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Program in 1979.  The BETT system functions by severing the nozzle at the interface with the booster case and then reacting the momentum of the nozzle through the extrusion of bolts (see fig. C18).  This action opens a large annular region between the nozzle and booster case to allow substantial pressure decay and subsequently to extinguish the solid propellant burn.  It is necessary to capture the nozzle, via bolt extension, to react this momentum against the very large pressure  times area force applied at the forward dome of the motor.  The booster, using the BETT system, is extinguished in approximately 0.03 second.  Figure C19 demonstrates the dynamic nature of the thrust decay for the BETT system.  This system was studied for implementation in the Space Shuttle system by the Space Shuttle systems contractor.  The results of their study indicated that it would take a decay time ranging between 0.34 and 0.85 second to avoid structural damage to the SRB-ET forward attach.  Therefore, the BETT system, having a reaction time of 0.03 second, is incompatible with the current Space Shuttle design.

The Langley Research Center has proposed a concept for controlled SRB thrust termination.  The concept consists of explosively cutting opposing vents in pairs in the sides of the SRM just above the nozzle as shown in figure C20.  Vent diameters are increased by consecutively cutting larger vents as the pressure decays.  The concept merits further study but is not of sufficient maturity at this time to be a candidate for a TT system for the Space Shuttle.

V. SUMMARY

A. Summery of the History

The Space Shuttle Program considered all known methods of providing Orbiter and/or crew recovery from emergency situations during the first stage of ascent flight.  These methods were either baselined and then deleted or considered and not implemented because of one or a combination of the following factors.

1.  Limited utility,
2.  Technical complexity,
3.  Lack of reaction time and appropriate cues,
4.  Cost and schedule, and/or
5.  Performance and mission objectives impact.

Because of these factors, the Space Shuttle Program adopted the philosophy that the reliability of first stage ascent must be assured through conservative design, testing, and certification to preclude time critical failures that prohibit the continuation of flight through SRB burnout.

On the other hand, Orbiter avionic software changes were pursued aggressively to enhance survivability for cases well beyond the accepted program risk level of Fail-Safe.  Many of these proposed

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