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