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Application of Probabilistic Methods for the Analysis of Space Shuttle Launch Area Risk (PSAM-0374)

Excerpt

In support of the Space Shuttle Return-to-Flight effort, the risk to the public from Shuttle launches has been reevaluated with the latest probability estimates and trajectory simulation results. The launch area risk is quantified by estimating the likelihood and outcome of possible failure events that can occur during first stage ascent. Probabilistic methods are employed to estimate the likelihood of each failure scenario applicable to this time period, which extends from liftoff through Solid Rocket Booster (SRB) separation. The resulting probability values and corresponding Shuttle trajectory results enable the Air Force Range Safety community to calculate casualty expectation (Ec) values.

For each vehicle element, the Shuttle Probabilistic Risk Assessment (SPRA) model identifies failure modes that can lead to a catastrophic event. The first stage failure modes are categorized based on the type of expected vehicle response. Failures of particular interest are ones that result in an uncontrolled trajectory deviation referred to as a malfunction turn. With the aid of system experts, malfunction turn failure effects are identified, modeled, and incorporated into a six degree-of-freedom ascent simulation. Trajectories are then simulated for each malfunction turn failure mode to determine the impact of the failure on controllability and vehicle integrity. The ascent trajectory results serve as initial conditions for free-flying SRB and vehicle debris trajectory simulations. In addition to the vehicle response categorization, failure modes are also classified as either time based or event based. Event-based failures can only occur during discrete events such as SRB ignition or SRB separation. For time-based failures, the probability is distributed across the duration of first stage ascent using one of four different time profiles developed for this task. Trajectory results are obtained for different failure times throughout the first stage, and the probability values associated with each trajectory vary depending on the time distribution that is selected for the failure mode being simulated. Once a probability value is obtained for each simulated trajectory, the results are incorporated into Air Force range safety risk models.

The results of the trajectory simulations and SPRA lay the foundation for obtaining launch area risk estimates, which can then be communicated to range safety managers for use in risk mitigation and policy decisions. The trajectory simulation techniques and probabilistic methods used during this analysis are presented along with an overview of the ascent simulation results and first stage probabilistic data. A brief comparison with the methods used during previous Shuttle launch area risk assessments is shown as well.

  • Summary/Abstract
  • Introduction
  • Failure Probability Estimation
  • Trajectory Simulation
  • Conclusion
  • Acknowledgments
  • References

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