In rocket systems, the re-entry speed to atmosphere is very high which leads to compression of air molecules and appearance of strong bow shock waves in the leading edge; consequently, this yields aerodynamic heating. Using ablating-dispensing materials on the leading edge surfaces, it is important to accurately determine heat flux on these moving boundaries. Measuring heat flux directly is very difficult or impossible in some situations. In the present study, the online Kalman filtering is used to determine heat flux accurately. Since the heat flux is estimated in online (non-iterative) fashion, the optimum location of temperature sensors can be effectively determined. In addition, the results of this study can be used to design heat flux sensors. In this paper, the optimum locations of three temperature sensors are calculated on the basis that the disturbances occur due to burning of sensors are reduced. More robust solutions are obtained for heat flux on the ablating surfaces.