The use of system modelling tools for design and analysis of liquid rocket engines is necessary during the pre-design activities; their flexibility makes them a fundamental in strument along the entire design period from pre-design phase to parametric studies for fine tuning of the engine parameters. For this purpose these tools have to be reliable and fast, in order to perform iterative engine design loops and parametric studies in a reason able time. Moreover, they should be able to perform off-design predictions and detailed analyses. The Steady State library presented within this paper enables to realize these purposes. Its object oriented structure ensures maximum flexibility and allows for studying any liquid rocket engine thermodynamic cycle (open or closed). In this paper the tool is validated with the RL-10 engine and its capability will be demonstrated by designing several engine cycles. The entire library has been implemented within the existing analysis software Ecosim Pro, designed to model various kinds of dynamic systems. The standard propulsion library ESPSS can be used to study both stationary states and transients of a propulsion system. Unfortunately its use for steady state applications is not trivial because of the complexity of the transient models there implemented. Therefore, simplified pre-design and parametric studies are difficult and time-consuming. The library described in this paper has been designed specifically for steady state pur poses and can be considered as a completion of the ESPSS library, providing a helpful and fast tool for the pre-design phase and off-design analyses. To this aim, the available fluid properties and combustion modelling functions of ESPSS have been implemented in an adequate form into the new libraries. Additionally, fluid dynamic, combustion and heat transfer models have been developed to simulate the physical steady state behaviour of the main components of a propulsion system, as pipes, valves, turbines, pumps, orifices, combustion chamber and nozzle. These components are suited for both launcher and spacecraft applications. The main components feature a flag that enables the user to switch from "design" mode to the "off-design" analysis mode to enhance the flexibility of the library and perform different simulations with the same model. After the description of the main components of the Steady State library, their inte gration and validation will be presented through different examples of liquid rocket engine cycles design.
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