SPECIAL SAFETY FEATURES

Some fuzes incorporate special safety features. The most important safety features are detonator safe, shear safe, and delay arming.

Detonator safe fuzes do not have the elements of their firing train in the proper position for firing until the fuze is fully armed. The elements remain firmly fixed and out of alignment in the fuze body while the fuze is unarmed. This increases safety during shipping, stowing, and handling of the fuze. The arming action of the fuze aligns the firing train.

A shear-safe fuze does not become armed if its arming mechanism is damaged or completely severed from the fuze body, The arming mechanism of the fuze protrudes from the bomb, and it might be severed from the fuze body if the bomb is accidentally dropped. Shear-safe fuzes give additional security for carrier operations and for externally mounted bombs.

Delay arming mechanically or electrically slows the arming of the fuze. It keeps a fuze in the safe condition until the bomb falls far enough away from or long enough from the aircraft to minimize the effects of a ^{premature explosion.} Delay arming helps to make carrier operations safe because a bomb accidentally released during landing or takeoff ordinarily will not have sufficient air travel, velocity, or time to fully arm the fuze.

MECHANICAL FUZES

LEARNING OBJECTIVE: Identify the various types of mechanical fuzes to include their physical description and functional operation.

There are many fuzes in use by the Navy today. Some of the commonly used fuzes are discussed in this TRAMAN. To keep up with current fuzes, you should refer to Aircraft Bombs, Fuzes, and Associated Components, NAVAIR 11-5A-17, and Airborne Bomb and Rocket Fuze Manual, NAVAIR 11-1F-2.

M904E2/E3/E4 MECHANICAL IMPACT NOSE FUZE

The M904 (series) fuze (fig. 1-1) is a mechanical impact nose fuze used in the Mk 80 (series) low-drag general-purpose (LDGP) bombs. The M904 (series) fuze is installed in the nose fuze well of the bomb and requires the use of an adapter booster. The fuze is detonator-safe, and it contains two observation windows through which you can determine the safe/arm condition of the fuze. There is no special locking feature designed into the fuze for shear safety if the bomb is accidentally dropped. However, detonation is unlikely if the collar (forward end of the fuze) is sheared off by the accidental drop before arming is complete.

Figure 1-1-Mechanical impact nose fuze M904 (series).

The fuze may be configured for a number of preselected arming and functioning delays needed by a mission. There are nine arming delays from 2 to 18 seconds in 2-second increments, and any combination of six functioning delays from instantaneous to 250 milliseconds (0.250 seconds) may be selected. An internal governor, driven by the permanently mounted arming vane, allows relatively constant arming times at release speeds ranging from 170 to over 525 knots.

Functioning times are determined by the installation of an M9 delay element. Anyone of six delay elements may be installed. Each delay element is identified by the functioning delay time stamped on the element body-NONDELAY (instantaneous), 0.01, 0.025, 0.05, 0.1, or 0.25 second.

Physical Description

The M904 (series) fuze contains approximately 1 1/2 ounces of tetryl in the booster, which is located at the base of the fuze body. The entire fuze weighs about 2 1/3 pounds and is 9 1/4 inches long.

The M904E4 is a thermally protected fuze. It is especially designed for use with the thermally protected Mk 80 (series) general-purpose bombs and the thermally protected M148E1 adapter booster. This significantly increases the cook-off time (table 1-1) of the bombs subjected to intense heat or flame.

ARMING DELAY TIMES.- Arming delay times are inscribed into the face of the forward nose retaining ring. A white indexing line is scribed on the knurled delay setting knob below the arming vane. The white indexing line must be matched to one of the indicated arming times to select the desired arming delay. To select the required arming delay time, depress the setting index locking pin and rotate the knurled arming delay setting knob until the white indexing line is aligned with the desired arming delay time stamped on the nose retaining ring. The 2- and 4-second arming times are for use with retarded weapons, and are only set by removing the stop screw located next to the setting index locking pin. Never try to reinstall the stop screw when either of these two settings are used. The stop screw may be reinstalled at any delay setting of 6 seconds or more.