**3.2 Application of graphite coating to expander cycle rocket engine**

The graphite coating has the possibility to improve an expander cycle rocket engine performance because temperature at an exit of regenerative cooling passage can be higher. That leads to increase the turbine power. Simple analysis of turbopump is considered in the present study. The specification of turbopump and rocket engine are considered as Table.2. The specification of turbopump and rocket engine are same as those studied by Schuff (2006). 90 percent of LNG fuel is used to drive the turbopump and the rest of them are bypassed.

The combustion chamber pressure, Pc, can be given by the power balance and pressure loss at fuel passage. although the detailed analysis is still necessary. Figure 18 show the combustion chamber pressure, Pc, versus Turbine Inlet Temperature (TIT). TIT is considered to be equal to the maximum allowable temperature at the exit of regenerative cooling passage.

The effects of TIT on Pc and vacuum Isp are indicated in Figure 18 and 19, respectively. Vacuum Isp is evaluated with chemical frozen flow in a nozzle and calculated by computational code by Gordon and Mcbride [6]. The higher TIT is, the greater vacuum Isp can be obtained, however, improvement of vacuum Isp is not so great. For example, in the case of nozzle expansion of 40, only 4 sec of vacuum Isp increment ( from 337.5 sec to 342.4 sec ) can be expected when TIT would be varied from 400 K to 900 K. However, combustion pressure can be increased to about twice as shown in Fig.18.

Therefore, practical application of graphite coating may be useful to minimize the combustion chamber size, rather than the improvement of Isp. In such cases, the size of combustion chamber will be depended on heat exchange between fuel and combustor. In our future study, more detailed and practical estimation should be conducted on heat transfer analysis and turbopump specification.
