Editing Atmospheric entry (section)

==History==
[[File:Blunt body reentry shapes.png|thumb|Early reentry-vehicle concepts visualized in [[Schlieren photography|shadowgraph]]s of [[high speed wind tunnel]] tests]]
The concept of the ablative [[heat shield]] was described as early as 1920 by [[Robert Goddard]]: "In the case of meteors, which enter the atmosphere with speeds as high as {{convert|30|mi}} per second, the interior of the meteors remains cold, and the erosion is due, to a large extent, to chipping or cracking of the suddenly heated surface. For this reason, if the outer surface of the apparatus were to consist of layers of a very infusible hard substance with layers of a poor heat conductor between, the surface would not be eroded to any considerable extent, especially as the velocity of the apparatus would not be nearly so great as that of the average meteor."<ref>{{cite web|last=Goddard|first=Robert H.|title=Report Concerning Further Developments|date=Mar 1920|url= http://siarchives.si.edu/history/exhibits/documents/goddardmarch1920.htm|publisher=The Smithsonian Institution Archives|access-date= 2009-06-29| archive-url= https://web.archive.org/web/20090626145252/http://siarchives.si.edu/history/exhibits/documents/goddardmarch1920.htm| archive-date= 26 June 2009 | url-status= live}}</ref>

Practical development of reentry systems began as the range, and reentry velocity of [[ballistic missiles]] increased. For early short-range missiles, like the [[V-2 rocket|V-2]], stabilization and aerodynamic stress were important issues (many V-2s broke apart during reentry), but heating was not a serious problem. Medium-range missiles like the Soviet [[R-5 Pobeda|R-5]], with a {{convert|1200|km|nmi|sp=us|adj=on|abbr=off}} range, required ceramic composite heat shielding on separable reentry vehicles (it was no longer possible for the entire rocket structure to survive reentry). The first [[ICBM]]s, with ranges of {{convert|8000|to|12000|km|nmi|sp=us|abbr=on}}, were only possible with the development of modern ablative heat shields and blunt-shaped vehicles.

In the United States, this technology was pioneered by [[H. Julian Allen]] and [[Alfred J. Eggers|A. J. Eggers Jr.]] of the [[National Advisory Committee for Aeronautics]] (NACA) at [[Ames Research Center]].<ref>Boris Chertok, "Rockets and People", NASA History Series, 2006</ref> In 1951, they made the counterintuitive discovery that a blunt shape (high drag) made the most effective heat shield.<ref>{{cite book |url=https://history.nasa.gov/SP-4305/sp4305.htm |last=Hansen |first=James R. |date=Jun 1987 |title=Engineer in Charge: A History of the Langley Aeronautical Laboratory, 1917–1958 |series=The NASA History Series |volume=sp-4305 |chapter=Chapter 12: Hypersonics and the Transition to Space |chapter-url=https://history.nasa.gov/SP-4305/ch12.htm |publisher=United States Government Printing |isbn=978-0-318-23455-7 |access-date=July 12, 2017 |archive-date=July 14, 2019 |archive-url=https://web.archive.org/web/20190714121746/https://history.nasa.gov/SP-4305/sp4305.htm |url-status=live }}</ref> From simple engineering principles, Allen and Eggers showed that the heat load experienced by an entry vehicle was inversely proportional to the [[drag coefficient]]; i.e., the greater the drag, the less the heat load. If the reentry vehicle is made blunt, air cannot "get out of the way" quickly enough, and acts as an air cushion to push the shock wave and heated shock layer forward (away from the vehicle). Since most of the hot gases are no longer in direct contact with the vehicle, the heat energy would stay in the shocked gas and simply move around the vehicle to later dissipate into the atmosphere.

The Allen and Eggers discovery, though initially treated as a military secret, was eventually published in 1958.<ref>
{{cite journal
 | url=http://naca.central.cranfield.ac.uk/reports/1958/naca-report-1381.pdf
 | last1=Allen | first1=H. Julian
 | last2=Eggers | first2=A. J. Jr.
 | title=A Study of the Motion and Aerodynamic Heating of Ballistic Missiles Entering the Earth's Atmosphere at High Supersonic Speeds
 | journal=NACA Annual Report | issue=NACA-TR-1381
 | volume=44.2 | pages=1125–1140 | year=1958
 | publisher=NASA Technical Reports
 | url-status=dead
 | archive-url=https://web.archive.org/web/20151013182336/http://naca.central.cranfield.ac.uk/reports/1958/naca-report-1381.pdf
 | archive-date=October 13, 2015
}}</ref>