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III. SURVIVAL PATHS FOR FIRST STAGE FAILURES

The intent of this section is to review: (1) the abort design requirements for Space Shuttle ascent, (2) a chronology of the abort enhancements added post-design, and (3) a list of potential improvements for consideration and their relative effects on the resultant survival paths.

The level I safety requirements for the Space Shuttle were specified to be at least Fail-Safe. This design requirement implied that specific means must be implements to return the Orbiter to a safe landing for the class or single-point failures that affected primary propulsion in such as way that orbit cannot be achieved. Therefore, the primary intact abort design requirements were for specific propulsion system failures. The most significant (in terms of design) was the loss or partial loss of one SSME. The Space Shuttle intact aborts were not designed to cope with other failures like structural failures, SRB failures, loss of control, or multiple system failures. To resolve the structural failures and the SRB failures, the program place emphasis on design analysis, conservative safety margins, and testing for improved reliability to eliminate these failures.

Although the safety specification only required a philosophy of Fail-Safe, a class of multiple failures could be resolved through the development of specific software. These cases had a very high benefit-to-cost ratio and, therefore, were aggressively pursued. Flight software and manual procedures were developed for selected contingency cases (multiple SSME thrust loss); however, safe return of the Orbiter, crew, and payload to a prepared runaway is not always possible. In most cases, contingency aborts will result in a water ditching which may or may not be survivable. 

A chronology of the abort enhancements that have been added is shown in figure C8. Enhancement primarily were flight software code changes that added additional targeting, sequencing, and control capability to exercise the various abort paths. Hardware changes for most vehicle capability were not normally approved. The addition of the TAL intact abort did provide a significant gain in abort capability and provided improved launch trajectory shaping for maximizing payload. Considerable changes have been made to provide additional propellant management capability to reduce abort landing weights.

Some potential improvements are described in figure C9. These range from simple flight code changes to complex hardware changes. Most contingency aborts lead to ditching the Orbiter in the water which obviously results in loss of the Orbiter and, potentially, loss of the crew. Consequently, any changes which could improve the chances of flying the Orbiter to an acceptable runway or that would allow the crew to bail out safely prior to a ditch would enhance flight safety for these cases.

A summary of the resultant survival paths and the approximate failure times for various improvements compared with the current capability is enclosed as figure C10. It is important to note that these paths are

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Transcription Notes:
This paper has been thoroughly reviewed and was only opened for editing to fix one error on the last line.