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Review on April 23, 1983, are included as figures C5, C6, and C7. The key decision parameters were that sufficient redundancy existed in many of the first stage ascent systems to allow for intact abort after SRB burnout, and that SRB burnthrough was a time critical failure that must be precluded by design.

TT was revisited several times during the development period. Space Shuttle Directive number S13141, signed October 1, 1979, instructed the system contractor to determine the thrust decay time necessary to reduce the dynamic loads at the forward SRB-ET attach structure to a point such that they were within the design envelope. The decay curves were given to NASA Marshall Space Flight Center (MSFC) to determine if they were achievable. MSFC's response was that there existed no practical approach to achieve the required pressure decay rates. 

On July 12, 1982, the Associate Administrator (AA) for Space Transportation Systems (STS's) requested a revisit of the utility of TT. Response to the request is contained in a letter from the Director of the Nasa Johnson Space Center (JSC) to the AA. The statement from that letter pertaining to TT is repeated below. 

"In our opinion, further study of a thrust terminator system for the SRB would not be productive. The potential failure modes which could result in a set of conditions requiring SRB thrust termination are either very remote, or a result of primary structural failure. The structural failure risk would normally be accepted as a part of the factor of safety verification by analysis or test. In addition, any thrust termination system is going to be extremely heavy, very costly, and at best, present some risk to the Orbiter and ET. Venting of hot gases, and the shock load or pressure spike, have the potential for being as great a hazard as the problem to be corrected. It does not appear that a practical approach exists for achieving the desired pressure decay rate without a major redesign of the motor."


3. Intact Abort

Intact abort was implemented as required. During the course of the development, a transatlantic abort landing (TAL) capability was added to cover the abort "gap" between the baselined RTLS and AOA capability. With this addition, the Space Shuttle system has complete coverage for loss of one Space Shuttle main engine (SSME) thrust and partial intact capability for loss of thrust from two SSME's. 

4. Ejection Seats

Ejection seats were implemented and removed after the test flights as planned.

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