Problem Statement
Liquid rocket engines based on gas-generator cycles represent a simple and reliable solution, but they sacrifice performance by discarding part of the propellants and may suffer from operational limitations related to combustion by-products. The SpaceX Vacuum Merlin engine, operating on an RP-1/LOX open cycle, is a clear example, as its fuel-rich gas generator produces soot that prevents cycle closure and limits overall efficiency. The increasing interest in methane motivates the investigation of alternative architectures. This work carried out at Novaeka Company based in Padova, investigates a theoretical redesign of the Vacuum Merlin engine by replacing RP-1 with LCH4 and adopting a fuel rich staged combustion closed cycle. A previous steady-state analysis demonstrated the thermodynamic feasibility. The present work extends the assessment to a full transient analysis, with particular focus on engine start-up. A detailed model was developed in EcosimPro, and used to simulate ignition in vacuum conditions, analyse valve sequencing, turbopump spin-up, and evaluate overall system stability. The model was further extended including the chill-down phase and a conceptual test bench for the sea level version, allowing the investigation of a realistic test sequence and its impact during hot-fire operation. In addition, a Design of Experiments approach was applied to quantify the sensitivity of engine performance and health parameters to propellant inlet temperatures and turbomachinery efficiency variations.
Overall, the results show that stable operating conditions can be achieved through a controlled ignition sequence, consistent with steady-state predictions. The integrated test bench simulations highlight the practical difficulty of maintaining ideal cryogenic conditions at engine inlet, also indicating that engine performance is primarily driven by oxidizer inlet temperature and turbine efficiency.
However, the achievable performance gains with respect to the original Merlin architecture remain limited, while system complexity, engine length and volume increase. Although technically feasible, the redesigned Merlin offers no decisive advantage over the existing architecture, providing insight into why such a solution has not been adopted in any operational launch systems.