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continuing on into a low, but safe, orbit, the choice between performing an AOA back to Edwards or performing a more favored abort mode known as abort-to-orbit (ATO) would be made. This decision, made after MECO, depends on when the engine failures occurred and how great the loss of performance was. This performance loss is measured as an "underspeed" below the velocity required to insert the Orbiter into a 105 by 105 nautical mile orbit. If the Orbiter had enough propellant to make up the underspeed and still be able to deorbit later, the decision is normally to perform the ATO. After the ATO is performed, the remaining propellant onboard is again evaluated to see whether the orbit can be raised further, still keeping enough propellant to deorbit and also to perform the on-orbit operations required during the mission. 

     A graphic representation of the intact abort modes available to STS 51-L is shown in figure 1.

     Early abort mode capability is based on a certain period of time, at the beginning of the abort mode, in which only a single SSME fails. For cases in which two, or all three, SSME's fail before achieving single engine capability, an intact abort may not be possible. In this case, a contingency abort would occur. Contingency aborts are always based on the assumption that two or three SSME's have failed, cannot be implemented before normal SRB jettison (same as intact abort), and usually result in a water landing or ditching. The operations community's general attitude towards ditching is that it does not have a high probability of crew survival. 

4. Range Safety

     The event timeline of the STS 51-L flight shows that data were lost at 73.6 seconds, the vehicle breakup showed on the Range Safety displays at 75 seconds, the range safety system (RSS) was armed at 102 seconds, and the destruct command was sent at 107 seconds. The key to an analysis of Range Safety actions on STS 51-L is the time of SRB no longer endanger (NLE). SRB NLE is a Range term that signifies that point in ascent at which, under certain assumptions, a free-flying SRB can no longer endanger a populated landmass. The critical assumption in the computation of SRB NLE time is that the SRB would tumble after separation. Under this assumption, the STS 51-L NLE time was 59 seconds. The Flight Rule regarding this case states that the Range Safety Officer (RSO) will initiate flight termination action after SRB NLE only if a flight termination line is violated.
     When the destruct command was sent during STS 51-L, the flight was beyond the SRB NLE time and no flight termination line was violated. The rationale for the decision to destruct was that the RSO had video coverage of the SRB's and had clearly observed they were not tumbling, thus undermining the key assumption in the SRB NLE computation and invalidating the Flight Rule. Without positive confirmation that both SRB's were being tracked and were headed downrange, the destruct action was deemed necessary. Had video coverage not been available, no Range Safety action would have been taken.
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