Blackout analysis of small reentry vehicles

The high temperatures associated with the hypersonic reentry process lead to an increase in the collisions between molecules, which may result in the disruption of the electronic structure, producing free electrons and ions. This production of free electrons and ions creates a plasma or ionized flow field around the vehicle that is known to degrade the quality of radio-wave signal propagation. As a result, radio frequency waves can be attenuated and/or reflected by electrons in the plasma leading to a loss of communication or "Black-Out." The vehicle subsequently loses all communication including GPS signals, data telemetry, and can travel for hundreds of miles during this loss of communication. For example, Apollo 13 experienced a blackout phase of 6 minutes and the Mars pathfinder experienced a 30-second radio blackout. One of the most promising approaches in reducing the blackout phase for vehicles is aerodynamic shaping. It is known that small, slender reentry vehicles approximately 20 cm in diameter will have less gas ionization on the surface resulting in a smaller blackout period. Thus, a primary goal of this project is to investigate the flow field of various geometries at different flight conditions to improve the design of airborne collision avoidance system (ACAS) technology. Hypersonic CFD simulations are conducted in the commercial software CFD++ where Dunn and Kang's reaction file is implemented to model the chemical non-equilibrium phenomenon of the flow-field. It is found that as the cone angle decreases, an increase in the electron density occurs further downstream. Due to the shift of the reaction zone towards the aft position for smaller cone angles, higher mach numbers are seen to have less of an impact in determining the location of an antenna. The 15° geometry is seen to be best in terms of communicating through the blackout period. Results are in good agreement with literature.