EML 4920 Professional Orientation

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EML 4920 Professional Orientation

• Safety, Reliability, and Risk Assessment

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Risk

• Three elements determine risk
• Initiating causes
• Hazardous condition
• Consequences

Risk Magnitude (Likelihood)X(Impact)
Risk
Probability
Consequences
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Probability and Consequence Estimates

• Fault tree analysis sets of failures leading to
  the top event
• Event tree analysis possible outcomes of a
  potential failure
• Source dispersion models
• Fire models
• Explosion models

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Risk Management

• Engineering control design out the risk (90
  effectiveness)
• Administrative control control procedures (50
  effectiveness)
• Personnel control training methods (30
  effectiveness)

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Risk Management Programs

• Program Definition define consequences and
  acceptable risk level
• Hazards Evaluation define scenarios, estimate
  likelihood and impact, thus risk
• Risk Reduction identify corrective actions to
  reduce risk levels
• Implementation verify actions, continue life
  cycle assessments, periodic review

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Space Missions
STS-115, September 9, 2006
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System Safety and Reliability

• Xi mission success during phase i
• xi mission is safe during phase i
• Rms probability of mission success
• Rcs probability of crew safety

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Mission Depends on Serial Success

• Assuming serial success of all the mission phases
  1lt i lt n, then the probabilities become
• Rms Pr(X1X2.Xn)
• Rc Pr(x1x2xn)

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Mission Phases are Independent

• Assume each of the mission phases are
  independent, in which case
• Rms Pr(X1)Pr(X2)Pr(Xn)
• Rcs Pr(x1)Pr(x2)Pr(xn)

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Top-level functional failures that could lead to
a loss-of-vehicle (LOV) event

• Propulsion failure
• Vehicle configuration failure
• Containment failure
• Vehicle environment failure
• Externally initiated failure

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Mission Phases for Two-stage Rocket Round Trip to
ISS
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Success of Mission Phases Depends on a Number of
Systems
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Assume All Systems are Independent and Must
Operate Successfully or Safely
For phase 1, the first stage ascent Pr(X1)
Pr(Y1) Pr(Y2) Pr(Y3) Pr(Y4) Pr(Y5) Pr(Y1)
Pr(y1) Pr(y2) Pr(y3) Pr(y4) Pr(y5) Yi success
of system i yi safe performance of system i
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Powered and Unpowered Phases
unpowered
Rms Pr(X4) Pr(X6) Pr(X7) Pr(X8)
Pr(X1) Pr(X2) Pr(X3) Pr(X5) Rms
R1R2
powered
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Bracket Propulsion System Reliability Low
Propulsion System Reliability
For low propulsion reliability set R11 and set
probabilities for powered phases equal Pr(X1)
Pr(X2) Pr(X3) Pr(X5)Plow Then Rms
(Plow)4 Plow (Rms)1/4
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Bracket Propulsion System Reliability Equal
Propulsion System Reliability
For propulsion reliability about the same as the
other systems and with all phases of the mission
about equally reliable Rms (Psame)8
Psame (Rms)1/ 8
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Relationship Between Mission Reliability and
Phase Reliability
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Reliabilities of past manned space missions
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The Hazard or Failure Rate
The hazard or failure rate is the fraction of
survivors at time t which fail per unit time
z(t) lim(failures over Dt)/(survivors at time
t)(Dt)
The reliability function
The mean time to failure
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Failure Rate Models Reliability Estimation
Assume a constant failure rate z(t) l (suitable
for a first-order approximate analysis)
z(t) l R(t) e-lt MTTF l-1
li constant failure rate for phase i titime
from beginning of phase i
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Reliability Estimation
Best reliability estimates are from experimental
data. For a constant failure rate, estimate l
by the ratio of failures to total operating
hours maximum likelihood estimate (MLE)