Reliability estimation methodology for Liquid Rocket Engines
Creators: name:  L. AYALA FERNANDEZ 
name:   V. BRAUN 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: SPACE
REUSABILITY
REUSABLE LAUNCH VEHICLE

description: The launch rate has steeply increased in the last few years. However, the launch success rate in 2020 was the lowest of the last 15 years. Launch failures do not only come at a great economic cost, but also at a large waste of resources on Earth. Moreover, failed upper stages can remain in orbit for long periods of time, adding up to the tones of space debris orbiting Earth and threatening future space missions. Consequently, ensuring the reliability of space launch vehicles is essential in order to protect both Earth and space environment, in addition to the missions being launched. Current methods used to assess the reliability of launch vehicles, such as Failure Modes, Effects and Criticality Analysis (FMECA), Fault Tree Analysis (FTA), Reliability Block Diagram (RBD) and Event Tree Analysis (ETA), are very time consuming and strongly rely on the expertise of the team performing the assessment. Even though these assessments are necessary, and they must always be performed before the system becomes operational, they are not practical in early stages of the design, when the design is still flexible and several system architectures might be available. However, the launcher design process can be greatly benefitted by early reliability assessments, as changes in the design are easier to make in these stages with a lower associated cost. The goal of this work is therefore to develop a simplified methodology that can be used for early reliability estimation, which would allow space launch vehicle designers to compare different system architectures and make trade-offs based on the reliability of different design options. The model is not intended to be more accurate than traditional methods. Indeed, it should be highlighted that the methods mentioned before are still required in later stages. However, the current work intends to provide a quicker and easier to use alternative, allowing reliability-based decisions in early stages of the design. Historical data has shown that more than half of launch vehicle failures originate in the propulsion subsystem, more specifically in Liquid Rocket Engines (LREs). Therefore, the modelling efforts have been focused on this subsystem. Previous studies have identified key drivers of the reliability of LREs. In this work, empirical and statistical relationships were used to build a model that estimates the reliability of a LRE as a function of these key drivers. The reliability drivers used include the engine cycle, the propellant used, the nominal thrust of the engine, the engine burn time, and the number of engines among others. These parameters are also key on the design of the propulsion system, so they are expected to be known already in the early design stages in which the model would be used. A total of 10 parameters have been included in the model. Finally, in order to show the usefulness of this reliability model, a benchmark mission has been chosen and several design options have been evaluated. It is therefore shown how the reliability assessment performed with the proposed reliability model can be used to perform trade-offs in the design.