Calculation of refractive index around a hypersonic vehicle with infrared sensors

The disturbed gas envelope around a hypersonic vehicle which carries an infrared sensor is shown. The envelope contains a compressed hot gas and plasma produced by the ionization of the gas constituents. Both gas and plasma surrounding the sensors have high densities, resulting in a sharp density gradient at the shock wave in front of the vehicle. The density gradients inside the envelope act like a multiple-lens system which refracts optical rays in the medium. Plasma effects are important only in the nose-cone region where temperatures and the densities of the compressed gas are high. In regions far from the nose-cone, plasma effects are negligible. The relative effects of the gas and the plasma on the refractive index are as follows. At low Mach numbers, the gas effects are more significant than the plasma effects. At high Mach numbers, this situation is reversed. By comparing the effects of plasma based on the ionizations obtained from Saha's equation and the CFD code, it was found that transport effects make use of the Saha equation less reliable for estimating the refractive index due to the plasma.<>