Editing Atmospheric entry (section)

===Passively cooled===
[[File:Mercury capsule HD.jpg|thumb|upright|left|The Mercury capsule design (shown here with its [[Launch escape system|escape tower]]) originally used a radiatively cooled TPS, but was later converted to an ablative TPS.]]
In some early ballistic missile RVs (e.g., the Mk-2 and the [[sub-orbital spaceflight|sub-orbital]] [[Project Mercury|Mercury spacecraft]]), ''radiatively cooled TPS'' were used to initially absorb heat flux during the heat pulse, and, then, after the heat pulse, radiate and convect the stored heat back into the atmosphere. However, the earlier version of this technique required a considerable quantity of metal TPS (e.g., [[titanium]], [[beryllium]], [[copper]], etc.). Modern designers prefer to avoid this added mass by using ablative and thermal-soak TPS instead.

Thermal protection systems relying on [[emissivity]] use high emissivity coatings (HECs) to facilitate [[radiative cooling]], while an underlying porous ceramic layer serves to protect the structure from high surface temperatures. High thermally stable emissivity values coupled with low thermal conductivity are key to the functionality of such systems.<ref name="rtps">{{cite journal | last1=Shao| first1=Gaofeng|display-authors=et al| title= Improved oxidation resistance of high emissivity coatings on fibrous ceramic for reusable space systems | journal= Corrosion Science | year=2019 | volume=146| pages= 233–246 | doi= 10.1016/j.corsci.2018.11.006 | arxiv=1902.03943 | bibcode=2019Corro.146..233S| s2cid=118927116}}</ref>

Radiatively cooled TPS can be found on modern entry vehicles, but [[reinforced carbon–carbon]] (RCC) (also called ''carbon–carbon'') is normally used instead of metal. RCC was the TPS material on the Space Shuttle's nose cone and wing leading edges, and was also proposed as the leading-edge material for the [[X-33]]. [[Carbon]] is the most refractory material known, with a one-atmosphere sublimation temperature of {{Convert|3825|C}} for graphite. This high temperature made carbon an obvious choice as a radiatively cooled TPS material. Disadvantages of RCC are that it is currently expensive to manufacture, is heavy, and lacks robust impact resistance.<ref>{{Cite web|url=https://history.nasa.gov/columbia/CAIB_reportindex.html|title=Columbia Accident Investigation Board|website=history.nasa.gov|access-date=July 12, 2017|archive-date=December 25, 2017|archive-url=https://web.archive.org/web/20171225231135/https://history.nasa.gov/columbia/CAIB_reportindex.html|url-status=live}}</ref>

Some high-velocity [[aircraft]], such as the [[SR-71 Blackbird]] and [[Concorde]], deal with heating similar to that experienced by spacecraft, but at much lower intensity, and for hours at a time. Studies of the SR-71's titanium skin revealed that the metal structure was restored to its original strength through [[annealing (metallurgy)|annealing]] due to aerodynamic heating. In the case of the Concorde, the [[aluminium]] nose was permitted to reach a maximum [[operating temperature]] of {{convert|127|°C|°F}} (approximately {{convert|180|C-change|F-change|0}} warmer than the normally sub-zero, ambient air); the metallurgical implications (loss of [[tempering (metallurgy)|temper]]) that would be associated with a higher peak temperature were the most significant factors determining the top speed of the aircraft.

A radiatively cooled TPS for an entry vehicle is often called a ''hot-metal TPS''. Early TPS designs for the Space Shuttle called for a hot-metal TPS based upon a nickel [[superalloy]] (dubbed [[René 41]]) and titanium shingles.<ref name="auto">{{Cite web|url=http://www.astronautix.com/s/spaceshuttle.html|title=Space Shuttle|website=www.astronautix.com|access-date=April 22, 2022|archive-date=March 18, 2022|archive-url=https://web.archive.org/web/20220318082348/http://www.astronautix.com/s/spaceshuttle.html|url-status=dead}}</ref> This Shuttle TPS concept was rejected, because it was believed a silica tile-based TPS would involve lower development and manufacturing costs.{{Citation needed|date=July 2010}} A nickel [[superalloy]]-shingle TPS was again proposed for the unsuccessful [[X-33]] [[single-stage-to-orbit]] (SSTO) prototype.<ref>{{Cite web |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040095922_2004100223.pdf |title=X-33 Heat Shield Development report |access-date=July 7, 2017 |archive-date=January 26, 2021 |archive-url=https://web.archive.org/web/20210126222704/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040095922_2004100223.pdf |url-status=live }}</ref>

Recently, newer radiatively cooled TPS materials have been developed that could be superior to RCC. Known as [[Ultra high temperature ceramic matrix composite|Ultra-High Temperature Ceramics]], they were developed for the prototype vehicle Slender Hypervelocity Aerothermodynamic Research Probe (SHARP). These TPS materials are based on [[zirconium diboride]] and [[hafnium diboride]]. SHARP TPS have suggested performance improvements allowing for sustained [[Mach number|Mach]] 7 flight at sea level, Mach 11 flight at {{convert|100000|ft|adj=on}} altitudes, and significant improvements for vehicles designed for continuous hypersonic flight. SHARP TPS materials enable sharp leading edges and nose cones to greatly reduce drag for airbreathing combined-cycle-propelled spaceplanes and lifting bodies. SHARP materials have exhibited effective TPS characteristics from zero to more than {{Convert|2000|C}}, with melting points over {{Convert|3500|C}}. They are structurally stronger than RCC, and, thus, do not require structural reinforcement with materials such as Inconel. SHARP materials are extremely efficient at reradiating absorbed heat, thus eliminating the need for additional TPS behind and between the SHARP materials and conventional vehicle structure. NASA initially funded (and discontinued) a multi-phase R&D program through the [[University of Montana]] in 2001 to test SHARP materials on test vehicles.<ref>{{cite web|url=http://hubbard.engr.scu.edu/docs/thesis/2003/SHARP_Thesis.pdf |title=SHARP Reentry Vehicle Prototype |access-date=2006-04-09 |url-status=dead |archive-url=https://web.archive.org/web/20051215231157/http://hubbard.engr.scu.edu/docs/thesis/2003/SHARP_Thesis.pdf |archive-date=2005-12-15 }}</ref><ref>{{Cite web|url=http://www.coe.montana.edu/me/faculty/cairns/sharp/sharp.htm|archive-url=https://web.archive.org/web/20151016071845/http://www.coe.montana.edu/me/faculty/cairns/sharp/sharp.htm |url-status=dead |title=sharp structure homepage w left<!-- Bot generated title -->|archive-date=October 16, 2015}}</ref>