The microwave attenuation in the vacuum plume of the liquid-propellant rocket engine was studied. Based on a sample of the bipropellant engine

the electron number densities in the chamber was calculated using a thermodynamic calculation code. The ionization and recombination processes in the nozzle were simulated under the assumption of frozen components or limited reaction rate. The electron number densities in the plume and the microwave attenuation of 5－30GHz were estimated. It is shown that the number densities are within the range of 1013-1014cm-3 and 1010-1011cm-3

respectively

for the chamber and the exit of the nozzle. The alkali metals

K and Na

are main contributors of electrons while Cl- and OH- are main electron absorber. The attenuation is more serious for the microwave of lower frequency and oblique path

as high as 4.5dB in case of 5 GHz.