>http://orbiter-forum.com/showthread.php?t=5603 Wow, rick, nice find! To summarize the above thread there are 2 more reasons why ascending is better. 1) Noctilucent clouds are more prominent further north and nasa was concerned about the ice particles possibly causing damage at mach 25 although it sounds like further research has deemed it safe for the shuttle but nasa probably didn't feel a strong need to change their policy. 2) More fuel is needed for descending path or alternatively higher insertion angle accuracy is needed. #2 is more complicated. When the shuttle first "hits" the atmosphere, because of the rotation of the earth, the farther north you hit the atmostphere the higher the speed differential because the earth moves more slowly at higher altitudes (more slowly away from the shuttle). This is supposedly 291 meters per second difference on ascending versus descending from the ISS although that number sounds suspect to me. If the shuttle "hits" the atmosphere at a higher velocity, then the angle you hit the air at has to be even more precise so you don't accidentally skip right out of the atmosphere again and have to start over but with possibly no propellant left or alternatively hit "too hard". This angle has a 0.1 degree tolerance. So they prefer to use more propellent to slow the shuttle down more on descending passes to keep this tolerance the same. I suspect this also has to do with the ancient software used to land the shuttle. Obviously the shuttle doesn't hit the air instantly - it is a gradual thing - but this explanation, although simplified, still holds on a gradual entering of the atmosphere. Although I doubt they save and sell the extra propellent - I suspect it is all vented away. So I don't see how this matters much. - George Roberts http://gr5.org _______________________________________________ Seesat-l mailing list http://mailman.satobs.org/mailman/listinfo/seesat-l
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