## 4 Results and Discussion  
In order to test ODREC, the results of the study carried out by [PERSON] et al. [39] were reproduced. In that study, the authors analyzed the regenerative cooling of a 30 kN thrust class LOX/LCH\({}_{4}\) engine demonstrator that operates at an oxidizer-to-fuel ratio of 3.4 and a combustion chamber pressure of 55 bar [40]. They adopted a numerical approach that had been experimentally validated in a breadboard campaign [41]. A 3-D CFD model was created for the conjugate heat transfer analysis of the channel/coolant side. Additionally, an input heat flux obtained through reactive CFD analysis was defined on the hot gas side of the engine wall. The cooling channel dimensions, thrust chamber profile, and calculated results were obtained by digitizing the plots in the reference study [42]. The same case was then analyzed using ODREC. Comparison of the results obtained using ODREC and the results of the reference study are shown in Figure 6 and 7 for the wall temperature and the wall heat flux and for the coolant temperature and pressure, respectively. As the results in Figure 6 suggest, the wall heat flux was somewhat over-predicted by ODREC compared to the reference study, but the overall trend was captured. Wall temperature, however, was severely over-predicted, despite displaying similar trends. On the other hand, results in Figure 7 show that the coolant pressure drop calculation was fairly accurate compared to the reference study. The coolant temperature was over-predicted as a result of the higher estimated wall temperatures as well. It can be concluded that such a simple 1-D approach is not sufficient to perfectly reproduce the results of an experimentally validated, intensive numerical method. This mainly results from the use of the approximations presented by the Bartz correlation, Haaland equation, and Dittus-Boelter correlation as discussed in [43]. Particularly for LOX/LCH\({}_{4}\) engines, the Bartz correlation is reported to over-predict [44]. This shows that the propellant combination also plays a role in increasing the accuracy of the correlations [2], where in ODREC the most general correlations are used. The software being open source provides the user the freedom to modify the particular correlations. Additionally, 1-D analyses  
Figure 6: Comparison of the hot gas side wall temperature and heat flux results with the reference study [39]. usually over-predict the temperature as also encountered in [13]. However, the authors of that study also rightfully state that the aim of a 1-D thermal analysis is to provide a rapid estimation of the trends rather than very accurate estimations. Similarly, the results produced by the current tool provide insight about the thermal behavior within a certain margin. Furthermore, the phase change phenomenon is easily integrated without a high computational power demand. Therefore, this tool could be utilized to accelerate the early design phase by conducting parametric studies to evaluate the effects of geometrical parameters on thermal behavior, comparing different coolants and further examining the optimum design parameters. A Regenerative Cooling Comparison of LOX/LCH\({}_{4}\) and LOX/LC\({}_{3}\)H\({}_{8}\) Engines  
Regenerative cooling analyses of two small-scale rocket engines, a LOX/LCH\({}_{4}\) engine with an OF ratio equal to 4 and a LOX/LC\({}_{3}\)H\({}_{8}\) engine with an OF ratio equal to 3.9, both operating at a combustion chamber pressure of 26 bar and delivering a thrust of 15 kN, were carried out using ODREC. Relevant input parameters that are not explained in the text are given in Table 1. Combustion chamber entry temperature inputs for liquid fuel and liquid oxidizer are given as dictated by the CEA program as mentioned in Section 3.1. In this context, the LOX, LCH\({}_{4}\), and LC\({}_{3}\)H\({}_{8}\) combustion chamber entry temperatures were set to 90.17 K, 111.64 K, and 231.076 K, respectively.