Active cooling in additively manufactured liquid rocket engines

Abstract

The heat fluxes in (liquid) rocket engines can go up to high values and they need active cooling to prevent the chamber wall from failing. Transpiration cooling is identified as a way to achieve lower wall temperatures than commonly used regenerative cooling and regenerative cooling with additional film cooling (referred to as film cooling from now on). This can lead to higher performance of the engine. However, transpiration cooled engines are not in use today and this is mainly attributed to problems with the required porous wall materials. Additive manufacturing (AM) is a promising solution for these material problems. Better cooling is especially useful for Inconel additively manufactured rocket engines as such engines experience higher wall temperatures due to the low thermal conductivity of the material.
This thesis has two purposes. Firstly, an analysis was performed to see if transpiration cooling actually performs better than regenerative and film cooling in total engine performance. Secondly, it was investigated if AM can be used to create the porous walls required for transpiration cooling.
To compare the cooling techniques, a simplified model for each cooling technique was developed. These models were verified and validated using data from literature. A new (transcritical) film cooling model was created by combining three existing film cooling models. The wall temperatures obtained from the three cooling methods were compared by applying them to a reference liquid rocket engine with either Inconel or copper as wall material. Subsequently, the losses in specific impulse and dry mass were determined. Then, a delta-v calculation for each cooling method was made to objectively compare them.
The conclusions on the comparison of the cooling techniques are that the regenerative cooled engine reaches wall temperatures above the material limits. Therefore, it is not a feasible to use this cooling method for the reference engine. Film and transpiration cooling can both achieve temperatures below the limit. Transpiration cooling requires less coolant than film cooling to achieve the same temperature. This will result in lower losses in specific impulse compared to film cooling. However, to achieve these low coolant mass flows, thick chamber walls are required to achieve the specified pressure drop over the wall. When comparing transpiration cooling to film cooling on the total delta-v achieved, it is found that the Inconel chambers perform better than the copper ones. However, it depends on the pore size if transpiration cooled engines outperform film cooled ones. A smaller pore size is better.
Additionally, it was found that the pore sizes producible with AM are an order of magnitude larger than required. Therefore, experiments were performed on the pressure drop over AM porous walls with different geometries. With these experiments, it was found that a new porous wall geometry made using AM techniques has a lower pressure drop than the geometry used in the calculations, being 1.32 lower. A geometry designed to achieve an as large as possible pressure drop increased the pressure drop 24.9 times. However, this geometry does not achieve a uniform coolant injection required for transpiration cooling, so further research is required.