Editing Atmospheric entry (section)

===Ablative===
[[File:Apollo 12 heat shield.JPG|thumb|left|Ablative heat shield (after use) on [[Apollo 12]] capsule]]
The [[Ablation|ablative]] heat shield functions by lifting the hot shock layer gas away from the heat shield's outer wall (creating a cooler [[boundary layer]]). The boundary layer comes from ''blowing'' of gaseous reaction products from the heat shield material and provides protection against all forms of heat flux. The overall process of reducing the heat flux experienced by the heat shield's outer wall by way of a boundary layer is called ''blockage''. Ablation occurs at two levels in an ablative TPS: the outer surface of the TPS material chars, melts, and [[Sublimation (physics)|sublimes]], while the bulk of the TPS material undergoes [[pyrolysis]] and expels product gases. The gas produced by pyrolysis is what drives blowing and causes blockage of convective and catalytic heat flux. [[Pyrolysis]] can be measured in real time using [[thermogravimetric analysis]], so that the ablative performance can be evaluated.<ref>Parker, John and C. Michael Hogan, "Techniques for Wind Tunnel assessment of Ablative Materials", NASA Ames Research Center, Technical Publication, August, 1965.</ref> Ablation can also provide blockage against radiative heat flux by introducing carbon into the shock layer thus making it optically opaque. Radiative heat flux blockage was the primary thermal protection mechanism of the [[Galileo Probe]] TPS material (carbon phenolic). 

Early research on ablation technology in the USA was centered at [[NASA]]'s [[Ames Research Center]] located at [[Moffett Field]], California. [[Ames Research Center]] was ideal, since it had numerous [[wind tunnels]] capable of generating varying wind velocities. Initial experiments typically mounted a mock-up of the ablative material to be analyzed within a [[hypersonic]] wind tunnel.<ref>Hogan, C. Michael, Parker, John and Winkler, Ernest, of [[NASA]] Ames Research Center,  "An Analytical Method for Obtaining the Thermogravimetric Kinetics of Char-forming Ablative Materials from Thermogravimetric Measurements", AIAA/ASME Seventh Structures and Materials Conference, April, 1966</ref> Testing of ablative materials occurs at the Ames Arc Jet Complex. Many spacecraft thermal protection systems have been tested in this facility, including the Apollo, space shuttle, and Orion heat shield materials.<ref>{{Cite web|title = Arc Jet Complex|url = http://www.nasa.gov/centers/ames/research/technology-onepagers/arcjetcomplex.html|publisher = NASA|website = www.nasa.gov|access-date = 2015-09-05|archive-date = October 5, 2015|archive-url = https://web.archive.org/web/20151005011405/http://www.nasa.gov/centers/ames/research/technology-onepagers/arcjetcomplex.html|url-status = live}}</ref>

[[File:Mars Pathfinder.jpg|thumb|upright|''[[Mars Pathfinder]]'' during final assembly showing the aeroshell, cruise ring and solid rocket motor]]

====Carbon phenolic====
Carbon phenolic was originally developed as a rocket nozzle throat material (used in the [[Space Shuttle Solid Rocket Booster]]) and for reentry-vehicle nose tips.
The [[thermal conductivity]] of a particular TPS material is usually proportional to the material's density.<ref name="Di Benedetto">{{cite book|last1=Di Benedetto|first1=A.T.|last2=Nicolais|first2=L.|last3=Watanabe|first3=R.|title=Composite materials : proceedings of Symposium A4 on Composite Materials of the International Conference on Advanced Materials – ICAM 91, Strasbourg, France, 27–29 May 1991|date=1992|publisher=North-Holland|location=Amsterdam|isbn=978-0444893567|page=111}}</ref> Carbon phenolic is a very effective ablative material, but also has high density which is undesirable.

The NASA [[Galileo Probe]] used carbon phenolic for its TPS material.<ref name=HSAE>{{cite journal |last=Milos |first=Frank S. |journal=Journal of Spacecraft and Rockets |issn=1533-6794 |year=1997 |doi=10.2514/2.3293 |title=Galileo Probe Heat Shield Ablation Experiment |volume=34 |issue=6 |pages=705–713 |bibcode=1997JSpRo..34..705M |url=https://zenodo.org/record/1235941 }}
</ref>

If the heat flux experienced by an entry vehicle is insufficient to cause pyrolysis then the TPS material's conductivity could allow heat flux conduction into the TPS bondline material thus leading to TPS failure. Consequently, for entry trajectories causing lower heat flux, carbon phenolic is sometimes inappropriate and lower-density TPS materials such as the following examples can be better design choices:

====Super light-weight ablator====
''SLA'' in ''SLA-561V'' stands for ''super light-weight ablator''. SLA-561V is a proprietary ablative made by [[Lockheed Martin]] that has been used as the primary TPS material on all of the 70° sphere-cone entry vehicles sent by NASA to Mars other than the [[Mars Science Laboratory]] (MSL). SLA-561V begins significant ablation at a heat flux of approximately 110&nbsp;W/cm<sup>2</sup>, but will fail for heat fluxes greater than 300&nbsp;W/cm<sup>2</sup>. The MSL aeroshell TPS is currently designed to withstand a peak heat flux of 234&nbsp;W/cm<sup>2</sup>. The peak heat flux experienced by the ''[[Viking 1]]'' aeroshell which landed on Mars was 21&nbsp;W/cm<sup>2</sup>. For ''Viking 1'', the TPS acted as a charred thermal insulator and never experienced significant ablation. ''Viking 1'' was the first Mars lander and based upon a very conservative design. The Viking aeroshell had a base diameter of 3.54&nbsp;meters (the largest used on Mars until Mars Science Laboratory). SLA-561V is applied by packing the ablative material into a honeycomb core that is pre-bonded to the aeroshell's structure thus enabling construction of a large heat shield.<ref>{{cite tech report
| first=Huy
| last=Tran
| author2=Michael Tauber
| author3=William Henline
| author4=Duoc Tran
| author5=Alan Cartledge
| author6=Frank Hui
| author7=Norm Zimmerman
| title=Ames Research Center Shear Tests of SLA-561V Heat Shield Material for Mars-Pathfinder
| number=NASA Technical Memorandum 110402
| institution=NASA Ames Research Center
| year=1996
| url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960049758_1996080506.pdf
| access-date=July 7, 2017
| archive-date=September 25, 2020
| archive-url=https://web.archive.org/web/20200925143246/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960049758_1996080506.pdf
| url-status=live
}}</ref>

====Phenolic-impregnated carbon ablator====
[[File:OSIRIS-REx Sample Return (NHQ202309240002).jpg|thumb|OSIRIS-REx Sample Return Capsule at USAF Utah Range.|left]]
''Phenolic-impregnated carbon ablator'' (PICA), a [[carbon fiber]] preform impregnated in [[phenolic resin]],<ref>{{cite conference |url=http://www.jeanlachaud.com/research/Lachaud2010_AbstractOFWS5.pdf |first1=Jean |last1=Lachaud |first2=Nagi |last2=N. Mansour |title=A pyrolysis and ablation toolbox based on OpenFOAM |conference=5th OpenFOAM Workshop |place=Gothenburg, Sweden |date=June 2010 |page=1 |access-date=August 9, 2012 |archive-date=September 12, 2012 |archive-url=https://web.archive.org/web/20120912052713/http://www.jeanlachaud.com/research/Lachaud2010_AbstractOFWS5.pdf |url-status=live }}</ref> is a modern TPS material and has the advantages of low density (much lighter than carbon phenolic) coupled with efficient ablative ability at high heat flux. It is a good choice for ablative applications such as high-peak-heating conditions found on sample-return missions or lunar-return missions. PICA's thermal conductivity is lower than other high-heat-flux-ablative materials, such as conventional carbon phenolics.{{Citation needed|date=February 2009}}

PICA was patented by [[NASA Ames Research Center]] in the 1990s and was the primary TPS material for the [[Stardust (spacecraft)|Stardust]] aeroshell.<ref>Tran, Huy K, et al., "Qualification of the forebody heat shield of the Stardust's Sample Return Capsule", AIAA, Thermophysics Conference, 32nd, Atlanta, GA; 23–25 June 1997.</ref> The Stardust sample-return capsule was the fastest man-made object ever to reenter Earth's atmosphere, at 28,000&nbsp;mph (ca. 12.5&nbsp;km/s) at 135&nbsp;km altitude. This was faster than the Apollo mission capsules and 70% faster than the Shuttle.<ref name=stardust>{{cite web|url=http://stardust.jpl.nasa.gov/cool.html|title=Stardust – Cool Facts|website=stardust.jpl.nasa.gov|access-date=January 9, 2010|archive-date=January 12, 2010|archive-url=https://web.archive.org/web/20100112063823/http://stardust.jpl.nasa.gov/cool.html|url-status=live}}</ref> PICA was critical for the viability of the Stardust mission, which returned to Earth in 2006. Stardust's heat shield (0.81&nbsp;m base diameter) was made of one monolithic piece sized to withstand a nominal peak heating rate of 1.2&nbsp;kW/cm<sup>2</sup>. A PICA heat shield was also used for the [[Mars Science Laboratory]] entry into the [[Martian atmosphere]].<ref name="N+SX_picaX"/>

=====PICA-X=====
An improved and easier to produce version called PICA-X was developed by [[SpaceX]] in 2006–2010<ref name="N+SX_picaX"/> for the [[SpaceX Dragon 1|Dragon]] [[space capsule]].<ref name=srdc20090223>{{cite web|url=http://www.spaceref.com/news/viewpr.html?pid=27612|title=SpaceX Manufactured Heat Shield Material Passes High Temperature Tests Simulating Reentry Heating Conditions of Dragon Spacecraft|website=www.spaceref.com|date=February 23, 2009 }}</ref> The first reentry test of a PICA-X heat shield was on the [[Dragon C1]] mission on 8 December 2010.<ref name=clog20101208>[https://web.archive.org/web/20101211200945/http://cosmiclog.msnbc.msn.com/_news/2010/12/08/5614525-dragon-could-visit-space-station-next Dragon could visit space station next], ''[[msnbc.com]]'', 2010-12-08, accessed 2010-12-09.</ref> The PICA-X heat shield was designed, developed and fully qualified by a small team of a dozen engineers and technicians in less than four years.<ref name="N+SX_picaX">
{{cite web
 |last=Chambers 
 |first=Andrew 
 |title=NASA + SpaceX Work Together 
 |url=http://www.nasa.gov/offices/oce/appel/ask/issues/40/40s_space-x_prt.htm
 |publisher=NASA 
 |access-date=2011-02-16 
 |author2=Dan Rasky 
 |date=2010-11-14 
 |quote=''SpaceX undertook the design and manufacture of the reentry heat shield; it brought speed and efficiency that allowed the heat shield to be designed, developed, and qualified in less than four years.''' 
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20110416170908/http://www.nasa.gov/offices/oce/appel/ask/issues/40/40s_space-x_prt.htm 
 |archive-date=2011-04-16 
}}</ref>
PICA-X is ten times less expensive to manufacture than the NASA PICA heat shield material.<ref name="a&s201201">{{cite news
| last=Chaikin
| first=Andrew
| title=1 visionary + 3 launchers + 1,500 employees = ? : Is SpaceX changing the rocket equation?
| url=http://www.airspacemag.com/space/is-spacex-changing-the-rocket-equation-132285884/?page=2
| access-date=2016-06-03
| newspaper=Air & Space Smithsonian
| date=January 2012
| quote=''SpaceX's material, called PICA-X, is one-tenth as expensive than the original [NASA PICA material and is better], ... a single PICA-X heat shield could withstand hundreds of returns from low Earth orbit; it can also handle the much higher energy reentries from the Moon or Mars.''
| archive-date=September 7, 2018
| archive-url=https://web.archive.org/web/20180907221220/https://www.airspacemag.com/space/is-spacex-changing-the-rocket-equation-132285884/?page=2
| url-status=live
}}</ref>

=====PICA-3=====
A second enhanced version of PICA—called PICA-3—was developed by SpaceX during the mid-2010s.  It was first flight tested on the [[Crew Dragon]] spacecraft in 2019 during the [[Crew Dragon Demo-1|flight demonstration mission]], in April 2019, and put into regular service on that spacecraft in 2020.<ref>[https://www.nasa.gov/multimedia/nasatv/#public NASA TV broadcast for the Crew Dragon Demo-2 mission departure from the ISS] {{Webarchive|url=https://web.archive.org/web/20200802031316/https://www.nasa.gov/multimedia/nasatv/#public |date=August 2, 2020 }}, NASA, 1 August 2020.</ref>

===== HARLEM =====
PICA and most other ablative TPS materials are either proprietary or classified, with formulations and manufacturing processes not disclosed in the open literature. This limits the ability of researchers to study these materials and hinders the development of thermal protection systems. Thus, the High Enthalpy Flow Diagnostics Group (HEFDiG) at the [[University of Stuttgart]] has developed an open carbon-phenolic ablative material, called the HEFDiG Ablation-Research Laboratory Experiment Material (HARLEM), from commercially available materials. HARLEM is prepared by impregnating a preform of a carbon fiber porous monolith (such as Calcarb rigid carbon insulation) with a solution of resole phenolic resin and [[polyvinylpyrrolidone]] in [[ethylene glycol]], heating to polymerize the resin and then removing the solvent under vacuum. The resulting material is [[Curing (chemistry)|cured]] and machined to the desired shape.<ref>{{Cite journal |last1=Poloni |first1=E. |last2=Grigat |first2=F. |last3=Eberhart |first3=M. |last4=Leiser |first4=David |last5=Sautière |first5=Quentin |last6=Ravichandran |first6=Ranjith |last7=Delahaie |first7=Sara |last8=Duernhofer |first8=Christian |last9=Hoerner |first9=Igor |last10=Hufgard |first10=Fabian |last11=Loehle |first11=Stefan |display-authors=3|date=12 August 2023 |title=An open carbon–phenolic ablator for scientific exploration |journal=Scientific Reports |volume=13 |issue=1 |page=13135 |article-number=13135|doi=10.1038/s41598-023-40351-x |doi-access=free|issn=2045-2322 |pmc=10423272 |pmid=37573464|bibcode=2023NatSR..1313135P }}</ref><ref>{{Cite journal |last1=Poloni |first1=E. |last2=Bouville |first2=Florian |last3=Schmid |first3=Alexander L. |last4=Pelissari |first4=Pedro I.B.G.B. |last5=Pandolfelli |first5=Victor C. |last6=Sousa |first6=Marcelo L.C. |last7=Tervoort |first7=Elena |last8=Christidis |first8=George |last9=Shklover |first9=Valery |last10=Leuthold |first10=Juerg |last11=Studart |first11=André R. |display-authors=1 |date=2022 |title=Carbon ablators with porosity tailored for aerospace thermal protection during atmospheric re-entry |journal=Carbon |volume=195 |pages=80–91 |doi=10.1016/j.carbon.2022.03.062 |doi-access=free|issn=0008-6223|arxiv=2110.04244 |bibcode=2022Carbo.195...80P }}</ref>

====SIRCA====
[[File:Ds 2.jpg|thumb|[[Deep Space 2]] [[Lander (spacecraft)|impactor]] aeroshell, a classic 45° sphere-cone with spherical section afterbody, enabling aerodynamic stability from atmospheric entry to surface impact]]
Silicone-impregnated reusable ceramic ablator (SIRCA) was also developed at NASA Ames Research Center and was used on the Backshell Interface Plate (BIP) of the ''[[Mars Pathfinder]]'' and [[Mars Exploration Rover]] (MER) aeroshells. The BIP was at the attachment points between the aeroshell's backshell (also called the afterbody or aft cover) and the cruise ring (also called the cruise stage). SIRCA was also the primary TPS material for the unsuccessful [[Deep Space 2]] (DS/2) Mars [[Lander (spacecraft)|impactor]] probes with their {{Convert|0.35|m|ft|-base-diameter|adj=mid|sp=us}} aeroshells. SIRCA is a monolithic, insulating material that can provide thermal protection through ablation. It is the only TPS material that can be machined to custom shapes and then applied directly to the spacecraft. There is no post-processing, heat treating, or additional coatings required (unlike Space Shuttle tiles). Since SIRCA can be machined to precise shapes, it can be applied as tiles, leading edge sections, full nose caps, or in any number of custom shapes or sizes. {{as of|1996}}, SIRCA had been demonstrated in backshell interface applications, but not yet as a forebody TPS material.<ref>Tran, Huy K., et al., "Silicone impregnated reusable ceramic ablators for Mars follow-on missions," AIAA-1996-1819, Thermophysics Conference, 31st, New Orleans, June 17–20, 1996.</ref>

====AVCOAT====
[[AVCOAT 5026-39|AVCOAT]] is a [[NASA]]-specified ablative heat shield, a glass-filled [[epoxy]]–[[novolac]] system.<ref name=nasa196808>[https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680021275_1968021275.pdf Flight-Test Analysis Of Apollo Heat-Shield Material Using The Pacemaker Vehicle System] {{Webarchive|url=https://web.archive.org/web/20200925144304/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680021275_1968021275.pdf |date=September 25, 2020 }} [[NASA]] Technical Note D-4713, pp. 8, 1968–08, accessed 2010-12-26. ''"Avcoat 5026-39/HC-G is an epoxy novolac resin with special additives in a fiberglass honeycomb matrix. In fabrication, the empty honeycomb is bonded to the primary structure and the resin is gunned into each cell individually. ... The overall density of the material is 32 lb/ft3 (512 kg/m3). The char of the material is composed mainly of silica and carbon. It is necessary to know the amounts of each in the char because in the ablation analysis the silica is considered to be inert, but the carbon is considered to enter into exothermic reactions with oxygen. ... At 2160O R (12000 K), 54 percent by weight of the virgin material has volatilized and 46 percent has remained as char. ... In the virgin material, 25 percent by weight is silica, and since the silica is considered to be inert the char-layer composition becomes 6.7 lb/ft3 (107.4 kg/m3) of carbon and 8 lb/ft3 (128.1 kg/m3) of silica."''</ref><!-- the AVCOAT article claims this material was used on the Apollo command module, but no citation is provided so the claim is not [[WP:V|verified]]. -->

NASA originally used it for the [[Apollo command and service module#Command Module (CM)|Apollo command module]] in the 1960s, and then utilized the material for its next-generation beyond low Earth orbit [[Orion (spacecraft)|Orion]] crew module, which first flew in a December 2014 test and then operationally in November 2022.<ref name=nasa20090407>[http://www.nasa.gov/home/hqnews/2009/apr/HQ_09-080_Orion_Heat_Shield.html NASA.gov NASA Selects Material for Orion Spacecraft Heat Shield] {{Webarchive|url=https://web.archive.org/web/20101124220318/http://www.nasa.gov/home/hqnews/2009/apr/HQ_09-080_Orion_Heat_Shield.html |date=November 24, 2010 }}, 2009-04-07, accessed 2011-01-02.</ref> The Avcoat to be used on Orion has been reformulated to meet environmental legislation that has been passed since the end of Apollo.<ref name=fg20090310>{{Cite web|url=http://www.flightglobal.com/articles/2009/03/10/323585/nasas-orion-heat-shield-decision-expected-this-month.html|title=Flightglobal.com NASA's Orion heat shield decision expected this month 2009-10-03, accessed 2011-01-02|access-date=January 2, 2011|archive-date=March 24, 2009|archive-url=https://web.archive.org/web/20090324160130/http://www.flightglobal.com/articles/2009/03/10/323585/nasas-orion-heat-shield-decision-expected-this-month.html|url-status=live}}</ref><ref>{{cite web|url=http://www.thefreelibrary.com/Company+Watch+-+NASA-a0198584187|title=Company Watch – NASA. – Free Online Library|website=www.thefreelibrary.com|access-date=January 2, 2011|archive-date=October 22, 2012|archive-url=https://web.archive.org/web/20121022003103/http://www.thefreelibrary.com/Company+Watch+-+NASA-a0198584187|url-status=live}}</ref>