Transpiration cooling is a thermodynamic process where

cooling Cooling is removal of heat, usually resulting in a lower temperature and/or phase change. Temperature lowering achieved by any other means may also be called cooling.ASHRAE Terminology, https://www.ashrae.org/technical-resources/free-resources/as ...

is achieved by a process of moving a liquid or a gas through the wall of a structure to absorb some portion of the

heat energy In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...

from the structure while simultaneously actively reducing the

convective Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...

and radiative heat flux coming into the structure from the surrounding space. One approach to transpiration cooling is to move liquid through small pores in the outer wall of a body leading to evaporation of the liquid to a gas via the physical mechanism of

evaporative cooling An evaporative cooler (also known as evaporative air conditioner, swamp cooler, swamp box, desert cooler and wet air cooler) is a device that cools air through the evaporation of water. Evaporative cooling differs from other air conditioning s ...

. Other approaches are possible.Transpiration Cooling
, Thermopedia.com, accessed 30 January 2019.

Applications

Transpiration cooling is used in the

aerospace Aerospace is a term used to collectively refer to the atmosphere and outer space. Aerospace activity is very diverse, with a multitude of commercial, industrial and military applications. Aerospace engineering consists of aeronautics and astr ...

industry, in jetTranspiration Cooling Systems for Jet Engine Turbines and Hypersonic Flight
, accessed 30 January 2019. and rocket engines. In 2018, researchers at the

University of Oxford , mottoeng = The Lord is my light , established = , endowment = £6.1 billion (including colleges) (2019) , budget = £2.145 billion (2019–20) , chancellor ...

were experimentally testing transpiration cooling as a

Thermal Protection System Atmospheric entry is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. There are two main types of atmospheric entry: ''uncontrolled entry'', such as the entr ...

for Hypersonic Vehicles such as rockets or

spaceplane A spaceplane is a vehicle that can fly and glide like an aircraft in Earth's atmosphere and maneuver like a spacecraft in outer space. To do so, spaceplanes must incorporate features of both aircraft and spacecraft. Orbital spaceplanes te ...

s. Transpiration cooling is one of a variety of cooling techniques that may be used to reduce

regenerative cooling Regenerative cooling is a method of cooling gases in which compressed gas is cooled by allowing it to expand and thereby take heat from the surroundings. The cooled expanded gas then passes through a heat exchanger where it cools the incoming comp ...

loads in

rocket engine A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accorda ...

s and subsequently reduce propellant requirements. Other techniques exist, such as film cooling,

ablative cooling Ablation ( la, ablatio – removal) is removal or destruction of something from an object by vaporization, chipping, erosive processes or by other means. Examples of ablative materials are described below, and include spacecraft material for a ...

, radiative cooling, heat sink cooling and dump cooling. Transpiration cooling is being considered for use in space vehicles reentering the Earth's atmosphere at hypersonic velocities where a transpirationally cooled outer skin could serve as a part of the

thermal protection system Atmospheric entry is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. There are two main types of atmospheric entry: ''uncontrolled entry'', such as the entr ...

of the reentering spacecraft.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], Mike Wall, space.com, 23 January 2019, accessed 30 January 2019.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], Eric Ralph, ''Teslarati News'', 23 January 2019, accessed 30 January 2019 SpaceX publicly mentioned such a system in 2019 for use on their Starship reusable second stage and orbital

spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...

to mitigate the harsh conditions of reentry. The design concept envisioned a double stainless-steel skin, with active coolant flowing between the two layers, with some areas additionally containing multiple small pores that would allow for transpiration cooling. After design and testing in terrestrial labs, SpaceX subsequently stated that although an alternative heat mitigation approach—using low-cost ceramic tiles on the windward side of Starship—was being developed,Could do it, but we developed low cost reusable tiles that are much lighter than transpiration cooling & quite robust
, Elon Musk, 24 September 2019, accessed 24 September 2019. transpiration cooling could be used in some areas. Few details on the design are expected to be publicly released, as

US law The law of the United States comprises many levels of codified and uncodified forms of law, of which the most important is the nation's Constitution, which prescribes the foundation of the federal government of the United States, as well as va ...

prevents SpaceX from releasing such information.Q: Are you still considering transpiration cooling for most vulnerable hotspots or are the heat shield tiles tough enough? A: It might be used in some areas. ITAR laws prevent us from being too specific about solutions.
, Elon Musk, 9 October 2020, accessed 10 October 2020.

References

{{Reflist, 33em Thermodynamic processes Spaceflight technology

Applications

See also

References