# Fault Tree Analysis (FTA): Concepts and Applications

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1 Fault Tree Analysis (FTA): Concepts and Applications Bill Vesely NASA HQ

2 Inductive and Deductive Modeling are the Two Basic Types of Modeling Inductive models forwardly induce the consequences of an event. Induce Forwards Event Forward Looking Logic Consequences Deductive models backwardly deduce the causes of an event. Deduce Backwards Event Backward Looking Logic Causes 2

3 An Inductive Model Defines Scenarios for an Initiating Event An initiating event is first defined which can have undesired consequences. Subsequent events are identified which define possible progressions of the initiating event. Possible realizations of the subsequent events are defined and linked to model scenarios. The consequence of each scenario is described. 3

4 A Deductive Model Resolves the Causes for an Event An event is first defined for which causes are to be resolved. The event is resolved into its immediate and necessary sufficient causal events. The event is related to the causal events using appropriate logic. This stepwise resolution of events into immediate causal events proceeds until basic causes (primary causes) are identified. 3

5 Fault Tree Analysis: a Systematic and Stylized Deductive Process An undesired event is defined The event is resolved into its immediate causes This resolution of events continues until basic causes are identified A logical diagram called a fault tree is constructed showing the logical event relationships 4

6 Benefits of Constructing a Fault Tree The fault tree explicitly shows all the different relationships that are necessary to result in the top event In constructing the fault tree, a thorough understanding is obtained of the logic and basic causes leading to the top event The fault tree is a tangible record of the systematic analysis of the logic and basic causes leading to the top event The fault tree provides a framework for thorough qualitative and quantitative evaluation of the top event 5

7 Elements of Fault Tree Analysis (FTA) FTA is a deductive analysis approach for resolving an undesired event into its causes FTA is a backward looking analysis, looking backward at the causes of a given event Specific stepwise logic is used in the process Specific logic symbols are used to to illustrate the event relationships A logic diagram is constructed showing the event relationships. 6

8 Why FTA is carried out To exhaustively identify the causes of a failure To identify weaknesses in a system To assess a proposed design for its reliability or safety To identify effects of human errors To prioritize contributors to failure To identify effective upgrades to a system To quantify the failure probability and contributors To optimize tests and maintenances 7

9 Role of FTA in System Safety Analysis FTA is used to resolve the causes of system failure FTA is used to quantify system failure probability FTA is used to evaluate potential upgrades to a system FTA is used to optimize resources in assuring system safety FTA is used to resolve causes of an incident FTA is used to model system failures in risk assessments 8

10 Role of FTA in PRA Mission Success Starts With Safety A Probabilistic Risk Assessment (PRA) models event scenarios An event scenario consists of an initiating event and subsequent system failures FTA is carried out to model the causes of the system failures Using data on the probability of the causes, the probability of system failure is determined The probability of the accident scenario is thereby determined 9

11 The Thought Process in FTA FTA is backward looking The end result is the analysis starting point The end result is then traced back one step at a time to its immediate causes The relationships of the causes, or events, are shown with logic symbols This backward tracing process continues until the basic causes are identified FTA systematizes and codifies the process 10

12 Comparison of FTA with Other Approaches FTA is not a Fishbone analysis which is a more informal depiction of event causes (informal deductive) FTA is not an FMEA which assesses different effects of single basic causes (inductive) FTA is not Event Tree Analysis which assesses the consequences of given initiating events (inductive) FTA is a formal approach for resolving the basic causes of a given undesired event (formal deductive) 11

13 FTA Operates in Failure Space Mission Success Starts With Safety Designers design for success Safety analysts analyze for failure There can be various degrees of success Thresholds for failure are identifiable Failure events can be more readily discretized Failure quantifications are simpler The failure mindset probes for weaknesses and gaps 12

14 Success Space Versus Failure Space 13

15 Different Failure and Success States for a Trip COMPLETE FAILURE MAXIMUM TOLERABLE FAILURE ACCIDENT (DEATH OR CRIPPLING INJURY) ACCIDENT (CAR DAMAGED; NO PERSONAL INJURY) MINOR ACCIDENT FLAT TIRE WINDSHIELD WIPERS INOPERATIVE (HEAVY RAIN) TRAFFIC JAM MAXIMUM ANTICIPATED FAILURE ARRIVES AT 9:00 WINDSHIELD WIPERS INOPERATIVE (LIGHT RAIN) TRAFFIC CONGESTION MINIMUM ANTICIPATED FAILURE ARRIVES AT 8:45 LOST HUBCAP WINDSHIELD WIPERS INOPERATIVE (CLEAR WEATHER) TOTAL SUCCESS ARRIVES AT 8:30 (NO DIFFICULTIES WHATSOEVER 14

16 A Fault Tree Models Failure Modes A failure mode is the failure state of the system or component Examples of failure modes are fail to start, fail to open, fail to shutdown In contrast, failure mechanisms are the processes by which failures occur Examples of failure mechanisms are corrosion, overpressure, and fatigue A failure mechanism is only included in the failure mode definition when detailed mechanisms are modeled 15

17 Illustration of Failure Mode Versus Failure Mechanism Description of Event System Subsystem Valve Actuator No flow from subsystem when required Mechanism Mode Effect Valve unable to open Mechanism Mode Effect Binding of actuator stem Mechanism Mode Corrosion of actuator stem Mechanism 16

18 Door Bell Example Differentiating Failure Modes and Failure Mechanisms 17

19 Failure Modes and Mechanisms of the Door Bell System Failure Effect Failure Mode Mechanism Switch fails to Contacts broken Mechanical shock make contact High contact resistance Corrosion Bell-solenoid unit fails to ring Low voltage from battery Clapper broken or not attached Clapper stuck Solenoid link broken or stuck Insufficient magneto-motive force No electrolyte Positive pole broken Shock Corrosion Open circuit in solenoid Short circuit in solenoid Leak in casing Shock 18

20 Failure Mechanisms and Failure Causes In some areas, failure mechanism and failure cause are differentiated A failure cause is defined as the initiator of a failure (example: valve fails to open because of stuck operator) A failure mechanism is defined as the process by which the failure occurs (e.g. a valve fails to open because of a stuck operator due to corrosion buildup) In FTA, what is important is that the failure mode be precisely define which is What and When describing the fault or failure 19

21 Review Questions Mission Success Starts With Safety 1. When should inductive modeling be considered? 2. When should deductive modeling be considered? 3. What are the advantages of working in failure space? Could we develop success-based models? 4. What characterizes FTA as a distinct, deductive modeling approach? 5. Can failure modes, failure mechanisms, and failure causes be defined at different levels? 6. Consider the Main Engine of the Space Shuttle. What are possible failure modes, failure causes and failure mechanisms? 20

22 The Fault Tree FTA produces a Fault Tree The fault tree is the logical model of the relationship of the undesired event to more basic events. The top event of the fault tree is the undesired event. The middle events are intermediate events. The bottom of the fault tree is the causal basic events or primary events. The logical relationships of the events are shown by logical symbols or gates. 21

23 Basic Fault Tree Structure Top Undesired Event Logic Gates Intermediate Events Basic Events 22

24 The Four Necessary Steps to Begin a Fault Tree 1. Define the undesired event to be analyzed (the focus of the FTA) 2. Define the boundary of the system (the scope of the FTA) 3. Define the basic causal events to be considered (the resolution of the FTA) 4. Define the initial state of the system 23

25 Illustration of the Steps of a FTA Define FTA Scope Identify FTA Objective Define FT Top Event Define FTA Resolution Construct FT Evaluate FT Interpret/ Present Results Define FTA Ground Rules 24

26 Basic Events of a Fault Tree Top Event or Intermediate Event Undeveloped Event Basic Event 25

27 Basic Gates of a Fault Tree OR gate- the above output event occurs if either of the input lower level events occur AND gate- the above output event occurs if all of the input lower level events occur TRANSFER gate transfer to/from another part of the fault tree 26

28 Simple Battery Powered Circuit (BPC) Switch Motor Battery 27

29 Specifications for the BPC FT Mission Success Starts With Safety Undesired top event: Motor does not start when switch is closed Boundary of the FT: The circuit containing the motor, battery, and switch Resolution of the FT: The basic components in the circuit excluding the wiring Initial State of System: Switch open, normal operating conditions 28

30 Start of BPC FT (1) Mission Success Starts With Safety Motor does not start when switch closed OR Motor Fails to Start When EMF applied No EMF applied to Motor OR Wire from Battery to Motor fails open No EMF from Battery 29 A

31 Continuation of the BPC FT (2) A No EMF from Battery OR Battery Fails to produce EMF No EMF to Battery OR Wire from Switch to Battery fails open No EMF across Switch 30 B

32 Continuation of the BPC FT (3) B No EMF across Switch OR Switch fails to contact Wire from Switch to Motor fails open 31

33 Fault Tree Exercise: Fuel Supply System Top Event: No Fuel to Motor When Requested BLOCK VALVE A CONTROL VALVE A BVA CVA FUEL SUPPLY MOTOR BVB CVB BLOCK VALVE B CONTROL VALVE B Control valves: initially closed, opened manually Block valves normally open 32

34 Symbols Used in FTA Software Programs Intermediate Event (Gate) gate logic type gate description gate identifier Primary Event (Basic Cause) event description event identifier primary event identifier 33

35 Fault Tree Software Representations D Fails Top event Logic Gate G1 A Fails B OR C Fail Intermediate event A G2 B Fails C Fails Gate or Event Identifier B C Basic events 34

36 OVERRUN OF ANY MOTOR AFTER TEST IS INITIATIED Mission Success Starts With Safety G019 EMF APPLIED TO MOTOR 1 FOR t>60 SEC EMF APPLIED TO MOTOR 2 FOR t>60 SEC EMF APPLIED TO MOTOR 3 FOR T>60 SEC G020 G021 G022 EMF REMAINS ON K5 COIL FOR T>60 SEC G025 KS RELAY CONTACTS REMAIN CLOSED FOR T>60 SEC G023 KS RELAY CONTACTS FAIL TO OPEN B026 K2 RELAY CONTACTS FAIL TO OPEN B028 K2 RELAY CONTACT FAILS TO OPEN WHEN K5 RELAY CONTACTS CLOSED FOR T>60 SEC G024 EMF NOT REMOVED FROM K2 RELAY COIL WHEN K5 CONTACTS CLOSED FOR t>60 SEC G030 A Typical Fault Tree K3 RELAY CONTACTS REMAIN CLOSED FOR T>60 SEC K1 RELAY CONTACTS FAIL TO OPEN WHEN K3 CONTACTS CLOSED FOR t>60 SEC K1 RELAY CONTACTS FAIL TO OPEN WHEN K5 CONTACTS CLOSED FOR t>60 SEC EMF TO K2 COIL THRU S1, KT1, KT2 AND KT3 CONTACTS G027 G028 G031 B032 TEST SIGNAL REMAINS ON K3 COIL FOR t>60 SEC K5 RELAY CONTACTS FAIL TO OPEN EMF TO K1 COIL THRU TIMER CIRCUIT WHEN K5 CONTACTS CLOSED FOR t>60 SEC EMF TO K1 COIL THRU S1 CONTACTS WHEN K5 CONTACTS CLOSED FOR t>60 SEC B042 B043 G097 G098 EMF NOT REMOVED FROM K1 RELAY COIL WHEN K5 CONTACTS CLOSED FOR t>60 SEC G048 S1 SWITCH INASDVERTENTLY CLOSES OR FAILS TO OPEN B100 RESET SIGNAL INADVERTENTLY APPLIED OR NOT REMOVED FROM SWITCH S1 B101 KT1 TIMER RESET B050 KT2 TIMER CONTACTS FAIL TO OPEN WHEN K5 CONTACTS CLOSED FOR t>60 SEC G090 KT3 TIMER RESET B075 KT2 TIMER CONTACTS FAIL TO OPEN B095 KT2 TIMER DOES NOT "TIME OUT" DUE TO IMPROPER INSTALLATION OR SETTING B096 35

37 The Top Event of the Fault Tree Mission Success Starts With Safety The top event should describe WHAT the event is and WHEN it happens The top event is often a system failure but can be any other event The top event is the specific event to be resolved into its basic causes Defining the wrong top event will result in wrong assessments and conclusions 36

38 Examples of Top Event Definitions Fire Suppression System Fails to Operate when actuated Fire Suppression System Inadvertently Activates during normal conditions Auxiliary Power System Fails to Continually Operate for the required time period Fuel Supply System Fails to Shutoff after the fueling phase Launch Vehicle Fails to Ignite at Launch Launch Vehicle Suffers a Catastrophic Failure at Launch 37

39 The OR Gate Mission Success Starts With Safety The OR Gate represents the logical union of the inputs: the output occurs if any of the inputs occur The OR gate is used when an event is resolved into more specific causes or scenarios The OR gate is used when a component failure is resolved into an inherent failure or a command failure The OR gate is used when an event is described in terms of equivalent, more specific events 38

40 An OR Gate Resolving A Component Failure into Specific Failures VALVE IS FAILED CLOSED G001 VALVE IS CLOSED DU E TO H ARDWARE FAILU RE VALVE IS CLOSED DU E TO TESTING B001 B002 VALVE IS CLOSED DU E TO HUMAN ERROR B003 39

41 The AND Gate Mission Success Starts With Safety The AND Gate represents the logical intersection of the inputs: the output occurs if all of the inputs occur The OR gate is used when an event is resolved into combinations of events that need to occur The AND gate is used when a redundant system is resolved into multiple subsystems that need to fail The AND gate is used when a system failure is resolved into conditions and events needed to occur 40

42 AND Gate for a Redundant Power Supply P O W E R U N A V A I L A B L E T O D C B U S G F U E L C E L L 1 I S F A I L E D B A T T E R Y IS F A IL E D B B F U E L C E L L 2 IS F A I L E D B

43 Summary of OR and AND Gates Mission Success Starts With Safety OR Gate (Logical Plus Gate) IN PU T A OUTPUT Q G001 + IN PU T B B001 B002 AND Gate (Multiplication Gate) INPUT A OUTPUT Q G001 INPUT B B001 B002 42

44 Linking OR and AND Gates Mission Success Starts With Safety Q OCCURS G001 A OCCURS AND THEN B OCCURS B OCCURS AND THEN A OCCURS G002 G003 A OCCURS B OCCURS GIVEN THE OCCURRENCE OF A B OCCURS A OCCURS GIVEN THE OCCURRENCE OF B B001 B002 B003 B004 43

45 Terminating Events in a Fault Tree The terminating events of a fault tree identify where the FTA stops Two fundamental terminating events are the basic event and the undeveloped event The basic event represents the lowest level event (cause) resolved in the fault tree The undeveloped event represents an event which is not further developed for causes 44

46 Expanded Types of Terminating Events Basic Causal Event- treated as a primary cause with no further resolution Condition Event- defines a condition which needs to exist Undeveloped Event- not further developed House Event- an event expected to occur. Sometimes used as a switch of True or False Transfer Symbol- transfer out of a gate or into a gate 45

47 Extended Gate Symbols Mission Success Starts With Safety PRIMARY EVENT SYMBOLS BASIC EVENT - A basic initiating fault requiring no further development CO NDITIO NING EVENT - Specific conditions or restrictions that apply to any logic gate (used prim arily with PRIO RITY AND and INHIBIT gates) UNDEVELO PED EVENT - An event which is not further developed either because it is of insufficient consequence or because inform ation is unavailable HO USE EVENT - An event which is norm ally expected to occur GATE SYMBOLS AND - O utput fault occurs if all of the input faults occur O R - O utput fault occurs if a least one of the input faults occurs n C O M B IN A T IO N - O u tp u t fa u lt o c cu rs if n o f th e in p u t fa u lts o cc u r EXCLUSIVE O R - O utput fault occurs if exactly one of the input faults occurs PRIO RITY AND - O utput fault occurs if all of the input faults occur in a specific sequence (the sequence is represented by a CO NDITIO NING EVENT drawn to the right of the gate) INHIBIT - O utput fault occurs if the (single) input fault occurs in the presence of an enabling condition (the enabling condition is represented by a CO NDTIO NING EVENT drawn to the right of the gate) TRANSFER SYMBOLS TRANSFER IN - Indicates that the tree is developed further at the occurrence of the corresponding TRANSFER O UT (e.g., on another page) TRANSFER O UT - Indicates that this portion of the tree m ust be attached at the corresponding TRANSFER IN 46

48 Illustration of the Inhibit Gate Mission Success Starts With Safety O-RING FAILURE T < T(critical) G002 B003 EXISTENCE OF TEM PERATURE T B004 47

49 Transfer Gates Mission Success Starts With Safety TRANSFER IN TRANSFER OUT 48

50 Review Questions Mission Success Starts With Safety 1. What is a FT constructed as part of the resolution process? 2. What is the basic paradigm of FTA? 3. Can the top event be a system success? 4. Can any relation be expressed by AND and OR gates? 5. Can the FT be terminated at events more general than basic component failures? 6. Can a FT be developed to a level below a basic component level, e.g. to a piecepart level? 7. Can an intermediate or basic event in the fault tree consist of non-failure of a component? 49

51 Developing the Fault Tree Mission Success Starts With Safety 1. Define the top event as a rectangle 2. Determine the immediate necessary and sufficient events which result in the top event 3. Draw the appropriate gate to describe the logic for the intermediate events resulting in the top event 4. Treat each intermediate event as an intermediate level top event 5. Determine the immediate, necessary and sufficient causes for each intermediate event 6. Determine the appropriate gate and continue the process 50

52 Advise in Developing the Fault Tree The system being analyzed for the undesired event needs to be studied and understood before the fault tree is constructed If an electrical or hydraulic system is being analyzed, the fault tree is constructed by tracing the causes upstream in the circuit to the basic causes For a generalized network or flow, the fault tree is similarly constructed by upstream tracing of the causes 51

53 Remember the Four Key Attributes of a Fault Tree Top Event- What specific event is being analyzed? Boundary- What is inside and outside the analysis? Resolution- What are the primary causes to be resolved to? Initial State- What is assumed for the initial conditions and states? 52

54 Defining the Boundary and Resolution of the Fault Tree The boundary defines what is inside the analysis and what is outside the analysis The resolution defines the basic causes to be resolved The boundary defines the interfaces to be included or excluded The resolution defines what types of events are modeled 53

55 Examples of Boundary Definitions All components shown in a system schematic with detailed system specifications All major systems identified to comprise an enterprise with detailed system descriptions and their interfaces The individual steps defined in a process with the detailed process description The individual processes involved in transforming given inputs into a finished product with detailed descriptions A software description including coding, flow charts, and detailed descriptions 54

56 Examples of Resolution Definitions Resolve basic causes to major components in the system with descriptions of the the included components Resolve basic causes to individual tasks in a process with specific listing of the tasks to be included Resolve basic causes to major system components, including interfaces among the systems, with detailed descriptions of the components and interfaces Resolve the basic causes of software failure to the individual statements in the software program Resolve basic causes to major components in the system but do not include interfaces to the system 55

57 The Initial State for the Fault Tree The initial state for the FTA defines the initial states of components, initial conditions, and initial inputs assumed The initial states for the components involve what components are assumed to be initially operational The initial state can also involve the past history description of the component Initial conditions include assumed environments and operational conditions Initial inputs include assumed initial commands, assumed failures existing, and assumed events that have occurred 56

58 A Fault Tree Distinguishes Faults Versus Failures The intermediate events in a fault tree are called faults The basic events, or primary events, are called failures if they represent failures of components It is important is to clearly define each event as a fault or failure so it can be further resolved or be identified as a basic cause Write the statements that are entered in the event boxes as faults; state precisely what the fault is and the conditions under which it occurs. Do not mix successes with faults. 57

59 A Fault Tree Distinguishes a Component Fault From System Fault For each event, ask the question whether the fault is a state of component fault or a state of system fault. The answer determines the type of gate to construct If the answer to the question, Is this fault a component failure? is Yes, classify the event as a state of component fault. If the answer is No, classify the event as a state of system fault. 58

60 Component Fault Versus System Fault (Continued) For a state of component fault the component has received the proper command For a state of system fault the proper command may have not been received or an improper command may have been received The event description needs to clearly define the conditions to differentiate these different faults 59

61 Gates for Component Versus System Faults For a state of component fault use an OR gate if the fault is not a failure (basic event) For a state of system fault the gate depends on the event description If the fault event is classified as state of component, add an OR-gate below the event and look for primary, secondary and command failure modes. If the fault event is classified as state of system, look for the minimum necessary and sufficient immediate cause or causes. A state of system fault event may require an AND-gate, an OR-gate, an INHIBIT-gate, or possibly no gate at all. As a general rule, when energy originates from a point outside the component, the event may be classified as state of system. 60

62 Example of Component Versus System Faults FAULT Switch fails to close when thumb pressure is applied. Switch inadvertently opens when thumb pressure is applied Motor fails to start when power is applied to its terminals. Motor ceases to run with power applied to terminals FAULT Switch inadvertently closes with no thumb pressure applied. Motor inadvertently starts. OPERATING STATE STANDBY STATE CLASSIFICATION State of component State of component State of component State of component CLASSIFICATION State of component State of system

63 Primary Failure Versus Secondary Failure A failure can be further resolved into a primary failure OR secondary failure A primary failure is a failure within design environments A secondary failure is a failure outside design environments Usually secondary failures are not included unless abnormal conditions are modeled If secondary failures are included then the secondary failure is resolved into the abnormal condition existing AND the failure occurring 62

64 A Primary-Secondary Failure Gate Primary failure under normal environment Secondary failure under abnormal environment 63 Abnormal condition exists

65 Secondary Failure Modeling Guidelines Include a secondary failure when an abnormal environment is of specific focus Include a secondary failure when an abnormal environment can have a non-negligible probability of existing Otherwise, as a general rule, do not include secondary failures in the fault tree since they can greatly compound the complexity of the fault tree 64

66 The No Miracle Rule Mission Success Starts With Safety Do not assume abnormal conditions will occur to prevent a fault from propagating In particular, do not assume a failure of another component will occur to prevent a fault from propagating If the norma l functioning of a component propagates a fault sequence, then it is assumed that the component functions normally. 65

67 Naming Schemes For the Fault Tree Each Gate and Event on the Fault Tree needs to be named The Name should ideally identify the Event Fault and the What and When Conditions Software packages have default names that can be used but are not descriptive Basic events should in particular be named to identify the failure mode What is important is that the same event be given the same name if it appears at different locations 66

68 Example of Simple Naming Scheme Component Type Component Failure Mode Description HX F Heat Exchanger Cooling Capability Fails HX J Heat Exchanger Tube Rupture HX P Heat Exchanger Plugs IN F Inverter No Output IR F Regulating Rectifier No Output IV F Static Voltage Regulator No Output LC D Logic Circuit Fails to Generate Signal LS D Level Switch Fails to Respond LS H Level Switch Fails High LS L Level Switch Fails Low 67

69 More Complex Naming Schemes Mission Success Starts With Safety S u b system C o m P R A F a ilu re D a ta C o m p o n e n t p o n e n t ID T y p e fa ilu re M o d e LH2 A_O_LH2_DISCVL_FTCM A_O_LH2_DISCVL_FTCE A_O_LO2_DISCVL_FTCE Valve, 17" Disconnect fails to close LDS E_O_LDS_ACTLUL_JAM E_O_LDS_ACTRUL_JAM E_O_LDS_ACTNUL_JAM Actuator, hyd uplock jams 68

70 Advise in Defining Ground Rules for an FTA 1. For FT quantification, model to the highest level for which data exists and for which there are no common hardware interfaces 2. Do not generally model wire faults because of their low failure rates 3. Do not generally model piping faults because of their low failure rates 4. Do not further develop an AND gate with three independent inputs if there are lower order contributing combinations 5. Do not further develop an event to an OR gate if there are higher probability input events 69

71 The Fault Tree Versus the Ishikawa Fishbone A fault tree is sometimes erroneously thought to be an example of an Ishikawa Fishbone Model The fishbone is a loosely-structured, brain-storming tool for listing potential causes of an undesired event Fault tree analysis is a stepwise formal process for resolving an undesired event into its immediate causes The fault tree displays the stepwise cause resolution using formal logic symbols 70

72 The Ishikawa Fishbone Diagram Personnel Material Machines Attribute Fault Attribute Fault Undesired Event Attribute Fault Attribute Fault Methodology Environment Management 71

73 Review Questions Mission Success Starts With Safety 1. What is the basic paradigm of FTA? 2. How is FTA different from a Fishbone Model? 3. Can all relations be expressed by AND and OR gates? 4. What are the four key attributes of an FTA? 5. What is the difference between a fault and a failure as defined in FTA? Is this distinction used in other areas? 6. How is a state of component fault modeled? 7. Why can t there be more definite rules for modeling a state of system fault? 72

74 Mono-propellant Propulsion System A mono-propellant propulsion system provides an example for FTA The system is pressure fed and provides thrust for a vehicle while in orbit Additional support systems are not considered Different fault trees can be constructed depending on the failure to be modeled 73

75 Defining the FT Key Attributes for the Monopropellant System Fault Tree Top Event Defined based on the specific system failure mode to be analyzed. Boundary Extracted from the system logic diagrams. Resolution Include the major components in the system diagram. Do not include wiring faults. Initial State Dependent on the system failure mode to be analyzed. 74

76 System Schematic and Boundaries S1 K2 K1 TK1 Inert gas p ressurizatio n tank R elief valve RV1 Inert gas iso latio n valve IV 1 K3 Inert gas pressure reg ulator R G 1 S3 Inert gas check valve CV1 P ro p ellant tank with bladder PT1 R elief valve RV2 Relief valve RV3 Thruster isolation valve IV 2 S2 Timer relay K6 R elief valve RV4 K4 K5 T hrust cham b er inlet valve IV 3 Cataly st Monopropellant Propulsion System 75

77 System Components for the FTA TK1 Propellant Storage Tank PT1- Propellant Tank 1 RV1 Relief Valve 1 K1 Arming Relay K1 RV2 Relief Valve 2 K2 Firing Protection Relay RV3 Relief Valve 3 K3 Arming Relay RV4 Relief Valve 4 K4 Firing Relay IV1 Isolation Valve 1 K5 Firing Relay IV2 Isolation Valve 2 K6 Timing Relay IV3 Isolation Valve 3 S1 Arming Switch RG1 Regulator 1 S2 Firing Switch CV1 Check Valve 1 S3 Emergency Cutoff Switch 76

78 System Description: Basic Operation The system consists of a reservoir TK1 of inert gas that is fed through an isolation valve IV1 to a pressure regulator RG1. The pressure regulator RG1 senses pressure downstream and opens or closes to control the pressure at a constant level. A check valve, CV1 allows passage of the inert gas to the Propellant Tank PT1. Separating the inert gas from the propellant is a bladder that collapses as propellant is depleted. Propellant is forced through a feed line to the Thruster Isolation Valve IV2 and then to the Thrust Chamber Inlet Valve IV3. For the Thruster to fire, the system must first be armed, by opening IV1 and IV2. After the system is armed, a command is sent to IV3, to open, allowing H 2 O 2 into the thrust chamber. As the propellant passes over the catalyst, it decomposes producing the byproducts and heat and the expanding gas that creates the thrust. The relief valves RV1-4 are available to dump propellant overboard should an overpressure condition occur in any part of the system. 77

79 System Description: Arming and Thrust The electrical command system controls the arming and thrusting of the propellant system. To arm, switch S1 is momentarily depressed, allowing electromotive force (emf) to activate relay switches K1, K2 and K3, and open valves IV1 and IV2. K1 closes and sustains the emf through the arming circuit. K2 momentarily opens to preclude the inadvertent firing of the system during the transition to the armed mode, and closes when S1 is released. K3 closes in the firing circuit. The system is now armed with power supplied to sustain IV1 and IV2 in the open position. When firing switch S2 is momentarily depressed, K4 closes sustaining the firing circuit. K5 closes completing the circuit for K6, which begins timing to a predetermined time for the thruster to fire. The completed circuit opens IV3 and thrusting begins. 78

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