Editing Atmospheric entry (section)

====Perfect gas model====
Almost all aeronautical engineers are taught the [[Ideal gas|perfect (ideal) gas model]] during their undergraduate education. Most of the important perfect gas equations along with their corresponding tables and graphs are shown in NACA Report 1135.<ref>{{cite journal |title=Equations, tables, and charts for compressible flow |publisher=NASA Technical Reports |issue=NACA-TR-1135 |year=1953 |url=http://www.nasa.gov/sites/default/files/734673main_Equations-Tables-Charts-CompressibleFlow-Report-1135.pdf |journal=NACA Annual Report |volume=39 |pages=613–681 |access-date=June 17, 2015 |archive-date=September 4, 2015 |archive-url=https://web.archive.org/web/20150904043857/http://www.nasa.gov/sites/default/files/734673main_Equations-Tables-Charts-CompressibleFlow-Report-1135.pdf |url-status=live }}</ref> Excerpts from NACA Report 1135 often appear in the appendices of thermodynamics textbooks and are familiar to most aeronautical engineers who design supersonic aircraft.

The perfect gas theory is elegant and extremely useful for designing aircraft but assumes that the gas is chemically inert. From the standpoint of aircraft design, air can be assumed to be inert for temperatures less than {{Convert|550|K}} at one atmosphere pressure. The perfect gas theory begins to break down at 550 K and is not usable at temperatures greater than {{Convert|2000|K}}. For temperatures greater than 2,000 K, a heat shield designer must use a ''real gas model''.