Characterisation of the radio transmission black-out — CFD Evaluation of Black-Out

Abstract

This document describes the DLR CFD based evaluation of the blackout phenome-non around a 30 cm spherical re-entry performed in the frame of the "Characterisation of the radio transmission black-out phenomenon due to plasma flow" project. Three different flow conditions from the reference trajectory have been simulated employing the DLR CFD code TAU in order to determine the plasma frequencies at around peak heating conditions of a spherical re-entry vehicle. The flow-field was discretized employing hybrid grids. For each flow condition the grid was adapted several times to obtain grid converged results. An 11 component model accounting for N2, O2, NO, N, O, N2+, O2+, NO+, N+, O+ and e- in combination with a two-temperature thermal nonequilibrium model accounting for a separate electron temperature was employed to determine the electron number density around the capsule accurately. The plasma frequencies were determined based on the computed electron number densities. The maximum plasma frequency which is considered the minimal transmission frequency for unhindered transmission was determined along radial straights. The results indicate radio transmission black-out for all of the investigated flow conditions. However, while the radio wave attenuation is 13 orders of magnitude at peak heating conditions it drops to a factor of 70 at peak dynamic pressure. Therefore, radio communication should be possible at slightly lower altitude.