Editing Atmospheric entry (section)

====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>