In-flight S-band telemetry attenuation by ionized solid rocket motor plumes at high altitude

During rocket flights, ionized exhaust plumes from solid rocket motors may interfere with radio frequency transmission under certain conditions. A computational fluid dynamics and finite-difference time-domain method coupling approach was applied for pre-flight prediction of S-band frequency attenuation of the Epsilon launch vehicles 3rd stage with a flight altitude of 240 km. As the latest Japanese all-solid launch vehicle, it lacked available in-flight data to predict the attenuation. And although the numerical method has already been demonstrated with the P-band at a flight altitude of 100 km, it has yet to be demonstrated at higher frequency like the S-band and at higher altitude; hence, the plume flow simulation at the 240 km altitude might suffer low accuracy due to rarefied effect. Thus, the accuracy of the numerical method was investigated by comparing the flight data of M-V launch vehicles, already retired all-solid vehicles before the Epsilon launch vehicle. S-band transmission has been successfully estimated, but the results confirmed that flight altitude affects attenuation and showed some discrepancy against flight data due to rarefied effect of the flow. The numerical prediction of S-band attenuation empirically considering the altitude effect based on the Knudsen number ultimately led to the successful flight of the Epsilon launch vehicle.