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Columbia Accident Investigation Board Public Hearing
Lt/Gen. A. G. Casey USAF (Ret)
March 25, 2003

Shuttle Reliability

  • It is demonstrated at .984
    • or about two failures for 100+ flights
  • .984 is about a factor of 2 better than most unmanned launchers
    • On average they have been about .95 or 5 failures per 100
    • That is pretty good considering it is at risk both in ascent and re-entry to land
      • Very high reliability is achieved by redundancy and margins
      • Redundancy has already been built in where practical
      • Margins have to be designed in and verified in qualification test
  • Not adequate for optional human flight operations

Redesign of Subsystems

  • Not really practical for the shuttle fleet
    • Extensive analysis has already been done on vulnerabilities and changes made where affordable
    • Impossible to identify the next most probable failure mode with any certainty
  • Redesign with greater margin is only practicable for the long term
    • Improved Safety, Reliability & Affordability should drive an organized system engineering study of a replacement system

Assembly, Test and Operational Controls

  • NASA has repeatedly demonstrated effective assembly despite the complexity of shuttle configuration
    • Hard to improve on the status quo for assembly
  • The Challenger failure was operation beyond the qualification of the seal
    • The margin was negative in that environment
  • You may find that a recurrence of the Columbia failure can be avoided by acceptance testing
    • If acceptance testing can preclude debris during ascent

Cause of Failure

  • High speed impacts on the shuttle wings are beyond the qualification envelop of the orbiter structure
    • The known debris from the tank hitting the left wing is incontrovertible
    • Regardless of the specific sequence of failure events it seems the remedy is to preclude debris from impacting critical systems during ascent
    • This was doubtless an original design requirement but it has not been achieved

Preclude Debris Impacts

  • One might consider reducing the amount of insulation on the tank.
    • There should be sufficient data now to know precisely how thick the insulator must be for adequate margin
    • It may be that excessive margin here is dangerous for the down vehicle subsystems
  • Testing should be developed to insure the integrity of the foam insulator and the bonded on segments [or any other potential debris].

Retain Margins

  • A concerted effort should be made to operate within design margins
    • A series of flight successes does not verify margins
    • Only rigorous qual testing verifies existence of design margin
    • Special effort should be made to preclude waivers or deviations in production, assembly, or pre-flight check out from reducing flight margins
    • Aging or repeated use may have eroded design margins
      • Aging and surveillance programs have been useful in Aircraft & ICBMs to protect margins and predict service life
  • Comprehensive system engineering effort is needed to define and protect margins

Return to Flight

  • It is important to return to flight soon
    • Long delays incur loss of people and skills as well as morale
      • All of the above may reduce reliability
    • All reasonable steps to preclude debris impact is the best approach to return to flight
    • In the short term higher reliability than .984 cannot be guaranteed thus:
      • Crew size should be minimum
      • Do not use Shuttle where ELVs & Robots can to do the job
March 25, 2003 Presentations

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