In ground test, the measured values include propulsion system wall temperature, wall pressure, thrust, fuel temperature, fuel pressure, etc. During a test, these can be used to inspect the working status of the propulsion system and to control fluid transfer system. Concretely, the measured values of fuel temperature and fuel pressure can be used to control fluid transfer system.

The fluid transfer system, such as one of injecting the hydrocarbon mixture into the propulsion system at both low and high temperature, must meet the transfer requirement over very different fluid densities. To reduce the pressure loss of high temperature fluid, one can let the fluid flows in passages (or holes) with larger area when its temperature is higher. This means the fluid transfer system can adjust the area of flow passages according to the fluid temperature.

The requirements for the fluid pressure just before the injecting holes, hereafter denoted as *p*inj, include three aspects. Firstly, the pressure of the hydrocarbon mixture during the circulation should not be markedly less than its critical value which is about 2.3MPa11. Secondly, steady combustion requires *p*inj should be higher than 1.5MPa. Lastly, the driving pressure of the fluid transfer system requires *p*inj should not be too high. Otherwise the circulation of the hydrocarbon mixture will be severely affected and the cooling structure will be destroyed. These requirements must be met by the fluid transfer system.

To meet the requirements for *p*inj and design the fluid transfer system, one should understand the following two things first. One is the relationship between the fluid pressure *p*inj and its temperature. The other is the matching between *p*inj and the area of injecting holes.

The fuel injecting holes, as shown in Fig. 5, are composed by many small holes which can restrict the fluid flow mass rate. The supercritical fluid flow across them can be approximated by the flow past a contracting passage and the maximum fluid speed is the local sound speed. The fluid flow before the injecting holes and passing them can be treated as a one dimensional isentropic flow passing different area if the three dimensional and dissipative effects are ignored. So the balance equations of mass, energy, and entropy can be adopted to solve the flow, which are

*puA = \({\dot {m}_0}\) *(1)

where, *h* is the enthalpy of the hydrocarbon mixture, *ρ* the density, *u* the fluid speed, \({\dot {m}_0}\) the mass flow rate, *a* the sound speed, *T* the temperature, *p* the pressure, *s* the entropy, *A* the total area of the injecting holes, and the subscript f means fluid and subscript t means the value at stagnation point. The area of the flow before the injecting holes is much bigger than *A*, so *p*f,t and *T*f,t can be treated as the values just before the injecting holes (*p*inj and *T*inj). Using these balance equations and the relationship between *ρ, T* and *p* (Fig. 2), the relationship between the mass flow rate and *p*f,t (*p*inj) can be determined after some iterations of algebraic calculation.

According to the above three balance equations, one can also increase the total area of the injecting holes (*A*) to decrease *p*f,t (*p*inj). And this is the regulating scheme of the total area of the injecting holes (*A*) for the fluid transfer system. To consider the operational performance and to limit the number of the fuel pipes, three groups of injecting holes are chosen to design the fluid transfer system, the regulating scheme of the area of fuel injecting holes is shown in Fig. 6.

The regulating criterion is the key to the regulating scheme. Inappropriate criterion may lead to a too low *p*inj and the extinction of propulsion system. Applying the three balance equations of mass, energy, and entropy, one can construct a modeling tool to calculate the relationships among *p*inj, *A* and *T*inj. For the chosen three groups of injecting holes, the relationships among *p*inj, *A* and *T*inj are shown in Fig. 7.