Assessing Model Uncertainty in the Conceptual Design of a Monopropellant Propulsion System

An assessment of model uncertainty via probabilistic methods is described. An important question that arises in conceptual design is how accurate do models have to be to be useful? That is to say, when do other uncertainties in a higher fidelity model counteract its accuracy when compared to a lower fidelity model faced with these same uncertainties? A detailed definition of model uncertainty is provided. As an example, the model uncertainty in the conceptual design of a monopropellant blowdown hydrazine propulsion system is investigated. A simple model that estimates the mass and performance of the propulsion system is summarized. A description of a higher fidelity model follows. The two models are compared for three recent missions. For the propulsion examples investigated, the high-fidelity model provides a significant benefit over the simple model in estimating the propulsion system dry mass. However, for estimating the propellant mass required, the high-fidelity and simple models are comparable. The analysis discussed in this paper is an integral part of ongoing research into propagating and mitigating the effect of all types of uncertainty in the design and development of complex multidisciplinary engineering systems.