Editing Atmospheric entry (section)

===Actively cooled===
Various advanced reusable spacecraft and hypersonic aircraft designs have been proposed to employ heat shields made from temperature-resistant metal [[alloy]]s that incorporate a refrigerant or cryogenic fuel circulating through them.

Such a TPS concept was proposed for the [[Rockwell X-30|X-30 National Aerospace Plane]] (NASP) in the mid-80s.{{cn|date=January 2024}} The NASP was supposed to have been a [[scramjet]] powered hypersonic aircraft, but failed in development.{{cn|date=January 2024}}

In 2005 and 2012, two unmanned [[lifting body]] craft with actively cooled hulls were launched as a part of the German [[Sharp Edge Flight Experiment]] (SHEFEX).{{cn|date=January 2024}}

In early 2019, [[SpaceX]] was developing an actively cooled heat shield for its [[SpaceX Starship|Starship]] spacecraft where a part of the thermal protection system will be a [[transpiration cooling|transpirationally cooled]] outer-skin design for the reentering spaceship.<ref name=sdc20190123>[https://www.space.com/43101-elon-musk-explains-stainless-steel-starship.html Why Elon Musk Turned to Stainless Steel for SpaceX's Starship Mars Rocket] {{Webarchive|url=https://web.archive.org/web/20190203064031/https://www.space.com/43101-elon-musk-explains-stainless-steel-starship.html |date=February 3, 2019 }}, Mike Wall, space.com, 23 January 2019, accessed 23 March 2019.</ref><ref name=trati20190123>[https://www.teslarati.com/spacex-ceo-elon-musk-starship-transpiring-steel-heat-shield-interview/ SpaceX CEO Elon Musk explains Starship's "transpiring" steel heat shield in Q&A] {{Webarchive|url=https://web.archive.org/web/20190124041422/https://www.teslarati.com/spacex-ceo-elon-musk-starship-transpiring-steel-heat-shield-interview/ |date=January 24, 2019 }}, Eric Ralph, ''Teslarati News'', 23 January 2019, accessed 23 March 2019</ref> However, SpaceX abandoned this approach in favor of a modern version of heat shield tiles later in 2019.<ref name="musk20190924">{{cite tweet |last=Musk |first=Elon |author-link=Elon Musk |user=elonmusk |number=1176561209971101696 |date=24 September 2019 |title=@OranMaliphant @Erdayastronaut Could do it, but we developed low cost reusable tiles that are much lighter than transpiration cooling & quite robust|access-date=9 May 2021 |archive-url=https://web.archive.org/web/20210427153543/https://twitter.com/elonmusk/status/1176561209971101696 |archive-date=27 April 2021 |url-status=live}}</ref><ref name="musk20190724">{{cite tweet |last=Musk |first=Elon |author-link=Elon Musk |user=elonmusk |number=1154229558989561857 |date=24 July 2019 |title=@Erdayastronaut @goathobbit Thin tiles on windward side of ship & nothing on leeward or anywhere on booster looks like lightest option|access-date=9 May 2021 |archive-url=https://web.archive.org/web/20210427154113/https://twitter.com/elonmusk/status/1154229558989561857 |archive-date=27 April 2021 |url-status=live}}</ref>

The [[Stoke Space Nova]] second stage, announced in October 2023 and not yet flying, uses a regeneratively cooled (by liquid hydrogen) heat shield.<ref>{{Cite web |last1=Volosín |first1=Trevor Sesnic |last2=Morales |first2=Juan I. |date=2023-02-04 |title=Full Reusability By Stoke Space |url=https://everydayastronaut.com/stoke-space/ |access-date=2023-02-05 |website=Everyday Astronaut |language=en-US}}</ref>

In the early 1960s various TPS systems were proposed to use water or other cooling liquid sprayed into the shock layer, or passed through channels in the heat shield. Advantages included the possibility of more all-metal designs which would be cheaper to develop, be more rugged, and eliminate the need for classified and unknown technology. The disadvantages are increased weight and complexity, and lower reliability. The concept has never been flown, but a similar technology (the plug nozzle<ref name="auto"/>) did undergo extensive ground testing.