Implications and Mitigation of Radio Frequency Blackout During Reentry of Reusable Launch Vehicles

Abstract

Reentering hypersonic vehicles become surrounded by a layer of dissociated plasma, consisting of ions and free electrons, during high-velocity passage through the upper atmosphere. The ionized layer or sheath can reflect and attenuate propagating electromagnetic waves severely, causing radio communications to be degraded or temporarily disrupted. This interruption in signal transmission and reception, or radio frequency (RF) blackout, is of concern for determination of vehicle position and vehicle control, key issues affecting public safety, especially for future manned flights where continuous contact with ground control will be crucial. Causes of RF blackout from plasma generation around vehicles during hypersonic reentry were reviewed. Methodologies for mitigating communications blackouts, applicable to reusable launch vehicles (RLVs) for commercial space, were surveyed. Interactions of RF signals with a known ionized layer, including reflection, attenuation, refraction, high-power breakdown limits, and effects of the plasma on antenna characteristics, were explored. RF blackout mitigation strategies fall into two general classes: passive and active. Passive approaches necessitate design of vehicle configurations to minimize plasma effects on communications signals. Active approaches entail manipulation of the plasma conditions and electron density in localized regions surrounding communication antennas to facilitate RF transmission. Examples of passive approaches include: using vehicles with leading edges aerodynamically shaped to minimize plasma generation, designing for communication at higher frequencies, and designing for radiating higher power from the vehicle. Examples of active approaches include: injection of electrophilic quenchants or droplets that evaporatively cool the plasma and application of magnetic fields. The most promising approaches for mitigating the interruption of communications due to interactions of plasma electrons with RF signals are aerodynamic shaping, injection of electrophilic quenchants, use of magnetic windows, and use of high frequencies within the limits imposed by atmospheric attenuation.