• orbital stages
• green propellants
• self-pressurisation
• tank emptying
• rocket performance

The objective of this work is to deliver a thorough evaluation of the most suitable technologies destined for the propulsion system of an Orbital Transfer Vehicle. The impact that such an emerging concept has on the space industry has been investigated, along with the main challenges associated with its propulsion system design.
The study focuses on chemical propulsion systems due to their short transfer times and suitability for orbital operations. Ground operations are highlighted as often overlooked but significant contributors to mission cost and complexity. Mitigating this impact is crucial, and the use of propellants that can simplify ground operations is advocated. In this regard, the drawbacks related to the toxicity of hydrazine-based compounds serve as a starting point for exploring innovative propellants with reduced hazards and risks in handling procedures.
Among such promising propellants, another physical property has been researched, known as self-pressurisation. A self-pressurising fuel or oxidizer provides itself with the required pressure for injection into the combustion chamber without the need for a heavy additional tank for pressurised gas, a turbo-pump, or other system devices such as pressure regulators and valves, which are renowned to be susceptible to failure. Therefore, self-pressurising propellants appear to offer a simplification of both ground operation and overall system design by reducing the number of system components and therefore the related complexity.
Throughout this study, propellants are initially evaluated by considering various physical characteristics, such as density, vapor pressure, chemical stability, and their compatibility with human interaction without the need for invasive protection devices. This latter requirement is hereafter referred to as ``green." The benefits of such a characteristic intend not only to minimise inherent risks during ground operations but also to reduce associated handling costs.
The comparison of propellants extends to their rocket performance, with a focus on specific impulse, volumetric specific impulse, and combustion temperature. This examination provides a comprehensive view of their efficacy in propulsion systems.
Finally, the research delves into the current state-of-the-art modelling techniques specifically tailored for self-pressurizing propellants. It particularly outlines the commonly used models to describe the dynamics of tank draining for saturated fluids and their associated mass flow rates. On the basis of such models, the time history of thermodynamic properties within the tank along with rocket performance has been computed over the burning time. The primary objective is to evaluate the optimal configuration for the propulsion system. Emphasis has been placed on the challenges of modelling a cavitating flow through the feedline and on the impact that the extracted propellant phase has on the engine thrust performance.