Mechanical & Electronic Fuze

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DELTA M137 MOD 3

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Chapter 1: AN INTRODUCTION TO MECHANICAL AND ELECTRONIC FUZES CONTENTS • 1.1.Introduction • 1.2.Functions of Fuzes • 1.3.History of Fuzes • 1.4.Classification of Fuzes • 1.5.Components of Fuzes • 1.6.Closure

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Chapter 2:

Design of Fuzes

Chapter 3:

Mechanical Fuzes

Chapter 4 :

Electronic Fuzes

Chapter 5 :

Testing of Fuzes

1.1 Introduction

• The safety mechanism should work until the

ammunition is launched and after the launch, the firing mechanism should take over.

• In order to achieve this requirement, an arming

mechanism is also required in the ammunition. All the abovementioned requirements of the

ammunition are fulfilled by the devices called

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1.2 Functions of Fuzes

• Fuzes are the devices attached to ammunition for the purpose of safing, arming and firing.

• Basic function of the fuze is to fire the ammunition when desired and ensure safety during other times. • Therefore, the fuze is also referred to as the brain of

the ammunition. The basic functions of the fuzes are listed below

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1.2 Functions of Fuzes

1. Safing: To ensure safety during storage, handling

(accidental mishandling), transportation and launching of the ammunition.

2. Arming: To sense the conditions of launch of ammunition

and to align explosive trains, close switches and establish other links to enable the firing of ammunition thereafter.

3. Firing: To initiate the detonation in the ammunition at

desired point in space or at preset time.

4. Fuze must be able to carry out abovementioned functional requirements reliably under extreme operational

conditions such as high velocity, spin, environmental variables, impact etc.

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1.3 History of Fuzes

• The history of fuzes dates back to First World War, when primitive Direct Action fuzes having simple mechanical construction were developed (1914-17). • Prior to 1942, fuze system where entirely

mechanical systems. Two types of proximity fuzes were developed during World War II, the radio and the photoelectric proximity fuzes.

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1.3 History of Fuzes

• Photoelectric fuzes required light for operation and would sometime function early when sun moved into and out the field of view of the photoelectric lens.

• For this reason, this fuze was discarded in 1943 and the term proximity or VT (Variable Time) fuze was used to refer to radio proximity fuzes.

• Prior to early 1960s, the types of fuze used were either

mechanical systems or hybrid systems containing electrical timing systems (vacuum tube units and other electrical

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1.3 History of Fuzes

• The first electronic hybrid (transistors and vacuum tubes) fuze, the M532, was made in early 1960s for a mortar round.

• The first fully transistorized fuze, the M429, was

made in the 1965-70 time-period for a 2.75” rocket. • The M514A1E1 (later named M728) was the first

fully transistorized artillery fuze and was made in the late 1960s to early 1970s.

• New electronic fuzes with multiple features came into existence in 1980s.

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1.4 Classification of Fuzes

• Fuzes can be classified into sub-categories based on their ammunition, tactical application, functioning, location etc.

Some of such classifications are discussed below. Based on Functioning

Based on the principle of operation, fuzes are classified as follows.

• (i) Impact or Percussion Fuzes: Impact fuze initiates the firing action when actual contact with the target takes place. Impact fuzes are further classified as follows

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1.4 Classification of Fuzes

a) Point Detonating Fuzes (PD):

Point detonating fuzes are located on the nose of the projectile and function upon the impact with the target or following impact with a timed delay. b) Base Detonating Fuzes (BD): Base detonating

fuzes are located on the base of the projectile and function with short delay after the initial impact at the nose of the fuze.

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1.4 Classification of Fuzes

c) Initiating, Base-Detonating (PIBD):

Point-initiating, base-detonating fuze has the

target-sensing element in the nose of the projectile and the functional part of the fuze is in the base

d) Delay Fuzes: These fuzes are designed to function after a long delay (minutes to days) after the initial impact. These fuze find their application in bombs, underwater mines etc.

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1.4 Classification of Fuzes

Point-detonating, Base-detonating, Point-initiating-Base-detonating and Delay fuzes are combined called as Direct

Action fuzes as they function when something reasonably

solid compresses the nose of the fuze.

e) Graze fuzes: Many times, the projectile may land at the

target at very low descent angles, resulting in a grazing action rather than a direct impact and the Direct Action fuzes may not function. Graze fuzes are designed for such cases. It detonates if the shell decelerates appreciably

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1.4 Classification of Fuzes

• (ii) Time Fuzes: Time fuzes function at the end of an elapsed time after arming or impact. Time fuzes may further be classified based on the mechanism to measure time and induce delay such as mechanical, electronic, pyrotechnic, chemical, radiological etc. Time fuzes are used for illumination projectile and special purpose mines, bombs and grenades.

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1.4 Classification of Fuzes

• In case the target is missed while firing from air-to-air or ground-to-air-to-air, it is desired to destroy the

projectile in the air itself. For such applications, Self-destruction feature in incorporated in fuzes known as Self Destruction (SD) fuzes, by using a suitable timing mechanism

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1.4 Classification of Fuzes

• (iii) Proximity Fuzes: These fuzes function when they sense that they are in the proximity to the

target. These fuzes are also called Influence fuzes. These fuzes are particularly effective in uses against personnel, light ground targets, aircrafts and

superstructures of ships. The proximity sensor generally functions on the principle of Doppler effect.

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1.4 Classification of Fuzes

• (iv) Command Fuzes: Command fuzes are remotely controlled devices. Command fuzes function through a signal communicated to the fuze from a remote point through electrical, mechanical, optical or other means.

• (v) Combination Fuzes: Fuzes involving more than one of the principles of operations discussed above are called

Combination Fuzes. These fuzes have multi-options so that same fuze can serve for different tactical applications, with one mode of operation as Principle action and others as

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1.4 Classification of Fuzes

1.4.2. Based on types of mechanisms

• Various mechanisms in the fuzes, such as safety, arming and firing, can be designed by using

mechanical linkages or electrical / electronic circuits etc. Based on the mechanisms in the fuze, they are classified Mechanical Fuzes, Electronic Fuzes,

Optical Fuzes, Chemical Fuzes etc. Mechanical and Electronic fuzes are discussed in detail in the

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1.4 Classification of Fuzes

1.4.3. Based on the Tactical Application

• Based on the tactical application, fuzes are classified as Air-to-Air, Air-to-Ground, Ground-to-Air and

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1.4 Classification of Fuzes

1.4.4. Based on the Purpose

Based on the purpose of the fuze or the target, fuzes are classified as follows.

• _{Antipersonnel (APERS)} • _{Armor-Piercing (AP)}

• Blast or High Explosive (HE) • _{Concrete-Piercing (CP)}

• _{High Explosive Anti-Tank (HEAT)} • _{High Explosive Plastic (HEP)}

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1.4 Classification of Fuzes

1.4.5. Based on Ammunition

• Based on ammunition with which the fuze is employed, fuzes are classified as Bomb fuze, Grenade fuze, Guided Missile fuze, Mine fuze, Mortar fuze, Projectile fuze, Rocket fuze etc.

• Type of ammunition and the fuzes used with them (based on the functioning) are given in the table below.

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1.4 Classification of Fuzes

.

04.

Direct Action Fuzes (PD), Time Fuzes (M, El), Proximity Fuzes, Combination Fuzes. Naval Guns

Naval Guns 03.

Direct Action Fuzes (PD), Time Fuzes (M, El), Proximity Fuzes, Combination Fuzes, Delay Fuzes (Illuminating and Smoke Bombs).

Mortars 02.

Direct Action Fuzes (PD), Time Fuzes (M, El), Proximity Fuzes, Combination Fuzes.

Artillery Guns 01.

Fuzes Used Ammunition

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1.4 Classification of Fuzes

Direct Action Fuzes (PD), Time Fuzes (Self

.

Air Target Missile 09.

Surface Target Missiles

08.

Time Fuzes (M, El), Delay Fuzes. Sea Mines

07.

Pressure Fuzes. Land Mines

06.

Bomb Fuzes 05.

Fuzes Used Ammunition

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1.4 Classification of Fuzes

Direct Action Fuzes (PD, BD), Proximity Fuzes, Combination Fuzes. Small Caliber Guns 10. Fuzes Used Ammunition S. No. •M – Mechanical Fuze, •El – Electronic fuze, •PD – Point Detonating, •BD – Base detonating,

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1.5 Components of Fuzes

• Many varieties of the fuze have been developed

over the years in order to achieve specific functional objectives.

• All of the fuses have some basic

mechanisms/modules in common, though the

working of mechanism may be markedly different. Some of the basic modules of the fuzes are

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1.5 Components of Fuzes

• _{1.5.1 Safety Mechanism: In order to provide}

adequate safety to the fuze, following safety

features are incorporated in all the fuzes, though through different means.

i) The present design philosophy entails the fuze to have at least two safing features, either one

capable of preventing unintended detonation. The concept involved is that there is low probability of both the safety features failing simultaneously.

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1.5 Components of Fuzes

ii) Fuzes are designed to be detonator safe i.e. even if the detonator functions, it cannot initiate the explosive train prior to launch. The basic method to achieve this is by an interrupted explosive train, which aligns itself only after the launch of the ammunition.

iii) Fuzes for artillery projectiles, mortar projectiles and rockets needs to be bore safe i.e. the detonator will not initiate a bursting charge while the projectile is in the

launching tube. This is achieved by preventing initiating pin (firing pin) from hitting the detonator by introducing some interruption in between.

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1.5 Components of Fuzes

iv) A delay mechanism may be separately

incorporated in some fuzes to delay the arming of the fuze until the ammunition has left the launcher. In other cases, the time taken by arming mechanism to arm the fuze or bore safety feature takes care of this requirement

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1.5 Components of Fuzes

• 1.5.2 Arming Mechanism: Fuzes are designed to be bore safe and detonator safe for safety requirements. These

feature needs to be disabled after the launch of the

ammunition to complete the firing circuit and this objective is fulfilled by arming mechanism.

• Arming mechanism mainly involves aligning explosive train elements or to remove the barriers in the explosive trains or to complete the firing circuit by closing switches. The energy to align the element and control the action time may be obtained from the forces experienced at the launch and during flight, or through any external source.

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1.5 Components of Fuzes

• 1.5.3 Explosive Train: Explosive train provides transition of a relatively feeble stimulus generated by initiating mechanism into the desired explosive output of the main charge. It consists of explosive elements arranged in the order of decreasing

sensitivity. The initial explosive component is known as initiator, which may be a primer or a detonator.

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1.5 Components of Fuzes

• A primer may not detonate itself but causes detonation of the subsequent element in the

explosive train whereas detonator detonates to generate an intense shock wave causing the

detonation of subsequent elements. Delay elements may be provided in the explosive train to delay the propagation of detonation to the booster charge so that the ammunition can penetrate in the target.

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1.5 Components of Fuzes

• Relay elements may be provided to pick up the explosive stimulus from detonator, augment it and transmit it to the next element.

• Leads transmit the detonation wave from detonator to the booster. Booster charge contains more

explosive and it amplifies the detonation wave to a sufficient magnitude or maintains detonating

conditions for a long enough time to initiate the main charge of the ammunition.

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1.5 Components of Fuzes

• 1.5.4 Firing Mechanism: Once the ammunition is armed, the fuze should initiate the explosion as per the performance desired of the ammunition. Ammunition may be required to explode after hitting the target or at a distance from the

target or at preset time after penetrating the target or after recognizing some specified external circumstances. The

firing mechanism of the fuze should initiate the explosion as per the design requirements. The target sensing mechanism of the fuze senses the target (or proximity to it) either due to impact or due to influence sensing (Doppler effect).

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1.5 Components of Fuzes

• A time fuze has a timer, which initiates the

explosion after a preset time varying from few seconds to few minutes. Command fuzes initiate their ammunition on impulses received after

launching. A combination of abovementioned methods is also used in fuzes for increased

effectiveness and/or self-destruction. Once the target is sensed, the detonator of the explosive train is

initiated by using firing pins or electrical stimulus generated by electrical or electronic circuits.

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1.6 Closure

• 1.6 Closure:

• Fuzes are one of the most important components of the ammunition. Functioning and reliability of the ammunition depends upon the performance of the fuze. Basic requirements of the fuze are safing,

arming and firing and it requires a great effort from the designer to incorporate the entire design feature within limited space. The design of fuze and steps involved in it are discussed in the next chapter.

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CHAPTER 2 DESIGN OF FUZES

• 2.1 Introduction:

• Fuze is an example of complex modern design.

• Design and development of fuze may initiate either because of requirement of the user (Defence

Forces),

• or because of some new brilliant idea or design developed by the designer resulting in better or improved fuzes for existing application

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2.1 Introduction

• In the first case, the designer is provided with the functional objectives intended in the fuze and it is designer’s job to decide whether the objectives can be met by the improvement of an existing design or it necessitates development of a new fuze.

• Designer has an idea of the physical parameters

required for the fuze and he has to develop the fuze within the constraints mentioned in the user’s

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2.1 Introduction

• In the latter case, the designer transforms his concept or design into a physical fuze, which

performs a new function or satisfies some functional objectives in a better way than the existing designs. • The product is communicated to the supplying

agency and the user, and based on their feedback and proofing results, the fuze is either developed further for production or is discarded.

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2.1 Introduction

In either case,

• The designer must be thoroughly clear about the design objectives of the fuze,

• The environmental condition in which it will work, • The safety features required,

• Tradeoffs permissible,

• Critical and non-critical design objectives, • Economics involved,

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2.2 Design Objectives for Fuzes

• The basic functional objectives of fuze may be broadly expressed as Safing, Arming and Firing, Arming

and performing the three functions reliably under all conditions.

• However, there are many supplementary design objectives, which are seldom expressed explicitly. Essential design objectives of fuze are listed below.

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2.2 Design Objectives for Fuzes

1. Reliability of action.

2, Safety and resistance to deterioration in handling, use and storage.

3. Simplicity of construction.

4. Adequate strength in use and for accidental mishandling.

5. Compactness

6. Safety and ease of manufacture and loading. 7. Economy in manufacture.

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2.3 Approach to Design of Fuzes

• Design of fuze is conceptually similar to design of any other device. Some of the essential ingredients involved in the design of fuze are as follows.

1.Detailed study of explicitly stated and implicitly involved design objectives and classification of objectives as critical (must required) and non-critical (desirable).

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2.3 Approach to Design of Fuzes

2. Developing of alternative means and systems to achieve design objectives.

3. Analysis of every alternative based on its

capability to fulfill design objectives, resources required, cost involved, manufacturability etc.

4. Development of a mathematical or logical model i.e. a set of relations among the objectives,

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2.3 Approach to Design of Fuzes

5.Selection of best possible design satisfying all the critical design objectives and most of non-critical design objectives and which are economically

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2.4 Steps involved in Fuze Design

• The first step in the design of fuze is to understand the fuze tactical requirements in detail and

considering possible mechanisms or

electrical/electronic circuits to meet them.

• Based on the design objectives and constraints, the explosive train is established and basic arming,

firing and safing mechanisms are selected.

• Preliminary sketches are prepared keeping in mind the functionality of the fuze and manufacturability.

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2.4 Steps involved in Fuze Design

• Next step is preparation of the drawings of every

mechanism and component, which can be utilized for detailed design analysis.

• All tactical, environmental, safety and design requirements of the fuze are reviewed critically. Forces acting on the

fuzes are calculated, material is selected and the design is further refined.

• Performance with this design is calculated and reliability of the design is estimated.

• Finally, detailed drawing of the fuze is made indicating all dimensions, tolerances, and view.

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2.4 Steps involved in Fuze Design

• Prototypes are made from the final drawing and are tested for their performance.

• Trials depend on the type of fuze, severity of requirement, available time and funds.

• The evaluation must be realistic and reliable. The components and sub-assemblies of the fuze are tested thoroughly individually and in assembled state.

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2.4 Steps involved in Fuze Design

• Final step in fuze development is proof testing and acceptance testing.

• A sample is selected from the pilot lot and is tested in actual ground conditions. The functioning and reliability of the fuze is assessed and it is either accepted for mass production or is returned to the designer for further development or is discarded.

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2.5 Planning for Design of Fuzes at OFB

• _{The design activity of fuzes at OFB can be}

planned in either or a combination of following modes

ii) Through Reverse Engineering.

a) In this method, an imported fuze is dissembled

into all its components either at OF Khamaria or at OF Chanda, who have boiling facilities.

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2.5 Planning for Design of Fuzes at OFB

a. The dissembled components are scanned to get the cloud points, which can be used to generate the

solid model of the component.

For this purpose, an agreement exists between OFB and CMERI, Durgapur, under which the

components can be forwarded to CMERI and they provide with the cloud points.

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2.5 Planning for Design of Fuzes at OFB

a. The CDDs (Center for Design and Development) at MPF, OFPM, OFC, RFI and OFAj are equipped with

Imageware, a reverse engineering software, through

which the output from CMERI is imported to obtain solid model in Unigraphics, which can give component

drawings.

b. The materials are assigned to the components by material testing of the components of the imported fuzes.

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2.5 Planning for Design of Fuzes at OFB

i. Through Re-engineering.

b. Formulation up to solid modeling is similar to the procedure mentioned above in reverse engineering. c. Materials are assigned by experience.

d. Testing loads are assigned to simulate and use.

e. After fixing “b” and “c”, finite element analysis is done by using analysis software ANSYS to find out the

components are safe and will function under actual load conditions (Inertial loads, Thermal loads,

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Electro-2.5 Planning for Design of Fuzes at OFB

iii Through Innovation over existing products

• _{In this case, various alternatives are thought for} achieving the intended function and the general scheme of an existing design can be altered.

iv Through de-novo new designs

• _{In this case, by using the existing knowledge base} and, the principle and concepts already available through collateral applications, a new design is prepared.

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2.6 Closure

• Fuze development is a continuous process. Even

when one design is finalized and accepted, it may be required to modify it to incorporate more features

into it or to increase its reliability.

• The pace of development of defence equipments today is extremely fast and new fuzes are being continuously developed to meet the extreme

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2.6 Closure

• Prior to 1960s, mechanical and electrical fuzes were the only fuzes present.

• From the last 3 decades, electronic fuzes are

replacing mechanical and electrical fuzes from many of their applications.

• Mechanical fuzes, their mechanism and their

features are discussed in next chapter, which will be followed by the discussion of electronic fuzes in the Chapter 4.

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CHAPTER 3 MECHANICAL FUZES

3.1 Introduction

• Mechanical fuzes have their safing, arming and

firing mechanisms consisting mostly of mechanical components, linkages and mechanisms.

• Due to their simplicity and ease of

conceptualization, mechanical fuzes were the

earliest fuzes developed and are still used with many types of ammunitions

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3.1 Introduction

• There can be various ways in which, mechanisms in a fuze can be designed. Which design is superior

depends on the function of the fuze and the perception of the designer.

• In the following sections, different mechanisms commonly used in mechanical fuzes are discussed.

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3.2 Fuze Initiation or Firing Mechanisms

• The function of fuze initiation mechanism or firing mechanism is to sense the target or environment,

and to initiate detonation in the explosive train when the pre-specified external condition has been

achieved. Therefore, fuzes have target sensors followed by initiating mechanism

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3.2.1 Target Sensing

• Target sensing depends on the task assigned to the ammunition. Ammunition may be required to fire at different locations or time depending on the desired purpose.

• It may be required to fire on impact, after

penetrating the target, after preset time, at a distance from target or any other condition.

• Target sensing methods used with mechanical fuzes are discussed below.

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(i) Sensing by contact

• In this mode, fuzes initiate their action by contact with the target. By modifying the mechanisms, the fuze can be made to initiate as soon as the impact takes place or after a time lag so that the

ammunition penetrates the target.

• Initiation of such fuzes is usually activated by the mechanical action resulting from contacting the target, for example, by moving a firing pin, by closing a switch etc.

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(i) Sensing by contact

• Contact sensing can be applied in variety of ways. It can be applied to initiate the burst on the target

surface itself when the fuze touches the target (Point Detonating).

• It can be used to initiate the firing from behind the ammunition when nose touches the target, so that

some time lag is introduced before the firing and the ammunition penetrates the target (Base Detonating).

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(i) Sensing by contact

• Contact sensing can also be used for initiating burst at a distance from the target, when nose senses the target, firing initiates at the base, for example, fuzes for High Explosive Anti-Tank (HEAT) ammunition (Point-Initiating, Base-Detonating).

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(ii) Presetting

• This type of sensing is achieved by a time fuze. The fuze is designed to initiate at a fixed time after the launch. A mechanical clockwork system is usually employed for the purpose, which measure the time after the launch and initiates the fuze after a

particular time has elapsed. However, the intervals of time are limited in mechanical fuzes depending on the clockwork mechanism used. The time at

which fuze should function is determined by the type of target, distance of the target etc.

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(iii) Combination and Self Destruction

• Combination fuze incorporates features of both impact fuze and time fuze. It functions either on impact to the target or after a particular time. The later option is also utilized for providing

self-destruction option especially for fuzes of ground-to-air and ground-to-air-to-ground-to-air application

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3.2.3 Initiating Mechanisms

• Once the target sensor informs the fuze to initiate the detonation, an initiating mechanism starts the detonation chain.

• Many mechanisms with different operating

principles have been developed for the purpose. These mechanisms require power to initiate the detonation chain and time delays.

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3.2.3 Initiating Mechanisms

• In mechanical fuzes, the contact sensing (impact) or presetting (clockwork mechanism) is converted

directly into mechanical movement of a firing pin, which in turn is driven either into or against the first element of the explosive chain.

• Functioning delays are usually obtained by

pyrotechnic delays, which form a part of explosive train.

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(i) Initiation by Stab

• The principle involved in initiation by stab is that if a pin punctures the primer case and enters a suitable explosive charge, an explosion can be initiated.

• The firing pin usually is made up steel and aluminum alloy and has the shape of a truncated cone.

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(i) Initiation by Stab

• A Stab detonator converts the mechanical impact of the initiator (firing pin) into detonating wave is

shown in the Figure 3.1.

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(ii) Initiation by Percussion

• Contrary to initiation by stab, the firing pin does not puncture the primer case in initiation by percussion. Instead, the firing pin dents the case and pinches the explosive between the case and a metal anvil

provided at the back.

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(ii) Initiation by Percussion

• As the explosive is squeezed between the case and the anvil, its granular structure fractures and

detonation wave is initiated. Percussion firing pins usually have a semi-hemispherical tip (Figure 3.2).

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(iii) Initiation by Adiabatic Compression

• This type of initiation does not require any firing pin. On impact with the target, an air column

undergoes adiabatic compression resulting in

temperature rise, which can be used to detonate the explosive charge (Figure 3.3)..

Figure 3.3 Initiation by

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(iii) Initiation by Adiabatic Compression

• Though simple in construction, this fuze is neither sensitive nor reliable at low velocities and thin

targets, and is therefore seldom used.

Figure 3.3 Initiation by

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(iv) Initiation by Friction

• Heat generated by friction can be used to detonate explosive charge in a fuze. The friction may be

generated by rubbing two surfaces together, an example of which is a wire coated with friction

composition pulled through an ignition mix (Figure 3.4)

Figure3.4 Initiation by

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Simple firing mechanism

• A simple firing mechanism of a mechanical direct action fuze is shown in the Figure 3.5. Due to

impact with the target, the firing pin extension moves downwards forcing the firing pin into the detonator and thus initiating the explosive train.

Figure 3.5 Simple Firing

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3.3 Explosive Train

• Explosive train amplifies a relatively weak stimulus by initiating mechanism to detonate the main

charge. It is an assembly of explosive elements arranged in the order of decreasing sensitivity

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3.3.1 Classification of Explosives

• Explosive materials are chemical substances, which can undergo rapid chemical change without an

outside supply of oxygen, and with the liberation of large quantities of energy generally accompanied by the evolution of hot gases. These are mixtures of

certain fuels of extremely high calorific value and oxidizers

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3.3.1 Classification of Explosives

• Explosives are classified as Low explosives and

High explosives. Low explosives are those in which the advance of chemical reaction into the unreacted explosive is less than the velocity of sound through the undisturbed material. Low explosives normally burn and deflagrate rather than detonate. Low

explosives may be gas producing or non-gas

producing. Due to their low rate of detonation, these are not usually employed in the fuze explosive

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3.3.1 Classification of Explosives

• On the other hand, High explosives are those in which the advance of chemical reaction into the unreacted explosive exceeds the velocity of sound through this explosive.

• High explosives are further classifies as Primary and Secondary. Primary high explosives are sensitive in initiation by both heat and shock (e.g. lead azide,

lead styphnate, diazodinitrophenol, hexanitromannite).

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3.3.1 Classification of Explosives

• Therefore, these explosive form the initial elements of the explosive trains. Secondary high explosive are not readily initiated by heat or shock but rather by an explosive shock from a primary explosive

(e.g. PETN, RDX, tetryl, TNT, picatrol etc.). These explosive form the delay elements, relay elements and booster charge in the fuze.

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3.3.2 Elements of an Explosive Train

• An explosive train consists of some initial elements (primer, detonator etc.), which amplify the weak

stimulus provided by the initiator, and other

explosive elements (delay elements, relay elements, booster charge etc.), which sustains and transmits the amplified stimulus to the explosives in the shell. • These elements of the explosive train are discussed

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3.3.3 Initial explosive components

• Initial explosive components of the explosive train are

generally primer and detonator, and are also referred to as initiators. A primer is a relatively small, sensitive explosive component, which serves as an energy transducer,

converting mechanical or electrical energy into explosive energy.

• The explosive output is relatively small and is further

amplified and sustained by later elements of the explosive train. Primer may not detonate itself but may induce

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3.3.3 Initial explosive components

• A detonator is a small sensitive component capable of reliably initiating high order detonation in the

next high explosive of the explosive train.

• It differs from the primer in that its output is an intense shock wave. It can be initiated directly by mechanical or electrical means, or by the output energy of the primer.

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3.3.3 Initial explosive components

• Typically, initiators and detonators have three

charges: a priming charge, an intermediate charge and a base charge, although two of these charges can be combined.

• Priming charge is similar to primer and is generally made of lead azide or lead styphnate. Intermediate charge is usually lead azide while the base charge can be lead azide, PETN, tetryl or RDX.

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3.3.3 Initial explosive components

• _{Based on the initiating mechanisms, initiators can} be of following types.

i) Stab initiators: It consists of a cup loaded with

explosives and covered with a closing disc (Figure 3.1). It is sensitive to mechanical energy and is

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3.3.3 Initial explosive components

ii) Percussion primers: It consists of a cup filled

with a thin layer of primer mix and an anvil on the other side (Figure 3.2). When the firing pin hits the cup, the primer mix is squeezed between the cup and the anvil, and the detonation starts. Thus,

percussion primers fire without puncturing the cup

Figure 3.2 Percussion

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3.3.3 Initial explosive components

• _{Flash detonators: They are similar to stab}

initiator but they are sensitive to heat and initiate due to heat generated by mechanical impact

• _{Electrical Primer and detonators: These}

initiators function as a result of detonation of a spot charge in the primer due to passage of

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3.3.3 Initial explosive components

• The input to the initiator may be mechanical energy, electrical energy or some other kind of energy input. • The output of the initiator may be a shock wave, a

flame, hot gases etc. Selection of a suitable initiator depends on the design features of the fuze and the initiation properties of the other elements of the explosive trains.

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3.3.4 Booster Charges

• This is the last element of the explosive train and it contains more explosive than any other element.

The booster charge is initiated by one or several leads or directly by a detonator. It amplifies the detonation wave to a sufficient magnitude and maintains detonating conditions for long enough time to initiate the main charge of the ammunition. The most common explosives for booster are Tetryl, RDX, granular TNT, RDX –wax mixture and

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3.3.5 Other explosive elements

• Some other explosive elements of the explosive

train are employed to sustain and amplify the output of the initiators, and to pass it to booster charge.

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3.3.5 Other explosive elements

• _{Delay elements:}

• _{Delay elements are incorporated into the explosive} train to enhance target damage by allowing the

missile to penetrate before exploding or to control the timing of sequential operations. Delay

elements are the components providing time lag in the explosive train. Generally, delay column burns like a cigarette i.e. they are ignited at one end and burn linearly.

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3.3.5 Other explosive elements

• Delay column are ignited by a suitable primer.

Explosives used in delay elements can be classified as gas producing (e.g. Black powder) and Gasless mixtures (e.g. metallic fuel plus oxidant). A typical delay element is shown in the Figure 3.6.

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3.3.5 Other explosive elements

(ii) Relays:

• Relays are small explosive elements used to

augment and transmit the weak explosive stimulus of the initiator or delay element to the next

components of the explosive train. Nearly all relays are loaded with Lead Azide, a primary explosive.

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3.3.5 Other explosive elements

(iii) Leads

• The purpose of the leads is to transmit the

detonation wave from the detonator to the booster. Tetryl and RDX are the most common explosives for leads. The efficiency of the lead depends upon explosive density, confinement length and diameter

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3.3.5 Other explosive elements

• A simple explosive train in the fuze consisting of detonator, lead and booster is as shown in the Figure 3.8.

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3.4 Safing Mechanisms

• The tactical requirements of a fuze necessitate the use of extremely sensitive explosive train, which responds to small impact forces, heat and electrical stimulus.

• This introduces a very important design

consideration while designing fuze: safety during manufacture, loading, transportation, storage, and assembly with the ammunition.

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3.4 Safing Mechanisms

• Safety enters into every facet of fuze design and

development. The present design philosophy entails the fuze to have at least two safing features, either one capable of preventing unintended detonation. • The concept is that there is low probability of both

the safety features failing simultaneously. The main safety features incorporated in mechanical fuzes and mechanisms to achieve them are discussed below.

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3.4 Safing Mechanisms

3.4.1 Detonator Safe:

Fuzes are designed to be detonator safe i.e. functioning of the detonator will not initiate

subsequent explosive train components prior to arming. The basic method to achieve this is by an interrupted explosive train by mechanical

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3.4 Safing Mechanisms

The interrupter should have a positive lock while in safe position. The detonator should be assembled in safe position so that fuze is safe during all final

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3.4 Safing Mechanisms

• An example of detonator safe fuze is shown in the Figure 3.9.

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3.4.2 Bore Safe

:

• Fuze must also be bore safe i.e. the detonator should not initiate the bursting charge while the projectile is in launching tube.

• This can be achieved by the time lag required by the arming mechanism to function or by adding a

separate time measuring device to delay the arming of the fuze until the ammunition has left the

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3.4.2 Bore Safe

:

• Figure 3.10 gives an

example of bore safe fuze by introducing a rotary shutter with a hole. The rotary shutter has a hole through which, the firing pin can strike the

detonator. In the safe state, the hole in the shutter is out of line with the firing

Figure 3.10 (a) Fuze in Safe state

(b) Fuze in armed state

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3.4.2 Bore Safe

:

• After the firing, the

shutter aligns itself with some mechanism to

bring the firing pin, hole in the shutter and

detonator in the same line. The fuze is now in armed state and will

function on impact

Figure 3.10 (a) Fuze in Safe state

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3.4.3 Partial Arming Proof:

• The fuze must never remain in the partially

armed state. As soon as the force that cause the partial arming is removed, the

fuze must return to the

unarmed state. One-way to avoid partial arming is as

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3.4.3 Partial Arming Proof:

• Consider a plate loaded with a spring to prevent its rotation and is

required to be rotated by 1800 to accomplish

arming. Figure 3.11(a)

shows the mechanism in unarmed state.

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3.4.3 Partial Arming Proof:

• An external force Fapp is applied to rotate the plate. Due to applied force, the plate rotates against the spring force and a

restoring force Fres also acts on the plate. When the plate turns partially say

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3.4.3 Partial Arming Proof:

• Due to the restoring spring force, the plate returns to its initial unarmed state (Figure 3.11 c). For arming, complete rotation of plate is required and therefore, applied force Fapp is applied throughout the rotation so that the plate cannot return back due to restoring spring

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3.4.4 Arming Indicator:

• An arming indicator is sometime provided in the fuze to indicate whether the fuze is in armed or unarmed state.

• An anti-insertion feature may also be provided by which, the fuze cannot be inserted in their

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3.5 Arming Mechanisms

• The arming process consists of aligning the

elements of explosive train and/or in removing the barriers along the train. The main considerations in selection of a suitable arming mechanism are that sufficient energy should be available for the arming purpose and the arming should take place at a safe distance from the launcher

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3.5 Arming Mechanisms

• The energy required for arming can be provided by the forces experienced by the fuze during launch or by some external energy source. These sources

depend on the ballistic environment experienced by the fuze. These ballistic environments and energy sources are discussed in some detail in the

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3.5 Arming Mechanisms

• The second objective can be met by incorporation of a time measuring device in the arming mechanism by which the arming can take place only after some time after the fuze has left the launcher and is at safe distance from it.

• Sometime, the time involved in mechanism to

function is itself large enough to satisfy the safety requirement during arming. Various mechanisms

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3.5.1 Ballistic Environments

• The energy available at the time of launch depends upon the ballistic conditions experienced by the

fuze, which may be of following types: • high acceleration,

• low acceleration

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3.5.1 Ballistic Environments

• (i) High Acceleration: Projectiles fired from small arms, guns, howitzers, mortars, and rifles etc.

experience acceleration of the order of ‘40,000g’ and are subjected to ballistic environment called High acceleration. Projectile launched with high acceleration may be either spin stabilized or fin stabilized.

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3.5.1 Ballistic Environments

• Spin stabilized high acceleration projectiles are subjected to setback force, centrifugal force,

tangential force and creep force. Fin stabilized projectiles are also subjected to all the forces

mentioned above except those resulting from spin of the projectile (centrifugal forces).

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3.5.1 Ballistic Environments

• (ii) Low Acceleration: Some projectiles (rockets) carry their own propellant and propellant is

consumed during the flight. These ammunitions are thus launched with relatively lesser force and are subjected to ballistic environment called Low

acceleration. There is not much setback force and hence, other forces are used for arming purposes.

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3.5.1 Ballistic Environments

• (iii) Gravity Acceleration: Airplane bombs are often dropped at a height and are subjected to

acceleration equal to that of gravity. Fuzes for such ammunition experiences aerodynamic and

barometric forces. So either of these force are utilized for arming or manual arming may be provide as in hand grenades

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3.5.2 Energy Source for Arming:

• Generally, the forces experienced during the launch of the ammunition (setback force) and flight (torque, centrifugal force, creep etc.) are large enough to

fulfill the energy requirements of arming

mechanisms. However, if the external forces are

small or if their effect is comparable to that created by rough handling, a separate power source is

provided in the fuze. Some of these energy sources are discussed below.

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3.5.2 Energy Source for Arming:

• _{Setback: It is the relative rearward movement of}

the component in the ammunition undergoing forward acceleration during launch)

• _{Creep: It is the tendency of the components of the}

fuze to move forward as the ammunition slows down due to drag force by air friction and

resistance during flight.

• Centrifugal force: This is the force experienced

due to rotation of the spin-stabilized projectile during the flight.

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3.5.2 Energy Source for Arming:

• _{Tangential force: This is the force experienced}

by spring-loaded weights under the application of angular velocity.

• Coriolis force: It is the force experienced by a

ball in a radial slot, which rotates at an angular velocity. This is seldom used for arming purpose.

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3.5.2 Energy Source for Arming:

• _{Torque: It is the effect of force acting at a} distance (by lever arm). It produces angular acceleration of a part.

• _{Forces of the Air Stream: This force is due to}

airflow past the propeller blades and is used to turn propellers in bombs and rockets.

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3.5.2 Energy Source for Arming:

viii) Ambient Pressure: Ambient pressure is very high at sea floors and can be used for arming in sea mines and depth charges.

ix) Setforward: Setforward force is the reaction force experienced when the ammunition is rammed into an automatic weapon. It is opposite to setback

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3.5.2 Energy Source for Arming:

• _{Sideways: Perfect alignment of projectile with the gun} cannot be achieved practically. Thus, during launch, the projectile tries to align itself with the gun resulting in force called sideways.

• _{Non-environmental energy sources: When none of the}

above mentioned force is strong enough to provide necessary energy for arming of fuze, auxiliary power

sources are provided. Springs may be used in compressed stage to store and deliver energy. Batteries may be used to turn a rotor or close a switch and cause arming.

Explosives burn to produce heat and gases and the pressure developed by them can also be utilized for arming purpose.

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3.5.3 Mechanical Arming Devices:

• _{Fuzes operated by mechanical devices make use} of mechanical linkages like springs, gears, sliders, rotors and plungers, or a combination of them.

Some of these mechanisms are discussed below

i) Springs: Springs provide a reservoir for stored

energy, which can be conveniently accommodated in the fuze and can be used over the 20-year shelf life required for the fuze.

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3.5.3 Mechanical Arming Devices:

• There are three general types of spring used in fuze arming mechanism.

• The Flat Leaf spring is a thin beam, which creates tensile and compressive stresses when it bends.

• The Flat Spiral spring wounds into a spiral and releases energy during unwinding.

• The Helical Coil spring is a wire coil, in which

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3.5.3 Mechanical Arming Devices:

(a) Belleville spring: This spring is normally used with

land mines. When a force is applied in one of its

equilibrium position, the spring flattens and moves rapidly to its other equilibrium position causing initiation.

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3.5.3 Mechanical Arming Devices:

(b) Power Springs: These are flat spiral springs used to

drive clockworks and are also called mainsprings. Springs are usually contained in a hollow case to which one end of the spring is attached and the other end is attached to an arbor.

(c) Hairsprings: These are special spiral springs which

differ from power springs on two counts (i) the number of coil is large and space in-between in small, (ii) the spring is small.

(d) Constant Force Springs: These are spiral springs, so

wound that they provide constant force caused while unwinding. These are also called negators.

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3.5.3 Mechanical Arming Devices:

ii) Sliders: Many fuze components such as

interrupters and lock pins are basically sliders,

which are moved by spring forces or inertia forces (setback, creep, centrifugal etc.). Sliders are usually held in their initial position using springs. Slider

may move along the axis the of the fuze or perpendicular to it or at an angle to it.

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3.5.3 Mechanical Arming Devices:

• _{When the slider is to move along the axis of the} fuze, it is held in initial position by a spring force and due to setback force experienced at launch, the slider moves within the ammunition against the spring force in accordance with the Newton’s law of conservation of momentum. When the

slider is to move at an angle to or perpendicular to the axis of the fuze, it is held in initial position by lock pins and is driven either by spring force or by centrifugal force.

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3.5.3 Mechanical Arming Devices:

iii) Rotary Devices: Rotary devices are pivoted so that they can turn through a specified angle only and the rotation may be caused by centrifugal forces, air stream effects or unwinding springs. These devices follow the general principle that the rotor turns until the moment of inertia of the rotor with respect to the ammunition spin axis is a maximum. Some of these devices are as follows

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a) Disk Rotor: It rotates in a plane perpendicular to

the axis of the fuze to align the firing pin and the detonator. (Figure 3.9)

Figure 3.9 (a) Fuze in Safe state

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3.5.3 Mechanical Arming Devices:

• _{Centrifugal Pendulum: It is a bar pivoted at its} center and rotates in a plane perpendicular to the axis of the fuze.

• Simple Plunger: This device operates by

centrifugal forces and due to its asymmetry about the pivot, it releases with a preferred orientation to cause arming

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3.5.3 Mechanical Arming Devices:

d) Sequential Arming Segment: This device

consists of series of pivoted segments held in

position by springs. When a sustained acceleration occurs, each segment rotates through an angle

causing the release of next segment and the rotation of the last segment disengages a spring held rotor to cause the arming action in the fuze

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3.5.3 Mechanical Arming Devices:

e) Rotary Shutter: It is an

unsymmetrical disc pivoted at the center of the

semicircular part and rotates due to centrifugal forces. Rotation of disc locates the hole such that the firing pin aligns with the detonator through the hole. An

example of such shutter is shown in Figure 3.10.

Figure 3.10 (a)

Fuze in Safe state

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3.5.3 Mechanical Arming Devices:

f) Ball Cam Rotor: It has a stationary part with a slot, a rotor with a

spirally cut slot and a ball. The ball engages the rotor with the stationary part and due to the centrifugal forces, the rotor moves with a fixed angular velocity. Thus, this mechanism can be used to introduce mechanical time delays in the fuze.

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3.5.3 Mechanical Arming Devices:

g) Ball Rotor: It is a ball having a detonator cavity,

initially held in unarmed state by detents such that the firing pin and detonator cavity are misaligned. The rotation of the ball against centrifugal forces causes the alignment of firing pin and detonator cavity and hence the fuze is armed.

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3.5.3 Mechanical Arming Devices:

iv) Clockworks: Clockworks are mechanisms used

to establish mechanical time delays in the fuzes. Clockworks have many parts but their principle

parts are escapements and gear trains. Escapements are the regulators of the mechanical time fuzes while the gear trains are their transducers. It consists of a toothed wheel actuated by applied torque, a pallet with two teeth and a spring mass mechanism

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3.5.3 Mechanical Arming Devices:

• _{When the escape wheel turns, one pallet tooth is} pushed along the escape wheel tooth and the other pallet engages the escape wheel. The same process is repeated every time the escape wheel turns and thus it acts as an oscillating system, which can be used as time counter. Clockwork mechanism can be used to arm the fuze only after it had travel safe distance from the launcher and for creating

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3.5.3 Mechanical Arming Devices:

v)Minor Mechanical Devices: Various minor

mechanical devices such as pins, detents, links, knobs, levers, pivots etc serves various important purposes in the fuze. Pins are used for locking

purposes and can be broken or moved for unlocking. Links are not desirable in the fuzes as they require space to operate but some links may be incorporated in fuzes to transmit motion from one part to another.

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3.5.3 Mechanical Arming Devices:

• Detents are short rods whose purpose is to restrict motion of other member by exerting their shear strength. Knobs are used to select or set fuze

functions. Levers are used to restrict the motion of another part by a locking action. Spiral unwinder

system provides arming delays in fuzes due to effect of projectile spin.

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3.6 Features of Mechanical Fuzes

:

The main features of mechanical fuzes are as follows. 2 Mechanical fuzes are simple in construction. Most

of the components are simple mechanical links and can be easily designed and manufactured.

3 Mechanical fuzes can reliably operated in electro-magnetic environment, where electronic fuze may malfunction due to electromagnetic interference (EMI).

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3.7 Limitations with Mechanical Fuzes

• _{Mechanical fuzes have some limitations associated} with it, which are discussed hereunder

1 Mechanical fuze cannot function as proximity

fuzes as there is no target sensing mechanism to sense a target at a distance and initiates the fuze.

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3.7 Limitations with Mechanical Fuzes

1 Mechanical fuze can be designed as time fuze, but the number of time interval options possible is limited. Besides, timing mechanisms are often of complicated

design and are therefore undesirable for mass production. Time mechanisms require manual setting, which is

undesirable for modern high firing rate weapons. Manual setting also introduces element of human error during

setting and reduces reliability of the fuze

2 Miniaturization is possible in mechanical fuze only to a limited extend, whereas the electronic fuzes can be

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3.8 Closure

• Mechanical fuze were the first fuze developed mainly due to their simple construction and functioning. They have been serving their purpose effectively and efficiently for last

century. However, the new requirements of modern warfare require intelligent fuzes, which can have more than one

mode of operation. Electronic fuzes are the present

generation fuzes that have replaced mechanical fuzes from many applications especially from time fuzes. These are discussed in detail in the next chapter

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CHAPTER 4 ELECTRONIC FUZES

4.1 Introduction

• Basic functions of fuze are safing, arming, target sensing and firing. Mechanical Fuzes achieve these objectives by less accurate and unreliable

mechanisms whereas Electronic fuze achieves these functions by means of electronic circuits or with a combination of both electronic circuits and

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4.1 Introduction

• In particular, safing and arming functions are

combinely achieved by electronic and mechanical mechanisms. The target sensing and firing are

achieved by electronic circuits. In this chapter, the advantages of electronic fuzes, basic elements of electronic fuzes and basics of different types of electronic fuzes are discussed.

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4.2 Advantages of electronic fuzes

• Necessity of electronic fuze basically arises from the limitations of other existing fuze systems. The

conventional mechanical fuzes contain all the

necessary systems for performing their function but the intelligence integrated in the fuze is absent.

• Integration of electronics into fuze adds the required intelligence to the system and makes it more

accomplished to do its job. Advantages of electronic fuzes over conventional mechanical fuzes are

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4.2 Advantages of electronic fuzes

i) Proximity fuze action is possible only with

electronic fuze. A great practical advantage of this type of fuze over conventional time fuze is that it relieves the gunner of the responsibility of fuze setting.

• _{He has to only ensure that ammunition passes}

within lethal range to the target and the fuze does the rest.