This slide presentation reviews the use of Probabilistic Risk Assessment (PRA) to assist in the decision making for the shuttle design and operation.  Probabilistic Risk Assessment (PRA) is a comprehensive, structured, and disciplined approach to identifying and analyzing risk in complex systems and/or processes that seeks answers to three basic questions: (i.e., what can go wrong? what is the likelihood of these occurring? and what are the consequences that could result if these occur?)  The purpose of the Shuttle PRA (SPRA) is to provide a useful risk management tool for the Space Shuttle Program (SSP) to identify strengths and possible weaknesses in the Shuttle design and operation. SPRA was initially developed to support upgrade decisions, but has evolved into a tool that supports Flight Readiness Reviews (FRR) and near real-time flight decisions.  Examples of the use of PRA for the shuttle are reviewed

Boyer, Roger L.

Hamlin, Teri, L.

NASA Technical Reports Server

SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
USE OF PROBABILISTIC RISK ASSESSMENT 
(PRA) IN THE SHUTTLE DECISION MAKING 
PROCESS
Roger L. Boyer                        Teri L. Hamlin
Analysis Branch Chief                          Shuttle PRA Lead
NASA Johnson Space Center
Safety & Mission Assurance 
Presented at 
PM Challenge 
February 9 - 10, 2011
Long Beach, CA
1
https://ntrs.nasa.gov/search.jsp?R=20110001613 2019-08-30T14:01:47+00:00Z
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
INTRODUCTION
Probabilistic Risk Assessment (PRA) is a comprehensive, 
structured, and disciplined approach to identifying and 
analyzing risk in complex systems and/or processes 
that seeks answers to three basic questions:
 What kinds of events or scenarios can occur (i.e., what 
can go wrong)?
 What are the likelihoods and associated uncertainties of 
the events or scenarios?
 What consequences could result from these events or 
scenarios (e.g., Loss of Crew and Loss of Mission)?
2
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
• The Space Shuttle Program (SSP) initiated the 
development of a Shuttle Probabilistic Risk 
Assessment (SPRA) in March 2001.  Prior to that 
there were a number of PRA estimates for the 
Shuttle, but none were sponsored by the SSP.  
– Chart on next page summarizes the Shuttle PRA evolution.
• The “consequence” or metric of concern selected for 
the SPRA is Loss of Crew and/or Vehicle (LOCV).  
• The risk contributors include hardware failures, 
external events, crew errors, software failures, and 
phenomenological events.  
3
BACKGROUND
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
SHUTTLE PRA EVOLUTION
• The advent of established NASA requirements, standards, and tools - as well 
as the development of a strong Shuttle program PRA team have resulted in 
significant recent progress
• Iteration 3.2 is the most comprehensive and used Shuttle PRA to date
4
SPRAT PRA 
Iteration 1.5
2003
Shuttle PRA
1998
Shuttle PRA
1995
Phase 1 
1993 
Galileo 
1988
1988
First 
somewhat 
integrated 
PRA
conducted on
the Space 
Shuttle. Done 
in support of 
Galileo 
Mission.
(Ascent Only). 
1993
Update of the 
Galileo study  
results to 
reflect then 
current test 
and 
operational 
base of the 
shuttle. 
(Ascent Only)
1995
First major 
integrated 
(multi phase) 
shuttle PRA. 
Done with 
input from 
prime 
contractors.
1998
Unpublished 
analysis using 
QRAS. No 
integration of 
elements. 
Limited to 
three Orbiter 
systems and 
the 
Propulsion 
elements
SPRAT PRA
Iteration 2.0
2004/2005 
2004/2005
Integrated 
PRA with all 
elements, 18 
Orbiter 
Systems, 
MMOD and 
human 
actions 
included. 
Peer 
reviewed.
1987
Proof of 
concept study 
for applying 
PRA to Space 
Shuttle.  
Scope was 
limited to 
APUs for 
Orbiter and 
SRB
1:70 1:611:55 1:73 1:131 1:234
2003
Integrated 
PRA with all 
elements, 18 
Orbiter 
Systems, 
MMOD and 
human 
actions 
included. 
Presented to 
Peer review 
Team.
1:78
SPRAT PRA
Iteration 2.1
2005
SPRAT PRA
Iteration 2.2
2006/2007 
SPRAT PRA
Iteration 3.0
2008 
2005
Integrated 
PRA with all 
elements, 18 
Orbiter 
Systems, 
MMOD and 
human 
actions 
included. 
Peer 
reviewed.
Updated Pre-
valve 
modeling
1:67
SPRAT PRA
Iteration 3.1
2009 
2006/2007
Updated 
SPRA 
iteration 2.1 
with 
Inspection 
with Repair 
and Crew 
Rescue.  
Updated 
MMOD and 
Ascent Debris 
Modeling
1:77 1:81
2008
Updated 
SPRA 
iteration 2.2 
with Abort 
modeling, 
Rendezvous 
and Docking.  
Updated 
Functional 
Data, MMOD 
and Ascent 
Debris
SPRAT PRA
Iteration 3.2
2010 
2009
Updated 
SPRA 
iteration 3.0 
with 
corrected 
APU 
Hydrazine 
Leak 
Probabilities
2010
Updated 
SPRA 
iteration 3.1 
with updated 
MMOD, 
Ascent 
Debris, 
Orbiter Flight 
Software, 
Incorporated 
Orbiter 
Review 
Summit 
Comments
1:85 1:89
Mean Probability of LOCV
SLEP Risk 
Trades
HST Manifest 
Decision
Examples of SPRA uses:
STS-128 
Flight 
Rationale
STS-131 
Flight 
Rationale
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
• The purpose of the SPRA is to provide a useful risk 
management tool for the SSP to identify strengths 
and possible weaknesses in the Shuttle design and 
operation. 
– SPRA was initially developed to support upgrade decisions, 
but has evolved into a tool that supports Flight Readiness 
Reviews (FRR) and near real-time flight decisions.
5
BACKGROUND
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
LEVELS OF ASSESSMENT
• Full Scope SPRA
– Establishes baseline risk associated with the overall 
mission by mission phase, as well as by vehicle elements 
and subsystems
– Documented end states, assumptions, approach, and risk 
drivers
• Focused PRA
– Answers specific question that doesn’t require full model, 
but benefits from it
• Insights
– Knowing relative risk contributors provides input for 
decisions without comprehensive PRA
6
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
KEY INFORMATION FOR MANAGEMENT
• Clear presentation of analysis
– if the audience doesn’t understand the analysis, the information will 
not be used
– Difficult because many different ways people process information
• Applicable assumptions and limitations
– PRA is only as good as the assumptions that go into the analysis, thus 
important to share for managers to understand the basis of the results
– Limitations should be understood, so that the results are not misused
• Estimates of uncertainty
– state of knowledge about the system being modeled (e.g. the real 
capability of the system to successfully respond to an event)
– randomness of the probabilistic parameters (e.g.  the uncertainty in 
estimating a failure probability of an event)
7
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
EXAMPLES
8
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
9
SPACE SHUTTLE PROGRAM
Space Shuttle SR&QA Office
NASA Johnson Space Center, Houston, Texas
Presenter
Date Page 101/20/2004Comparison of Upgrades
Current 
Estimated 
Shuttle Risk (1)
Current 
Estimated Risk 
Contribution
Proposed 
Upgrade 
Estimated Risk 
Contribution
Overall Shuttle 
Risk Estimate 
With Proposed 
Upgrade
Percent 
Change from 
Current 
Estimate
AHMS 1.28E-02 1.14E-03 6.94E-04 1.24E-02 -3.5
AHPS 1.28E-02 1.22E-03 4.50E-06 1.16E-02 -9.5
SSME 
CWN (2) 1.28E-02 1.20E-04 4.78E-05 1.27E-02 -0.6
Helium 
APU 1.28E-02 2.34E-04 9.05E-05 1.27E-2 -1.1
(1) Estimate of Loss of Crew / Vehicle risk based on version 1.5 of shuttle PRA
(2) Estimates based on values used for Rocketdyne baseline analysis
Shuttle Service Life Extension Program (SLEP)
Assessed the 
risk of each 
proposed 
upgrade and
compared 
relative 
changes in risk
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
10
SPACE SHUTTLE PROGRAM
Space Shuttle SR&QA Office
NASA Johnson Space Center, Houston, Texas
Presenter
Date Page 11/26/04
Autonomous Shuttle Risk 
Evaluation
• Preliminary Shuttle Probabilistic 
Risk Assessment (SPRA) results 
show  crew actions during entry are 
a risk  driver.
– Contributions were developed with 
the assistance of the Astronaut Office 
(Dom Gorie).
– Results / methods are currently 
undergoing an independent review.
• These actions are or could be
automated, potentially reducing the
risk of entry.
External Events
44%
Functional Failures
18%
Common Cause Failures
6%
Phenomenological
17%
Human Reliability
• Of the approximately 200 crew 
actions modeled, the top four 
contribute about 11% of the 15% 
human reliability total.
1) Crew fails to deploy landing gear
2) Crew Brakes at the Wrong Time
3) Crew Improperly Performs Pre-flare
4) Crew Lands too Hard
Orbiter Risk Estimates
Shuttle Service Life Extension Program (SLEP)
Showed that 
~70% of 
calculated risk 
due to crew 
error occurs 
during entry, 
descent, and 
landing
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
Engine Cutoff (ECO) Sensors
 Assessed the risk of 
changing the Launch 
Commit Criteria (LCC) 
for these ECO sensors 
from requiring four of 
four sensors to only 
requiring three of four 
sensors. 
 Pointed out the need to 
better understand the 
other side of the risk 
trade when a launch is 
scrubbed due to ECO 
sensor failures, i.e., 
scrub turnaround risk.
11
FOUR LO2  ECO 
POINT 
SENSORS  
FOUR  LH2  ECO
POINT SENSORS
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
Probability of LH2 Low Level Cutoff (STS-122)
 Shuttle Program 
Manager requested 
and used   
 Model used 
historical data in a 
simulation model
 Shuttle Program 
Manager could see 
it impact of adding 
Ascent Performance 
Margin (APM) on 
risk
12
Simulated Mission 
ΔLH2 usage
Unusable
APM
Uncertainty Protection
Mission 
Requirements
ΔLH2 300 lbs
ΔLH2 1500 lbs
ΔLH2 -300 lbs
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
Solid Rocket Booster Power Bus Isolation Supply Analysis
 Emphasized the need to 
implement a design change that 
would eliminate the failure in 
future flights
13
Median Reliability Derived From 105 No Failure PBIS 
Events
0.965
0.97
0.975
0.98
0.985
0.99
0.995
1
0 2 4 6 8 10 12 14 16
Age of a PBIS (Flights)
R
el
ia
bi
lit
y 
 .
  
  
  
  
R
el
ia
bi
lit
y 
 .
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
Main Propulsion Flow Control Valve
 Shuttle Program used these risk estimates as 
supporting flight rationale for STS-119, combined 
with FCV inspection and impact testing
14
0
5
10
15
20
25
30
35
-10.0 10.0 30.0 50.0 70.0 90.0 110.0
Data
Fit
Probability of 
LOCV due to 
Venting
Failure Rate
3 poppet failures are 
assumed to vent if  occur 
before 120 seconds
Poppet Break Size 
Distribution
Thresholds
5,000,000 
replications
INPUTS
Dual-Failure Vent Threshold
0
20
40
60
80
100
120
80 105 130 155 180
Size (Degrees)
Ti
m
e
 (
Se
co
n
d
s)
T2*
Old Fit
Piecewise Linear
Chance of 
multiple 
failures
# of failures, 
size and time 
Compared 
to 
Thresholds
Exponential Rate Distribution
0.0E+00
5.0E-06
1.0E-05
1.5E-05
2.0E-05
2.5E-05
0.0E+00 5.0E+04 1.0E+05 1.5E+05 2.0E+05
Time (Seconds)
f(
x)
Pressure Dispersion
200
220
240
260
280
300
320
340
360
380
-10 10 30 50 70 90 110
Time (Seconds)
D
eg
re
es
Data
Fit
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
Oct 27, 2006
Ascent Debris
1:821
Other Ascent
1:457
SSME  1:667
HST Without
Crew Rescue
HST with
Crew Rescue
2008 Predicted ISS MissionSTS-115
0.00E+00
2.00E-03
4.00E-03
6.00E-03
8.00E-03
1.00E-02
1.20E-02
1.40E-02
1.60E-02
1.80E-02
2.00E-02
Pr
ob
ab
ili
ty
 o
f L
O
C
1:76
Risk Comparisons
32%
15%
1:65
Ascent Debris
1:392
Other Ascent
1:457
Ascent Debris
1:855
Other Ascent
1:457
SSME  1:667
Other Ascent
1:457
SSME  1:667
Ascent Debris
1:421
1:77
1:8711%
SSME  1:667
Other Entry
1:401
MMOD
1:205
Other Orbit 
1:1230
Other Entry
1:334
MMOD
1:192
Other Orbit
1:1080
MMOD
1:281
Other Entry
1:402
Other Orbit
1:1230
MMOD
1:297
Other Entry
1:402
Other Orbit
1:1230
• Expected HST risk is similar to the STS-115 accepted 
mission risk if Crew Rescue is available
- Reduction in ascent debris risk may partially offset 
the increased MMOD risk for the HST mission
15
Hubble Space Telescope (HST) Manifest Decision
Analysis 
compared HST 
risk with and 
without crew 
rescue to other 
Shuttle missions 
in order to help 
NASA 
Administrator 
decide whether 
or not the HST 
mission was an 
acceptable risk
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
Oct 27, 2006 HST SM4 Manifesting Review – Pre-Decisional  For Internal Use only
RISK REDUCTION COMPARISON
SSME Block 
I to Block II
PGME Added 
to WSB
Wire to Wire Short 
(Inadvertent thruster fire)
0
2
4
6
8
10
12
14
Crew 
Rescue
AHMS
Pe
rc
en
t R
is
k 
R
ed
uc
tio
n
• For an HST mission there are no single 
system changes that would result in a 
mission risk reduction as significant as 
LON/Crew Rescue.
15%
11%
1%
<1%
<<1%
16
Hubble Space Telescope (HST) Manifest Decision (2)
Risk reduction 
with crew rescue 
was compared to 
risk reductions 
from 
implemented 
Shuttle upgrades
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
Probability of Launch on Need
 Assisted the Shuttle 
Program Manager with 
making an informed 
decision not to release the 
HST rescue vehicle
17
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
18
8/23/09
STS-128 PCA FAILURE RATE RESULTS 
Failure rates between 2.0E-06 and 2.1E-05 per cycle 
KEY ASSUMPTIONS
• Assumes 0.5 cycles for AC inverter contactor, 1.5 cycles for RJD 
contactor and 1.5 cycles for  ODS and Payload contactors for STS-
128
• Analysis assumes failure rate based upon contactor cycles
• 5 broken contactor failures are used in the analysis
• Assumes contactor failure will result in inadvertent “off” or failure 
to turn “on”
• Non-latching contactors are not included in the analysis
• Contactor cycles based upon engineering judgment
Probability of a Broken Contactor on STS-128
Mean – 1:7400
95th- 1:5500
5th – 1:10000
Using a Random failure rate the mean probability of a 
broken contactor on STS-128 is: 1:4100
The probability of a SAIL contactor of ~15700 cycles 
old breaking in a 6 week period (Assuming 15 
contactor s and 2 cycles per day) is: ~1:20
Probability of a Broken Contactor on the Ground
The probability of a vehicle inverter contactor of ~4700 
cycles old breaking in a 6 week period (Assuming 27 
contactors and 4 cycles per week) is: ~1:100
Low Risk due to limited # 
of cycles in flight
1
S/N ASSEMBLE Cycles P(f) 5th 95th
FPCA-1  V070-763320
K1 AC Inverter 1, Phase A 127 4/16/1982 6100 1.8E-05 8.4E-06 3.3E-05
K2 AC Inverter 1, Phase B 128 4/16/1982 6100 1.8E-05 8.4E-06 3.3E-05
K3 AC Inverter 1, Phase C 126 4/16/1982 6100 1.8E-05 8.4E-06 3.3E-05
K11 RJDF Bus A 092 11/14/1979 1245 3.6E-06 1.6E-06 6.6E-06
FPCA-2 V070-763340
K1 AC Inverter 2, Phase A 096 1/20/1981 6300 1.9E-05 8.7E-06 3.5E-05
K2 AC Inverter 2, Phase B 112 1/20/1981 6300 1.9E-05 8.7E-06 3.5E-05
K3 AC Inverter 2, Phase C 117 1/20/1981 6300 1.9E-05 8.7E-06 3.5E-05
K13 RJDF-1 Bus B PWR (RPC#36) 111 1/20/1981 1245 3.6E-06 1.6E-06 6.6E-06
FPCA-3 V070-763360
K-1 AC Inverter 3, Phase A 212 10/12/1978 6900 2.1E-05 9.5E-06 3.8E-05
K-2 AC Inverter 3, Phase B 214 10/12/1978 6900 2.1E-05 9.5E-06 3.8E-05
K-3 AC Inverter 3, Phase C 215 10/12/1978 6900 2.1E-05 9.5E-06 3.8E-05
K-6 RJDF-2B Manif F4/F5 Drivers 216 12/10/1985 1245 3.6E-06 1.6E-06 6.6E-06
MPCA-1 V070-764400
K4 SPARE 221 7/11/1989 700 2.0E-06 9.1E-07 3.6E-06
K5 ODS/ECLSS 228 7/11/1989 1180 3.4E-06 1.6E-06 6.2E-06
MPCA-2 V070-764430
K4 SPARE 103 3/31/1980 700 2.0E-06 9.1E-07 3.6E-06
K5 ODS/ECLSS 106 3/31/1980 1180 3.4E-06 1.6E-06 6.2E-06
APCA-1 V070-765310
K1 Reaction Jet Driver Bus A 138 11/10/1982 1245 3.6E-06 1.6E-06 6.6E-06
APCA-2 V070-765320
K1 Aft Payload Bay Power B 137 3/29/1982 700 2.0E-06 9.1E-07 3.6E-06
K2 RJDA Manif Drivers Bus B 180 2/9/1984 1245 3.6E-06 1.6E-06 6.6E-06
APCA-3 V070-765330
K1 Aft Payload Bay Power C 072 10/10/1979 700 2.0E-06 9.1E-07 3.6E-06
K2 RJDA Manif Drivers 079 10/10/1979 1245 3.6E-06 1.6E-06 6.6E-06
OV103 Weibull (β=2.024, η=25538)
-032 / 266775
-013 / J12867
-013 / J43296
-019 / EJ3166
-039 / ER1634
-033 / F71099
-003 / AM6520
-009 / F66222
STS-128 Power Controller Assembly Risk Presented at L-2
Analysis was used 
to help Shuttle 
Managers decide 
that PCA risk was 
acceptable for flight
Analysis showed 
that it was much 
more likely to have 
a broken contactor 
on the ground
Important to 
inform 
managers of 
the analysis 
assumptions
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
19
2
FAILURE SCENARIO RISK UNCERTAINTIES
• Given the failed helium isolation valve failed open, the identified risk scenarios have various 
mission impacts as shown in backup chart 6.
• Loss of Right RCS Function is failure of both regulators and assumes a mission time of 48 
hours (prior to reaching 82% which is expected late FD2, early FD3) and results in NPLS
• Overpressurization of the Propellant System is failure of both regulators and failure of either 
the burst disc or the relief valve and uses 314 hours (STS-131 mission time)
• Loss of RCS Control is failure of both regulators and either cross-feed or LRCS failure and uses 
48 hours (prior to reaching 82% which is expected late FD2, early FD3)
• Each scenario is developed to the point where the mission impact is reached.   
• No change of state in the failed isolation valve is assumed.
• If both helium isolation valves are assumed to be failed open, the calculated risk for regulator 
fail open will double, which will impact all of the risk estimates.
1:4250
1:93200
1:45000
1:30300
1.E-06 1.E-05 1.E-04 1.E-03
RRCS Fail (NPLS)
Overpress of Propellant System (LOCV)
Loss of RCS Control (LOCV)
Total LOCV
STS-131 Helium Isolation Valve Risk
Analysis was 
used to 
support STS-
131 flight 
rationale at 
the HQ Flight 
Readiness 
Review
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
20
RIGHT RCS HELIUM SYSTEM RELIABILITY
Pri Reg 
Fail OP
(>Burst Disk)
Failure
Sec Reg 
Fail CL
Pri Reg 
Fail CL
Sec Reg 
Fail OP
(>Burst Disk)
Switch Regs
A/B
Fuel Tk 
Overpress &
Structural Fail
BD/RV 
Fail CL
Overboard Vent 
thru BD/RV
Loss of Verns
Impacts on:
Mated Control
Mission Content
OR
2nd
3rd
Failure
NEOM
Probable
NEOM
O
ps  w
orkarounds
R RCS Fuel 
He Isol  
Fail OP
Rely on Sec Reg
1st
1:936 based upon 218 
hours of docked time
1:768 based upon 266 
hours (T-0 to undock)
1:93,200
based upon 314 
hours (STS-131 
mission time) 
1:1950 Either reg valve failure 314 
hours (STS-131 mission time) 
>1:100 based 
on flight history 
1:373 using 314 hours 
(STS-131 mission time) 
1:655 Both reg valves fail, using 314 
hours (STS-131 mission time) 
3
Pri Reg 
Creep High
(<Burst Disk)
Sec Reg 
Creep High
(<Burst Disk)
Both reg valves failure results in  
Loss of RRCS causing NPLS if 
failure occurs in first 48 hours –
1:4250
 Analysis results 
combined with 
graphical display 
to help 
communicate to 
Management at 
HQ Flight 
Readiness Review 
Right RCS Helium System Reliability
SPACE SHUTTLE PROGRAM
Space Shuttle Safety and Mission Assurance Office
NASA Johnson Space Center, Houston, Texas
SUMMARY
• Showed various ways of communicating  and using 
PRA findings in the Shuttle Program
• Stated that it is important to provide management:
– Clear presentation of analysis
– Applicable assumptions and limitations
– Estimates of uncertainty
• Maintain consistency and accuracy across the 
program to make it relevant
• Used various levels of PRA to answer the mail
• The Shuttle Program has benefited from using PRA 
and others can too
21


Use of Probabilistic Risk Assessment in Shuttle Decision Making Process

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