Study on the thermal and life prediction characteristics of the regenerative cooling system for the methane liquid rocket engine

Abstract

Now that methane is pro-environment, pro-economy and higher specific impulse compared with other hydrocarbon fuels, methane rocket engines have been researched in Russia, USA, Japan and EU as the center of the countries and domestic research was carried out at some companies from mid 1990s. It was very difficult to acquire pure methane in the domestic market and liquefied natural gas was inevitably used instead of pure methane, because liquefied natural gas could be purchased in the market. So, the characteristics as a rocket fuel of liquefied natural gas should be researched and confirmed in order to figure out if liquefied natural gas could be acceptable for a rocket fuel Rocket Thermal Analysis (hereinafter, RTA) was newly developed for rapid and more accurate prediction of wall temperautre in the cooling channels. Main characteristic of RTA was heat flux boundary condition on the gas-side chamber wall, contrary to the existing researches dealing mainly with experimental equations of convective heat transfer coefficient such as Bartz equation in the Western countries and Korea. Moreover, new concepts on periodical variation of OF mixture ratio near chamber wall and curtain cooling were suggested in order to improve the accuracy of RTA that was considerably related to thermal stability and prohibition of coking generation. Meanwhile, the reusability of the engines should be evaluated quantitatively in order to overcome the limited R&D budget and prevent the damage of the engines and test facility and injury of the test engineers caused by the rupture of the cooling channel. Therefore, the possible number of firing tests was researched numerically and experimentally from a viewpoint of low cycle fatigue regime. Also, inelastic structural analysis with FEM took much time to converge their results and a simple procedure for low cycle fatigue life prediction was requested practically. So, a simple procedure using Neubers correlation and Miners rule was newly proposed and the time to get low cycle fatigue life could be reduced greatly. Methane content in the liquefied natural gas was core factor to determine the characteristics as a rocket fuel. Minimal volumetric methane content was thought to be 90%, almost the same with the methane content among commercial liquefied natural gas, in order that liquefied natural gas could be used as a fuel for the regeneratively-cooled rocket engines. Cooling performance of liquefied natural gas was 20 % lower than pure methane and specific impulse difference between them was about 1%. Though maximum characteristic velocity of pure methane appeared at normalized OF mixture ratio of 0.75, that of liquefied natural gas was 0.72 theoretically and 0.75 experimentally. It was very difficult and impossible to measure gas and coolant-side wall temperature. So, the verification of RTA was carried out by the use of coolant temperature increase between inlet and outlet of the cooling channel. Bartz equation method predicted much higher coolant temperature increase than RTA and RTE. But, RTE and RTA calculated similar coolant temperature increase. The result that RTA had small discrepancy of about 4.2% from RTE proved the validity of RTA algorithm with rapid calculation time. Meanwhile, coolant temperature increase predicted by RTA as well as RTE was 40~64% higher than that of water-cooled firing tests. So, periodical variation model of OF mixture ratio near chamber wall was introduced newly to improve the accuracy of wall temperature prediction. The periodic variation model function was assumed to be a 3rd-order polynomial function in the minimum symmetric region and OF mixture ratio with this periodic variation model varied from 1 to 60% higher value than the design point. In conclusion, the difference of coolant temperature increase between RTA and firing test result could be reduced to 20% or so and more accurate temperature calculation was available. Low thrust rocket engines had much difficulty owing to absolutely small fuel mass. Therefore, OF mixture ratio could be reduced from 2.4 at the core region to 1.46 and then, combustion gas temperature was also lowered considerably to 34% by injection of more fuel less than 0% of total fuel in the exterior region, called curtain cooling region in this stud, while specific impulse was decreased by only 2.8%. Since lower OF mixture ratio in the curtain cooling region could lower wall temperature by more than hundreds K, the possibility of coking formation could be controlled with additional fuel mass in the exterior region and thermal stability of the combustion chamber was increased dramtically. After nonlinearity of material properties and geometrically large deformation were considered with the use of total effective strain from inelastic structural analysis with ABAQUS and low cycle fatigue test result of the material properties evaluation research, low cycle fatigue life of the cooling channel in this study was calculated to be 56~112 cycles to failure. As a low cycle fatigue life prediction method to figure out the possibility number of firing tests, a new simple procedure was proposed using stress and strain caused by thermal expansion, Neuber correlation, Coffin-Manson relation and Miner rule. In conclusion, low cycle fatigue life with a simple procedure was 94. This result could suggest the theoretical background of 36 to 48 times firing test implementation with the particular combustion chamber.