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weather are poor; hence, weather forecasts must be made conservatively to protect against weather violations. The time from a TAL weather final launch commit decision to actual landing is about half that of the nominal EOM.

The TAL abort mode capability was developed to provide maximum nominal performance and to reduce the risk of an intact abort gap during ascent. There are methods which could be used to reduce the exposure to TAL at the cost of payload weight; however, for some payloads, for example like the Tracking and Data Relay Satellite (TDRS) it is very unlikely that the TAL exposure can be eliminated by special trajectory shaping techniques. For some flights, abort shaping and cargo weight reduction of from 5 to 10,000 pounds can reduce the single SSME failure TAL exposure to zero.

The TAL abort mode is not certified but is qualified by flight design assessment. TAL was not originally an intact abort requirement on the Space Shuttle vehicle thermal design and, hence, if a TAL is performed, some vehicle margins are approached or slightly exceeded. These probable exceedances have been assessed and are believed to result in TPS surface temperature violations which will result in localized tile slumping. Hence, thermal exceedances become a refurbishment issue rather than a flight safety problem.

Additionally, there is an increased risk over RTLS from post separation rupture of the external tank. The fragmentation velocity and proximity of tank at time of rupture is a concern. The tank may be as close as 2 miles away with post rupture fragments having relative velocities up to 1100 feet per second. This data is based upon ET rupture test results performed on previous flights. That flight data has shown two forms of rupture. One has appeared as a non-violent structural breakup, and the other seems to appear explosive in nature. Further tests will be conducted; however, this concern is not deemed sufficient to not perform a TAL abort when the situation demands one.

TAL was developed in the STS 2 timeframe and was included in the flight software on STS 3. Nearly all flights have been flown with a TAL exposure as can be seen in figure 1.

In response to a question concerning the criticality rating of the brakes, an analysis of a Dakar landing was performed for a major ingle point hydraulic or structural brake failure losing 25 percent of braking capability. The analysis used current launch commit Flight Rules and the rollout prediction techniques used preflight to calculate rollout margins for actual day launch day wind conditions. The analysis indicated a range of headwind speeds (0 to 4 knots) for which current Flight Rules and the rollout analysis would indicate that Dakar conditions are acceptable; but if a single brake failed, the Orbiter would exceed the maximum braking energy limits or use a portion of the protected rollout margin.

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