The ideal working temperature of a solar thermal propulsion (STP) system is generally above 2200 K, and at this temperature, ammonia (NH3) can dissociate; therefore, it is not accurate to use a single ammonia propellant for the calculation and analysis of thruster performance, and the components of the mixture after ammonia dissociation should be considered. The mixture after ammonia dissociation is composed of atoms and molecules. Since the temperature usually does not exceed 3000 K (it is difficult for high-temperature resistant materials to achieve this temperature), the existence of ionic components can be completely ignored. For vibrational excitation, only molecular nitrogen and hydrogen are considered, which have stable vibrational excitation levels. This chapter focuses on the effect of chemical reactions on the propellant temperature and thruster performance. In an actual 3D flow field, the dissociation characteristics and temperature variation of the ammonia propellant mixture are completely different from those of lumped parameters. The 3D flow field distribution and component distribution pattern are obtained through numerical simulation. This chapter mainly discusses the flow and composition change pattern of the ammonia propellant mixture inside of the heat exchanger core and the nozzle.