An energy balance in inductively coupled radio frequency electrothermal thruster using water as a propellant was investigated with the objective of improving the thrust performance. Absorption efficiency was estimated by means of an I-V sensor, enthalpy transfer efficiency was estimated by means of a spectrometer and thrust efficiency was estimated by means of a thrust stand. The absorption efficiency using water is 0.75, which is worse than that using argon. This is because the number density using water is less than that of argon due to the low ionization coefficient. The enthalpy transfer efficiency using water is 0.14, which is superior to that using argon. This is because the temperature using water is lower than that of argon. The thrust efficiency using water is 0.05. The left energy would be dispersed as frozen flow loss and nozzle loss. Overall, the thrust performance, thrust, specific impulse and thrust efficiency are 3.6 mN, 340 sec, and 0.05, respectively, at mass flow rate =1.1 mg/s, and Incident power = 100 W.
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