Optimal Liquid Engine Architecture by Performance-Cooling Tradeoff Analysis

Abstract

We perform physics-based theoretical analyses on the tradeoff between the overall performance of specific impulse and the thermal protection ability of a coolant liquid film to identify the optimal configuration of a liquid rocket engine. A bipropellant thruster is set as the target, which uses a hypergolic propellant mixture of nitrogen tetroxide as the oxidant and monomethyl hydrazine as the fuel. By considering the practical nonuniform distribution of the local mixture ratio produced in the thrust chamber, the maximum specific impulse is achieved when the fuel and oxidizer spray widths become identical, allowing for the specification of the diameter of the impinging-type injector. The heat balance between the convective heat transfer from the combustion gas and the latent heat of the three-dimensionally wavy liquid film provides the optimal diameter of the combustion chamber. The longest liquid film is found to be achieved when half of the initial film is entrained by the fast gas, correspondingly mitigating the heat transfer area due to the liquid film waviness. We successfully demonstrate the optimal architecture of the liquid engine based on the tradeoff, in which an improvement of specific impulse by 1 s shortens the film length by 2 mm.