact)

U.S. EXPLOSIVE ORDNANCE

PART 6 - BOMBS AND BOMB FUZES

Chapter 20 - BOMB FUZES

Section 3 - NAVY-DESIGNED BOMB FUZES

Mk 243 Mod 0 and Mk 244 Mods 0 and 1 (Nose Mechanical Impact)

Bombs	500-lb. G.P. AN-M64 and 64A1 500-lb. G.P. Mk 12 Mod 2 1,000-lb. G.P. AN-M65 and 65A1 1,000-lb. G.P. Mk 13 Mod 2 2,000-lb. G.P. AN-M66 and M66A1
Functioning	Mk 243 Mod 0 - Delay of 0.025 second
Armed condition	When space between vane cup and vane-cup support is 5/16 inch
Fuzes used with	Tail Hydorstatic AN-Mk 230 or Mk 229 (set for 25 feet in high-speed glide bombing); or AN-M100A2 series (with 0.24-second delay for high-angle drops)
Arming time	130 vane revolutions

General: The fuze resembles the Nose Mechanical Impact Fuze AN-M103 in appear-ance, and has been designed specifically to be used against submarines or ships, since it will not function on water or at an impact angle of less then 45°. The blunt firing pin me-rely rests loosely over the primer in the cavity cup, and, when struck by the striker shoulder, is driven against the primer, initiating the delay. It is intended as a replacement for the hydrostatic fuzes.

Operation: As the vanes rotate, the pinion gear revolves around the upper movable gear and lower stationary gear. Since the upper gear has 23 teeth and the lower gear has 22 teeth, the upper gear is pushed around clockwise with the pinion by the amount of one tooth per revolution of the vanes. The lower gear is prevented from rotating by the lower-gear stop arm protruding into the striker. As the upper gear rotates, the arm-ing screw unthreads from the striker, allowing the arming stem spring to lift the arming stem free of the detonator slider. After 130 vane revolutions, the vanes fall away and the arming stem clears the detonator slider, which is moved across the fuze body by its spring and locked under the delay element by a detent and the slider locking pin. Om im-pact with a hard surface, the striker body is forced inward, shearing both the locating pin and the shear threads and forcing the shoulder of the striker against the blunt firing pin. The firing pin sets off the primer delay of 0.025 second, the detonator, the booster lead-in, and the booster in succession.

Remarks: When using this fuze, check the vanes by simply turning back and forth a few times in order to determine that the arming mechanism operates easily. The fuze will not function on water impact from altitudes of release up to 2,000 feet.

The Nose Mechanical Impact Fuze Mk 244 Mod 0 incorporates a 4-seond (minimum of 4, maximum of 5) delay instead of the 0.025-second delay of the Mk 243. Except for the delay, the two fuzes are identical. "4-Sec. Delay" is stamped in black letters on both si-des of the vane cup support of the Mk 244 Mod 0, to provide easy visual identifiaction of the fuze.

To activate the Fuze Mk 243 Type on soft ground, a striker plate has been added and the number of shear threads reduced 50% in the Mk 244 Mod 1, which retains the delay of the Mk 244 Mod 0. The modification has been made to allow use of this type of fuze against ground targets in the event that no marine targets are encountered on a miss-ion. The minimum dropping altitude over land is 1,000 feet.

Figure 371. Nose Impact Fuzes Mk 243 Mod 0 and Mk 244 Mod 1