|MadSci Network: Astronomy|
Hello Jessica, The shuttle does experience heating during both launch and when returning. The heating is much less, for several reasons, during the launch phase. This heating results from friction between the air molecules and the shuttle. The amount of friction is strongly dependent on velocity. The heat load is dependent on the length of time of the heating. LAUNCH The launch phase of a mission is much faster so less heat builds up because the shuttle rises almost vertically. The shuttle launch starts from zero velocity (no friction) and quickly increases and this generates heat. However, as this is occurring the friction is decreasing because the density of the atmosphere (number of gas molecules per cubic centimeter) is decreasing. Also the temperature in the atmosphere is falling and this helps carry away the heat from the thermal tiles. RE-ENTRY The re-entry starts with the shuttle moving very fast in the upper atmosphere. As the shuttle's altitude decreases the atmosphere's density is increasing and the result is more friction. If the re-entry angle is too steep (fast) the shuttle will penetrate the atmosphere too quickly with the result that the temperature build-up will exceed the limits of the heat shield tiles. If the re-entry angle is too shallow (slow) then the thermal tiles will experience high heat loads due to the long re-entry time and the heat conduction through the tiles will cause failure because of the temperatures in excess of the structure's design limits. Only a small range of re-entry conditions is acceptable and this still requires thermal protection to protect the shuttle from the atmospheric friction heat load generated during the re-entry. More information can be found at: Shuttle Thermal Protection System Why do reentering bodies experience extreme heating? With your search engine look for the 1979 paper presented at 14th AIAA Thermophysics Conference titled "Effects of Aerodynamic Heating and TPS Thermal Performance Uncertainties on the Shuttle Orbiter" by Goodrich, Derry and Maraia John Haberman NASA/Goddard Space Flight Center
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