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]

For other uses, see Transpirational cooling (biological).

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] In 2018, researchers at the University of Oxford were experimentally testing transpiration cooling as a Thermal Protection System for Hypersonic Vehicles such as rockets or spaceplanes.[7][3]

Transpiration cooling is 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 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 of the reentering spacecraft.[8][9][1][10] SpaceX publicly mentioned 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 between the two layers, 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 subsequently stated that although an alternative heat mitigation approach—using low-cost ceramic tiles on the windward side of Starship—was being developed,[12][13] transpiration cooling could be used in some areas. Few details on the design are expected to be publicly released, as US law prevents SpaceX from releasing such information.[14]

See also

References
  1. https://www.teslarati.com/spacex-ceo-elon-musk-starship-transpiring-steel-heat-shield-interview/ Archived 2019-01-24 at the Wayback Machine 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 Archived 2019-01-30 at the Wayback Machine, Thermopedia.com, accessed 30 January 2019.
  3. Ifti, Hassan Saad; Hermann, Tobias; McGilvray, Matthew (15 September 2018). "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. ISBN 978-1-62410-577-7. S2CID 139312401.
  4. Transpiration Cooling Systems for Jet Engine Turbines and Hypersonic Flight Archived 2020-08-02 at the Wayback Machine, accessed 30 January 2019.
  5. Shine, S.R.; Nidhi, S.Shri (2018). "Review on film cooling of liquid rocket engines". Propulsion and Power Research. 7: 1–18. doi:10.1016/j.jppr.2018.01.004.
  6. Keener, D.; Lenertz, J.; Bowersox, R.; Bowman, J. (1995). "Transpiration cooling effects on nozzle heat transfer and performance". Journal of Spacecraft and Rockets. 32 (6): 981–985. Bibcode:1995JSpRo..32..981K. doi:10.2514/3.26718.
  7. Hermann, T.; Ifti, H. S.; McGilvray, M.; Doherty, L.; Geraets, R.P. (26 November 2018). Mixing characteristics in a hypersonic flow around a transpiration cooled flat plate model. Archived from the original on 10 October 2020. Retrieved 10 October 2020.
  8. Gebhardt, Chris (3 April 2019). "Starhopper conducts Raptor Static Fire test". NASASpaceFlight.com. Archived from the original on 4 April 2019. 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 Archived 2019-02-03 at the Wayback Machine 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. "SpaceX's steel Starship glows during Earth reentry in first high-quality render". 26 March 2019. Archived from the original on 2019-03-26. Retrieved 2019-03-26.
  11. SpaceX Starship Will "Bleed Water" From Tiny Holes, Says Elon Musk Archived 2019-01-24 at the Wayback Machine. 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 Archived 2019-09-24 at the Wayback Machine, 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. Archived from the original on 24 September 2019. Retrieved 8 September 2019.
  14. 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. Archived 2020-10-10 at the Wayback Machine, Elon Musk, 9 October 2020, accessed 10 October 2020.
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