Transpiration cooling

Transpiration cooling is a thermodynamic process where cooling 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 from the structure while simultaneously actively reducing the convective and radiative[1] heat flux coming into the structure from the surrounding space.[2]

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. Other approaches are possible.[2][3]

Applications

Transpiration cooling is used in the aerospace industry, in jet[4] and rocket engines.[5][6] Researchers at the University of Oxford are experimentally testing transpiration cooling as a Thermal Protection System for Hypersonic Vehicles such as rockets or Spaceplanes[3][7].

Transpiration cooling is just one of a variety of cooling techniques that may be used to reduce regenerative cooling loads in rocket engines and subsequently reduce propellant requirements. Other techniques exist, such as film cooling, ablative cooling, radiative cooling, heat sink cooling and dump cooling.[5]

Transpiration cooling has been 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 of the reentering spacecraft.[8][9][1][10] SpaceX considered such a system in 2019 for use on their Starship reusable second stage and orbital spacecraft to mitigate the harsh conditions of reentry. The design concept envisioned a double stainless-steel skin with active coolant flowing in between the two layers and with some areas additionally containing multiple small pores that would allow for transpiration cooling.[8][11][1] After design and testing in terrestrial labs, SpaceX ultimately determined to not use transpiration cooling and use ceramic tiles on the windward side of Starship.[12][13]

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See also

References
  1. 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
  2. Transpiration Cooling, Thermopedia.com, accessed 30 January 2019.
  3. Ifti, Hassan Saad; Hermann, Tobias; McGilvray, Matthew (2018-09-15), "Flow Characterisation of Transpiring Porous Media for Hypersonic Vehicles", 22nd AIAA International Space Planes and Hypersonics Systems and Technologies Conference, AIAA SPACE Forum, American Institute of Aeronautics and Astronautics, doi:10.2514/6.2018-5167, retrieved 2020-07-08
  4. Transpiration Cooling Systems for Jet Engine Turbines and Hypersonic Flight, accessed 30 January 2019.
  5. Review on film cooling of liquid rocket engines, S.R. Shinen, S. Shri Nidhi, 21 March 2016, Propulsion and Power Research, 2018;7(1):1–18
  6. Transpiration cooling effects on nozzle heat transfer and performance, 1995, D. Keener, J. Lenertz, R. Bowersox, and J. Bowman, Journal of Spacecraft and Rockets, Vol. 32, No. 6, pp. 981-985. https://doi.org/10.2514/3.26718
  7. Ifti, H. S.; Hermann, T.; McGilvray, M. (2019). "Transpiration cooling at Mach 5 employing porous UHTC". Cite journal requires |journal= (help)
  8. Gebhardt, Chris (3 April 2019). "Starhopper conducts Raptor Static Fire test". NASASpaceFlight.com. Retrieved 4 April 2019. Transpiration cooling is the act of pumping coolant – in this case, liquid methane – through the skin structure of Starship to absorb additional heat in particular areas of the vehicle during reentry.
  9. 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.
  10. https://www.teslarati.com/spacex-starship-official-render/
  11. SpaceX Starship Will "Bleed Water" From Tiny Holes, Says Elon Musk. Kristin Houser, Futurism. 22 January 2019.
  12. 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.
  13. Ralph, Eric. "SpaceX tests ceramic Starship heat shield tiles on Starhopper's final flight test". Teslarati. Retrieved 8 September 2019.


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