Burt Rutan's White Knight and SpaceShip One, Photo Courtesy of Scaled Composites
Thermodynamics and Propulsion

19. Radiation Heat Transfer (Heat transfer by thermal radiation)

All bodies radiate energy in the form of photons moving in a random direction, with random phase and frequency. When radiated photons reach another surface, they may either be absorbed, reflected or transmitted. The behavior of a surface with radiation incident upon it can be described by the following quantities:

  • $ \alpha$ = absorptance - fraction of incident radiation absorbed
  • $ \rho$ = reflectance - fraction of incident radiation reflected
  • $ \tau$ = transmittance - fraction of incident radiation transmitted.
Figure 19.1 shows these processes graphically.

Figure 19.1: Radiation surface properties
Image fig11SurfaceProperties_web

From energy considerations the three coefficients must sum to unity

$\displaystyle \alpha+ \rho + \tau= 1.$

Reflective energy may be either diffuse or specular (mirror-like). Diffuse reflections are independent of the incident radiation angle. For specular reflections, the reflection angle equals the angle of incidence.