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7. Thunderstorms are not acceptable within 50 miles of KSC or 30 miles of any other site. In addition, the approach path to the landing site must show at least 2 miles above and 5 miles horizontal clearance to assure that the Orbiter does not receive a lightning strike, even though analysis indicates that the Orbiter should survive a lightning strike.  The 50-mile limit at KSC includes an extra 20 miles of conservatism due to a perceived difference in weather stability at KSC.  For RTLS approaches the limits are reduced since the forecast time is reduced. 

8.  At least 1000 feet of runway must be available beyond the threshold when using the high wind aimpoint to assure that the Orbiter does not land short. 

9.  Brake energy used for normal end of mission rollout computation is 42 000 000 footpounds applied to a single brake taking 60 percent of the total energy load on the upward crosswind side.  This value is raised to 55 000 000 footpounds for the abort landings.  Using the actual forecast winds, density altitude and these brake energies, each landing opportunity is evaluated in real time and the rollout margin is computed.  Nosewheel steering is assumed to be failed so that crosswind must be controllable by differential braking.  For nominal end of mission, a 3000-foot margin is required.  For abort, 2000 feet is required.  The runway is no go if these conditions cannot be met. 

VIII. WEATHER FORECASTING

Weather forecasting is a primary ingredient in making the end-of-mission landing site and runway selection and in giving the final approval for the deorbit burn.  On the day that the landing field has marginal (with respect to ability to satisfy the Flight Rules) weather, the decision to commit to a landing attempt is difficult no matter which landing field is planned for return.  Data was presented showing the climatology of EAFB and KSC.  In addition, a "forecasting difficulty" assessment was made for the 90-minute forecast required for the end of mission. 

The KSC climatology review showed that landings in the afternoon in the spring and summer are not very likely to be acceptable and are typically difficult to forecast accurately for 90 minutes due to the tropical nature of the weather.  For landings occurring just after sunrise, the before sunrise weather forecast is difficult to make to sufficiently protect visibility (fog) and should be avoided if possible.  Late night and early morning landings in late fall and winter are subject to a high probability of waveoff.  For midmorning landings at KSC in the summer, the probabilities of acceptable conditions are good and the forecasting job is normally acceptable.  When combined with the conservatism that must be added to account for forecasting difficulty and current availability of forecasting tools, the KSC waveoff probability may run as high as 50 percent and as low as 25 percent.  

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