# Fay-Riddell equation

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Relation in aerospace engineering

The **Fay-Riddell equation** is a fundamental relation in the fields of [aerospace engineering](/source/Aerospace_engineering) and [hypersonic flow](/source/Hypersonic_speed), which provides a method to estimate the [stagnation point](/source/Stagnation_point) heat transfer rate on a blunt body moving at hypersonic speeds in dissociated air.[1] The [heat flux](/source/Heat_flux) for a spherical nose is computed according to quantities at the wall and the edge of an equilibrium [boundary layer](/source/Boundary_layer).

q ˙ w = 0.763 ⋅ Pr − 0.6 ( ρ e μ e ) 0.4 ( ρ w μ w ) 0.1 ( d u e d x ) s ( h 0 , e − h w ) [ 1 + ( Le 0.52 − 1 ) ( h D h 0 , e ) ] {\displaystyle {\dot {q}}_{w}=0.763\cdot {\text{Pr}}^{-0.6}(\rho _{e}\mu _{e})^{0.4}(\rho _{w}\mu _{w})^{0.1}{\sqrt {\left({\frac {du_{e}}{dx}}\right)_{s}}}(h_{0,e}-h_{w})\left[1+({\text{Le}}^{0.52}-1)\left({\frac {h_{D}}{h_{0,e}}}\right)\right]}

where Pr {\displaystyle {\text{Pr}}} is the [Prandtl number](/source/Prandtl_number), Le {\displaystyle {\text{Le}}} is the [Lewis number](/source/Lewis_number), h 0 , e {\displaystyle h_{0,e}} is the [stagnation enthalpy](/source/Stagnation_enthalpy) at the boundary layer's edge, h w {\displaystyle h_{w}} is the wall [enthalpy](/source/Enthalpy), h D {\displaystyle h_{D}} is the enthalpy of dissociation, ρ {\displaystyle \rho } is the [air density](/source/Density_of_air), μ {\displaystyle \mu } is the [dynamic viscosity](/source/Dynamic_viscosity), and ( d u e / d x ) s {\displaystyle (du_{e}/dx)_{s}} is the velocity gradient at the stagnation point. According to Newtonian hypersonic flow theory, the velocity gradient should be: ( d u e d x ) s = 1 R 2 ( p e − p ∞ ) ρ e {\displaystyle \left({\frac {du_{e}}{dx}}\right)_{s}={\frac {1}{R}}{\sqrt {\frac {2(p_{e}-p_{\infty })}{\rho _{e}}}}} where R {\displaystyle R} is the nose radius, p e {\displaystyle p_{e}} is the [pressure](/source/Pressure) at the edge, and p ∞ {\displaystyle p_{\infty }} is the free stream pressure. The equation was developed by James Fay and Francis Riddell in the late 1950s. Their work addressed the critical need for accurate predictions of aerodynamic heating to protect spacecraft during [re-entry](/source/Atmospheric_entry), and is considered to be a pioneering work in the analysis of chemically reacting [viscous flow](/source/Viscous_flow).[2]

## Assumptions

The Fay-Riddell equation is derived under several assumptions:

1. **Hypersonic Flow**: The equation is applicable for flows where the [Mach number](/source/Mach_number) is significantly greater than 5.

1. **Continuum Flow**: It assumes the flow can be treated as a [continuum](/source/Continuum_mechanics), which is valid at higher altitudes with sufficient air density.

1. **Thermal and Chemical Equilibrium**: The gas is assumed to be in [thermal](/source/Thermal_equilibrium) and [chemical equilibrium](/source/Chemical_equilibrium), meaning the energy modes (translational, rotational, vibrational) and chemical reactions reach a steady state.

1. **Blunt Body Geometry**: The equation is most accurate for blunt body geometries where the leading edge radius is large compared to the boundary layer thickness.

## Extensions

While the Fay-Riddell equation was derived for an equilibrium boundary layer, it is possible to extend the results to a chemically frozen boundary layer with either an equilibrium [catalytic](/source/Catalysis) wall or a noncatalytic wall.[2] q ˙ w = 0.763 ⋅ Pr − 0.6 ( ρ e μ e ) 0.4 ( ρ w μ w ) 0.1 ( d u e d x ) s × { ( h 0 , e − h w ) [ 1 + ( Le 0.63 − 1 ) ( h D h 0 , e ) ] , ( Equilibrium Catalytic ) ( 1 − h D h 0 , e ) , ( Noncatalytic ) {\displaystyle {\dot {q}}_{w}=0.763\cdot {\text{Pr}}^{-0.6}(\rho _{e}\mu _{e})^{0.4}(\rho _{w}\mu _{w})^{0.1}{\sqrt {\left({\frac {du_{e}}{dx}}\right)_{s}}}\times {\begin{cases}(h_{0,e}-h_{w})\left[1+({\text{Le}}^{0.63}-1)\left({\frac {h_{D}}{h_{0,e}}}\right)\right],&({\text{Equilibrium Catalytic}})\\\left(1-{\frac {h_{D}}{h_{0,e}}}\right),&({\text{Noncatalytic}})\end{cases}}}

## Applications

The Fay-Riddell equation is widely used in the design and analysis of [thermal protection systems](/source/Thermal_protection_system) for re-entry vehicles.[3][4][5] It provides engineers with a crucial tool for estimating the severe aerodynamic heating conditions encountered during atmospheric entry and for designing appropriate thermal protection measures.

## See also

- [Aerodynamic heating](/source/Aerodynamic_heating)

- [Atmospheric entry](/source/Atmospheric_entry)

- [Hypersonic flight](/source/Hypersonic_flight)

- [Stagnation point](/source/Stagnation_point)

## References

1. **[^](#cite_ref-1)** Fay, J. A.; Riddell, F. R. (1958). ["Theory of Stagnation Point Heat Transfer in Dissociated Air"](https://arc.aiaa.org/doi/10.2514/8.7517). *Journal of the Aerospace Sciences*. **25** (2): 73–85. [doi](/source/Doi_(identifier)):[10.2514/8.7517](https://doi.org/10.2514%2F8.7517). [ISSN](/source/ISSN_(identifier)) [1936-9999](https://search.worldcat.org/issn/1936-9999).

1. ^ [***a***](#cite_ref-:0_2-0) [***b***](#cite_ref-:0_2-1) Anderson, Jr., John D. (2019). *Hypersonic and High-Temperature Gas Dynamics*. AIAA Education Series (3rd ed.). American Institute of Aeronautics and Astronautics. pp. 754–764. [ISBN](/source/ISBN_(identifier)) [978-1-62410-514-2](https://en.wikipedia.org/wiki/Special:BookSources/978-1-62410-514-2).

1. **[^](#cite_ref-3)** Li, Peng; Gao, Zhenxun (2014-08-04). ["An Engineering Method of Aerothermodynamic Environments Prediction for Complex Reentry Configurations"](https://arc.aiaa.org/doi/10.2514/6.2014-4414). *AIAA SPACE 2014 Conference and Exposition*. AIAA SPACE Forum. American Institute of Aeronautics and Astronautics. [doi](/source/Doi_(identifier)):[10.2514/6.2014-4414](https://doi.org/10.2514%2F6.2014-4414). [ISBN](/source/ISBN_(identifier)) [978-1-62410-257-8](https://en.wikipedia.org/wiki/Special:BookSources/978-1-62410-257-8).

1. **[^](#cite_ref-4)** Zuppardi, Gennaro; Verde, Gianpaolo (1998). ["Improved Fay-Riddell Procedure to Compute the Stagnation Point Heat Flux"](https://arc.aiaa.org/doi/10.2514/2.3342). *Journal of Spacecraft and Rockets*. **35** (3): 403–405. [Bibcode](/source/Bibcode_(identifier)):[1998JSpRo..35..403Z](https://ui.adsabs.harvard.edu/abs/1998JSpRo..35..403Z). [doi](/source/Doi_(identifier)):[10.2514/2.3342](https://doi.org/10.2514%2F2.3342). [ISSN](/source/ISSN_(identifier)) [0022-4650](https://search.worldcat.org/issn/0022-4650).

1. **[^](#cite_ref-5)** Papadopoulos, Periklis; Subrahmanyam, Prabhakar (2005-05-16). ["Computational Investigation and Simulation of Aerothermodynamics of Reentry Vehicles"](https://arc.aiaa.org/doi/10.2514/6.2005-3206). *AIAA/CIRA 13th International Space Planes and Hypersonics Systems and Technologies Conference*. American Institute of Aeronautics and Astronautics. [doi](/source/Doi_(identifier)):[10.2514/6.2005-3206](https://doi.org/10.2514%2F6.2005-3206). [ISBN](/source/ISBN_(identifier)) [978-1-62410-068-0](https://en.wikipedia.org/wiki/Special:BookSources/978-1-62410-068-0).

## External links

- [Stagnation Point Heating](https://tfaws.nasa.gov/TFAWS12/Proceedings/Aerothermodynamics%20Course.pdf)

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Adapted from the Wikipedia article [Fay-Riddell equation](https://en.wikipedia.org/wiki/Fay-Riddell_equation) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Fay-Riddell_equation?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.