This chapter will introduce you to some of the common terms and acronyms associated with fuzes used in the Navy. Basic fuze theory, general classes of fuzes, and the various types of fuzes are also discussed in this chapter.

In its simplest form, a mechanical fuze is like the hammer and primer used to fire a rifle or pistol. A mechanical force (in this case, the bomb impacting the target) drives a striker into a sensitive detonator. The detonator ignites a train of explosives, eventually firing the main or filler charge. A mechanical bomb fuze is more complicated than the simple hammer and primer. For safe, effective operation, any fuze (mechanical or electrical) must have the following design features:

• Depending upon the type of target, the fuze may be required to delay the detonation of the bomb after impact for a preset time (functioning delay). Functioning delay may vary from a few milliseconds to many hours.

• It should not detonate the bomb if the bomb is accidentally released or if the bomb is jettisoned in a safe condition from the aircraft. To provide these qualities, a number of design features are used. Most features are common to all types of fuzes.

Electrical fuzes have many characteristics of mechanical fuzes. They differ in fuze initiation. An electrical impulse is used to initiate the electrical fuze rather than the mechanical action of arming vane rotation. An electrical pulse from the delivery aircraft charges capacitors in the fuze as the bomb is released from the aircraft. Arming and functioning delays are produced by a series of resistor/capacitor networks in the fuze. The functioning delay is electromechanically initiated, with the necessary circuits closed by means of shock-sensitive switches. The electric bomb fuze remains safe until it is energized by the electrical charging system carried in the aircraft. Because of the interlocks provided in the release equipment, electrical charging can occur only after the bomb is released from the rack or shackle and has begun its separation from the aircraft; however, it is still connected electrically to the aircraft's bomb arming unit. At this time, the fuze receives an energizing charge required for selection of the desired arming and impact times.

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

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. 

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. 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. Any one 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. 

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. 

IDENTIFICATION OF ARMED FUZES

There are three conditions of the M904 fuze—safe, partially armed, and fully armed. You can verify the fuze conditions by looking through the two observation windows in the fuze body (fig. 1-1). To check the fuze condition, hold the fuze vertically and look through the windows perpendicular to the fuze body. Look at table 1-2. It shows you what you would see through the observation windows of the M904E3/4 fuze at various time settings and fuze conditions. Also, check the M904E4 to make sure the thermal sleeve is firmly bonded to the fuze collar and is not cracked. NOTE: If the safe condition of any fuze is in doubt, explosive ordnance disposal (EOD) personnel should be notified immediately. 

The M904 fuze arms and functions by the rotation of the arming vane and alignment of its internal components. When the fuze is released from the aircraft, the fuze arming wire is withdrawn from the fuze arming vane, and the arming vane is rotated by the airstream. Arming vane rotation is controlled by the constant arming action of the governor in the fuze. The arming vane continues to rotate until the preselected arming delay period (2 to 18 seconds) elapses (ends). Once the arming delay period elapses, the firing train is in full alignment and ready to function. t impact, the forward part of the fuze body drives the striker body and firing pin down into the M9 delay element. After the proper delay, the M9 delay ignites the relay, detonator, lead, and booster,which sets off the main charge.

1. Fuze safe/arm indicator 2. Timer starting pin 3. Timer starting pin bracket 4. Option time setting adjustment 5. Primary time setting adjustment 6. Band release stud and nut 7. Fuze arming wire 8. Safety pin assembly 9. Impeller 10. Impeller band assembly 11. Time setting observation window 12. Option time pin 13. Indicator observation window 14. Nose fairing (reference) 15. Option time wire 16. Retainer slide 17. Clip 18. Safety pin assembly guide tube

The Mk 339 Mod 1 mechanical time fuzes (fig.1-2) are used with dispenser weapons and have the following characteristics: 

• Nose-mounted

• Air-enabling

• Detonator-safe

Physical Description

The following paragraph describes the sequence of events that must occur for Mk 339 fuze to function.

MOD 1 (WITH OPTION TIME WIRE).—This fuze is physically and functionally the same as the fuze  described in the preceding paragraph except that an option wire is installed. If the pilot selects the primary mode of delivery when the weapon is released from the aircraft, only the arming wire is pulled out and the primary mode of the fuze is initiated. If the pilot selects the option mode of delivery, both the arming wire and the option wire are pulled out, initiating the option time mode of the fuze. If only the option time wire is pulled out on airborne release, the fuze will dud. Both the fuze arming wire and option wire must be pulled out for the fuze to function in the option mode.

The Mk 376 (fig. 1-6) electric bomb fuze provides an all-electric capability for the Mk 80 (series) bombs with either conical or retarding fins, thermally protected bombs, and laser-guided bombs (LGB). Electric fuzes require an electric pulse from the aircraft fuze function control (FFC) system. The FFC gives in-flight selection of function delay and arming delay times. The 376 fuze is used with the Mk 43 target-detecting device for airburst capability.

The Mk 376 Mod 0 electric tail fuze is detonator safe. The booster contain 4.3 ounces of tetryl explosive. This fuze is classified HERO SAFE, and no unusual RADHAZ precautions are required under normal operating conditions. Four discreet dc voltages for in-flight selection of functioning delay times are used in the Mk 376 fuze. The Mk 31 safety device automatically selects arming delay times.

The Mk 31 safety device is used to adapt the fuze to the fuze well of the bomb, provide mechanical safing of the fuze, and unlock the timer-decelerometer. The safety device contains a pop-out pin that locks the fuze in an unarmed condition. The spring-loaded pin is held in the safe position by either a safety cotter pin or an arming wire. When the weapon is released from the aircraft, the arming wire is pulled from the pop-out pin, allowing the pin to pop out, unlock the decelerometer, thus initiating the arming time. When the free-fall mode of delivery is used the Mk 376 fuze, arming is completed 10.0 seconds after release from the aircraft. If the Mk 31 safety device senses weapon deceleration (Snakeye fins open), the internal circuits of the fuze are switched, and the fuze becomes armed in 2.6 seconds. The quicker arming time is required to ensure the fuze is fully armed for low-altitude delivery. If deceleration is not sensed by 2.6 seconds, the fuze arming delay continues to the 10.0-second arming time.

The following description applies specifically to the Mk 376 fuze. Two arming delays are used in the Mk 376 fuze—2.6 seconds for retarded delivery and 10.0 seconds for unretarded delivery. The appropriate arming delay is automatically selected by the fuze according to the actual delivery mode of the weapon. That is, if the weapon does not retard, whether intentionally or unintentionally, the fuze automatically provides a 10.0-second arming delay. At release, the arming wire is withdrawn and a charging voltage (+300, +195, -195, or -300 Vdc) is applied to the fuze. The pilot selects the voltage in flight by the fuze function control set located in the cockpit.  The fuze polarity and level of the fuze charging voltage is important only with respect to functioning delay.  Arming is the same in any case. A regulator in the fuze converts the applied voltage to the required level and polarity. It is then applied to the energy storage unit and the 2.6-second timer. If the weapon decelerates, the Mk 31 safety device senses the deceleration and causes the retard switch to close. At 2.6 seconds, the timer completes its cycle and transfers the voltage to the rotor-actuating bellows. The bellows operate and turn the rotor to the armed position. If the weapon does not decelerate, the retard switch does not close. The 2.6-second timer continues to run. At 3.8 seconds, the Mk 31 safety device causes the voltage to transfer to the input of the rotor-actuating bellows. At 10.0 seconds, the bellows operates and turns the rotor to the armed position.

The FMU-152/B is an advanced fuze system for use in general purpose and penetrating unitary warheads. The FMU-152/B provides safing, in-flight cockpit selection, and multifunction and multiple delay arming and fuzing functions. The FMU-152/B is a multifunction; multiple delay fuze system with hardened target capabilities that provide arming and fuzing functions for general purpose and penetrating, unitary warheads. The FMU-152/B system operates in three fuze mission phases: the "pre-release," "pre-arm," and "post-arm" phases. The "pre-release" phase includes all fuze functions performed prior to the point at which the weapon is released from the delivery aircraft. The "pre-arm" phase includes all fuze functions occurring between weapon release and weapon arming. The "post-arm" phase includes all fuze functions after the weapon is armed.

The FMU-143E/B fuze (fig. 1-7) is used with the GBU-24B/B. It is initiated by the FZU-32B/B initiator, which is used to generate and supply power to arm the fuze. The safe condition is verified by the presence of a safety pin or arming wire through the pop-out pin (gag rod). 

The FMU-139 (series) electronic bomb fuze (fig. 1-8) is an electronic impact or impact-delay fuze. It is used in Mk 80 series general-purpose bombs, including laser-guided bombs. The arming times are in-flight selectable, and the functioning delay must be set during weapon assembly. There are three arming times (2.6, 5.5, and 10.0 seconds) and four functioning delay settings (10, 25, and 60 milliseconds, and instantaneous). Only 2.6/60, 2.6/25, 2.6/10, and 2.6/inst high drag arm/delay switch positions are authorized for Navy/Marine Corps use. The low drag arm time switch should always be in theXposition. The low drag arm time rotary switch is positioned at X for shipping, storage, and all FFCS (fuze function control set) use. The FMU-139 fuze differs from the Mk 376 fuze in that the gag rod and arming wire housing are located in the center of the faceplate (fig. 1-9).

FMU-140/B DISPENSER PROXIMITY FUZE

The FMU-140/B (fig. 1-13) has an optional arm and fire (timer) mode. It is used with the Rockeye II and Gator weapons. The FMU-140/B is a self-powered doppler radar device acting as a radar altimeter. Arming times and functioning altitudes are variable and are preflight selectable on the fuze faceplate, located on the side of the fuze. In the proximity mode, if the preselected altitude is reached before the fuze has had time to arm, or the dispenser is released below the pre-selected altitude, the fuze will have the potential to function anytime after arming. If the fuze has not functioned by the time it reaches an altitude of 300 ±25 feet, it will function at that time. 

SAFETY PRECAUTIONS

Most fuzes contain a charge of high explosives and must be handled carefully. When compared to the burster charge, the amount of explosives is small; however, the explosives in fuzes are much more sensitive. Fuzes are manufactured to meet all safety requirements when used properly. However, the safety features designed in any weapon or explosive  component are only as reliable as the person using them. Fuzes are packed in sealed, moisture proof containers, and should not be unsealed until they are required for use. Fuzes unpacked and not used should be returned to their original condition, repacked, and dated. Once the hermetic seal is broken, these fuzes should be used before those that are still hermetically sealed. Mk 376 electric fuzes that have been removed from their sealed containers and whose shipping caps have been removed and repacked for more than 30 days must be disposed of according to current directives. Additionally, dispose of electric fuzes exposed to excessive moisture, regardless of length of time. Fuzes must be handled carefully at all times and should never be dropped, tumbled, dragged, or thrown. They should not be struck with a hammer or any tool, either to open the container or to align them in a stowage rack.