Implementation of microwave transmissions for rocket plume diagnostics

This paper presents an experiment that relates the electron number density of a solid rocket exhaust plume to its propagation constant. Estimated values of the collision frequency and electron number density are taken from a numerical simulation of a rocket plume and are used to guide the design of the experimental apparatus. The apparatus transmits a radio sweep between 4 GHz and 7 GHz through the exhaust plume of a J-class solid rocket motor. Data are range-gated to a 10 cm resolution. Microwave transmissions are used to measure the scattering parameters of the exhaust plume. While diffraction of the transmission around the exhaust makes precise analysis difficult, measurements of the signal attenuation across the plume imply an electron number density between 10¹⁴ m^-3 and 10¹⁵ m^-3.