Analysis and optimization of chill-down processes in cryogenic liquid propellant rocket engines

For Liquid Rocket Engines which employs cryogenic fluids, chill-down process represents a fundamental phase before turbopumps operation. The propellant, before being sent to the combustion chamber, is employed to perform cooling function of both fuel and oxidizer fluidic transfer lines. In this way, pumps cavitation and correct ignition during engine operation are ensured. This thesis, carried out at the Avio Company based in Colleferro, has the main objective of analysing and optimizing chill-down processes in cryogenic LREs, using EcosimPro software. The thermal conditioning process of cryogenic propellant transfer lines is essential due to the high temperature difference existing between the cryogenic liquid in the storage tanks and the wall of ducts at the ambient temperature. The chill-down process requires an amount of propellant which will cool the solid walls of the pipes, bringing them to the same temperature of the liquid fluid and establishing a continuous cryogenic propellant flow in liquid phase. Important parameters of this process are chill-down time, defined as the time required to complete the desired cooling of the feed lines, and propellant consumption, which must be minimized because unusable for propulsive purposes. Following the exposition in chapter 1 of the general principles on cryogenic LREs, in ch. 2 the generalities on the chill-down process are described and the behaviour of the cryogenic fluid in the feed lines is explained. Moreover, the influence of the main parameters, such as the inlet driving pressure and the gravity acceleration, as well as the various possible methodologies suitable to successfully complete chill-down process, are detailed. To pursue the main objective of this thesis, a preliminary analysis of several chill-down techniques, going deeper in each strategy advantages and drawbacks understanding, is performed, and subsequently analyzed with the EcosimPro software. For this reason, the goal of ch. 3 is to realize an initial sorting of different chill-down methods to evaluate a preliminary trade-off, as compromise between low propellant consumption and low cooling time. In ch. 4, the focus is to reproduce, simulate and analyse various experimental test cases on chill-down process, available in literature, to validate EcosimPro models and to identify software weakness points in the simulation of this kind of phenomena. This activity leads to the conclusion that the software is not able to correctly simulate the chill-down, therefore appropriate margins shall be considered in the results employing standard components of ESPSS EcosimPro Library. In ch. 5, the heart of the experimental work of this thesis is the analysis and optimization of the open loop full flush flow chill-down technique for the cryogenic oxidant feed line, for application in the framework of VEGA-E program. The aim is to ensure, assigned the boundary conditions, the cooling of the lines within a certain target time, with the lowest propellant consumption. Eventually, in ch. 6, the design of a closed-loop chill-down method follows for the recovery of the propellant used in the lines cooling, which otherwise would be expelled into the environment and therefore lost. It is an innovative design, not yet used in present main launchers, for reasons related to the costs, complexity, and feasibility of the process. However, this is a topic that will largely find its way into future developments, since launcher mass budget reduction leads to a further increase in payload mass.