Reliability Module V1

7/23/2019 Reliability Module V1

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Space Systems Engineering: Reliability Module

Reliability Module

Space Systems Engineering, version 1.0




Module Purpose: Reliability

♦ To understand the importance of reliability as a

engineering discipline within systems engineering,particularly in the aerospace industry.

♦ To understand key reliability concepts, such as

constant failure rate, mean-time-between failure, and

“bathtub” curve.

♦ To introduce different forms of system redundancy,

including fault tolerance, functional redundancy, and

fault avoidance.

♦ Review ways to calculate reliability and the use of

block diagrams.




“It appears incontrovertible tat understanding

!ailure plays a "ey role in error#!ree design o! all

"inds, and tat indeed all success!ul design is te

proper and complete anticipation o! $at can go

$rong.%

Henry etroski

!esign aradigms

"ase Histories of #rror and $udgment in #ngineering



Space Systems Engineering: Reliability Module %

Ris" Pilosopy & ' (ey )esign )river

• &ome e'pressions you will hear in

the aerospace community() Reliability of *.+++

) o single point failure mode design

) &ingle thread design

) ust not fail

) /raceful degradation is 01

) 2ully redundant system

) "ritical function redundancy only

) 2aster, better, cheaper

• 3hat do they mean4



Space Systems Engineering: Reliability Module 5

Reliability )e!initions

♦ Reliability is the probability that the system-of-interest will not failfor a given period of time under specified operating conditions.

• Reliability is an inherent system design characteristic.• Reliability plays a key role in determining the system6s cost-

effectiveness.

• Reference( 7&7 &ystems #ngineering Handbook definition 89++5version:

♦ Reliability engineering is a specialty discipline within the systemsengineering process. Reflected in key activities(• Design - including design features that ensure the system can

perform in the predicted physical environment throughout themission.

• Trade studies - reliability as a figure of merit. 0ften traded with cost.

•Modeling - reliability prediction models, reflecting environmentalconsiderations and applicable e'perience from previous pro;ects.

• Test - making independent predictions of system reliability for testplanning<program= sets environmental test re>uirements andspecifications for hardware >ualification.



Space Systems Engineering: Reliability Module ?

Reliability Relationsips

ame &ymbol athematical Relationships 2ailures as random

Ha@ard Rate0r

2ailure Rate

λ8t: A -89<R: dR<dt A f8t: < 89 - 28t:: A λ

Reliability R8t:A ∫

t f8λ:d λ

A 9 - 28t:A e8- λt: A e8- t<TB2:

"umulative

2ailure

robability

28t:A ∫ * f8λ:d λ

A 9 - R8t:

2ailurerobability

!ensity

f8t: A - dR8t:<dt A λ8t:R8t:

t

For systems that must operate continuously, it is common to express their reliability in

terms of the Mean Time Between Failure (MTBF), where MTBF = 1/


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*onstant +ailure Rate!ource" Blanchar# an# Fabryc$y, !ystems %n&ineerin& an# 'nalysis, rentice all, 1**+

onstant Failure Rate") robability !istribution of reliability is an e'ponential function.) 7lthough an individual component may not have an e'p reliability distribution, in

a comple' system with many components the overall reliability may appear as a

series of random events and the system will follow an e'ponential reliabilitydistribution.


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e “-attub% +ailure Rate *urve

Burn-in or

#ebu&&in&

perio#

.seful life perio#l# a&e

perio#

(or cycles)

Because of burn-in failures an#/or ina#e0uate 0uality assurance practices, the failure

rate is initially hi&h, but &ra#ually #ecreases #urin& the infant perio# urin& the useful

life perio#, the failure rate remains constant, reflectin& ran#omly occurrin& failures

2ater, the failure rate be&ins to increase because of wear-out failures


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Redundancy

2ault Tolerance

♦ 2ault tolerance is a system design characteristic associatedwith the ability of a system to continue operating after acomponent failure has occurred.

♦ Dt is implemented by having design redundancy and a faultdetection response capability.

♦ !esign redundancy can take several forms( parallel, stand-by,

and cross-strapped 8see upcoming block diagram slide:.

2unctional Redundancy

♦ 2unctional redundancy is a system design and operationscharacteristic that allows the system to respond to component

failures in a way sufficient to meet mission re>uirements.♦ This usually involves operational work-arounds and the use of

components in ways that were not originally intended.• /alileo high-gain antenna e'ample

• 7pollo 9E e'ample


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ays to 'cieve Reliability in Space System

7lso known as “2ault 7voidance”

♦ rovide ample environmental and design margins, or useappropriate de-rating guidelines.

♦ Fse high->uality, carefully selected, screened parts whereneeded.

• Reliability for "lass & 8space >ualified: parts are typically 9* times

that of good commercial parts. "lass & parts tend to be e'pensiveand with long delivery times.

• 3arning on "ommercial-0ff-The-&helf 8"0T&: parts.

♦ Fse rigorously controlled assembly procedures conducted invery clean environments.

♦ "onduct formal inspections of manufacturing facilities,processes and documentation.

• 3hy is documentation of all steps in the process important4

♦ erform acceptance testing or inspections on all parts whenpossible.



Reliability *alculations Section


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-loc" )iagrams

a

b

a

b

a

Two units in parallel

R = Ra 3 Rb - RaRb

a ba ba

Two units in series

R = Ra 4 Rb

5ou may combine series an# parallel operations intoarbitrarily complex bloc$ #ia&rams


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*omputing Event Probability

♦ &uppose historical data demonstrates the number of failures per9** launches of a particular launch vehicle.

♦ 3hat is the probability of launching G* times without failure4

9 failure < 9** launches success A e'p8 -G*89<9**: : A *.C9+

5 failure < 9** launches success A e'p8 -G*85<9**: : A *.E?C

9* failure < 9** launches success A e'p8 -G*89*<9**: : A *.9E5

Recall from before that R8t: A e'p8 -λt :


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E/ample Reliability Problem

♦ 7 human-rated space launch system has a reliability, or probability ofsuccess, of *.+C. 7n abort system for the crew module is provided and

has a reliability of *.+5.• 3hat is the overall probability of crew survival4

Iet 7 A event of crew death

B9 A event of launch vehicle success

BG A event of launch vehicle failure

8B9: A *.+C 87< B9: A * 8abort system not needed:

8BG: A *.*G 87< BG: A *.*5 8abort system fails:

Then from the Iaw of "onditional robabilities,

87: A 8B9:87< B9: J 8BG:87< BG: A 8*.+C:8*: J 8*.*G:8*.*5: A *.**9

The reliability of crew survival is then

Rs A 9 - 87: A *.+++

The crew has a ++.+K chance of survival, even though neither the launchvehicle nor the abort system is anywhere close to being ++.+K reliable.


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E/ample Reliability Problem

♦ 7 human-rated space launch system has a reliability, or probability ofsuccess, of *.+C. 7n abort system for the crew module is provided and

has a reliability of *.+5.• 3hat is the overall probability of crew survival4

a

b

a

b

a

R = Ra 3 Rb 6 RaRb

R = 7*+ 3 7*8 6 7*+47*8 = 7***

!ame as before9

Ra = reliability of launch system = 7*+

Rb = reliability of abort system = 7*8


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E/ample: 'pollo M 'scent Engine

♦ "onsider the 7pollo Iunar odule ascent engine. This systemincluded three valves in the o'idi@er lines and three valves in the fuel

lines. 2or the system to function properly, at least one of the valves ineach set must work. The reliability of each valve is Rv A *.+.

♦ This system may be e'pressed using the following block diagram.

♦ 3hat is the probability of the entire system working4

R:

R:

R:

R:

R:

R:



'dditional Pause and earn pportunity

The #vent Tree methodology 8introduced in the Risk

odule: can also be used to calculate reliability. Lou

can redo the e'ample problems in this lecture for the

launch system or the 7pollo ascent engine using event

trees, and show the students that you get the sameresult.

Lou can also show additional e'ample problems using

the file #'ampleMReliabilityMroblems.pdf.



Module Summary: Reliability

♦ Reliability is a key attribute of space systems, influencingsystems engineering activities such as design, trade studies,

modeling, and test.♦ The reliability function, R8t:, is determined from the probability

that a system will be successful for at least some specified time.

♦ The Bathtub curve e'presses the failure rate as it depends onthe age of the system. #arly and late in life of the system

8similar to the human body: significantly higher failure ratesoccur called “infant mortality” and “old age” regions. Betweenthese regions normally lies an e'tended period of appro'imatelyconstant failure rate. The reliability of systems operating in thisregion can be simply characteri@ed by an e'ponential function.

♦ 3ays to achieve reliability include fault tolerance, functionalredundancy and fault avoidance.

♦ Block diagrams and event trees are useful tools in calculatingreliability. 7n understanding of probability basics is re>uired.



-ac"up Slides

!or Reliability Module

2ault Tree 7nalysis is included in the Risk odule,

however, it could also be addressed in the Reliability

odule. Here are some additional slides related to

fault tree analysis.


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+ault ree 'nalysis

♦ 7n analytical techni>ue, whereby

• 7n undesired state of the system is specified

• &ystem is analy@ed to find all credible ways that this state can occur

♦ odeled in a top-down fashion using symbolic logic.

♦ Iooks at failure domain only.

♦ rovides a >ualitative model that can be evaluated >uantitatively

using probabilistic assessment.

♦ Fsed in system design to understand what elements might

cause loss of mission 8or loss of crew:.

♦ Fsed in the analysis of nuclear reactor safety.

•

Fault Tree Handbook , FR#/-*%+G, F.&. uclear Regulatory "ommission,9+C9.

♦ 7lso used in accident investigations.• e.g., ars "limate 0rbiter and ars olar Iander, lost in 9+++.



2ault tree analysis is a graphical

representation of the combination

of faults that will result in theoccurrence of some 8undesired:

top event .

Dn the construction of a fault tree,

successive subordinate failure

events are identified and logicallylinked to the top event.

The linked events form a tree

structure connected by symbols

called gates.

+ault ree 'nalysis



Re!er to 2'S' Re!erence Publication 1345:

&ystem #ngineering “Toolbo'” for

!esign-0riented #ngineers

&ection E.?( 2ault Tree 7nalysis

8Handout:

articular points( 7nd<0r /ates e'planation

#'ample 2ault Tree 82ig E-G*:

Reliability Module V1

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Transcript of Reliability Module V1